HomeMy WebLinkAboutCity and County of Honolulu - Storm Water BMP Guide_2017Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
1
STORM WATER BMP GUIDE
FOR NEW AND REDEVELOPMENT
for the City and County of Honolulu
Permit No. HI S000002
Prepared by
City and County of Honolulu,
Department of Planning and Permitting
July 2017
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Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
i
Table of Contents
TABLE OF CONTENTS .............................................................................................................................i
LIST OF TABLES .......................................................................................................................................v
LIST OF FIGURES ..................................................................................................................................vii
LIST OF APPENDICES ............................................................................................................................ix
LIST OF ACRONYMS AND ABBREVIATIONS ..................................................................................xi
GLOSSARY..............................................................................................................................................xiii
ACKNOWLEDGEMENTS .....................................................................................................................xix
1. INTRODUCTION .....................................................................................................................1-1
1.1. Purpose and Scope ...........................................................................................................1-1
1.2. Users of the Manual .........................................................................................................1-1
1.3. Organization of the Manual .............................................................................................1-2
1.4. Low Impact Development and Storm Water Quality ......................................................1-2
1.5. Regulatory Requirements ................................................................................................1-6
1.5.1. Federal Programs ...............................................................................................1-6
1.5.2. State Programs ...................................................................................................1-6
1.5.3. Other Relevant Regulatory Programs ................................................................1-7
1.5.4. Total Maximum Daily Loads .............................................................................1-7
1.5.5. Endangered Species Act .....................................................................................1-7
1.5.6. Clean Water Act Section 404 Dredge and Fill Permits ......................................1-7
1.5.7. Clean Water Act Section 401 Water Quality Certification .................................1-8
2. APPLICATION OF PERMANENT BMP REQUIREMENTS IN THE CITY AND
COUNTY OF HONOLULU ......................................................................................................2-1
2.1. City and County of Honolulu Plan Review Requirements ..............................................2-3
2.1.1. Storm Water Quality Strategic Plans .................................................................2-3
2.1.2. Storm Water Quality Reports .............................................................................2-3
2.1.3. Storm Water Quality Checklists .........................................................................2-4
2.1.4. Operations and Maintenance Plan .....................................................................2-5
2.1.5. BMP Certification and Recording ......................................................................2-5
2.1.6. Variances ............................................................................................................2-6
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3. STORM WATER QUALITY PLANNING FOR NEW DEVELOPMENT AND
REDEVELOPMENT .................................................................................................................3-1
3.1. Assess Site Conditions .....................................................................................................3-2
3.1.1. Understand Hydrologic Conditions of Concern .................................................3-2
3.2. Site Design Strategies ......................................................................................................3-4
3.2.1. Conserve Natural Areas, Soils, and Vegetation .................................................3-4
3.2.2. Minimize Disturbances to Natural Drainages ....................................................3-4
3.2.3. Minimize Soil Compaction ..................................................................................3-5
3.2.4. Direct Runoff to Landscape Areas and Reduce Directly Connected
Impervious Areas ................................................................................................3-5
3.2.5. Minimize Impervious Surfaces ............................................................................3-6
3.3. Control Sources of Pollutants ........................................................................................3-10
3.4. Treat Runoff ...................................................................................................................3-12
4. SITE AND FACILITY DESIGN FOR WATER QUALITY PROTECTION ......................4-1
4.1. Infiltration ........................................................................................................................4-1
4.2. Retention and Detention ..................................................................................................4-3
4.3. Biofilters ..........................................................................................................................4-4
4.4. Street Design ....................................................................................................................4-5
4.5. Street Trees ......................................................................................................................4-7
4.6. Parking Lots .....................................................................................................................4-7
4.6.1. Hybrid Parking Lots ...........................................................................................4-7
4.6.2. Parking Grove ....................................................................................................4-9
4.6.3. Overflow Parking ...............................................................................................4-9
4.6.4. Porous Pavement with Subsurface Infiltration .................................................4-10
4.7. Driveways ......................................................................................................................4-10
4.8. Landscape and Open Space ...........................................................................................4-12
4.9. Multiple Small Basins ....................................................................................................4-12
4.10. Planning Development Strategies in Practice ................................................................4-13
5. TREATMENT CONTROL BMP DESIGN .............................................................................5-1
5.1. Best Management Practices Selection .............................................................................5-1
5.1.1. Determine Drainage Management Areas ...........................................................5-1
5.1.2. Evaluate Pollutants of Concern ..........................................................................5-2
5.1.3. Identify Candidate BMPs ....................................................................................5-2
5.1.4. Consider Operation and Maintenance Requirements ........................................5-5
5.1.5. Other Considerations .........................................................................................5-7
5.2. Numeric Sizing Criteria ...................................................................................................5-7
5.2.1. Volume-Based Best Management Practice Design ............................................5-7
5.2.2. Flow-Based BMP Design ...................................................................................5-8
5.2.3. Combined Volume-Based and Flow-Based BMP Design .................................5-10
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5.3. Infiltration Requirements ...............................................................................................5-10
5.3.1. Soil Types and Textures ....................................................................................5-10
5.3.2. Field Investigations ..........................................................................................5-12
5.3.3. Design Infiltration Rates ..................................................................................5-13
5.4. Technology Certification ...............................................................................................5-13
5.5. Feasibility Criteria .........................................................................................................5-14
5.5.1. Infiltration Feasibility .......................................................................................5-14
5.5.2. Harvest/Reuse Feasibility .................................................................................5-14
5.5.3. Biofiltration Feasibility ....................................................................................5-15
6. CONSIDERATIONS FOR CONSTRUCTION TREATMENT CONTROL BMPS ...........6-1
7. OPERATIONS AND MAINTENANCE OF BMPS ................................................................7-1
7.1. Consideration when Selecting Treatment BMPs .............................................................7-1
7.1.1. Sediment and Oil Removal and Disposal ...........................................................7-1
7.1.2. Maintenance Costs .............................................................................................7-1
7.2. Developing the O&M Plan ..............................................................................................7-2
7.3. Maintenance Agreements, Certification, and Modifications ...........................................7-2
7.4. Inspection Requirements .................................................................................................7-3
7.5. Minimum Maintenance Requirements ............................................................................7-3
8. REFERENCES ............................................................................................................................8-1
Table of Contents
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List of Tables
Table 1.1: Pollutants and Impacts on Water Quality .................................................................................1-5
Table 2.1: Summary of Post-Construction Requirements for New and Redevelopment ..........................2-3
Table 3.1: Estimated C-Factor for Various Surfaces during Small Storms ...............................................3-8
Table 3.2: Conventional Paving Surface Small Storm C-Factor versus Alternative Paving C-Factors ..3-10
Table 4.1: Comparison of Three Alternatives ..........................................................................................4-16
Table 5.1: Typical Pollutants Associated with Priority Projects ...............................................................5-3
Table 5.2: Treatment Control BMP Categories .........................................................................................5-3
Table 5.3: Treatment Control BMP Expected Pollutant Removals ...........................................................5-4
Table 5.4: Recommended Selection of Permanent BMPs .........................................................................5-5
Table 5.5: Summary of Operation and Maintenance Effort for BMPs ......................................................5-5
Table 5.6: Maintenance Rank Definition ...................................................................................................5-6
Table 5.7: Runoff Coefficients for Water Quality Flow Calculations .......................................................5-9
Table 5.8: Typical Soil Infiltration Ratesa ...............................................................................................5-11
Table 5.9: Test Pit/Boring Requirements for Infiltration .........................................................................5-13
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List of Figures
Figure 1.1: Document Organization ..........................................................................................................1-3
Figure 1.2: Impacts of Urbanization ..........................................................................................................1-4
Figure 2.1: Implementation of the CCH Water Quality Rules ..................................................................2-2
Figure 3.1: Project Life Cycle ....................................................................................................................3-1
Figure 3.2: Hydraulic Altercation after Certain BMPs are Implemented ..................................................3-3
Figure 3.3: Directly Connected Impervious Area versus Directing Runoff to Landscaped Areas ............3-5
Figure 3.4: Parking Lot Directing Runoff to Landscape Areas .................................................................3-6
Figure 3.5: Use of Self-Mitigating Areas ..................................................................................................3-9
Figure 3.6: Self-Mitigating Areas Treatment Volume ...............................................................................3-9
Figure 3.7: Example of Source Control Design for Outdoor Material Storage .......................................3-11
Figure 4.1: Infiltration Basin ......................................................................................................................4-1
Figure 4.2: Typical Infiltration Facility Schematic ....................................................................................4-2
Figure 4.3: Typical Bioretention Schematic ..............................................................................................4-2
Figure 4.4: Simple Detention System ........................................................................................................4-3
Figure 4.5: Retention System .....................................................................................................................4-3
Figure 4.6: Vegetated Swale ......................................................................................................................4-4
Figure 4.7: Schematic of Planter Box with Down Spout Connection and Underdrain .............................4-5
Figure 4.8: Comparison of Street Cross-Section (Two-Way, Residential Access Streets) .......................4-6
Figure 4.9: Hybrid Parking Lot ..................................................................................................................4-8
Figure 4.10: Hybrid Parking Lot (Honolulu, HI) .......................................................................................4-8
Figure 4.11: Turf Blocks ............................................................................................................................4-8
Figure 4.12: Permeable Joints ....................................................................................................................4-8
Figure 4.13: Parking Grove 1 .....................................................................................................................4-9
Figure 4.14: Parking Grove 2 .....................................................................................................................4-9
Figure 4.15: Overflow Parking ..................................................................................................................4-9
Figure 4.16: Subsurface Infiltration System ............................................................................................4-10
Figure 4.17: Unit Pavers ..........................................................................................................................4-11
Figure 4.18: Crushed Aggregate ..............................................................................................................4-11
Figure 4.19: Paving only under Wheels ...................................................................................................4-11
Figure 4.20: Alternative 1 - Conventional ...............................................................................................4-14
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Figure 4.21: Alternative 2 - Hybrid/Best Practices ..................................................................................4-15
Figure 4.22: Alternative 3 - Neo-Traditional ...........................................................................................4-15
Figure 5.1: DMA Delineation ....................................................................................................................5-1
Figure 5.2: Unit Water Quality Volume for 1 and 1.5 inch Runoff Depth ................................................5-8
Figure 5.3: Unit Water Quality Flow .......................................................................................................5-10
Figure 5.4: USDA Soils Textural Triangle ..............................................................................................5-11
List of Figures
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List of Appendices
APPENDIX A: SOURCE CONTROL BMP FACT SHEETS ...........................................................A-1
SC-01: Landscaped Areas ............................................................................................................A-3
SC-02: Roof Runoff Controls ......................................................................................................A-7
SC-03: Automatic Irrigation System .........................................................................................A-11
SC-04: Storm Drain Inlet ...........................................................................................................A-15
SC-05: Alternative Building Materials ......................................................................................A-19
SC-06: Vehicle/Equipment Fueling ...........................................................................................A-23
SC-07: Vehicle/Equipment Repair ............................................................................................A-27
SC-08: Vehicle/Equipment Cleaning .........................................................................................A-29
SC-09: Loading Dock ................................................................................................................A-33
SC-10: Outdoor Trash Storage ..................................................................................................A-37
SC-11: Outdoor Material Storage ..............................................................................................A-41
SC-12: Outdoor Work Areas .....................................................................................................A-45
SC-13: Outdoor Process Equipment Operations .......................................................................A-49
SC-14: Parking Areas ................................................................................................................A-53
APPENDIX B: TREATMENT CONTROL BMP FACT SHEETS....................................................B-1
TC-01: Infiltration Basin ..............................................................................................................B-3
TC-02: Infiltration Trench ............................................................................................................B-9
TC-03: Subsurface Infiltration ....................................................................................................B-15
TC-04: Dry Well .........................................................................................................................B-17
TC-05: Bioretention Basin ..........................................................................................................B-21
TC-06: Permeable Pavement ......................................................................................................B-27
TC-07: Green Roof .....................................................................................................................B-33
TC-08: Vegetated Bio-Filter .......................................................................................................B-37
TC-09: Enhanced Swale .............................................................................................................B-43
TC-10: Vegetated Swale .............................................................................................................B-49
TC-11: Vegetated Buffer Strip ...................................................................................................B-55
TC-12: Harvest/Reuse .................................................................................................................B-59
TC-13: Detention Basin ..............................................................................................................B-67
TC-14: Manufactured Treatment Device ....................................................................................B-73
TC-15: Sand Filter ......................................................................................................................B-75
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APPENDIX C: O&M FACT SHEETS .................................................................................................C-1
OM-01: Bio-Retention Basin ........................................................................................................C-3
OM-02: Detention Basin ...............................................................................................................C-5
OM-03: Green Roof ......................................................................................................................C-7
OM-04: Infiltration Trench/Basin .................................................................................................C-9
OM-05: Manufactured Treatment Device ..................................................................................C-11
OM-06: Pervious Pavement ........................................................................................................C-13
OM-07: Rainwater Harvesting ....................................................................................................C-15
OM-08: Sand Filter .....................................................................................................................C-17
OM-09: Vegetated Biofilter ........................................................................................................C-19
OM-10: Vegetated Swale/Strip ...................................................................................................C-21
List of Appendices
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List of Acronyms and Abbreviations
§Section
%Percent
ACQ Ammoniacal Copper
Quarentary
BMP Best Management Practice
CASQA California Stormwater Quality
Association
CCA Chromated Copper Arsenate
City City and County of Honolulu
(or CCH)
COE See USACE
CORP See USACE
cu-ft Cubic Feet (or feet3)
CWA Clean Water Act
CWB Clean Water Branch,
Department of Health, State of
Hawaii
CWPPP Certified Water Pollution Plan
Preparer
CZMA Coastal Zone Management
Act
DCIA Directly Connected
Impervious Area
DFM Department of Facility
Maintenance, City and County
of Honolulu
DLNR State of Hawaii, Department
of Land and Natural
Resources
DMA Drainage Management Area
DOFAW Division of Forestry and
Wildlife, Department of Land
and Natural Resources, State
of Hawaii
DOH Department of Health, State of
Hawaii
DPP Department of Planning and
Permitting, City and County
of Honolulu
ft foot (or feet)
HAR Hawaii Administrative Rules
HDPE High-Density Polyethylene
hr Hour
HSG Hydrologic Soil Group
in Inch (or inches)
IPM Integrated Pest Management
LID Low Impact Development
MEP Maximum Extent Practicable
min Minute
mph Miles per Hour
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List of Acronyms and Abbreviations
MS4 Municipal Separate Storm
Sewer System
NJCAT New Jersey Department of
Environmental Protection,
New Jersey Corporation for
Advanced Technology
NJDEP New Jersey Department of
Environmental Protection
Protocols
NOC Notice of Cessation
NOI Notice of Intent
NPDES National Pollutant Discharge
Elimination System
O&M Operations and Maintenance
PCB Polychlorinated Biophenyls
POC Pollutants of Concern
RV Recreational Vehicle
SC Source Control
sec second (or seconds)
SPCC Spill Prevention Control and
Countermeasure
sq-ft Square Feet (of feet2)
SSBMP Site-Specific Best
Management Practice Plan
State State of Hawaii
Strategic
Plan
Storm Water Quality Strategic
Plan
SUSMP Standard Urban Stormwater
Mitigation Plan
SWMPP Storm Water Management
Program Plan
SWPCP Storm Water Pollution Control
Plan
SWQ Storm Water Quality Branch,
Department of Facility
Maintenance, City and County
of Honolulu
SWQR Storm Water Quality Report
TAPE Washington State Department
of Ecology, Technology
Assessment Protocol -
Ecology
TMDL Total Maximum Daily Load
TSS Total Suspended Solids
UIC Underground Injection
Control
USACE United States Army
Corporation of Engineers (also
CORP or COE)
USDA United States Department of
Agriculture
USEPA United States Environmental
Protection Agency
USFWS United States Fish & Wildlife
Services
Water
Quality
Rules
City and County of Honolulu,
Rules Relating to Water
Quality (or WQR)
WQF Water Quality Flow
WQMP Water Quality Management
Plan
WQV Water Quality Volume
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Glossary
As used in this document, the following definitions shall apply unless the context indicates otherwise:
303(d) Listed: Water bodies listed as impaired as per Section 303(d) of the 1972 Clean Water Act.
Best Management Practices or BMPs: means schedules of activities, prohibitions of practices,
maintenance procedures, management practices, treatments, and temporary or permanent Structures or
devices that are intended and designed to eliminate and Minimize the discharge of pollutants, directly or
indirectly, to receiving waters, to the maximum extent practicable.
Biofiltration: A pollution control technique that uses living material to capture, and absorb or biologically
degrade pollutants.
Catch Basin (Also known as Inlet): Box-like underground concrete structure with openings in curbs and
gutters designed to collect runoff from streets and pavement.
Check Dams: Small temporary dams constructed across a swale or drainage ditch. Check dams reduce
the velocity of concentrated storm water flows, thereby reducing erosion of the swale or ditch. The dams
also decrease water velocity to increase sediment capture.
Clean Water Act (CWA): (33 U.S.C. 1251 et seq.) Requirements of the NPDES program are defined
under Sections 307, 402, 318 and 405 of the CWA.
Construction Activity: Includes clearing, grading, excavation, and contractor activities that result
in soil disturbance. Construction activities are regulated by the NPDES General Permit Coverage
Hawaii Administrative Rules (HAR) Chapter 11-55 Water Pollution Control, Appendix C-Storm Water
Associated with Construction Activities, effective October 22, 2007.
Denuded: Land stripped of vegetation or land that has had its vegetation worn down due to the impacts
from the elements or humans.
Detention: The capture and subsequent release of storm water runoff from the site at a slower rate than it
is collected, the difference being held in temporary storage.
Development: The sum of any and all actions that are undertaken to alter the natural or existing condition
of real property or improvements on real property if a building, electric, grading, grubbing, plumbing,
stockpiling or trenching permit is required for the Project. Development also includes Redevelopment and
changes in land use that may result in different or increased Pollutant discharges to the MS4 or Receiving
Waters. Development does not include work that does not involve any Land Disturbing Activity, the
installation of signs and traffic control devices, the construction of individual bus shelters, the installation
of temporary BMPs, emergency work necessary to repair surfaces that are in immediate need of
stabilization, the marking of improved surfaces with striping or signage, and minor and ordinary repairs to
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Glossary
existing improvements in the City's right of way, provided that the work will not increase the impervious
surface area of the Project Site or involve replacing 50 percent or more of the on-Site impervious surfaces
area.
Discharge: A release or flow of storm water or other substance from a conveyance system or storage
container. Broader discharges includes release to storm drains, etc.
Disturbed Area: Any and all portions of Project Site affected by Land Disturbing Activities. Disturbed
Areas include, but are not limited to, soils and surface areas affected by excavation, areas that are graded,
grubbed, or clearing by uprooting vegetation, areas affected by the demolition of foundations, areas used
for equipment staging, materials, or staging, and areas affected by heavy pedestrian or vehicular traffic
that disrupts ground covers or surface soil conditions.
Effluent Limits: Limitations on amounts of pollutants that may be contained in a discharge. Can be
expressed in a number of ways including as a concentration, as a concentration over a time period
(i.e., 30-day average must be less than 20 milligram/liter), or as a total mass per time unit, or as a
narrative limit.
Erosion: The wearing-away of land surface by wind or water. Erosion occurs naturally from weather
or runoff but can be intensified by land-clearing practices related to farming, new development,
redevelopment, road building, or timber cutting.
Evapotranspiration: The combined loss of water into the atmosphere by evaporation (water changing
from a liquid to a vapor from soil, water, or plant surfaces) and transpiration (water that is taken up by
plant roots and transpired through plant tissue and leaves).
Facility: Is a collection of industrial processes discharging storm water associated with industrial activity
within the property boundary or operational unit.
Flood or Flooding: The inundation to a depth of three inches or more of any property not ordinarily
covered by water. The terms do not apply to inundation caused by tsunami wave action.
Grading: Any excavation or fill, or combination thereof.
Hazardous Waste: A waste or combination of wastes that, because of its quantity, concentration, or
physical, chemical or infectious characteristics, may either cause or significantly contribute to an increase
in mortality or an increase in serious irreversible illness; or pose a substantial present or potential
hazard to human health or the environment when improperly treated, stored, transported, disposed of or
otherwise managed. Possesses at least one of four characteristics (ignitability, corrosivity, reactivity, or
toxicity) or appears on special USEPA or state lists. Regulated under the federal Resource Conservation
and Recovery Act and the California Health and Safety Code.
Illicit Discharges: Any discharge to a municipal separate storm sewer that is not in compliance with
applicable laws and regulations as discussed in this document.
Impervious Surface: A surface covering or pavement of a developed parcel of land that prevents the
land’s natural ability to absorb and infiltrate rainfall/storm water. Impervious surfaces include, but are
not limited to, rooftops; walkways; patios; driveways; parking lots; storage areas; impervious concrete
and asphalt; and any other continuous watertight pavement or covering. Landscaped soil and pervious
pavement, underlain with pervious soil or pervious storage material, are not impervious surfaces.
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Glossary
Industrial General Permit: A National Pollutant Discharge Elimination System (NPDES) Permit issued
by the State of Hawaii Department of Health Clean Water Branch for discharge of storm water associated
with industrial activity.
Industrial Park: A land development that is set aside for industrial development. Industrial parks are
usually located close to transport facilities, especially where more than one transport modalities coincide:
highways, railroads, airports, and navigable rivers. It includes office parks, which have offices and light
industry.
Infiltration: Practices which capture and temporarily store a design storm volume of water before
allowing it to infiltrate into the soil.
Inlet: An entrance into a ditch, storm drain, or other waterway.
Integrated Pest Management (IPM): An ecosystem-based strategy that focuses on long-term
prevention of pests or its damage through a combination of techniques such as: biological control, habitat
manipulation, modification of cultural practices, and use of resistant varieties. Pesticides are used only
after monitoring indicates they are needed according to established guidelines, and treatments are made
with the goal of removing only the target organism.
Land Disturbing Activity or Land Disturbance: Any action, activity, or land use that alters the
integrity, structure, texture, density, permeability, contents, or stress conditions of soil or ground surfaces
if a building, electric, grading, grubbing, plumbing, stockpiling or trenching permit is required for
the Project. Land disturbing activities include, but are not limited to actions that result in the turning,
penetration, or moving of soil, the resurfacing of pavement that involves the exposure of the base course
or subsurface soils, and the use of portions of a Project Site as staging areas or base yards.
Low Impact Development or “LID”: Systems and practices that use or mimic natural processes that
result in the infiltration, evapotranspiration or use storm water in order to protect water quality and the
aquatic habitat. At both site and regional scales, LID aims to preserve, restore, and create green space
using soils, vegetation, and rain harvest techniques.
Maximum Extent Practicable or MEP: Economically achievable measures for the control of the
addition of pollutants from existing and new categories of nonpoint sources of pollution, which reflect the
greatest degree of pollutant reduction achievable through the application of the best available nonpoint
source pollution control practices, technologies, processes, siting criteria, operating methods or other
alternatives.
Municipal Separate Storm Sewer System (MS4): The City’s drainage infrastructure that is designed or
intended to collect and convey storm water and includes, but is not limited to, City roads with drainage
improvements, City streets, catch basins, curbs, gutters, ditches, man-made channels, and storm drains.
National Pollutant Discharge Elimination System Permit or “NPDES Permit”: The permit issued to
the City pursuant to Title 40, Code of Federal Regulations, Part 122, Subpart B, Section 122.26(a) (1)
(iii), for storm water discharge from the City’s separate storm sewer systems; or the permit issued to a
person or property owner for a storm water discharge associated with industrial activity pursuant to Title
40, Code of Federal Regulations, Part 122, Subpart B, Section 122.26(a) (1) (ii), or other applicable
section of Part 122; or the permit issued to a person or property owner for the discharge of any pollutant
from a point source into the state waters through the City's separate storm sewer system pursuant to
Hawaii Administrative Rules, Chapter 11-55, “Water Pollution Control.”
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New Development: Land disturbing activities; structural development, including construction or
installation of a building or structure, the creation of impervious surfaces; and land subdivision.
Non-Storm Water Discharge: Any discharge to municipal separate storm sewer that is not composed
entirely of storm water.
Nonpoint Source Pollution: Pollution that does not come from a point source. Nonpoint source pollution
originates from aerial diffuse sources that are mostly related to land use.
Notice of Intent (NOI): A formal notice to the State of Hawaii Clean Water Branch submitted by the
owner of an industrial site or construction site that said owner seeks coverage under a General Permit
for discharges associated with industrial and construction activities. The NOI provides information on
the owner, location, type of project, and certifies that the owner will comply with the conditions of the
construction General Permit.
Notice of Cessation (NOC): Formal notice to the State of Hawaii Clean Water Branch submitted by
owner/developer that a construction project is complete.
Outfall: The end point where storm drains discharge water into a waterway.
Point Source: Any discernible, confined, and discrete conveyance from which pollutants are or may be
discharged. This term does not include return flows from irrigated agriculture or agricultural storm water
runoff.
Pollutant: Any dredge, spoil, solid refuse, incinerator residue, sewage, garbage, sewage sludge,
munitions, chemical waste, biological materials, radioactive materials, heat, wrecked or dismantled
equipment, rock, sand, soil, sediment, dirt, industrial, municipal, or agricultural waste and substances of
similar nature.
Pollution Prevention: Practices and actions that reduce or eliminate the generation of pollutants.
Precipitation: Any form of rain.
Pretreatment: Treatment of waste stream before it is discharged to a collection system.
Reclaim (Water Reclamation): Planned use of treated effluent that would otherwise be discharged
without being put to direct use.
Redevelopment: The creation, addition, and/or replacement of impervious surface on improved real
property. Redevelopment does not include trenching and resurfacing associated with utility work,
resurfacing and reconfiguring existing impervious surfaces, the repair of sidewalks or pedestrian ramps,
pothole repair, ordinary road maintenance, or the marking of vehicular or pedestrian lanes on existing
roads.
Retail Mall: One or more buildings that house or form a complex of retail stores with interconnecting
walkways. Retail and Commercial malls include, but are not limited to, mini-malls, strip malls, retail
complexes, and enclosed shopping malls or shopping centers.
Retention: The storage of storm water to prevent it from leaving the development site.
Reuse (Water Reuse): (see Reclaim)
Glossary
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Runoff: Water originating from rainfall and other sources (i.e., sprinkler irrigation) that flows over the
land surface to drainage facilities, rivers, streams, springs, seeps, ponds, lakes, and wetlands.
Run-on: Offsite storm water surface flow or other surface flow which enters your site.
Scour: The erosive and digging action in a watercourse caused by flowing water.
Secondary Containment: Structures, usually dikes or berms, surrounding tanks or other storage
containers, designed to catch spilled materials from the storage containers.
Sedimentation: The process of depositing soil particles, clays, sands, or other sediments that were picked
up by runoff.
Sediments: Soil, sand, and minerals washed from land into water, usually after rain, that collect in
reservoirs, rivers, and harbors, destroying fish nesting areas and clouding the water, thus preventing
sunlight from reaching aquatic plants. Farming, mining, and building activities without proper
implementation of BMPs will expose sediment materials, allowing them to be washed off the land after
rainfalls.
Self-Mitigating Area: A natural or landscaped area, including green roofs, which retains and/or treats
rainfall within its perimeter without accepting runoff from other areas. Self-Mitigating Areas must retain
all collected storm water or drain directly to the MS4.
Significant Materials: Includes, but not limited to, raw materials; fuels; materials such as solvents,
detergents, and plastic pellets; finished materials such as metallic products; raw materials used in food
processing or production; hazardous substances designed under Section 10 1(14) of CERLCA; any
chemical the facility is required to report pursuant to Section 313 of Title III of SARA; fertilizers;
pesticides; and waste products such as ashes, slag, and sludge that have the potential to be released with
storm water discharges.
Significant Quantities: The volume, concentrations, or mass of a pollutant in storm water discharge that
can cause or threaten to cause pollution, contamination, or nuisance that adversely impact human health
or the environment and cause or contribute to a violation of any applicable water quality standards for
receiving water.
Site Design Strategies: LID design techniques that are intended to maintain or restore the site’s
hydrologic and hydraulic functions with the intent of minimizing runoff volume and preserving existing
flow paths.
Source Control BMPs: Low-technology practices designed to prevent pollutants from contacting storm
water runoff or to prevent discharge of contaminated runoff to the storm drainage system.
Source Reduction (also Source Control): The technique of stopping and/or reducing pollutants at their
point of generation so that they do not come into contact with storm water.
Storm Drains: Above- and below-ground structures for transporting storm water to streams or outfalls
for flood control purposes.
Storm Water: Storm water runoff, surface runoff, street wash, or drainage and may include discharges
from fire fighting activities.
Glossary
xviii Storm Water BMP Guide for New and Redevelopment
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Storm Water Discharge Associated with Industrial Activity: Discharge from any conveyance which
is used for collecting and conveying storm water from an area that is directly related to manufacturing,
processing, or raw materials storage activities at an industrial plant.
Storm Water Hotspots: Storm water hotspots are areas where land use or activities generate highly
contaminated runoff, with concentrations of pollutants exceeding those typically found in storm water.
Storm Water Pollution Control Plan (SWPCP): A written plan that documents the series of phases and
activities that, first, characterizes your site, and then prompts you to select and carry out actions which
prevent the pollution of storm water discharges. For construction activities NOI, the Department of Health
Clean Water Branch has renamed SWPCP to site-specific BMPs plan.
Storm Water Retrofit: A storm water retrofit is a storm water management practice (usually structural)
put into place after development has occurred, to improve water quality, protect downstream channels,
reduce flooding, or meet other specific objectives.
Treatment Control BMPs: Engineered technologies designed to remove pollutants from storm water
runoff prior to discharge to the storm drain system or receiving waters.
Toxicity: Adverse responses of organisms to chemicals or physical agents ranging from mortality to
physiological responses such as impaired reproduction or growth anomalies.
Glossary
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Acknowledgements
The Storm Water Best Management Practice (BMP) Manuals are adaptations of products of the California
Stormwater Quality Association (CASQA) and other municipalities and guidance documents.
The development of the City Storm Water BMP Guide was guided by Technical Contributions from
representatives of regulatory agencies (water quality and health), industry, transportation, and consulting.
The quality of this manual is a result of the diverse expertise and experience of the committee and the
workgroup.
xx Storm Water BMP Guide for New and Redevelopment
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Storm Water BMP Guide for New and Redevelopment
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1-1
1. Introduction
Storm water runoff is part of a natural hydrologic process. However, human activities particularly
urbanization and agriculture, can alter natural drainage patterns and add pollutants to lakes, and streams as
well as coastal bays and estuaries, and ultimately, the ocean. Numerous studies have shown urban runoff
to be a significant source of water pollution, causing declines in fisheries, restrictions on swimming,
and limiting our ability to enjoy many of the other benefits that water resources provide. Urban runoff
in this context includes all flows discharged from urban land uses into storm water conveyance systems
and receiving waters and includes both dry weather non-storm water sources (i.e., runoff from landscape
irrigation, etc.) and wet weather storm water runoff. In this manual, urban runoff and storm water runoff
are used interchangeably.
For many years the effort to control the discharge of storm water focused on quantity (i.e., drainage
and flood control) and only to a limited extent on quality of the storm water (i.e., sediment and erosion
control). However, in recent years awareness of the need to improve water quality has increased. With
this awareness, Federal, State, and City programs have been established to pursue the ultimate goal
of reducing pollutants contained in storm water discharges to our waterways. The emphasis of these
programs is to promote the concept and the practice of preventing pollution at the source, before it can
cause environmental problems (United States Environmental Protection Agency [USEPA], 1992). Other
BMPs to reduce or eliminate post-project runoff should also be implemented. However, where further
controls are needed, treatment of polluted runoff may be required.
1.1. Purpose and Scope
The City and County Rules Relating to Water Quality (Water Quality Rules or WQR) specifies that
regulated new development and redevelopment projects include Low Impact Development (LID) Site
Design Strategies, Source Control BMPs (best management practices), and Post-Construction Treatment
Control BMPs to reduce the pollution associated with storm water runoff. This document provides
planning and design guidelines to support implementation of the Water Quality Rules. Presented in this
manual are the minimum design and technical criteria for the analysis and design of storm drainage
facilities and water quality. This document also provides guidance for storm water quality during
the planning phase and Operations and Maintenance (O&M) guidance. Implementing structural and
operational controls that go beyond the minimum is encouraged.
1.2. Users of the Manual
This manual provides guidance suitable for use by individuals involved in development or redevelopment
site water pollution control and planning. Each user of the manual is responsible for working within their
capabilities obtained through training and experience, and for seeking the advice and consultation of
appropriate experts at all times.
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Introduction
The target audience for this manual includes:
• Developers (including their planners and engineers);
• Contractors and subcontractors (including their engineers, superintendents, foremen, and
construction staff);
• City agencies involved in site development and redevelopment (including their engineers,
planners, and construction staff);
• Regulatory agencies (including permit and planning staff); and
• General public with an interest in storm water pollution control.
This manual also references the necessary forms and worksheets that developers, designers, consultants,
contractors, and other applicants need to complete and have approved by the City and County of Honolulu
(City or CCH).
1.3. Organization of the Manual
The manual is organized to assist the user in selecting and implementing BMPs to reduce impacts of
storm water and non-storm water discharges on receiving waters. Sections of this manual are displayed in Figure 1.1.
1.4. Low Impact Development and Storm Water Quality
When rain falls in natural, undeveloped areas, the water is absorbed and filtered by soil and plants.
However, for the past decades, typical urban development and storm drain systems have been designed
using impermeable materials to convey storm water away from developed areas as quickly and efficiently
as possible. Excess rainfall, or the portion of rainfall that is not abstracted by interception, infiltration,
or depression storage, becomes surface runoff. Large areas of connected impervious cover and changes
in land use often found in urban environments dramatically increase the volume and rate of storm water
discharges.
Under natural and undeveloped conditions, surface runoff can range from 10 to 30 percent (%) of the total
annual precipitation (Figure 1.2). Depending on the level of development and the site planning methods
used, the alteration of physical conditions can result in a significant increase of surface runoff to over 50%
of the overall precipitation. Alteration in site runoff characteristics can cause an increase in the volume
and frequency of runoff flows (discharge) and velocities that cause flooding, accelerated erosion, and
reduced groundwater recharge and contribute to degradation of water quality and the ecological integrity
of streams.
Additionally, storm water runoff naturally contains numerous constituents, however, urbanization and
urban activities including development and redevelopment typically increase constituent concentrations
to levels that impact water quality. Urban activities can also result in the generation of new dry-weather
runoff that may contain many of the pollutants listed in Table 1.1.
Pollutants associated with storm water include sediment, nutrients, bacteria and viruses, oil and grease,
metals, organics, pesticides, and trash (floatables). In addition, nutrient-rich storm water runoff is an
attractive medium for vector production when it accumulates and stands for more than 72 hours (hrs).
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Introduction
Figure 1.1: Document Organization
Section 1: Introduction
Section 2: Application of Permanent BMP Requirements in the City and County of
Honolulu
Section 3: Storm Water Quality Planning for New Development and Redevelopment
Section 4: Site and Facility Design for Water Quality Protection
Section 5: Treatment Control BMP Design
Section 6: Considerations for Construction Treatment Control BMPs
• This section provides a general review of the Regulatory Background and an overview of LID.
• This section provides an overview of the requirements for New and Redevelopment projects to
comply with the City and County of Honolulu, Department of Planning and Permitting's, "Rules
Relating to Water Quality."
• This section describes planning principles for New and Redevelopment projects including site
assessment, site design strategies, and source control design.
• This section describes how infiltration BMPs, retention/detention basins, biofilters, and structural
controls can be incorporated into common site features.
• This section describes BMP selection, feasibility criteria, numerical sizing criteria, and infiltration
testing for the design of Treatment Control BMPs.
• This section outlines construction sequencing and considerations to protect permanent BMPs
during construction.
Section 7: Operations and Maintenance of BMPs
• This section provides guidance for the operation and maintenance of BMPs to ensure BMP
effectiveness.
Section 8: References
• This section provides a list of reference documents.
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Introduction
Figure 1.2: Impacts of Urbanization
LID reduces peak runoff and improves water quality by allowing rainwater to infiltrate into the ground,
allowing rainwater to evaporate and transpire, or collecting rainwater as a resource for irrigation and other
methods of reuse. Rather than moving storm water off-site through a conveyance system, the goal of LID
is to restore the natural ability of a developed site to absorb storm water, resulting in an area more closely
resembling pre-development hydrology. Importantly, the LID strategy seeks to control storm water quality
at its source, using a range of small-scale, economical devices such as native landscaping and constructed
green spaces, bioretention facilities, vegetated swales, infiltration through permeable pavement, and green
roofs.
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Introduction
Table 1.1: Pollutants and Impacts on Water Quality
Pollutant Impact
Sediment Sediment is a common component of storm water, and can be a pollutant. Sediment
can be detrimental to aquatic life (primary producers, benthic invertebrates, coral
reefs and fish) by interfering with photosynthesis, respiration, growth, reproduction,
and oxygen exchange in water bodies. Sediment can transport other pollutants that
are attached to it including nutrients, trace metals, and hydrocarbons. Sediment is the
primary component of total suspended solids (TSS), a common water quality analytical
parameter.
Nutrients Nutrients including nitrogen and phosphorous are the major plant nutrients used for
fertilizing landscapes, and are often found in storm water. These nutrients can result in
excessive or accelerated growth of vegetation, such as algae, resulting in impaired use
of water in lakes and other sources of water supply.
Bacteria and Viruses Bacteria and viruses are common contaminants of storm water. For separate storm
drain systems, sources of these contaminants include animal excrement and sanitary
sewer overflow. High levels of indicator bacteria in storm water have led to the closure
of beaches, lakes, and streams to contact recreation such as swimming.
Oil and Grease Oil and grease includes a wide array of hydrocarbon compounds, some of which are
toxic to aquatic organisms at low concentrations. Sources of oil and grease include
leakage, spills, cleaning and sloughing associated with vehicle and equipment engines
and suspensions, leaking and breaks in hydraulic systems, restaurants, and waste oil
disposal.
Metals Metals including lead, zinc, cadmium, copper, chromium, and nickel are commonly
found in storm water. Many of the artificial surfaces of the urban environment (i.e.,
galvanized metal, paint, automobiles, or preserved wood) contain metals, which enter
storm water as the surfaces corrode, flake, dissolve, decay, or leach. Over half the trace
metal load carried in storm water is associated with sediments. Metals are of concern
because they are toxic to aquatic organisms, can bioaccumulate (accumulate to toxic
levels in aquatic animals such as fish), and have the potential to contaminate drinking
water supplies.
Organics Organics may be found in storm water in low concentrations. Often synthetic organic
compounds (adhesives, cleaners, sealants, solvents, etc.) are widely applied and may
be improperly stored and disposed. In addition, deliberate dumping of these chemicals
into storm drains and inlets causes environmental harm to waterways.
Pesticides Pesticides (including herbicides, fungicides, rodenticides, and insecticides) have been
repeatedly detected in storm water at toxic levels, even when pesticides have been
applied in accordance with label instructions. As pesticide use has increased, so too
have concerns about adverse effects of pesticides on the environment and human
health. Accumulation of these compounds in simple aquatic organisms, such as
plankton, provides an avenue for biomagnification through the food web, potentially
resulting in elevated levels of toxins in organisms that feed on them, such as fish and
birds.
Gross Pollutants Gross Pollutants (trash, debris, and floatables) may include heavy metals, pesticides,
and bacteria in storm water. Typically resulting from an urban environment, industrial
sites and construction sites, trash and floatables may create an aesthetic “eye sore”
in waterways. Gross pollutants also include plant debris (such as leaves and lawn-
clippings from landscape maintenance), animal excrement, street litter, and other
organic matter. Such substances may harbor bacteria, viruses, vectors, and depress the
dissolved oxygen levels in streams, lakes, and estuaries sometimes causing fish kills.
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Introduction
1.5. Regulatory Requirements
The Federal Water Pollution Control Act of 1972 also known as the Clean Water Act (CWA), as amended
in 1987, is the principal legislation for establishing requirements for the control of storm water pollutants
from urbanization and related activities. However, other Federal, State, and City requirements deal
directly or indirectly with controlling storm water discharges. Requirements for storm water under some
of these programs are evolving: Coastal Zone Management Act (CZMA), State of Hawaii Administrative
Rules (HAR), City Ordinances, Total Maximum Daily Loads (TMDLs), 401 Water Quality Certifications
and Endangered Species Act. The user is advised to contact local regulatory and/or City officials for
further information.
1.5.1. Federal Programs
In 1972, provisions of the CWA were amended so that discharge of pollutants to waters of the United
States from any point source is effectively prohibited, unless the discharge is in compliance with a
National Pollutant Discharge Elimination System (NPDES) permit. The 1987 amendments to the
CWA added Section 402(p), which established a framework for regulating municipal, industrial, and
construction storm water discharges under the NPDES program. On November 16, 1990, USEPA
published final regulations that established application requirements for storm water permits for municipal
separate storm sewer systems (MS4s) serving a population of over 100,000 (Phase I communities) and
certain industrial facilities, including construction sites greater than five (5) acres.
On December 8, 1999, USEPA published the final regulations for communities under 100,000 (Phase II
MS4s) and operators of construction sites between one (1) and five (5) acres.
1.5.2. State Programs
The statutory framework for the NPDES program requires that all point sources that discharge pollutants
in the waters of the United States must obtain an NPDES permit from the USEPA or an authorized State
(Hawaii is a delegated state). Storm water is regulated under the NPDES program. There has been a
phased approach to regulation of storm water. Phase I, in 1990, regulated discharges from Medium and
Large MS4s, industrial activity, and construction sites greater than or equal to five (5) acres. Phase II
became effective March 10, 2003 and regulated discharges from Small MS4s and construction sites from
one (1) acre to five (5) acres. Large, Medium, and Small MS4s were defined by the size of the population
that the system serves. The regulations required the issuance of permits to regulated dischargers.
In Hawaii, Small MS4s, industrial facilities, and construction activities greater than or equal to one
acre are normally covered by general permits. However if such facilities discharge storm water into
sensitive water bodies designated as Class AA marine, or Class 1 inland State waters, or areas restricted in
accordance with the State’s “No Discharge” policy, then those facilities must be covered by an individual
permit. Also, Small MS4s and industrial facilities could be covered under an individual permit issued to a
Large MS4.
Regulatory emphasis is placed on pollution prevention by regulating “end of pipe” discharges in lieu
of setting effluent limits. Prevention is accomplished through the development and implementation
of plans such as the MS4 Storm Water Management Program Plan (SWMPP), Industrial Storm Water
Pollution Control Plans (SWPCPs), and erosion control plans and Site-Specific BMP Plans (SSBMPs) for
construction sites.
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Introduction
Projects designated by the City as “Priority Projects” have been identified to have a greater potential for
activities which may contribute sources for pollutants. These projects, described in Section 2, have been
developed to reduce the risk of pollutant sources exposure to storm water runoff.
1.5.3. Other Relevant Regulatory Programs
In addition to meeting City storm water program requirements under CWA section 402(p), municipalities
are increasingly subject to other regulatory drivers that relate to the protection of surface water quality
and beneficial uses of water bodies in their communities. Several other regulatory programs that can
significantly affect new development and redevelopment planning and design are:
• TMDLs
• Endangered Species Act
• CWA Section 404 Dredge and Fill Permits
• Section 401 Water Quality Certification (regulated under HAR, Chapter 11-54)
In the coming years, these regulatory drivers will likely have at least as much impact on the design and
implementation of municipal storm water programs and BMP selection and maintenance as current storm
water regulations.
1.5.4. Total Maximum Daily Loads
TMDLs are a regulatory mechanism to identify and implement additional controls on both point and
non-point source discharges in water bodies that are impaired from one or more pollutants and are not
expected to be restored through normal point source controls. States identify impairments and pollutants
by putting impaired water bodies on a list as required under Section 303(d) of the CWA.
Storm water or urban runoff is listed as a suspected source for many of the water body pollutant
combinations in the current 303(d) list. Storm water programs must be designed not only to be in
compliance with the storm water NPDES permit regulations, but they must also be designed to implement
TMDLs in which storm water or urban runoff is named as a source.
1.5.5. Endangered Species Act
Like TMDLs, Endangered Species Act (Hawaii Revised Statutes Title 12) issues are becoming
increasingly important to storm water program design and implementation. The presence or potential
presence of an endangered species impacts storm water management programs and the selection and
maintenance of BMPs. The Department of Land and Natural Resources (DLNR) Division of Forestry
and Wildlife (DOFAW) and the US Fish & Wildlife Service (USFWS) provides information on the
designation of critical habitat in Hawaii.
The developers or public agency intending to conduct activities in or discharge to an area that serves as
a critical habitat must contact resource agencies such as DLNR, DOFAW, and the USFWS to learn about
specific compliance requirements and actions.
1.5.6. Clean Water Act Section 404 Dredge and Fill Permits
In 1972, Section 404 of the CWA was passed prohibiting the discharge of dredged or filled material into
U.S. waters without a permit from the Army Corps of Engineers (USACE or CORP or COE). Subsequent
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Introduction
court rulings and litigation further defined “Waters of the U.S.” to include virtually all surface waters,
including wetlands. Activities in waters of the United States that are regulated under this Act include fills
for development, water resource projects (such as dams and levees), infrastructure development (such as
highways and airports), and conversion of wetlands to uplands for farming and forestry.
The basic premise of the CWA is that no discharge of dredged or fill material is permitted if a practicable
alternative exists that is less damaging to the aquatic environment or if the nation’s waters would be
significantly degraded. When applying for a permit, it must be shown that:
• Steps have been taken to avoid wetland impacts where practicable.
• Potential impacts to wetlands have been minimized.
• Compensation for any remaining, unavoidable impacts through activities has been provided to
restore or create wetlands.
An individual permit is usually required for potentially significant impacts. However, for most discharges
that will have only minimal adverse effects, the USACE often grants up-front general permits. These may
be issued on a nationwide or state basis for particular categories of activities (for example, minor road
crossings, utility line backfill, and bedding) as a means to expedite the permitting process.
1.5.7. Clean Water Act Section 401 Water Quality Certification
Anyone proposing to conduct a project that requires a Federal permit (404) or involves dredge or fill
activities that may result in a discharge to U.S. surface waters and/or “Waters of the State” is required to
obtain a CWA Section 401 Water Quality Certification and/or Waste Discharge Requirements (Dredge/ Fill
Projects) from the State of Hawaii (State) Department of Health (DOH) Clean Water Branch (CWB),
verifying that the project activities will comply with state water quality standards (HAR Chapter 11- 54).
The rules and regulations apply to all “Waters of the State,” including isolated wetlands and stream
channels that may be dry during much of the year, have been modified in the past, look like a depression
or drainage ditch, have no riparian corridor, or are on private land.
Section 401 of the CWA grants each state the right to ensure that the state’s interests are protected on
any federally permitted activity occurring in or adjacent to “Waters of the State.” In Hawaii, the CWB is
the agency mandated to ensure protection of the State’s waters. If a proposed project requires a USACE,
CWA Section 404 permit and has the potential to impact Waters of the State, the CWB through HAR
Chapter 11-54 will regulate the project and associated activities through a Water Quality Certification
determination (Section 401), as part of the 404 process.
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2. Application of Permanent BMP
Requirements in the City and County of
Honolulu
According the Water Quality Rules, the following projects are required to implement post-construction
BMPs for water quality:
Priority A Priority B
• All new development and redevelopment, including any incremental development, that proposes land disturbing activities of 1 acre or more.
• Any project that may have significant water quality impacts due to its location or associated land use activities, including but not limited to the development or redevelopment of: ▪Retail gas outlets ▪Automotive repair shops ▪Restaurants ▪Parking lots with 20 stalls or more ▪Buildings greater than 100 feet in height ▪Retail malls ▪Industrial park
Priority B is further divided into Priority B1 and Priority B2. Priority B1 is any Priority Project with
5,000 square feet (sq-ft) or greater of impervious area. Priority B2 is any Priority Project with less than
5,000 sq-ft of Impervious Area.
The Water Quality Rules criteria must be met for Priority A, B1, and B2 projects as follows:
1. Incorporate appropriate LID Site Design Strategies to the maximum extent practicable (MEP).
2. Incorporate appropriate Source Control BMPs to the MEP.
In addition, the following criteria must be met for Priority A and B1 projects as follows:
1. Retain on-site by infiltration, evapotranspiration or harvest/reuse, as much of the Water Quality
Volume (WQV) as feasible, with appropriate LID Retention Post-Construction Treatment Control
BMPs.
2. Biofilter the remaining portion of the WQV that is not retained on-site with appropriate LID
Biofiltration Post-Construction Treatment Control BMPs as much as feasible.
3. If it is demonstrated to be infeasible to retain and/or biofilter the WQV, one of the following
alternative compliance measures is required:
a. Treat (by detention, filtration, settling, or vortex separation) and discharge with appropriate
Alternative Compliance Post-Construction Treatment Control BMPs, any portion of the WQV
that is not retained on-site or biofiltered.
a. Retain or biofilter at an offsite location, the volume of runoff equivalent to the difference
between the project’s WQV and the amount retained on-site or biofiltered. Offsite mitigation
projects must be submitted for City approval.
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Application of Permanent BMP Requirements in CCH
Figure 2.1: Implementation of the CCH Water Quality Rules
A flowchart of implementation of the CCH Water Quality Rules is shown in Figure 2.1.
Site Design, Source Control, and Treatment Control BMPs are discussed throughout this document.
Table 2.1 summarizes the requirements of the Water Quality Rules and objectives of each requirement.
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Application of Permanent BMP Requirements in CCH
Table 2.1: Summary of Post-Construction Requirements for New and Redevelopment
BMP Category Applicable Projects Pollution Prevention Objective
LID Site Design Required for all Priority projects;
recommended for all others. See
Section 3.2.
Strategies integrated into the facility design
to maintain or restore the natural hydrologic
functions of a site to reduce the rate of
runoff, filter out its pollutants, and facilitate
the infiltration of water into the ground.
Source Control Required for all Priority projects;
recommended for all others. See
Source Control BMPs in Section
3.3 and Appendix A.
Low-technology practices designed to
prevent pollutants from contacting storm
water runoff or to prevent discharge of
contaminated runoff to the storm drainage
system.
Treatment Control
LID Retention Required for Priority A and B1
projects if feasible; recommended
for all others if feasible. See
Treatment Control BMPs in
Section 5 and Appendix B.
Engineered technologies designed to
retain runoff on-site by infiltration,
evapotranspiration, or reuse.
LID Biofiltration Required for Priority A and B1
projects if LID Retention is
infeasible; recommended for all
others if feasible. See Treatment
Control BMPs in Section 5 and
Appendix B.
Engineered technologies designed to
remove pollutants from runoff by filtration,
adsorption, and biological uptake.
Alternative Compliance BMPs
Required for Priority A and B1
projects when retention and
biofiltration are infeasible. See
Treatment Control BMPs in
Section 5 and Appendix B.
Engineered technologies designed to
remove pollutants from runoff by detention,
filtration, settling, or vortex separation.
2.1. City and County of Honolulu Plan Review Requirements
The City’s Water Quality Rules requires several documents to be prepared and submitted prior to permit
issuance.
2.1.1. Storm Water Quality Strategic Plans
Storm Water Quality Strategic Plans (Strategic Plans) should be submitted with or within the Master
Development Plan. This requirement ensures that project developers, planners, and designers are
considering storm water management early in the planning process. The Strategic Plan must include a
written description of the proposed development, expected activities and pollutants that will be generated
by activities at the site, and LID site design strategies that will be used to comply with the Water Quality
Rules. The Strategic Plan must also include a development schedule.
2.1.2. Storm Water Quality Reports
An effective mechanism for documenting the incorporation of storm water quality controls into new
development and redevelopment projects on a site or watershed basis is to develop a written plan known
as a Storm Water Quality Report (SWQR). An effective SWQR clearly sets forth the means and methods
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Application of Permanent BMP Requirements in CCH
for long-term storm water quality protection. The SWQR will also prove to be useful during ownership
transitions to convey critical storm water quality control information to subsequent owners.
SWQRs must be submitted with either a building permit application or, for projects which require a
grading, grubbing, stockpiling or trenching permit, the SWQR must be submitted with construction plans
for review. SWQRs must be prepared by a Certified Water Pollution Plan Preparer (CWPPP). A CWPPP
must be an Architect, Engineer, Land Surveyor, or Landscape Architect licensed in the State of Hawaii.
Certification can be obtained from the DPP by the successful completion of an on-line training and test.
A template is provided by the City on the Department of Planning and Permitting (DPP) website. SWQRs
must contain the following information:
1. Project Name;
2. Master Plan Development Name;
3. Project Address;
4. Project Size (acres);
5. Tax Map Key;
6. The name, address, and telephone number of the owner(s)/developers of the property;
7. A description of site characteristics including drainage patterns, soils, vegetation, and steep or
unstable slopes that may be of concern;
8. A description of the future activities at the site including those that would require Source Control
BMPs
9. A description of the pollutants of concern (POC) expected to be generated at the site; and
10. A description of the BMPs that will be implemented including Site Design and Self-mitigating
Areas, Source Control, Retention, Biofiltration, and Alternative Compliance and which POCs are
addressed by those BMPs.
The following reports and plans should be included as attachments as applicable.
1. Location Map and Site Plans;
2. Existing and Proposed Drainage Plans;
3. Permanent BMP Plan including locations of all Source Control and Treatment Control BMPs and
a clear and definite delineation of areas covered by vegetation or trees that will be saved;
4. Treatment Control BMP Sizing Spreadsheets;
5. Infiltration Testing Results/Geotechnical Reports;
6. Operation and Maintenance Plan;
7. Proprietary Treatment Device Information; and/or
8. Evidence or explanation for any infeasibility criteria claimed in order to comply with the
requirements for infiltration, harvest/reuse, and biofiltration. Infeasibility criteria is discussed
more in Section 5.5 and must be documented on the form provided by DPP which can be
accessed on their website.
2.1.3. Storm Water Quality Checklists
Storm Water Quality Checklists (SWQCs) are required for Priority B2 Projects. An SWQC is similar to an
SWQR but requires less information. A template is provided by on the DPP website.
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Application of Permanent BMP Requirements in CCH
SWQCs must contain the following information:
1. Project Name;
2. Master Plan Development Name;
3. Project Address;
4. Project Size (acres of square feet);
5. Impervious Area; ;
6. Tax Map Key
7. The name, address, and telephone number of the owner(s)/developers of the property;
8. BMPs that will be implemented including Site Design Strategies, Self Mitigating Areas, and
Source Control BMPs.
The following reports and plans shall be included as attachments:
1. Permanent BMP Plan including locations of all Site Design Strategies, Source Control BMPs and
vegetated or landscaped areas; and
2. Operations and Maintenance Plan.
2.1.4. Operations and Maintenance Plan
The Water Quality Rules require new development and redevelopment projects that implement Source
Control and Treatment Control BMPs to regularly inspect and maintain installed BMPs to ensure they
operate as designed. Permanent BMPs are also subject to annual inspections by the City's Department
of Facility Maintenance (DFM). The owner/developer is required to keep records of inspection and
maintenance activities for a minimum of five (5) years and be made available to the City upon request.
The owner of the property on which a permanent structural BMP is located must submit a BMP
maintenance plan with the SWQR for all permanent structural BMPs. Modifications to the O&M
Plan after DPP acceptance are permitted before closing applicable building and/or grading, grubbing,
stockpiling, or trenching permits.
A template has been provided by the City and is available on the DPP website. O&M Plans shall include:
1. Name, phone number, and mailing address for the owner of the property.
2. Name and phone number for the individual(s), association, or management company responsible
ensuring maintenance is being performed.
3. Maintenance activities for each BMP.
4. Inspection frequencies for each BMP.
5. A post-construction BMP plan showing the location of each BMP with a summary of the
maintenance activities and inspection schedule for each BMP.
2.1.5. BMP Certification and Recording
Approved post-construction record drawings and the accepted O&M Plan must be recorded with the
drainage connection permit in the deed for the real property on which the permanent BMP will be located.
One copy of the drainage connection permit and recorded O&M Plan shall be submitted to the Director of
DFM prior to closing the building and/or grading, grubbing, stockpiling, or trenching permits.
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Application of Permanent BMP Requirements in CCH
A CWPPP shall inspect to confirm that the permanent post-construction BMPs have been installed in
conformance with the approved construction plans and submit the signed Certificate of Completion form
prior to closing the building and/or grading permits. The certification of completion form is available on
the DPP website.
2.1.6. Variances
Whenever there are practical difficulties involved in carrying out the LID Regulations, the DPP Director
has the authority to grant modifications to the provisions for individual cases. These must be filed for
individually and the applicant must demonstrate that the situation is unique, not caused by their own
actions, and will not result in unreasonable threat of Pollutant Discharges to the MS4 or State Waters.
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3. Storm Water Quality Planning for New
Development and Redevelopment
The City’s program requires BMPs to be implemented by developers, property owners, and public
agencies engaged in new development or redevelopment activities. Understanding new development and
redevelopment in the context of the project life cycle is important for proper selection and implementation
of BMPs as shown in Figure 3.1. The concept, planning, and design phases of a project may be spread
over a period of months to many years. BMPs incorporated into the concept, planning, and design phase
are much more cost-effective than the retrofit of developed projects with BMPs.
Figure 3.1: Project Life Cycle
Planning for storm water quality protection employs a multi-level strategy. To comply with the Water
Quality Rules, the strategy consists of: 1) Site design strategies to reduce or eliminate post-project runoff;
2) Controlling sources of pollutants; and 3) Treating the remaining storm water runoff before discharging
it to the storm drain system or to receiving waters.
This section describes how Elements 1 and 2 of the strategy can be incorporated into the site and facility
planning and design process, and by doing so, eliminating or reducing the amount of storm water runoff
that may require treatment at the point where storm water runoff ultimately leaves the site. It also
describes considerations when selecting treatment to achieve Element 3.
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Element 1 is referred to as a “site design strategy” because it utilizes strategies during site design to
reduce or eliminate runoff.
Element 2 may be referred to as “source controls” because they emphasize reducing or eliminating
pollutants in storm water runoff at their source through runoff reduction and by keeping pollutants and
storm water segregated.
Element 3 of the strategy is referred to as “treatment control” because it utilizes treatment mechanisms to
remove pollutants that have entered storm water runoff. Treatment controls integrated into and throughout
the site usually provide enhanced benefits over the same or similar controls deployed only at the “end of
the pipe” where runoff leaves the project site.
These principles are consistent with the permit and the program requirements for Priority Projects that
require a combination of site design, source control BMPs (that reduce or eliminate runoff and control
pollutant sources) and treatment control BMPs with specific quantitative standards. The extent to which
projects can incorporate site design strategies that reduce or eliminate post project runoff will depend
upon the land use and local site characteristics of each project. Reduction in post project runoff offers a
direct benefit by reducing the required size of treatment controls to meet the numeric standard included in
the Water Quality Rules. Therefore, project developers can evaluate tradeoffs between the incorporation
of alternative site design and source control techniques that reduce runoff and pollutants, and the size of
required treatment controls.
3.1. Assess Site Conditions
Site and watershed assessment includes assessing and describing the pre- and post-development site
conditions and how the site fits into the overall watershed or drainage area. The assessment should include
sufficient detail to allow for assessment of the need for and application of storm water BMPs. Information
typically required is listed below and can be refined during the detailed design process.
Site Information:
• Historic features
• Existing features and vegetation
• Planned features and activities
• Topography and drainage patterns
• Discharge locations
• Soil types
• Subsurface hydrology characterization
• General climate including average
precipitation and microclimates including
areas of full or partial shade
• Setbacks and buffer requirements
Vicinity Information:
• Major roadways
• Geographic features or landmarks
• Area surrounding the site
• General topography
• Area drainage
Watershed or Drainage Area Information:
• Receiving waters
• Watershed drainage
• Depth to groundwater
3.1.1. Understand Hydrologic Conditions of Concern
Development of impervious areas changes the landform and therefore the runoff hydrograph.
Modifications to the runoff hydrograph change downstream hydrology. New development typically
results in more runoff volume and higher rates of runoff. Many BMPs, such as detention basins, which
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detain volume, effectively remove the top part of the hydrograph, but extend the duration of flow. See
Figure 3.2.
Recent findings indicate that while such actions mitigate peak flows, the increased duration associated
with these actions has impacts as well. Problems include washing out habitat, eroding streambed and
banks, and changing downstream ecosystems. In addition to volume, rate, and duration, other factors such
as the amount of energy in the water and peak flow impact downstream conditions.
A comprehensive understanding of these factors is necessary to develop meaningful storm water
management plans. To be effective, these solutions must be done on an individual watershed basis.
Ideally, the runoff hydrograph that exists after construction would parallel the pre-construction
hydrograph. It is difficult to ask upstream developers to be concerned about what is happening several
miles below them in a watershed. On the other hand, storm water planners and policy makers must ask
what can be done to make the watershed more stable, and what enhancements are needed to balance
impacts to the watershed from development. Storm water quality planning for new development and
redevelopment can be used to make qualitative predictions concerning channel impacts due to changes in
runoff or sediment loads from the watershed.
The best way to resolve the watershed stability and balance issues is through a comprehensive drainage
water master plan. A formal drainage study considers the project area’s location in the larger watershed,
topography, soil and vegetation conditions, percent impervious area, natural and infrastructure drainage
features, and any other relevant hydrologic and environmental factors. A drainage study is typically
prepared by a licensed civil engineer. As part of the study, the drainage report includes:
• Field reconnaissance to observe downstream conditions.
• Computed rainfall and runoff characteristics including a minimum of peak flow rate, flow
velocity, runoff volume, time of concentration and retention volume.
• Establishment of site design, source control and treatment control measures to be incorporated
and maintained to address downstream conditions of concern.
Figure 3.2: Hydraulic Altercation after Certain BMPs are Implemented
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3.2. Site Design Strategies
The goal of LID Site Design is to reduce the hydrologic impact of development and to incorporate
techniques that maintain or restore the site’s hydrologic and hydraulic functions. The optimal LID site
design minimizes runoff volume and preserves existing flow paths.
The City requires five (5) Site Design strategies:
1. Conserve natural areas, soils, and vegetation.
2. Minimize disturbances to natural drainages.
3. Minimize soil compaction.
4. Direct runoff to landscaped areas and reduce directly connected impervious areas (DCIA).
5. Minimize impervious surfaces.
3.2.1. Conserve Natural Areas, Soils, and Vegetation
The conservation of natural areas, soils, and vegetation helps to retain numerous functions of
predevelopment hydrology, including rainfall interception, evapotranspiration, and infiltration.
Maximizing these functions will thereby reduce the amount of runoff that must be treated. Protection
of mature trees and vegetation provides habitat, prevents erosion, captures significant rainfall, provides
summer shading, and reduces runoff volume and velocity which protects and enhances downstream water
quality. Specific measures are:
• Preserve/protect riparian buffers.
• Preserve/protect wetlands.
• Preserve/protect natural flow pathways.
• Preserve/protect steep slopes.
• Preserve/protect sensitive environmental areas.
• Preserve/protect undisturbed vegetated areas/corridors.
• Preserve native trees and restrict disturbance of soils beneath tree canopies.
• Limit construction activities and disturbances to areas with previously disturbed soils.
• Avoid disturbing vegetation and soil on slopes and near surface waters.
• Leave an undisturbed buffer along both sides of natural streams.
Refer to Section 6 regarding considerations and sequencing practices to preserve existing vegetation
during construction.
3.2.2. Minimize Disturbances to Natural Drainages
Natural drainages offer a benefit to storm water management as the soils and habitat already function
as a natural filtering/infiltrating swale. Minimizing disturbances to natural drainage patterns preserves
the predevelopment timing, rate, and duration of runoff as well as preserving streamside habitats. When
determining the development footprint of the site, natural drainages should be avoided. By keeping the
development envelope set back from natural drainages, the drainage can retain its water quality benefit to
the watershed. Specific measures are:
• Limit site disturbance, clearing, and grading to the smallest areas necessary.
• Maintain surface flow patterns of undeveloped sites.
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• Maintain existing water body alignments, sizes, and shapes.
• Minimize and control construction traffic areas.
• Minimize and control construction stockpiling and storage areas.
• Use construction fencing to mark where no disturbances will be allowed.
3.2.3. Minimize Soil Compaction
Clearing, grading and compaction by construction traffic reduces the natural absorption and infiltration
capacities of the native soils. Soil compaction damages soil structure, reduces infiltration rates, limits root
growth and plant survivability, and destroys soil organisms. Subsequent tilling and/or addition of soil
amendments such as compost can help, but will not restore the original infiltration capacity of the soils.
By protecting native soils and vegetation in appropriate areas during the clearing and grading phase of
development the site can retain some of its existing beneficial hydrologic function. Specific measures are:
• Protect soils against compaction and rutting in areas where traffic is unavoidable.
• Minimize the size of construction easements and material storage areas.
• Limit areas of heavy equipment.
• Prohibit working on wet soils with heavy equipment.
• Restore compacted open space areas with tilling and soil amendments.
• Avoid extensive and unnecessary clearing and stockpiling of topsoil.
• Avoid/minimize soil compaction in open space, landscaped, and proposed LID BMP areas.
• Prepare soil amendments off-site.
3.2.4. Direct Runoff to Landscape Areas and Reduce Directly Connected Impervious Areas
Any impervious surface that drains into a catch basin, area drain, or other conveyance structure is a
DCIA. Impervious areas directly connected to the storm drain system are the greatest contributor to non-
point source pollution. The first effort in site planning and design for storm water quality protection is to
minimize the DCIA as shown in Figure 3.3.
Figure 3.3: Directly Connected Impervious Area versus Directing Runoff to Landscaped Areas
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Storm Water Quality Planning for New Development and Redevelopment
As storm water runoff flows across parking lots, roadways, and paved areas, the oils, sediments, metals,
and other pollutants are collected and concentrated. If this runoff is collected by a drainage system and
carried directly along impervious gutters or in closed underground pipes, it has no opportunity for filtering
by plant material or infiltration into the soil. It also increases in speed and volume, which may cause
higher peak flows downstream, and may require larger capacity storm drain systems, increasing flood
and erosion potential. Solutions that reduce DCIA prevent runoff, detain or retain surface water, attenuate
peak runoff rates, benefit water quality and convey storm water. Specific measures are:
• Design roof drains to flow to vegetated areas.
• Direct flow from paved areas to stabilized landscaped/vegetated areas (See Figure 3.4).
• Grade paved areas to achieve sheet flow to landscaped areas.
• Break up flow directions from large paved surfaces.
Figure 3.4: Parking Lot Directing Runoff to Landscape Areas
3.2.5. Minimize Impervious Surfaces
The principle of runoff reduction starts by recognizing that developing or redeveloping land within a
watershed inherently increases the imperviousness of the areas and therefore the volume and rate of
runoff and the associated pollutant load.
The extent of impervious land covering the landscape is an important indicator of storm water quantity
and quality and the health of urban watersheds. Studies have demonstrated a direct correlation between
the degree of imperviousness of an area and the degradation of its receiving waters. Impervious land
coverage is a fundamental characteristic of the urban and suburban environment; rooftops, roadways,
parking areas, and other impenetrable surfaces cover soils that allowed rainwater to infiltrate before
development.
Without these impervious coverings, inherent watershed functions would naturally filter rainwater and
prevent receiving water degradation. Impervious surfaces associated with urbanization can cause adverse
receiving water impacts in four (4) ways:
1. Rainwater is prevented from filtering into the soil, adversely affecting groundwater recharge and
reducing base stream flows.
2. Because it cannot filter into the soil, more rainwater runs off, and runs off more quickly, causing
increased flow volumes, accelerating erosion in natural channels, and reducing habitat and other
stream values. Flooding and channel destabilization often require further intervention. As a result,
riparian corridors are lost to channelization, further reducing habitat values.
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3. Pollutants that settle on the impervious pavements and rooftops are washed untreated into storm
sewers and nearby stream channels, increasing pollution in receiving water bodies.
4. Impervious surfaces retain and reflect heat, increasing ambient air and water temperatures.
Increased water temperature negatively impacts aquatic life and reduces the oxygen content of
nearby water bodies.
These techniques include actions to:
• Manage watershed impervious area.
• Include self-mitigating areas.
• Consider runoff reduction areas.
Manage Watershed Impervious Area
Land use planning on the watershed scale is a powerful tool to manage the extent of impervious land
coverage and is especially applicable to larger scale community development and master planning.
First, identify open space and sensitive resource areas and target growth to areas that are best suited
to development, and second, plan development that is compact to reduce overall land conversion to
impervious surfaces and reliance on land-intensive streets and parking systems.
Water resource protection is becoming more complex. A wide variety of regulatory agencies, diverse
sources of non-point source pollution, and a multitude of stakeholders make it difficult to achieve a
consistent, easily understandable strategy for watershed protection. Impervious land coverage is a
scientifically sound, easily communicated, and practical way to measure the impacts of new development
on water quality.
Impervious area reductions also provide additional benefits such as reduced urban heat island effect,
resulting in less energy use to cool structures and more efficient irrigation use by plants. Reductions have
also been attributed to more human-scale landscaping and higher property values.
Strategies for reducing impervious land coverage include:
• Cluster buildings so that they require less driveways and pathways.
• Taller narrower buildings rather than lower spreading ones.
• Sod or vegetative “green roofs” rather than conventional roofing materials.
• Pervious pavement for light duty roads, parking lots, and pathways.
• Use open space or hybrid street plan instead of grid and curvilinear.
• Maximize utilization of compact car spaces in parking areas.
• Reduce driveway sizes.
Many of these strategies are discussed in Section 3. Two (2) of the strategies to Minimize Impervious
Surfaces result in direct benefit to treatment control requirements: Self-Mitigating Areas and Runoff
Reduction Areas. These two (2) strategies result in a smaller WQV requiring treatment under the City’s
Water Quality Rules.
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Include Self-Mitigating Areas
Developed areas may provide “self-mitigation” of runoff if properly designed and drained.
A self-mitigating area is a natural or landscape area which retain and/or treat rainfall over the footprint of
the self-mitigating area but do not accept runoff from other areas. Self-mitigating areas can drain directly
offsite or to the public storm drain system without being treated by a structural BMP.
Self-mitigating areas might include:
• Conserved natural spaces
• Landscaped areas (including parks and lawns)
• Grass/vegetated swales
• Green roofs
The infiltration and biotreatment inherent to such areas provides the treatment control necessary. These
areas therefore act as their own BMP, and no additional BMPs to treat runoff are required.
As illustrated in Figure 3.5, site drainage designs must direct runoff from self-treating areas away from
other areas of the site that require treatment of runoff. Otherwise, the volume from the self-mitigating area
will only add to the volume requiring treatment from the impervious area as demonstrated in Figure 3.6.
Likewise, under this philosophy, self-mitigating areas receiving runoff from treatment-required areas
would no longer be considered self-mitigating, but rather would be considered as the BMPs in place
to treat that runoff. These areas could remain as self-mitigating, or partially self-mitigating areas, if
adequately sized to handle the excess runoff addition.
Consider Runoff Reduction Areas
Using alternative surfaces with a lower coefficient of runoff or “C-Factor” can reduce runoff from
developed areas. The C-Factor is a representation of the surface’s ability to produce runoff. Surfaces
that produce higher volumes of runoff are represented by higher C-Factors, such as impervious surfaces.
Surfaces that produce smaller volumes of runoff are represented by lower C-Factors, such as more
pervious surfaces. See Table 3.1 for typical C-Factor values for various surfaces during small storms.
Table 3.1: Estimated C-Factor for Various Surfaces during Small Storms
Paving Surfaces C-Factor Paving Surfaces C-Factor
Concrete 0.80 Permeable interlocking concrete pavement 0.10
Stone, brick, or concrete pavers with
mortared joints and bedding 0.80 Grid pavements with grass or aggregate
surface 0.10
Asphalt 0.70 Crushed aggregate 0.10
Stone, brick, or concrete pavers with sand
joints and bedding 0.70 Grass 0.10
Pervious concrete 0.10 Grass over porous plastic 0.05
Porous asphalt 0.10 Gravel over porous plastic 0.05
Note: C-Factors for small storms are likely to differ (be lower) than C-Factors developed for large, flood control volume size
storms. The above C-Factors were produced by selecting the lower end of the best available C-Factor range for each paving
surface. These C-Factors are only appropriate for small storm treatment design, and should not be used for flood control sizing.
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Figure 3.5: Use of Self-Mitigating Areas
Figure 3.6: Self-Mitigating Areas Treatment Volume
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Site design techniques that incorporate pervious materials may be used to reduce the C-Factor of a
developed area, reducing the amount of runoff requiring treatment. These materials include:
• Pervious concrete
• Pervious asphalt
• Turf block
• Brick (un-grouted)
• Natural stone
• Concrete unit pavers
• Crushed aggregate
• Cobbles
• Wood mulch
Table 3.2 compares the C-Factors of conventional paving surfaces to alternative; lower C-Factor paving
surfaces. By incorporating more pervious, lower C-Factor surfaces into a development (see Figure 3.7);
lower volumes of runoff may be produced. Lower volumes and rates of runoff translate directly to lower
treatment requirements.
Table 3.2: Conventional Paving Surface Small Storm C-Factor versus Alternative Paving C-Factors
Conventional Paving Surface C-Factors Reduced C-Factor for Paving Alternatives
• Concrete Patio/Plaza (0.80)
• Asphalt Parking Area (0.70)
• Decorative unit Pavers on Sand (0.10)
• Turf Block Overflow Parking Area (0.10)
• Pervious Concrete (0.10)
• Pervious Asphalt (0.10)
• Crushed Aggregate (0.10)
Other site design techniques such as disconnecting impervious areas, preservation of natural areas, and
designing concave medians may be used to reduce the overall C-Factor of development areas.
3.3. Control Sources of Pollutants
There are a number of items that can be routinely designed into a project that function as source controls
once a project is completed. Design of BMPs to control exposure to pollutants is guided by two (2)
general principles:
1. Prevent water from contacting work areas. Work and storage areas should be designed to prevent
storm water runoff from passing through shipping areas, vehicle maintenance yards, and other
work places before it reaches storm drains. The objective is to prevent the discharge of water
laden with grease, oil, heavy metals and process fluids to surface waters or sensitive resource
areas.
2. Prevent pollutants from contacting surfaces that come into contact with storm water runoff.
Precautionary measures should be employed to keep pollutants from contacting surfaces that
come into contact with runoff. This means controlling spills and reviewing operational practices
and equipment to prevent pollutants from coming into contact with storm or wash water runoff.
Examples of structural source controls include covers, impermeable surfaces, secondary containment
facilities, runoff diversion berms, and diversions to wastewater treatment plants. See Figure 3.7.
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The Water Quality Rules require that Source Control BMPs are required for all Priority A and B projects
for the following activities and areas:
Activity or Area Required to Implement Source Control BMP Fact Sheet (Appendix A)
Landscaped Areas SD-01
Automatic Irrigation System SD-03
Storm Drain Inlets SD-04
Vehicle/Equipment Fueling SD-06
Vehicle/Equipment Repair SD-07
Vehicle/Equipment Cleaning SD-08
Loading Docks SD-09
Outdoor Trash Storage SD-10
Outdoor Material Storage SD-11
Outdoor Work Areas SD-12
Outdoor Process Equipment Operations SD-13
Parking Areas SD-14
Source control fact sheets are provided in Appendix A. In addition, fact sheets are provided for Roof
Runoff Controls (SD-02) and Alternative Building Materials (SD-05) which are not required by the Water
Quality Rules.
The following information is provided for each of the above-listed BMPs:
• Brief Description/Approach
• Suitable Applications
• Design Considerations
Figure 3.7: Example of Source Control Design for Outdoor Material Storage
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• Design Guidelines
• Examples
• Operations & Maintenance Recommendations
3.4. Treat Runoff
Today’s drainage systems must meet multiple purposes: protect property from flooding, control stream
bank erosion, and protect water quality. To achieve this, designers must integrate conventional flood
control strategies for large, infrequent storms with storm water quality control strategies.
There are several basic water quality strategies for treating runoff:
• Infiltrate runoff into the soil.
• Capture, store, and reuse runoff on site.
• Convey runoff slowly through vegetation.
• Treat runoff on a flow-through basis using various treatment technologies.
• Retain/detain runoff for later release with the detention providing treatment.
Solutions should be based on an understanding of the water quality and economic benefits inherent in
construction of systems that utilize or mimic natural drainage patterns. Site designs should be based
on site conditions and use these as the basis for selecting appropriate storm water quality controls. The
drainage system design process considers variables such as local climate, the infiltration rate and erosivity
of the soils, and slope.
Unlike conveyance models, which are assessed by simple quantitative measures (flood control volumes
and economics), water quality designs must optimize a complex array of both quantitative and qualitative
standards, including engineering worthiness, environmental benefit, horticultural sustainability, aesthetics,
functionality, maintainability, economics and safety.
Treatment Control BMPs are discussed in more detail in Section 5.
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4. Site and Facility Design for Water Quality
Protection
Many common site features can achieve storm water management goals by incorporating one or more
basic elements, either alone or in combination, depending on site and other conditions. The basic elements
include infiltration, retention/detention, biofilters, and structural controls. This section first describes these
basic elements, and then describes how these elements can be incorporated into common site features.
4.1. Infiltration
Infiltration is the process where water enters the ground and moves downward through the unsaturated
soil zone. Infiltration is ideal for management and conservation of runoff because it filters pollutants
through the soil and restores natural flows to groundwater and downstream water bodies. See Figure 4.1.
Figure 4.1: Infiltration Basin
The infiltration approach to storm water management seeks to “preserve and restore the hydrologic
cycle.” An infiltration storm water system seeks to infiltrate runoff into the soil by allowing it to flow
slowly over permeable surfaces (see Figure 4.2 and Figure 4.3).
The slow flow of runoff allows pollutants to settle into the soil where they are naturally mitigated. The
reduced volume of runoff that remains takes a long time to reach the outfall, and when it empties into a
natural water body or storm sewer, its pollutant load is greatly reduced.
Infiltration basins can be either open or closed. Open infiltration basins include ponds, swales and other
landscape features and are usually vegetated to maintain the porosity of the soil structure and to reduce
erosion. Infiltration trenches and dry wells can also fall into this category although not typically vegetated.
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Closed infiltration basins can be constructed under the land surface with open graded crushed stone,
leaving the surface to be used for parking or other uses. Subsurface closed basins are generally more
difficult to maintain and more expensive than open filtration systems, and are used primarily where high
land costs demand that the land surface be reclaimed for economic use.
Infiltration systems are often designed to capture the “first flush” storm event and used in combination
with a detention basin to control peak hydraulic flows. They effectively remove suspended solids,
particulates, bacteria, organics and soluble metals and nutrients through the vehicle of filtration,
absorption and microbial decomposition. Groundwater contamination should be considered as a potential
adverse effect and should be considered where shallow groundwater is a source of drinking water. In
cases where groundwater sources are deep, there is a very low chance of contamination from normal
concentrations of typical urban runoff.
Bioretention facilities have the added benefit of aesthetic appeal and small scale applicability. These
systems can be placed in parking lot islands, landscaped areas surrounding buildings, perimeter parking
lots, and other open space sections. Placing bioretention facilities on land that City regulations require
developers to devote to open space efficiently uses the land. An experienced landscape architect can
choose plant species and planting materials that are easy to maintain, aesthetically pleasing, and capable
of effectively reducing pollutants in runoff from the site.
Figure 4.2: Typical Infiltration Facility Schematic
Figure 4.3: Typical Bioretention Schematic
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Figure 4.4: Simple Detention System
Figure 4.5: Retention System
4.2. Retention and Detention
Retention and detention systems differ from infiltration systems primarily in intent. Retention and
detention systems may release runoff slowly enough to reduce downstream peak flows to their pre-
development levels, allow fine sediments to settle, and uptake dissolved nutrients in the runoff where
wetland vegetation is included. Detention systems are designed to capture and retain runoff temporarily
and release it to receiving waters at predevelopment flow rates (see Figure 4.4). Permanent pools of water
are not held between storm events. Pollutants settle out and are removed from the water column through
physical processes. Detention systems are allowed under the Water Quality Rules for the portions of the
WQV that are infeasible to treat with infiltration, harvest/reuse, and Biofiltration BMPs.
Retention systems capture runoff and retain it between storms as shown in Figure 4.5. While infiltration
does occur, water held in the system is displaced by the next significant rainfall event. Pollutants settle
out and are thereby removed from the water column. Because water remains in the system for a period of
time, retention systems benefit from biological and biochemical removal mechanisms provided by aquatic
plants and microorganisms.
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Constructed wetland systems retain and release storm water in a manner that is similar to retention or
detention basins. The design mimics natural ecological functions and uses wetland vegetation to filter
pollutants. The system needs a permanent water source to function properly and must be engineered to
remove coarse sediment, especially construction related sediments, from entering the pond. Storm water
has the potential to negatively affect natural wetland functions and constructed wetlands can be used to
buffer sensitive resources.
4.3. Biofilters
Biofilters can consist of vegetated swales and filter strips, green roofs, and engineered or proprietary
biofiltration devices such as planter boxes and tree box filters.
Swales are vegetated slopes and channels designed and maintained to transport shallow depths of
runoff slowly over vegetation (see Figure 4.6). Swales are generally effective if flows are slow
[1 ft / second (sec) maximum] and depths are shallow (4 inch maximum). The slow movement of runoff
through the vegetation provides an opportunity for sediments and particulates to be filtered and degraded
through biological activity. In most soils, the biofilter can also provide an opportunity for storm water
infiltration, which further removes pollutants and reduces runoff volumes.
Swales intercept both sheet and concentrated flows and convey these flows in a concentrated, vegetation-
lined channel. Grass filter strips intercept sheet runoff from the impervious network of streets, parking
lots, and rooftops and divert storm water to a uniformly graded meadow, buffer zone, or small forest.
Typically, the vegetated swale and grass strip-planting palette can comprise a wide range of possibilities
from dense vegetation to turf grass. Grass strips and vegetated swales can function as pretreatment
systems for water entering bioretention systems or other BMPs. If biofilters are to succeed in filtering
pollutants from the water column, the planting design must consider the hydrology, soils, and
maintenance requirements of the site.
Figure 4.6: Vegetated Swale
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Site and Facility Design for Water Quality Protection
Appropriate plants not only improve water quality, but provide habitat and aesthetic benefits. Selected
plant materials must be able to adapt to variable moisture regimes. Turf grass is acceptable if it can be
watered in the dry season, and if it is not inundated for long periods.
Biofilters also include engineered or proprietary systems such as planter boxes, tree box filters, or similar.
They function as soil and plant-based filtration systems, similar to swales that remove pollutants through
a variety or physical, biological and chemical treatment processes. The components normally consist of a
ponding area, mulch layer, planting soils, plantings, and either a pervious or impervious bottom layer with
underdrain (see Figure 4.7). These devices can work hand in hand with down spout disconnections.
Figure 4.7: Schematic of Planter Box with Down Spout Connection and Underdrain
4.4. Street Design
Street design is mandated by City standards. More than any other single element, street design has a
powerful impact on storm water quality. Street and other transportation-related structures typically can
comprise between 60 to 70% of the total impervious coverage in urban areas and, unlike rooftops, streets
are almost always directly connected to an underground storm water system.
Recognizing that street design can be the greatest factor in development’s impact on storm water quality,
it is important that designers and developers employ street standards that reduce impervious land
coverage. Directing runoff to biofilters or swales rather than underground storm drains produces a street
system that conveys storm water efficiently while providing both water quality and aesthetic benefits.
On streets where a more urban character is desired, or where a rigid pavement edge is required, curb and
gutter systems can be designed to empty into drainage swales. These swales can run parallel to the street,
in the parkway between the curb and the sidewalk, or can intersect the street at cross-angles, and run
between residences, depending on topography or site planning. Runoff travels along the gutter, but instead
of being emptied into a catch basin and underground pipe, multiple openings in the curb direct runoff into
surface swales or infiltration/detention basins.
In recent years, new street standards have been gaining acceptance that meets the access requirements of
local residential streets while reducing impervious land coverage. These standards create a new class of
street that is narrower and more interconnected than the current local street standard, called an “access”
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Site and Facility Design for Water Quality Protection
street. An access street is at the lowest end of the street hierarchy and is intended only to provide access to
a limited number of residences.
A street standard that allows an interconnected system of narrow access streets for residential
neighborhoods has the potential to achieve several complimentary environmental and social benefits.
A hierarchy of streets sized according to average daily traffic volumes yields a wide variety of benefits:
improved safety from lower speeds and volumes, improved aesthetics from street trees and green
parkways, reduced impervious land coverage, less heat island effect, and lower development costs. If the
reduction in street width is accompanied by a drainage system that allows for infiltration of runoff, the
impact of streets on storm water quality can be greatly mitigated.
A comparison of street cross-sections is shown in Figure 4.8.
Figure 4.8: Comparison of Street Cross-Section (Two-Way, Residential Access Streets)
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Site and Facility Design for Water Quality Protection
4.5. Street Trees
Trees improve water quality by intercepting and storing rainfall on leaves and branch surfaces, thereby
reducing runoff volumes and delaying the onset of peak flows. A single street tree can have a total
leaf surface area of several hundred to several thousand sq-ft, depending on species and size. This
aboveground surface area created by trees and other plants greatly contributes to the water holding
capacity of the land. They attenuate conveyance by increasing the soil’s capacity to filter rainwater and
reduce overland flow rates. By diminishing the impact of raindrops on un-vegetated soil, trees reduce soil
erosion. Street trees also have the ability to reduce ambient temperature of storm water runoff and absorb
surface water pollutants.
4.6. Parking Lots
In any development, storage space for stationary vehicles can consume many acres of land area, often
greater than the area covered by streets or rooftops. In a neighborhood of single-family homes, this
parking area is generally located on private driveways or along the street. In higher density residential
developments, parking is often consolidated in parking lots.
The space for storage of the automobile, the standard parking stall, occupies only 160 sq-ft, but when
combined with aisles, driveways, curbs, overhang space, and median islands, a parking lot can require
up to 400 sq-ft per vehicle, or nearly one acre per 100 cars. Since parking is usually accommodated on
an asphalt or concrete surface with conventional underground storm drain systems, parking lots typically
generate a great deal of DCIA.
There are many ways to both reduce the impervious land coverage of parking areas and to filter runoff
before it reaches the storm drain system.
4.6.1. Hybrid Parking Lots
Hybrid lots work on the principle that pavement use differs between aisles and stalls. Aisles must
be designed for speeds between 10 and 20 miles per hour (mph), and durable enough to support the
concentrated traffic of all vehicles using the lot. The stalls, on the other hand, need only be designed for
the 2 or 3 mph speed of vehicles maneuvering into place. Most of the time the stalls are in use, vehicles
are stationary. Hybrid lots reduce impervious surface coverage in parking areas by differentiating the
paving between aisles and stalls, and combining impervious aisles with permeable stalls, as shown in
Figures 4.9 and 4.10.
If aisles are constructed of a more conventional, impermeable material suitable for heavier vehicle use,
such as asphalt, stalls can be constructed of permeable pavement. This can reduce the overall impervious
surface coverage of a typical double loaded parking lot by 60% and avoid the need for an underground
drainage system.
Permeable stalls can be constructed of a number of materials including pervious concrete, unit pavers
such as brick or stone spaced to expose a permeable joint and set on a permeable base, crushed aggregate,
porous asphalt, turf block, and cobbles in low traffic areas. Turf blocks and permeable joints are shown in
Figures 4.11 and 4.12.
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Site and Facility Design for Water Quality Protection
Figure 4.11: Turf Blocks Figure 4.12: Permeable Joints
Figure 4.9: Hybrid Parking Lot Figure 4.10: Hybrid Parking Lot (Honolulu, HI)
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Site and Facility Design for Water Quality Protection
4.6.2. Parking Grove
A variation on the permeable stall design, a grid of trees and bollards can be used to delineate parking
stalls and create a “parking grove.” If the bollard and tree grids are spaced approximately 19 ft apart,
two (2) vehicles can park between each row of the grid. This 9.5 ft stall spacing is slightly more generous
than the standard 8.5 to 9 ft stall, and allows for the added width of the tree trunks and bollards. A benefit
of this design is that the parking grove not only shades parked cars, but also presents an attractive open
space when cars are absent. Examples of parking groves are shown in Figures 4.13 and 4.14.
4.6.3. Overflow Parking
Parking lot design is often required to accommodate peak demand, generating a high proportion of
impervious land coverage of very limited usefulness. An alternative is to differentiate between regular
and peak parking demands, and to construct the peak parking stalls of a different, more permeable,
material. This “overflow parking” area can be made of a turf block, which appears as a green lawn when
not occupied by vehicles, or crushed stone or other materials (see Figure 4.15). The same concept can
be applied to areas with temporary parking needs, such as emergency access routes, or in residential
applications, recreational vehicle (RV), or trailer parking.
Figure 4.15: Overflow Parking
Figure 4.13: Parking Grove 1 Figure 4.14: Parking Grove 2
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Site and Facility Design for Water Quality Protection
4.6.4. Porous Pavement with Subsurface Infiltration
In some cases, parking lots can be designed to perform more complex storm water management functions.
Constructing a stone-filled reservoir below the pavement surface and directing runoff underground by
means of perforated distribution pipes can achieve subsurface storm water storage and infiltration as
shown in Figure 4.16. Subsurface infiltration basins eliminate the possibilities of mud, mosquitoes
and safety hazards sometimes perceived to be associated with ephemeral surface drainage. They also
can provide for storage of large volumes of runoff, and can be incorporated with roof runoff collection
systems.
Figure 4.16: Subsurface Infiltration System
4.7. Driveways
Driveways can comprise up to 40% of the total transportation network in a conventional development,
with streets, turn-arounds, and sidewalks comprising the remaining 60%.
Driveway length is generally determined by garage setback requirements and width by city codes and
land use ordinances. Driveways to City streets shall have a minimum width of 12 ft excluding flares
(reference: City Standard Details for Public Works Construction). If garages are setback from the street,
long driveways are required, unless a rear alley system is included to provide garage access. If parking
for two vehicles side by side is required, a 20 ft minimum width is required. Thus, if a 20 ft setback and
a two-car-wide driveway are required, a minimum of 400 sq-ft of driveway will result, or 4% of a typical
10,000 sq-ft residential lot. If the house itself is compact, and the driveway is long, wide, and paved with
an impervious material such as asphalt or concrete, it can become the largest component of impervious
land coverage on the lot.
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Site and Facility Design for Water Quality Protection
An option to reduce the area dedicated to driveways is to allow for tandem parking (one vehicle in front
of another on a narrow driveway). In addition, if shared driveways are permitted, then two (2) or more
garages can be accessed by a single driveway, further reducing required land area. Rear alley access to
the garage can reduce driveway length, but overall impervious surface coverage may not be reduced if the
alleys are paved with impervious materials and the access streets remain designed to conventional city
standards.
Alternative solutions that work to reduce the impact of water quality problems associated with impervious
land coverage on city streets also work on driveways. Sloping the driveway so that it drains onto an
adjacent turf or groundcover area prevents driveways from draining directly to storm drain systems.
Use of turf-block or unit pavers on sand creates attractive, low maintenance, permeable driveways that
filter storm water (see Figure 4.17). Crushed aggregate can serve as a relatively smooth pavement with
minimal maintenance as shown in Figure 4.18. As shown in Figure 4.19, paving only under wheels is
a viable, inexpensive design if the driveway is straight between the garage and the street, and repaving
temporary parking areas with permeable unit pavers such as brick or stone can significantly reduce the
percentage of impervious area devoted to the driveway.
Figure 4.19: Paving only under Wheels
Figure 4.17: Unit Pavers Figure 4.18: Crushed Aggregate
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Site and Facility Design for Water Quality Protection
4.8. Landscape and Open Space
In the natural landscape, most soils infiltrate a high percentage of rainwater through a complex web of
organic and biological activities that build soil porosity and permeability. Roots reach into the soil and
separate particles of clay, insects excavate voids in the soil mass, roots decay leaving networks of macro
pores, leaves fall and form a mulch over the soil surface, and earthworms burrow and ingest organic
detritus to create richer, more porous soil. These are just a few examples of the natural processes that
occur within the soil.
Maintenance of a healthy soil structure through the practice of retaining or restoring native soils where
possible and using soil amendments where appropriate can improve the land’s ability to filter and slowly
release storm water into drainage networks. Construction practices such as decreasing soil compaction,
storing topsoil on-site for use after construction, and chipping wood for mulch as it is cleared for the land
can improve soil quality and help maintain healthy watersheds. Practices that reduce erosion and help
retain water on-site include incorporating organic amendments into disturbed soils after construction,
retaining native vegetation, and covering soil during revegetation.
Subtle changes in grading can also improve infiltration. Landscape surfaces are conventionally graded to
have a slight convex slope. This causes water to run off a central high point into a surrounding drainage
system, creating increased runoff. If a landscape surface is graded to have a slightly concave slope, it
will hold water. The infiltration value of concave vegetated surfaces is greater in permeable soils. Soils
of heavy clay or underlain with hardpan provide less infiltration value. In these cases, concave vegetated
surfaces must be designed as retention/detention basins, with proper outlets or under drains to an
interconnected system.
4.9. Multiple Small Basins
Biofiltration, infiltration, and retention/detention basins are the basic elements of a landscape designed
for storm water management. The challenge for designers is to integrate these elements creatively and
attractively in the landscape – either within a conventional landscape aesthetic or by presenting a different
landscape image that emphasizes the role of water and drainage.
Multiple small basins can provide a great deal of water storage and infiltration capacity. These small
basins can fit into the parkway planting strip or shoulders of street rights-of-way. If connected by culverts
under walks and driveways, they can create a continuous linear infiltration system. Infiltration and
retention/detention basins can be placed under wood decks, in parking lot planter islands, and at roof
downspouts. Outdoor patios or seating areas can be sunken a few steps, paved with a permeable pavement
such as flagstone or gravel, and designed to hold a few inches of water collected from surrounding
rooftops or paved areas for a few hours after a rain.
All of these are examples of small basins that can store water for a brief period, allowing it to infiltrate
into the soil, slowing its release into the drainage network, and filtering pollutants. An ordinary lawn can
be designed to hold a few inches of water for a few hours after a storm, attracting birds and creating a
landscape of diversity. Grass/vegetated swales can be integrated with landscaping, providing an attractive,
low maintenance, linear biofilter. Extended detention basins (dry ponds) store water during storms,
holding runoff to predevelopment levels. Pollutants settle and are removed from the water column before
discharging to streams. Wet ponds serve a similar purpose and can increase property values by providing
a significant aesthetic, and passive recreation opportunity.
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Site and Facility Design for Water Quality Protection
Plant species selection is critical for proper functioning of infiltration areas. Proper selection of plant
materials can improve the infiltration potential of landscape areas. Deep-rooted plants help to build soil
porosity. Plant leaf-surface area helps to collect rainwater before it lands on the soil, especially in light
rains, increasing the overall water-holding potential of the landscape.
A large number of plant species will survive moist soils or periodic inundation. These plants provide a
wide range of choices for planted infiltration/detention basins and drainage swales. Most inundated plants
have a higher survival potential on well-drained alluvial soils than on fine textured shallow soils or clays.
4.10. Planning Development Strategies in Practice
The importance of site planning in storm water quality protection is illustrated in the following
examples of development strategies: conventional residential subdivision (Figure 4.20: Alternative 1),
conventional subdivision employing BMPs (Figure 4.21: Alternative 2), and a mixed-use transit-
oriented development (Figure 4.22: Alternative 3). All three (3) examples are intended to accommodate
approximately 660 housing units on a 220-acre site adjacent to a stream.
The conventional residential subdivision (Figure 4.20: Alternative 1) accommodates 660 single-family
homes on individual lots. One-sixth acre lots are accessed by a network of 40 ft wide cul-de-sac streets,
with 5 ft sidewalks adjacent to the curb on each side of the street. The street and sidewalks are located
within a 60 ft right-of-way, which is covered with a 40 ft wide street and two (2)- 5 ft sidewalks, or 50 ft
of pavement, 100% impervious land coverage (streets only), and no room for street trees. No variation
exists in housing types (all single-family).
Both the streets and the open space features lack structure or hierarchy. The few direct connections
through the neighborhood result in long stretches of overly wide streets that discourage walking.
Conventional development design does not use the recreational or storm water benefits of the available
open space and does not respond to natural and topographic features. Preservation of open space is a low
priority, and the setback between the development and the stream is minimal. The remaining open space
character is remnant space offering residents no stream access or parks. Storm water travels through
a 15,000 ft network of drainpipes and in the absence of current permit requirements would discharge
untreated runoff directly into the stream. However, applying typical permit requirements, the development
would still be required to incorporate runoff treatment for the water quality design volume defined in
the local permit or MS4 new development program. For example, if the permit required treatment of
the runoff from 0.75 inches of rainfall, the development as planned had an overall percent impervious
value of 45%, and the designer was considering the use of an extended detention basin for treatment, this
would require a treatment volume of approximately 8.3 acre-ft. Based on typical detention basin design
practices, this could result in the need to dedicate approximately 2 to 3 acres of land, or the equivalent of
approximately 12 to 18 lots to incorporate the basin into the development near the point where drainage
enters the stream. Alternatively, if a watershed master plan for water quality had been adopted in which
the development could participate financially, the project would contribute financially based on its
required treatment volume and the cost allocation plan for the watershed program.
The hybrid/best practices subdivision (Figure 4.21: Alternative 2) illustrates a conventional
neighborhood that applies some storm water management practices. This attempt accommodates 660
single-family homes on individual lots. Streets are narrower, with the interior access streets at 28 ft
wide, while internal neighborhood collectors are 32 ft wide. All streets have detached sidewalks that
accommodate street trees planted between the sidewalk and the curb. This development sets the houses
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Site and Facility Design for Water Quality Protection
100 ft back from the stream and offers residents 12 acres of access to open space and parks. The
overall imperviousness has been reduced to about 41%, thereby reducing the volume to be treated to
approximately 7.5 acre-ft. A detention basin has been created in open space within the development.
Nearly 25% of the 13,000 ft network of piped storm water drains to a detention pond.
By employing a hierarchy of narrower streets this neighborhood requires 1,475 sq-ft of street per housing
unit, a reduction of 19% relative to the conventional sub-division.
The neo-traditional mixed-use neighborhood is illustrated as Alternative 3 (Figure 4.22). This
neighborhood includes 660 housing units, but also introduces other uses: retail, office, and live-work,
within a network of tree-lined streets and open space. The neighborhood drains to an open space
park adjacent to the stream, naturally and efficiently filtering storm water before it enters the stream.
Bioswales along key streets capture and treat storm water en-route to the stream, providing aesthetic
appeal and recreational opportunities. Alternative 3 requires 965 sq-ft of street per housing unit, a
reduction of 47% relative to the conventional sub-division. A strategically located transit system stops
near shops and higher density housing makes transit feasible. Every dwelling unit in the neighborhood
is within a 5-minute (min) walk from shops or transit. The overall imperviousness of this site has been
reduced to approximately 36%, further reducing the treatment volume. In addition, there are a variety of
opportunities to incorporate treatment for all of the remaining runoff within the open space park without
the need to dedicate any additional developable land.
A comparison of the three (3) alternatives is shown in Table 4.1.
Figure 4.20: Alternative 1 - Conventional
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Site and Facility Design for Water Quality Protection
Figure 4.21: Alternative 2 - Hybrid/Best Practices
Figure 4.22: Alternative 3 - Neo-Traditional
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Site and Facility Design for Water Quality Protection
Table 4.1: Comparison of Three Alternatives
Alternative 1 Alternative 2 Alternative 3
Total Site (acre)220 220 220
Number of Housing Units 660 660 660
Parks and Open Space (acre)0 12 12
Stream Setback (feet)0 100 500
Impervious Land Coverage - Street (acre)28 22 15
Percentage of Site that is Impervious - Street only 13%10%7%
Percentage of Site that is Impervious - Street only (relative to
conventional)
100%81%53%
Linear feet of Pipe 15,000 13,000 10,000
Linear feet of Swale 0 0 4,700
Width of Major Streets (feet)40 32 32
Width of Minor Streets (feet)None 28 28
Typical lots in Alternatives 2 and 3 are illustrated in three (3) forms: street loaded, alley fed and rural.
In the street-loaded form, lot size is still approximately 1/6 acre, but the lot is narrower and deeper,
thus reducing the amount of street frontage per household. The two-car garage is accessed from a front
driveway. This front-loaded street accounts for 63% impervious land coverage in the 60 ft right-of-way.
Looking at a typical street, the traditional residential neighborhood reduces the number of feet of
street and sidewalk per housing unit by nearly 40% compared to the conventional subdivision. This is
accomplished by two (2) means: a narrower street width (28 ft versus 40 ft), and narrower, deeper lots
(60 ft versus 65 ft wide). Narrower lots mean less street frontage per lot.
In the alley-loaded form, the street right-of-way is narrowed to 50 ft, leaving 4 ft for trees between the
sidewalk and curb. This form also employs the narrower street, achieving a 40% reduction in pavement
dedicated to street and sidewalk. A 16 ft-wide alley is provided in the back to access a garage at the
rear of each lot. Additional pavement for the alley is balanced by elimination of pavement for the front
driveway. This model assumes an impervious asphalt or concrete alley. Gravel alleys are feasible, and
improve permeability. In this form, narrower, deeper lots are employed to accommodate the depth
required for the alley.
The rural street form dramatically reduces impervious land coverage. The street is 19 ft wide with gravel
shoulders for trees and parking. Pedestrians walk on the gravel shoulder or share the street with slow-
moving cars.
Looking at a typical street, the rural form provides the greatest reduction in impervious land coverage.
Only 570 sq-ft of pavement of street is required per housing unit, a reduction of 62% compared to the
conventional sub-division.
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5. Treatment Control BMP Design
Treatment Control BMPs are engineered technologies designed to remove pollutants from storm water
runoff prior to discharge to the storm drain system or receiving waters. This section addresses BMP
numeric sizing criteria, guidance for infiltration testing BMPs, and individual BMP fact sheets to support
compliance with the Water Quality Rules.
5.1. Best Management Practices Selection
5.1.1. Determine Drainage Management Areas
Drainage management areas (DMAs) provide an important framework for feasibility screening, BMP
prioritization, and storm water management system configuration. BMP selection, sizing, and feasibility
determinations must be made at the DMA level; therefore, delineation of DMAs is highly recommended
at the conceptual site planning phase and is mandatory for completing the project design and meeting
submittal requirements. This section provides guidance on delineating DMAs that is intended to be used
when choosing BMPs.
DMAs are defined based on the proposed drainage patterns of the site and the BMPs to which they drain.
During the early phases of the project, DMAs shall be delineated based onsite drainage patterns and
possible BMP locations identified in the site planning process. DMAs should not overlap and should be
similar with respect to BMP opportunities and feasibility constraints. More than one DMA can drain to
the same BMP. However, because the BMP sizes are determined by the runoff from the DMA, a single
DMA may not drain to more than one (1) BMP (see Figure 5.1).
Figure 5.1: DMA Delineation
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Treatment Control BMP Design
In some cases, it may be appropriate in early planning phases to generalize the proposed treatment
plan by simply assigning a certain BMP type to an entire planning area and calculating the total sizing
requirement without identifying the specific BMP locations at that time. This planning area would be later
subdivided for design-level calculations.
BMPs must be sized to treat the WQV or Water Quality Flow (WQF) from the total area draining to
the BMP, including any offsite or onsite areas that combine with project runoff and drains to the BMP.
To minimize offsite flows treated by project BMPs, consider diverting upgradient flows subject to local
drainage and flood control regulation.
5.1.2. Evaluate Pollutants of Concern
BMPs selected for each DMA should address anticipated pollutants of concern. Pollutants identified in
the USEPA 303(d) list for specific water bodies in Hawaii include metals, nitrogen, nutrients (without
specifying nitrogen or phosphorus), indicator bacteria (i.e., fecal coliform), pesticides, and trash. Less
commonly cited pollutants include suspended solids, polychlorinated biophenyls (PCBs), and ammonium.
With respect to metals, typically, only the general term is used. In some cases, lead is identified.
The BMP plan development process typically includes consideration of:
• Receiving water quality (including pollutants for which receiving waters are listed as impaired
under CWA section 303(d)).
• Land use type of the development project and pollutants associated with that land use type.
• Pollutants expected to be present on site.
• Changes in storm water discharge flow rates, velocities, durations, and volumes resulting from the
development project.
• Sensitivity of receiving waters to changes in storm water discharge flow rates, velocities,
durations, and volumes.
It is important to realize that pollutants of concern for a water body can extend beyond those pollutants
listed in the 303(d) list as causing impairment. For example, trash is a pollutant of concern in most
communities, yet only a few water bodies are presently listed as impaired by trash. The key is to
remember that a pollutant does not need to cause an immediate impairment for it to be considered when
developing a BMP Plan.
Table 5.1 summarizes pollutants typically associated with Priority Project land uses and can be used as
general guidance when selecting BMPs.
5.1.3. Identify Candidate BMPs
Selecting BMPs based on pollutants of concern is a function of site constraints, constituents of concern,
BMP performance, stringency of permit requirements, and watershed specific requirements such as
TMDLs. Pollutants of concern are especially important in water limited stream segments and must be
carefully reviewed in relationship to BMP performance.
To facilitate comparison of the BMP characteristics and selection, a summary of the BMP categories,
expected pollutants, and BMP performance is presented in Tables 5.2, 5.3 and 5.4, respectively.
Developers shall consider the expected pollutants that could be generated at the site when choosing
BMPs.
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Table 5.1: Typical Pollutants Associated with Priority Projects
Priority Project Categories
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Priority A: Residential Development > one acre X X X X X X
Priority A: Commercial Development >one acre P(1)P(1)X P(3)P(5)X X P(2)
Priority B: Industrial X X X X X
Priority B: Automotive Repair Shops X X X X(4)(5)
Priority B: Restaurants X X P(1)X
Priority B: Parking Lots P(1)P(1)X P(1)X X
Priority B: Retail Gasoline Outlets X X X X
Priority B: Buildings taller than 100 ft in height X X X X X X
(All) Streets, Highways & Freeways P(1)X X X P(1)X X X(4)
X = anticipated P = potential
(1) A potential pollutant if landscaping exists onsite.(2) A potential pollutant if the project includes uncovered parking areas.(3) A potential pollutant if land use involves food or animal waste products.
(4) Including petroleum hydrocarbons.
(5) Including Solvents
Table 5.2: Treatment Control BMP Categories
BMP Retention Biofiltration Other
Infiltration Basin
Infiltration Trench
Subsurface Infiltration
Dry Well
Bioretention Basin
Permeable Pavement
Harvesting/Reuse
Green Roof
Vegetated Bio-filter1
Enhanced Swale
Downspout Disconnection
Vegetated Swale
Vegetated Buffer Strip
Detention Basin
Manufactured Treatment Device
Sand Filter
1 Includes both proprietary and non-proprietary systems.
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Table 5.3: Treatment Control BMP Expected Pollutant Removals
Priority Project Categories
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Infiltration Basin H H H H H H H H
Infiltration Trench H H H H H H H H
Subsurface Infiltration H H H H H H H H
Dry Well H H H H H H H H
Bioretention Basin H H H H H H H H
Permeable Pavement H H L H H H H H
Green Roof M H H M M H M M
Vegetated Bio-Filter M H H M U H H H
Enhanced Swale M H H U U M M U
Vegetated Swale L M L L U M M U
Vegetated Buffer Strip L M M L U M M M
Manufactured Tree Filter M H H M U H H H
Harvesting/Reuse H H L H H H H H
Detention Basin L M H L U M L/M U
Manufactured Treatment Device L M/H H L L M/H L L
Sand Filter L/M H H M U H M/H M/H
H = High, M = Medium, L = Low, U = Unknown
By using BMPs that are applicable and feasible, the project can achieve benefits of these practices, while
not incurring unnecessary expenses (associated with using practices that do not apply or would not be
effective) or creating undesirable conditions (for example, infiltration-related issues, vector concerns
including mosquito breeding, etc.).
For example, on Oahu, many high-density, ultra-urban districts are in tidally influenced coastal areas
and frequently encounter high ground water where infiltration may not be feasible. These projects
would most typically fall into the redevelopment category rather than new development, which presents
additional challenges with space constraints and soil quality. In these cases, Developers are encouraged
to utilize multiple BMPs in combination or series to meet the LID retention and treatment criteria. While
infiltration may not be possible, the Rules prioritize harvest and reuse systems to capture roof runoff
and use the water onsite. For the remaining runoff, biofiltration BMPs can be integrated into the overall
facility landscaping which may include green roofs, aboveground planter boxes, and tree box filters.
Configuration of these BMPs as either aboveground systems or with shallow footprints can overcome the
challenges associated with high ground water levels so that underdrain inverts are compatible with the
surrounding drainage infrastructure.
Table 5.4 presents several development types and BMPs that are generally appropriate to meet the
requirements of the Water Quality Rules for the various priority project types.
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Treatment Control BMP Design
Table 5.4: Recommended Selection of Permanent BMPs
Priority Development Type
In
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a
t
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n
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e
a
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Gr
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a
t
i
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Ha
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t
/
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e
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t
De
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i
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e
So
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c
e
C
o
n
t
r
o
l
A
Single Family Residential H H NR1 H M H H
Commercial/Institutional/Mixed Use H H M1 H H H H
Parks and Open Space H H N/A H N/A M N/A
A or B High Density/Ultra-urban, Buildings Greater than
100 feet tall, Retail Malls M2 H H H H H H
A or B Parking Lots H H N/A H N/A H H
B Industrial, Automotive Repair Shops, Restaurants,
Retail Gasoline Outlets M3 M3 M H M H H
H= Highly Appropriate; M= May be Appropriate; NR= Not Recommended; N/A= Not Applicable
1. Green roofs are not recommended for single family homes.
2. High density/ultra-urban redevelopment projects may frequently encounter space constraints and high ground water levels
which may limit the appropriateness or feasibility of infiltration BMPs.3. Infiltration BMPs are not recommended for areas with high potential for concentrated pollutants or chemical spills. Source Control BMPs should be implemented in those areas.
5.1.4. Consider Operation and Maintenance Requirements
Once BMPs have been selected based on performance and appropriateness for site conditions, both
installation cost and operation and maintenance cost can become an important differentiator in BMP
selection. Treatment Control BMP costs vary depending on the type of BMP installed. Table 5.5 shows
maintenance costs compiled by the County of San Diego which looked at the maintenance time and
costs for large, medium, and small BMPs. In general, the more time consuming maintenance resulted in
higher costs. While permanent BMP maintenance is the responsibility of the owner, the City recommends
that BMPs be chosen with consideration to maintenance requirements when possible. For instance,
underground vault-type devices may provide valuable space savings, but there are concerns that regular
maintenance will not be provided, as well as the worker safety and public safety risks presented during
maintenance activities.
Table 5.5: Summary of Operation and Maintenance Effort for BMPs
BMP Small BMP Medium BMP Large BMP
Annual
Hours
Annual
Cost
Annual
Hours
Annual
Cost
Annual
Hours
Annual
Cost
Bioretention Area 32.0 $ 3,174 44.0 $ 4,078 68.0 $ 5,877
Flow-Through Planter 24.0 $ 2,367 30.0 $ 2,882 42.0 $ 3,781
Cistern With Bioretention 33.2 $ 3,186 38.2 $ 3,505 48.2 $ 4,255
Dry Detention Basin with Grass/
Vegetated Lining
25.8 $ 2,433 32.6 $ 3,204 53.0 $ 4,734
Source: San Diego Stormwater Urban Mitigation Plan website: http://www.sandiegocounty.gov/content/dam/sdc/dpw/
WATERSHED_PROTECTION_PROGRAM/susmppdf/bmp_om_cost_2012.xlsx. Costs in Hawaii may be higher.
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Treatment Control BMP Design
BMP Small BMP Medium BMP Large BMP
Annual
Hours
Annual
Cost
Annual
Hours
Annual
Cost
Annual
Hours
Annual
Cost
Dry Detention Basin with
Impervious Lining
15.8 $ 1,412 20.6 $ 2,067 35.0 $ 3,147
Underground Vault 19.8 $ 1,819 24.6 $ 2,418 39.0 $ 3,498
Cistern 15.2 $ 1,423 17.2 $ 1,830 23.2 $ 2,280
Infiltration Basin 28.6 $ 2,679 33.0 $ 3,297 46.2 $ 4,287
Infiltration Trench 17.8 $ 1,564 25.0 $ 2,418 46.6 $ 4,037
Wet Pond/Basin (Permanent Pool)113.8 $ 10,037 214.6 $ 17,081 517.0 $ 39,752
Constructed Wetland 109.8 $ 9,708 210.6 $ 16,763 513.0 $ 39,433
Vegetated Swale 15.3 $ 1,476 20.9 $ 2,203 26.5 $ 2,623
Austin Sand Filter 19.6 $ 1,712 24.4 $ 2,374 38.8 $ 3,454
Delaware Sand Filter 19.6 $ 1,712 24.4 $ 2,374 38.8 $ 3,454
Multi-Chambered Treatment Train 27.9 $ 2,587 32.7 $ 3,114 47.1 $ 4,194
Tree-Pit-Style Unit 15.6 $ 1,491 15.6 $ 1,491 15.6 $ 1,491
Vault Based Filtration with Replaceable Cartridges 23.8 $ 2,050 28.6 $ 2,690 43.0 $ 3,769
Swirl Concentrator 15.0 $ 2,062 15.0 $ 2,062 15.0 $ 2,062
Catch Basin Insert 15.0 $ 1,493 15.0 $ 1,493 15.0 $ 1,493
Catch Basin Insert with
Hydrocarbon Boom
17.0 $ 1,707 17.0 $ 1,707 17.0 $ 1,707
Permeable Pavements 7.8 $ 808 13.6 $ 1,206 23.2 $ 1,926
Self-Retaining 6.8 $ 598 6.8 $ 598 6.8 $ 598
Vegetated Roof 17.0 $ 1,975 21.0 $ 2,252 33.0 $ 3,152
Source: San Diego Stormwater Urban Mitigation Plan website: http://www.sandiegocounty.gov/content/dam/sdc/dpw/
WATERSHED_PROTECTION_PROGRAM/susmppdf/bmp_om_cost_2012.xlsx. Costs in Hawaii may be higher.
For the purpose of this document, the average annual maintenance time for a BMP device used to
determine the Maintenance Ranks are defined in Table 5.6. These maintenance ranks are specified on
each treatment control fact sheet (Appendix B).
Table 5.6: Maintenance Rank Definition
Maintenance Rank Definition
Low BMPs with an average maintenance requirement of less than 25 hours per year.
Medium BMPs with an average maintenance requirement of between 25 and 50 hours per year.
High BMPS with an average maintenance requirement of over 50 hours per year.
Long-term O&M guidelines are presented in Section 7.
Table 5.5: Summary of Operation and Maintenance Effort for BMPs (Continued)
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Treatment Control BMP Design
5.1.5. Other Considerations
Vector Breeding Considerations
The potential of a BMP to create vector breeding habitat and/or harborage should also be considered when
selecting BMPs. Mosquito and other vector production is a nuisance and public health threat.
Mosquitoes can breed in standing water almost immediately following a BMP installation and may
persist at unnaturally high levels and for longer seasonal periods in created habitats. BMP siting, design,
construction, and maintenance must be considered in order to select a BMP that is least conducive
to providing habitat for vectors. Tips for minimizing vector-breeding problems in the design and
maintenance of BMPs are presented in the BMP fact sheets. Certain BMPs, including ponds and wetlands
and those designed with permanent water sumps, vaults, and/or catch basins (including below ground
installations), may require routine inspections and treatments.
Threatened and Endangered Species Considerations
The presence or potential presence of threatened and endangered species should also be considered when
selecting BMPs. Although preservation of threatened endangered species is crucial, treatment BMPs are
not intended to supplement or replace species habitat except under special circumstances. The presence of
threatened or endangered species can hinder timely and routine maintenance, which in turn can result in
reduced BMP performance and an increase in vector production. In extreme cases, rights to the treatment
BMP and surrounding land may be lost if threatened or endangered species utilize or become established
in the BMP.
When considering BMPs where there is a presence or potential presence of threatened or endangered
species, early coordination with DLNR and USFWS is essential. During this coordination, the
purpose and the long-term operation and maintenance requirements of the BMPs need to be clearly
established through written agreements or memorandums of understanding. Absent firm agreements or
understandings, proceeding with BMPs under these circumstances is not recommended.
5.2. Numeric Sizing Criteria
Based on the selected BMPs, the capacity and primary design sizing criteria must be established using a
combination of local hydrology, project drainage characteristics (i.e., percent imperviousness or runoff
coefficient), and numerical sizing requirements. BMPs will be volume-based, flow-based, or demand-
based, and must be able to effectively treat the design quantity. Peak storm event flows must also be taken
into account if the BMP is a flow-based BMP, or a volume-based BMP that must also safely pass the
design storm (i.e., an in-line detention basin).
This section presents the methodology for calculating the WQV and WQF Rate, which are used to size the
majority of the Treatment Control BMPs. Calculations for the WQV and WQF should not include DMAs
that are considered Self-Mitigating (refer to Site Design Strategies).
5.2.1. Volume-Based Best Management Practice Design
Volume-based BMP design standards apply to BMPs whose primary mode of pollutant removal depends
on the volumetric capacity of the BMP. Examples of BMPs in this category include detention basins,
retention basins, and infiltration. Typically, a volume-based BMP design criteria calls for the capture and
infiltration or treatment of a certain percentage of the runoff from the project site.
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Treatment Control BMP Design
The WQV is calculated using the following equation:
WQV = PCA G 3630
Where WQV =Water Quality Design Volume [cubic feet (cu-ft)]
P =Design Storm Runoff Depth (in)
C =Volumetric Runoff Coefficient
A =Tributary Drainage Area (acres)
As specified in the Water Quality Rules, a design storm runoff depth of 1 inch shall be used for LID
retention BMPs and 1.5 inches shall be used for all treat and release BMPs including LID biofiltration and
alternative compliance BMPs. In previous analysis, the City determined that the 1-inch design storm was
equal to or exceeded the 24-hr 85th percentile storm for most of Oahu.
The volumetric runoff coefficient shall be calculated using the following equation as developed by
USEPA for smaller storms in urban areas:
C = 0.05 + 0.009I
Where C =Volumetric Runoff Coefficient
I =Percent of Impervious Cover (percent)
A graph presenting the relationship between the percent of impervious cover and the unit water quality
design volume for a 1-inch and 1.5-inch runoff depth is shown in Figure 5.2.
Figure 5.2: Unit Water Quality Volume for 1 and 1.5 inch Runoff Depth
5.2.2. Flow-Based BMP Design
Flow-based BMP design standards apply to BMPs whose primary mode of pollutant removal depends on
the rate of flow of runoff through the BMP. Examples of BMPs in this category include swales, screening
devices, and many proprietary products. Typically, flow-based BMP design criteria calls for the capture
and infiltration or treatment of the flow runoff produced by rain events of a specified magnitude.
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Treatment Control BMP Design
The design WQF rate is calculated using the Rational Formula:
WQF = 1.5 G CiA
Where WQF =Water Quality Design Flow Rate (cu-ft/sec)
C =Runoff Coefficient
i =Peak Rainfall Intensity (in/hr)
A =Tributary Drainage Area (acres)
As specified in the Water Quality Rules, a peak rainfall intensity of 0.4 inches per hour shall be used. The
runoff coefficient shall be determined from Table 5.7. For drainage areas containing multiple land uses,
the following formula may be used to compute a composite weighted runoff coefficient:
n
CC = ( ∑CiAi )/At i=1
Where CC =Composite Weighted Runoff Coefficient
C1,2,..n =Runoff Coefficient for each Land Use Cover Type
A 1,2,..n =Drainage Area to each Land Use Cover Type (acres)
At =Total Drainage Area (acres)
Table 5.7: Runoff Coefficients for Water Quality Flow Calculations
Type of Drainage Area Runoff Coefficient Type of Drainage Area Runoff Coefficient
Roofs 0.90 Permeable interlocking
concrete pavement
0.10
Concrete 0.80 Grid pavements with grass or aggregate surface 0.10
Stone, brick, or concrete
pavers with mortared joints
and bedding
0.80 Crushed aggregate 0.10
Asphalt 0.70 Grass 0.10
Stone, brick, or concrete
pavers with sand joints and
bedding
0.70 Grass over Porous Plastic 0.05
Pervious Concrete 0.10 Gravel over Porous Plastic 0.05
Porous asphalt 0.10
Note that these C-factors are only appropriate for small storm treatment design and are not appropriate for flood control
facilities.
A graph presenting the relationship between the weighted runoff coefficient and the unit water quality
design flow rate is shown in Figure 5.3.
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Treatment Control BMP Design
Figure 5.3: Unit Water Quality Flow
5.2.3. Combined Volume-Based and Flow-Based BMP Design
Volume-based BMPs and flow-based BMPs do not necessarily treat precisely the same storm water
runoff. For example, an on-line volume-based BMP such as a detention basin will treat the design runoff
volume and is essentially unaffected by runoff entering the basin at an extremely high rate, say from a
very short, but intense storm that produces the design volume of runoff. However, a flow-based BMP
might be overwhelmed by the same short, but intense storm if the storm intensity results in runoff rates
that exceed the flow-based BMP design flow rate. By contrast, a flow-based BMP such as a swale will
treat the design flow rate of runoff and is essentially unaffected by the duration of the design flow, say
from a long, low intensity storm. However, a volume-based detention basin subjected to this same rainfall
land runoff event will begin to provide less treatment or will go into bypass or overflow mode after the
design runoff volume is delivered.
Therefore, there may be some situations where designers need to consider both volume-based and flow
based BMP design criteria. An example of where both types of criteria might apply is an off-line detention
basin. For an off-line detention basin, the capacity of the diversion structure could be designed to comply
with the flow-based BMP design criteria while the detention basin itself could be designed to comply with
the volume-based criteria.
When both volume-based and flow based criteria apply, the designer should determine which of the
criteria apply to each element of the BMP system, and then size the elements accordingly.
5.3. Infiltration Requirements
LID Retention BMPs rely on the soil’s ability to infiltrate storm water runoff. This section outlines the
design requirements applicable to all infiltration facilities.
5.3.1. Soil Types and Textures
The soil types within the subsoil profile, extending a minimum of 3 ft below the bottom of the proposed
facility, should be identified to verify the infiltration rate or permeability of the soil. The infiltration
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Treatment Control BMP Design
rate, or permeability, measured in inches per hour, is the rate at which water passes through the soil
profile during saturated conditions. Although the units of infiltration rate and hydraulic conductivity of
soils are similar, there is a distinct difference between these two (2) quantities. They cannot be directly
related unless the hydraulic boundary conditions are known, such as hydraulic gradient and the extent of
lateral flow of water, or can be reliably estimated. Minimum and maximum infiltration rates establish the
suitability of various soil textural classes for infiltration. Each soil texture and corresponding hydrologic
properties within the soil profile are identified through analysis of a gradation test of the soil boring
material. Table 5.8 presents a list of the infiltration rates for the soil textures of the U.S. Department of
Agriculture (USDA) Textural Triangle, presented in Figure 5.4.
Table 5.8: Typical Soil Infiltration Ratesa
Texture Class Hydrologic Soil
Group
Infiltration Rate
(inches/hour)
Sand A 8.00
Loamy Sand A 2.00
Sandy Loam B 1.00
Loam B 0.50
Silt Loam C 0.25
Sandy Clay Loam C 0.15
Clay Loam D 0.09
Silt Clay Loam D < 0.09
Clay D < 0.05
a - Source: ASCE, 1998
Figure 5.4: USDA Soils Textural Triangle
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Treatment Control BMP Design
Soil textures acceptable for use with infiltration systems include those with infiltration rates equal to
or above 0.50 inches per hour (a soil texture indicative of loam). Soil textures with rates less than 0.50
inches per hour are not suitable as it increases the risk of the BMP not draining properly and creating
localized areas of standing water. It is important to note however, that Hydrologic Soil Group (HSG)
“D” soils (e.g., clay loam, silty clay loam, and silty clay) in Oahu have been shown to perform better
than their counterparts in the Continental United States. As a result, locations with HSG “D” soils should
not be automatically rejected as candidate sites for infiltration BMPs without the opinion of a licensed
professional engineer with geotechnical expertise.
5.3.2. Field Investigations
According to the Water Quality Rules, infiltration can be considered infeasible if the soil at the site is
HSG “C” or “D,” or if the infiltration rate is less than 0.5 in/hr. For HSG “A” and “B” soils, and those
sites who wish to use infiltration even with HSG C or D soils, soil investigations and infiltration tests are
required for infiltration facilities to accurately determine the local soil characteristics and capacity for
infiltration.
Soil Lithology and Depth to Groundwater
An initial soil investigation is recommended to adequately evaluate soil lithology and determine if there
are potential problems in the soil structure that would inhibit the rate or quantity of infiltration desired; or
if there are potential adverse impacts to structures, slopes or groundwater that could result from locating
the device nearby.
Geotechnical test pits or borings should be dug to a minimum of 5 ft deep below the proposed device
invert, or as determined by the licensed professional engineer with geotechnical expertise. A test pit
allows visual observation of the soil horizons and overall soil conditions both horizontally and vertically
in that portion of the site. Although the use of soil borings is permitted at the recommendation of a
geotechnical professional, it is discouraged as a substitute for test pits as visual observation is narrowly
limited in a soil boring and the soil horizons cannot be observed in-situ, but must be observed from the
extracted borings.
The soil profiles should be carefully logged to determine variations in the subsurface profile. Samples
should be collected from the soil profiles at different horizons and transported to a laboratory for soil
indices testing, plasticity, and chemical testing. In addition, the test pits or samples from borings should
be examined for other characteristics that may adversely affect infiltration. These include evidence of
significant mottling (indicative of high groundwater), restrictive layer(s), and significant variation in soil
types, either horizontally or vertically.
An initial indication of the seasonal high groundwater water table elevation should be determined by
using a piezometer or other accepted geotechnical means. The piezometer should be installed to a depth
of at least 20 ft below the proposed device invert using the direct push or other suitable method. Initial
groundwater levels shall be recorded at least 24 hrs after installation. The geotechnical professional will
make a determination whether the groundwater elevation determined after 24 hrs can be considered to be
a reasonable indication of the seasonal high water table for the site.
Permeability Testing
Infiltration rate tests are used to help estimate the maximum sub-surface vertical infiltration rate of the soil
below a proposed infiltration facility (e.g., infiltration trench or infiltration basin). The tests are intended
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Treatment Control BMP Design
to simulate the physical process that will occur when the facility is in operation; therefore a saturation
period is required to approximate the soil moisture conditions that may exist prior to the onset of a runoff
event. Laboratory tests are strongly discouraged, as a homogeneous laboratory sample does not represent
field conditions. Infiltration tests should be conducted in the field. Tests should not be conducted in the
rain or within 24 hrs of significant rainfall events (greater than 0.5 inches).
There are a variety of infiltration field test methodologies to determine the infiltration rate of a soil,
the two most coming being the Falling Head Percolation Test and the Double-Ring Infiltrometer Test.
The actual testing protocols and methods used for a specific project should be determined by a licensed
professional engineer with geotechnical expertise.
Table 5.9: Test Pit/Boring Requirements for Infiltration
Facility Recommended # of Permeability Tests
• Infiltration Basin
• Subsurface Infiltration
• Dry Well
• Bioretention Basin
• Permeable Pavement
1 test per 2,500 sq-ft
• Infiltration Trench 1 test per 100 linear ft
5.3.3. Design Infiltration Rates
To account for uncertainties and inaccuracies in testing, a correction (i.e., safety) factor shall be applied to
the assumed or measured infiltration rate to produce a design infiltration rate for BMP sizing calculations.
The minimum safety factor for infiltration facilities is 2.
5.4. Technology Certification
This manual does not endorse specific proprietary products, although many are described. It is left to
each developer to determine which proprietary products may be used, and under what circumstances.
When considering a proprietary product, the Water Quality Rules require that the developer consider
performance data, determined by established protocols. The City accepts certifications of product
performance by the Washington State Department of Ecology [Technology Assessment Protocol- Ecology
(TAPE)], for the New Jersey's Department of Environmental Protection [New Jersey Corporation for
Advanced Technology (NJCAT)].
For proprietary biofiltration systems, the BMP must be certified for general use by TAPE for Enhanced
Treatment (for the treatment of dissolved metals), Phosphorous Treatment, or Oil Treatment, according to
the predominant pollutant(s) of concern at that site.
For alternative compliance, the device must provide, at minimum, a TSS removal rate of 80%,
certified for general use by TAPE or verified by NJCAT consistent with the New Jersey Department of
Environmental Protection (NJDEP) protocols.
It can be expected that subsequent to the publishing of this manual, new public-domain technologies
will be proposed (or design criteria for existing technologies will be altered) by development engineers.
As with proprietary products, it is advised that new public-domain technologies be considered only if
performance data are available and have been collected following a widely accepted protocol.
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Treatment Control BMP Design
5.5. Feasibility Criteria
Individual BMPs may not be feasible at the site due to site constraints or activities. The City specified
feasibility criteria for each BMP in an effort to protect ground water quality, archaeological resources,
facilities from inadequate drainage, and avoid compromising the geotechnical/structural integrity of
surrounding properties. Infeasibility must be documented using the Feasibility Screening Worksheet
(Appendix F of the Water Quality Rules).
5.5.1. Infiltration Feasibility
Infiltration BMPs are infeasible and must not be used if any of the following conditions are met:
1. Soils beneath the BMP invert have measured infiltration rates of less than 0.5 in/hr or are USDA
HSG “C” or “D” as reported by the USDA Natural Resources Conservation Service.
2. The seasonally high groundwater table is within 3 ft from the BMP invert.
3. There is a documented concern that there is a potential onsite for soil pollutants, ground water
pollutants, or pollutants associated with industrial activities to be mobilized.
4. There are geotechnical concerns at the site.
5. Excavation for the installation of the BMP would disturb iwi kupuna or other archeological
resources.
6. The BMP cannot be built within the following setbacks:
Distance From the nearest...
10 ft private property line
20 ft building foundation at the project site
35 ft septic system
50 ft drinking water well
100 ft down-gradient building foundation
5.5.2. Harvest/Reuse Feasibility
Harvest/Reuse is considered infeasible for the any of the flowing reasons:
1. The demand is inadequate to reuse the required volume of water.
2. The technical requirements cause the harvesting system to exceed 2% of the total project cost.
3. The site where a cistern must be located is at a slope greater than 10%.
4. There is no available space to locate a cistern of adequate size to harvest and use the required
amount of water.
5. The cistern cannot be built within the following setbacks:
Distance From the nearest...
5 ft private property line or building foundation
10 ft septic system
6. The project includes a reclaimed water system and demand for a harvest/reuse system cannot be
met.
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5.5.3. Biofiltration Feasibility
Biofiltration BMPs must be evaluated individually for feasibility as each has different applications. Only
if all biofiltration BMPs are infeasible can alternative compliance be used.
1. Vegetated Biofilters are infeasible for any of the following reasons:
a. Excavation would disturb iwi kupuna or other archaeological resources.
b. The invert of underdrain layer is below seasonally high groundwater table.
c. The site does not receive enough sunlight to support vegetation.
d. The site lacks sufficient hydraulic head to support BMP operation by gravity.
e. Unable to operate off-line with bypass and unable to operate in-line with safe overflow
mechanism.
2. Green Roofs are infeasible for any of the following reasons:
a. The roof is for a single family residential dwelling.
b. Roof space is unavailable due to renewable energy, electrical, and/or mechanical systems.
c. Slope on roof exceeds 25% (14 degrees).
3. Dry Swales or Enhanced Swales are infeasible for any of the following reasons:
a. Excavation would disturb iwi kupuna or other archaeological resources.
b. The invert of underdrain layer is below seasonally high groundwater table.
c. The site lacks sufficient head to support BMP operation by gravity.
d. Unable to operate off-line with bypass and unable to operate in-line with safe overflow
mechanism.
4. Vegetated Swales are infeasible for any of the following reasons:
a. The excavation would disturb iwi kupuna or other archaeological resources.
b. The site does not receive enough sunlight to support vegetation.
c. Unable to operate off-line with bypass and unable to operate in-line with safe overflow
mechanism.
5. Vegetated filter strips are infeasible for any of the following reasons:
a. Excavation would disturb iwi kupuna or other archaeological resources.
b. The site does not receive enough sunlight to support vegetation.
c. Unable to operate off-line with bypass and unable to operate in-line with safe overflow
mechanism.
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As described in Section 1 of this document, the Water Quality Rules requires that a licensed engineer
inspect BMPs during construction and must certify that they were built according to plan. Precautions
must be taken during construction to ensure that permanent BMPs are installed as designed and function
as intended.
The first and most important step in protecting permanent BMPs during construction is to utilize
phasing to minimize the exposure of these structures to sediment. The following is a typical construction
sequence. Depending on the BMP and design variation, alterations may be necessary. Note that Erosion
and Sediment Control methods must adhere to the Water Quality Rules requirements throughout the
duration of construction.
1. Protect future infiltration areas from compaction prior to installation. Clearly mark the existing
vegetated areas to be preserved and future infiltration facilities with flags or temporary fencing.
2. Stabilize the entire area draining to the infiltration facility before construction of the infiltration
facility begins. Or, construct a diversion berm around the perimeter of the infiltration site to
prevent sediment transport during construction.
3. Excavate Structural Infiltration facilities to a uniform, level, uncompacted subgrade, free from
rocks and debris. Excavation should be performed with the lightest practical equipment and
should be placed outside the limits of the infiltration facility. If the use of heavy equipment
on the base of the facility cannot be avoided, the infiltrative capacity must be restored by soil
amendments or aerating prior to placing the infiltrative bed.
4. Complete final grading to achieve proposed design elevations, leaving space for upper layer of
compost, mulch or topsoil as specified on plans.
5. Plant vegetation according to planting plan. Erosion and sediment control measures, such as
temporary seeding and erosion control mats, should be used on vegetated slopes if appropriate.
6. Where pervious pavement is to be installed, installation of the pavement shall be scheduled as the
last installation at a development site. Vehicular traffic should be prohibited for at least two (2)
days following installation. Site materials should not be stored on pervious pavement.
7. Continue to use erosion and sediment control BMPs such as inlet protection and perimeter control
to protect permanent BMPs until they are ready to be brought online. Once the drainage area is
completely and permanently stabilized, the system can be brought online.
8. Consider re-verifying permeability prior to acceptance of the BMP.
For proprietary systems, the City recommends to follow manufacturers’ installation guidelines while
applying the basic principles outlined above.
Guidance on maintenance of BMPs after construction is presented in Section 7.
6. Considerations for Construction
Treatment Control BMPs
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7. Operations and Maintenance of BMPs
Once built, BMPs require ongoing, long-term inspection and maintenance to ensure that BMPs are
meeting the specified design criteria for storm water flow rate, volume and water quality. If the BMPs are
not properly maintained the effectiveness of the BMP decreases and may impact water quality. Routine
maintenance will also help avoid more costly rehabilitative maintenance to repair damages that may occur
when BMPs have not been adequately maintained on a regular basis. For these reasons, the City requires
project applicants to develop and record an O&M Plan as part of the permitting process.
7.1. Consideration when Selecting Treatment BMPs
The long-term performance of BMPs hinges on ongoing and proper maintenance. Consideration of the
time and funding necessary to support long term maintenance should be included as part of the process
when selecting treatment BMP(s). Maintenance costs will also include disposal of accumulated residuals.
Residuals include trash, oil and grease, filter media and fine sediments.
7.1.1. Sediment and Oil Removal and Disposal
Over time, BMPs will accumulate sediment, which will need to be removed to prevent clogging and
reduction in effectiveness. Routine maintenance activities need to include measuring the depth of
sediment as part of routine maintenance events and included in the maintenance log. Sediment should be
removed when sediment accumulation reaches thirty (30) percent of total capacity. Thirty (30) percent of
a facility’s capacity is calculated by multiplying the total capacity by 0.30.
Some facilities may require professional assistance, such as underground facilities and manufactured
facilities that have confined spaces. These types of facilities require proper certifications to enter or
must be cleaned by a vactor truck. Vacuuming with a vactor truck or street sweeping equipment may be
required for certain components, such as collection basins, piping or pervious pavement systems.
The owner is responsible for properly disposing accumulated residuals. Current research generally
indicates that residuals are not hazardous wastes and can be disposed of residuals the same way any
uncontaminated soil would be dispose of, following dewatering. If the BMP treats runoff from areas
where chemical or hazardous wastes could come into contact with storm water and an oily sheen, odor,
discoloration or other signs of pollution is observed, hire a professional laboratory or sampling firm to
assess whether the material needs specialized hauling, treatment, or disposal. If you need assistance
deciding whether the solids must be managed as hazardous waste, contact the City Department of
Environmental Services - Solid Waste Division.
7.1.2. Maintenance Costs
The City requires the applicant to include a description of the funding mechanism for long-term operation
and maintenance in the SWQR. For homeowner associations, this could be a portion of homeowner fees
or a specific assessment.
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Operations and Maintenance of BMPs
Maintenance costs need to include both routine (i.e., inspection, debris removal, or vegetation
management) and non-routine (i.e., sediment removal, facility component repair/replace or major
replanting) maintenance activities. The annual routine maintenance costs are typically between five (5)
to ten (10) percent of the facility’s total capital cost. For non-routine maintenance costs (i.e.: sediment
removal or vegetative replacement), the owner should set aside a percentage of the non-routine
maintenance costs per year based on the needed frequency. For example, if the facility needs sediment
removal every five (5) years, twenty (20) percent of the total cost for sediment removal must be put aside
each year. An additional three (3) to five (5) percent of the capital costs should also be incorporated into
the overall maintenance costs for eventual facility replacement. The life expectancy for most treatment
BMPs is between twenty-five (25) to fifty (50) years. The owner is responsible for replacing BMPs at the
end of their lifecycle.
7.2. Developing the O&M Plan
Detailed O&M plans are required under the Water Quality Rules Section (§) 20-3-53. The O&M Plan is
required to identify the specific maintenance activities and frequencies for each type of BMP. In addition,
these should include indicators for assessing when “as needed” maintenance activities are required.
An O&M plan should be prepared by the project proponents and submitted to the DFM Storm Water
Quality Branch (SWQ) prior to closure of any building, grading, grubbing, trenching, or stockpiling
permits for Priority A and B projects.
The Water Quality Rules require the following as part of the O&M Plan
• Name, phone number and mailing address for the owner of the property.
• Name and phone number for the individual(s), association, or management company responsible
ensuring maintenance is being performed.
• Maintenance activities for each BMP.
• Inspection frequencies for each BMP.
• A post-construction BMP plan showing the location of each BMP with a summary of the
maintenance activities and inspection schedule for each BMP.
An O&M Template is available from the CCH and is available on their website. General maintenance
activities and frequencies that should be included in the O&M plan are provided in Appendix C.
7.3. Maintenance Agreements, Certification, and Modifications
An O&M plan is particularly valuable during ownership transitions; for example, when a developer
transitions maintenance to a homeowners association, or when a developer turns over maintenance to
a new owner. The BMP maintenance plan is also important when evaluating properties for acquisition,
allowing long-term costs associated with BMPs to be factored into the property purchase agreement.
Because of the long-term nature of these BMPs, the City requires that the BMP plan and O&M plan be
recorded in the State of Hawaii Land Court or Bureau of Conveyances for privately-owned Real Property.
A copy of the recorded O&M plan shall be submitted to the Department and Director of the DFM prior to
closing the building and/or grading, grubbing, stockpiling or trenching permit(s).
Another valuable factor for ensuring BMP effectiveness is ensuring the BMP is installed according to
plan. The City requires the owner to retain/hire a Licensed Engineer in the State of Hawaii and certified
Storm Water BMP Guide for New and Redevelopment
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Operations and Maintenance of BMPs
by the DPP to observe the installation of BMPs during construction and submit a signed Certificate
of Completion Form to the City prior to closing the building and/or grading, grubbing, stockpiling or
trenching permit(s).
Any modifications to the O&M plan after permit closure must be approved by DFM. Modifications to the
O&M plan will not be accepted if it reduces the level of protection from pollutant discharge.
7.4. Inspection Requirements
The owner is responsible for operation and maintenance inspections under the Water Quality Rules. The
owner is required, at a minimum, to conduct an annual inspection of the installed BMP(s) and retain
maintenance and inspection records for at least five (5) years. Furthermore, the owner is required to allow
the City access to the BMPs for annual inspections to confirm compliance with the approved O&M plan.
The actual maintenance needs may be more frequently than annually. The need for maintenance depends
on the type of BMP, amount and quality of runoff delivered to the structural BMP and any pretreatment
facilities for that BMP. Maintenance should be performed on a routine basis and whenever needed, based
on maintenance indicators. The optimum maintenance frequency is each time the maintenance threshold
for removal of materials (sediment, trash, debris or overgrown vegetation) is met. If this maintenance
threshold has been exceeded by the time the structural BMP is inspected, the BMP has been operating
at reduced capacity. This would mean it is necessary to inspect and maintain the structural BMP more
frequently.
During the first year of normal operation of a structural BMP (i.e., when the project is fully built out and
occupied), inspection by the property owner's representative is recommended at least monthly. Inspection
during a storm event is also recommended. It is during and after a rain event when one can determine if
the components of the BMP are functioning properly. The inspection and maintenance frequency can be
adjusted, based on the results of the inspections performed during the first year.
7.5. Minimum Maintenance Requirements
There are many different variations of structural BMPs, and structural BMPs may include multiple
components. For the purpose of maintenance, the structural BMPs have been grouped into categories
based on common maintenance requirements. The following fact sheets are available in Appendix C:
• Bioretention Basin
• Detention Basin
• Green Roof
• Infiltration Trench/Basin
• Manufactured Treatment Device
• Pervious Pavement
• Rainwater Harvesting
• Sand Filter
• Vegetated Biofilter
• Vegetated Swale/Strip
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Operations and Maintenance of BMPs
The project civil engineer is responsible for tailoring the maintenance activities and frequency based
on the components of the structural BMP, and identifying the applicable maintenance indicators. The
factsheets are intended to be general guidance and are meant to help prepare the maintenance plan, more
explicit maintenance activities may be necessary to ensure proper operation and maintenance.
Storm Water BMP Guide for New and Redevelopment
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8. References
Alameda Countywide Clean Water Program. www.cleanwaterprogram.org/businesses_home.htm
Arizona Department of Transportation. 2009. ADOT Post-Construction Best Management Practices
Manual. http://www.azdot.gov/inside_adot/OES/Water_Quality/Stormwater/Manuals.asp
ASCE. 1998. Urban Runoff Quality Management, Manual and Report of Engineering Practice 87.
Reston, Virginia.
Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical
Handbook. http://www.georgiastormwater.com/
California Stormwater Quality Association. 2003. Stormwater Best Management Practice Handbook, Industrial and Commercial. http://www.cabmphandbooks.com/
California Stormwater Quality Association. 2003. Stormwater Best Management Practice Handbook,
New Development and Redevelopment. http://www.cabmphandbooks.com/
City and County of Honolulu. 2012. Green Infrastructure for Homeowners. https://www.honolulu.gov/
rep/site/dfmswq/library/Green_Infrastructure_for_Homeowners-FINAL.pdf
City and County of Denver. 2013. Storm Drainage Design and Technical Criteria. https://www.
denvergov.org/Portals/711/documents/StormMasterPlan/StormDrainageDesignTechnicalCriteria.
pdf
City of Boulder, Colorado. Partners for a Clean Environment. www.bouldercolorado.gov/www/pace/
government/index.html
City of Los Angeles, California. Stormwater Program, Best Management Practices for Businesses and
Commercial Industries. www.lastormwater.org/Siteorg/businesses/bmpbusiness.htm
County of Maui, Hawaii, Document Center, Department of Water. Water Resource Planning Division.
www.co.maui.hi.us/DocumentCenterii.aspx
County of Orange, California. OC Watersheds Industrial/Commercial Businesses Activities. www.
ocwatersheds.com/IndustrialCommercialBusinessesActivities.aspx
County of Santa Barbara, California. Storm Water Management Program. www.sbprojectcleanwater.org/
swmp.html
County of Suffolk, New York. Stormwater Management Program. www.co.suffolk.ny.us/stormwater/
bmps_businesses.html
8-2 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
References
City of Long Beach Development Services. 2012. Low Impact Development (LID) Best Management
Practices (BMP) Design Manual. http://www.lbds.info/civica/filebank/blobdload.
asp?BlobID=3855
City of Portland. 2014. Stormwater Management Manual. http://www.portlandoregon.gov/bes/47952
City of San Diego. 2016. Storm Water Standards. https://www.sandiego.gov/stormwater/regulations
Clean Water Services. 2009. Low Impact Development Approaches Handbook. http://www.
cleanwaterservices.org/Content/Documents/Permit/LIDA%20Handbook.pdf
Connecticut Department of Environmental Protection. 2004. Connecticut Stormwater Quality Manual.
http://ct.gov/dep/cwp/view.asp?a=2721&q=325704&depNav_GID=1654
County of Los Angeles Department of Public Works. 2014. Low Impact Development (Standards
Manual). http://dpw.lacounty.gov/DES/design_manuals/
County of San Diego Department of Public Works. 2012. Operation and Maintenance Cost Table for
Treatment Control BMPs. http://www.sandiegocounty.gov/dpw/watersheds/susmp/susmp.html
County of San Diego. 2014. Low Impact Development Handbook, Stormwater Management Strategies.
http://www.sandiegocounty.gov/content/dam/sdc/dplu/docs/LID_Handbook_2014.pdf
County of Santa Clara. 2012. C.3. Stormwater Handbook. http://www.scvurppp-w2k.com/c3_
handbook_2012.shtml
Ekern, P.C., and Chang, J.H. 1985. Pan Evaporation: State of Hawaii, 1894-1983. State of Hawaii,
Department of Land and Natural Resources, Division of Water and Land Development.
Idaho Department of Environmental Quality. 2005. Catalog of Stormwater Best Management Practices
for Idaho Cities and Counties. http://www.deq.idaho.gov/water/data_reports/storm_water/
catalog/
Iowa State University Institute for Transportation. 2009. Iowa Stormwater Management Manual. http://
www.intrans.iastate.edu/pubs/stormwater/index.cfm
King County Department of Natural Resources and Parks. 2009. King County, Washington Surface
Water Design Manual. http://www.kingcounty.gov/environment/waterandland/stormwater.aspx
Maine Coastal Program, State Planning Office. 2007. LID Guidance Manual for Maine Communities.
http://www.maine.gov/dep/blwq/docwatershed/materials.htm
Maine Department of Environmental Protection. 2006. Stormwater Management for Maine, Volume
III BMPs Technical Design Manual. http://www.maine.gov/dep/blwq/docstand/stormwater/
stormwaterbmps/index.htm
Maryland Department of the Environment. 2000. Maryland Stormwater Design Manual.
http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/
MarylandStormwaterDesignManual/
New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual,
Volume 2, Post-Construction Best Management Practices Selection and Design. http://des.
nh.gov/organization/divisions/water/stormwater/
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
8-3
References
New Jersey Department of Environmental Protection, Division of Watershed Management. 2009. New
Jersey Stormwater Best Management Practices Manual. http://www.state.nj.us/dep/stormwater/
North Carolina State University. 2009. Low Impact Development, A Guidebook for North Carolina.
http://www.ces.ncsu.edu/depts/agecon/WECO/lidguidebook/
National Oceanic Atmospheric Association Coral Reef Conservation Program. 2014. Storm Water
management in Pacific and Caribbean Islands: A practitioner’s guide to Implementing LID.
http://data.nodc.noaa.gov/coris/library/NOAA/CRCP/project/1906/Feb2014_IslandBMPGuide_
wAppendix.pdf
Pennsylvania Department of Environmental Protection. 2006. Pennsylvania Stormwater Best
Management Practices Manual. http://www.elibrary.dep.state.pa.us/dsweb/View/Collection-8305
Prince George’s County, Maryland, Department of Environmental Resource. 1999. Low
Impact Development Design Strategies, An Integrated Design Approach. http://www.
lowimpactdevelopment.org/publications.htm
Prince George's County, Maryland, Department of Environmental Resources. 2007. Bioretention
Manual. http://www.princegeorgescountymd.gov/der/esg/bioretention/bioretention.asp
Rawls, W. J., D. L. Brakensiek and K. E. Saxton. 1982. Estimation of Soil Water Properties.
Transactions ASAE, 25(5)1316-1320, 1328.
Riverside County Flood Control and Water Conservation District. 2006. Riverside County Stormwater Quality Best Management Practice Design Handbook. http://www.floodcontrol.co.riverside.
ca.us/downloads/Planning/BMP%20Handbook%20(draft%208).pdf
San Francisco Public Utilities Commission. Rainwater Harvesting System Sizing Calculator. http://
sfwater.org/msc_main.cfm/MC_ID/17/MSC_ID/404
Southeast Michigan Council of Governments. 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. http://www.semcog.org/lowimpactdevelopment.
aspx
State of California Department of Transportation. 2010. Stormwater Quality Handbooks, Project
Planning and Design Guide. http://www.dot.ca.gov/hq/oppd/stormwtr/
State of Hawaii, Department of Health, Wastewater Branch. 2009. Guidelines for the Reuse of Gray Water. http://hawaii.gov/wastewater/pdf/graywater_guidelines.pdf
State of Hawaii Department of Transportation. 2007. Storm Water Permanent Best Management
Practices Manual. http://stormwaterhawaii.com/pdfs/PermanentManual.pdf
State of Hawaii Office of Planning, Coastal Zone Management Program. 2006. Hawaii Low Impact
Development, A Practitioner’s Guide. http://hawaii.gov/dbedt/czm/initiative/lid.php
State of New York Department of Environmental Conservation. 2010. New York State Stormwater
Management Design Manual. http://www.dec.ny.gov/chemical/29072.html
8-4 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
References
University of California Cooperative Extension, California Department of Water Resources. 2000. A
Guide to Estimating Irrigation Water Needs of Landscape Plantings in California. http://www.
water.ca.gov/wateruseefficiency/docs/wucols00.pdf
University of Hawaii at Manoa, College of Tropical Agriculture and Human Resources. 2010.
Guidelines on Rainwater Catchment Systems for Hawaii. http://www.ctahr.hawaii.edu/Site/
PubList.aspx?key=Forest%20and%20Natural%20Resource%20Management
Urban Drainage and Flood Control District. 2010. Urban Storm Drainage Criteria Manual, Volume 3 -
Best Management Practices. http://www.udfcd.org/downloads/down_critmanual.htm
U.S. Department of Agriculture, Natural Resources Conservation Service. 2006. Soil Data Mart. http://
soildatamart.nrcs.usda.gov/
U.S. Department of Commerce National Oceanic and Atmospheric Administration, National Climatic
Data Center. 2002. Climatography of the United States No. 81, Monthly Normals of
Temperature, Precipitation, and Heating and Cooling Degree Days, 1971-2000, Hawaii. http://
cdo.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl
U.S. Environmental Protection Agency. 2004. Stormwater Best Management Practice Design Guide,
Volume 2, Vegetative Biofilters. http://www.epa.gov/ORD/NRMRL/pubs0402.html
U.S. Environmental Protection Agency. 2004. Stormwater Best Management Practice Design Guide,
Volume 3, Basin Best Management Practices. http://www.epa.gov/ORD/NRMRL/pubs0402.html
U.S. Soil Conservation Service. 1986. Urban Hydrology for Small Watersheds, Technical Release 55.
http://www.wsi.nrcs.usda.gov/products/w2q/H&H/Tools_Models/other/TR55.html
Ventura Countywide Stormwater Quality Management Program. 2010. Ventura County Technical
Guidance Manual for Stormwater Quality Control Measures. http://www.vcstormwater.org/
technicalguidancemanual.html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design
Specification No. 3, Grass Channels. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design
Specification No. 5, Vegetated Roof. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design Specification No. 7, Permeable Pavement. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.
html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design
Specification No. 8, Infiltration Practices. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.
html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design
Specification No. 9, Bioretention. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html
Virginia Department of Conservation and Recreation. 2011. Virginia DCR Stormwater Design
Specification No. 10, Dry Swales. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
8-5
References
Washington State University Extension. 2012. Low Impact Development Technical Guidance Manual for
Puget Sound. http://www.psp.wa.gov/downloads/LID/20121221_LIDmanual_FINAL_secure.pdf
Western Regional Climate Center, National Oceanic and Atmospheric Administration. Average Pan
Evaporation Data by State. http://www.wrcc.dri.edu/htmlfiles/westevap.final.html
Western Regional Climate Center, National Oceanic and Atmospheric Administration. Comparative Data
for the Western States. http://www.wrcc.dri.edu/htmlfiles/
8-6 Storm Water BMP Guide for New and Redevelopment
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Appendix A: Source Control BMP Fact
Sheets
This section describes specific Source Control (SC) BMPs to be considered for incorporation into newly
developed public and private infrastructure, as well as retrofit into existing facilities to meet storm water
management objectives.
Source Control BMPs are required for all Priority A and B projects for the following activities and areas:
• Landscaped Areas
• Automatic Irrigation Systems
• Storm Drain Inlets
• Vehicle/Equipment Fueling
• Vehicle/Equipment Repair
• Vehicle/Equipment Washing/Cleaning
• Loading Docks
• Outdoor Trash Storage
• Outdoor Material Storage
• Outdoor Work Areas
• Outdoor Process Equipment Operations
• Parking Areas
The following fact sheets are included in this guidance manual and recommended by CCH but are not
required by the Water Quality Rules.
• Alternative Building Materials
• Roof Runoff Controls
The following information is provided for each of the above-listed BMPs:
• Brief Description/Approach
• Suitable Applications
• Design Considerations
• Design Guidelines
• Examples
• Operations & Maintenance Recommendations
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Appendix A: Source Control BMP Fact Sheets
Description
Each project site possesses unique topographic, hydrologic, and vegetative features, some of which are
more suitable for development than others. Integrating and incorporating appropriate landscape planning
methodologies into the project design is the most effective action that can be done to minimize surface
and groundwater contamination from storm water.
Approach
Landscape planning should couple consideration of land suitability for urban uses with consideration
of community goals and projected growth. Project plan designs should conserve natural areas to the
maximum extent possible, maximize natural water storage and infiltration opportunities, and protect
slopes and channels.
Suitable Applications
Appropriate applications include residential, commercial and industrial areas planned for development or
redevelopment.
Design Considerations
Design requirements for site design and landscapes planning should conform to applicable standards and
specifications of agencies with jurisdiction and be consistent with applicable General Plan and Local Area
Plan policies.
Design Guidelines
• Conserve natural areas to the extent possible.
• Maximize natural water storage and infiltration opportunities to the extent possible.
• Limit runoff from landscaped areas to impervious areas.
• Protect slopes and channels.
SC-01: Landscaped Areas
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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Appendix A: Source Control BMP Fact Sheets
Designing New Installations
Begin the development of a plan for the landscape unit with attention to the following general principles:
• Formulate the plan on the basis of clearly articulated community goals. Carefully identify
conflicts and choices between retaining and protecting desired resources and community growth.
• Map and assess land suitability for urban uses. Include the following landscape features in the
assessment: wooded land, open un-wooded land, steep slopes, erosion-prone soils, foundation
suitability, soil suitability for waste disposal, aquifers, aquifer recharge areas, wetlands,
floodplains, surface waters, agricultural lands, and various categories of urban land use. When
appropriate, the assessment can highlight outstanding local or regional resources that the
community determines should be protected (i.e., a scenic area, recreational area, threatened
species habitat, farmland, fish run). Mapping and assessment should recognize not only these
resources but also additional areas needed for their sustenance.
Project plan designs should conserve natural areas to the extent possible, maximize natural water storage
and infiltration opportunities, and protect slopes and channels.
Conserve Natural Areas during Landscape Planning
If applicable, the following items are required and must be implemented in the site layout during the
subdivision design and approval process, consistent with applicable General Plan and Local Area Plan
policies:
• Cluster development on least-sensitive portions of a site while leaving the remaining land in a
natural undisturbed condition.
• Limit clearing and grading of native vegetation at a site to the minimum amount needed to build
lots, allow access, and provide fire protection.
• Maximize trees and other vegetation at each site by planting additional vegetation, clustering tree
areas, and promoting the use of native and/or drought tolerant plants.
• Promote natural vegetation by using parking lot islands and other landscaped areas.
• Preserve riparian areas and wetlands.
Maximize Natural Water Storage and Infiltration Opportunities within the Landscape Unit
• Promote the conservation of forest cover. Building on land that is already deforested affects
basin hydrology to a lesser extent than converting forested land. Loss of forest cover
reduces interception storage, detention in the organic forest floor layer, and water losses by
evapotranspiration, resulting in large peak runoff increases and either their negative effects or the
expense of countering them with structural solutions.
• Maintain natural storage reservoirs and drainage corridors, including depressions, areas
of permeable soils, swales, and intermittent streams. Develop and implement policies and
regulations to discourage the clearing, filling, and channelization of these features. Utilize them in
drainage networks in preference to pipes, culverts, and engineered ditches.
• Evaluating infiltration opportunities by referring to the storm water management manual for
the jurisdiction and pay particular attention to the selection criteria for avoiding groundwater
contamination, poor soils, and hydro-geological conditions that cause these facilities to fail.
If necessary, locate developments with large amounts of impervious surfaces or a potential to
produce relatively contaminated runoff away from groundwater recharge areas.
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Appendix A: Source Control BMP Fact Sheets
Protection of Slopes and Channels during Landscape Design
• Convey runoff safely from the tops of slopes.
• Avoid disturbing steep or unstable slopes.
• Avoid disturbing natural channels.
• Stabilize disturbed slopes as quickly as possible.
• Vegetated slopes with native or drought tolerant vegetation.
• Control and treat flows in landscaping and/or other controls prior to reaching existing natural
drainage systems.
• Stabilize temporary and permanent channel crossings as quickly as possible, and ensure that
increases in run-off velocity and frequency caused by the project do not erode the channel.
• Install energy dissipaters, such as riprap, at the outlets of new storm drains, culverts, conduits,
or channels that enter unlined channels in accordance with applicable specifications to minimize
erosion. Energy dissipaters shall be installed in such a way as to minimize impacts to receiving
waters.
• Line on-site conveyance channels where appropriate, to reduce erosion caused by increased flow
velocity due to increases in tributary impervious area. The first choice for linings should be grass
or some other vegetative surface, since these materials not only reduce runoff velocities, but
also provide water quality benefits from filtration and infiltration. If velocities in the channel are
high enough to erode grass or other vegetative linings, riprap, concrete, soil cement, or geo-grid
stabilization are other alternatives.
• Consider other design principles that are comparable and equally effective.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. Redevelopment includes, but is not limited to:
• Expansion of a building footprint.
• Addition to or replacement of a structure.
• Replacement of an impervious surface that is not part of a routine maintenance activity.
• Land disturbing activities related to structural or impervious surfaces.
Redevelopment may present significant opportunity to add features which had not previously been
implemented. Examples include incorporation of depressions, areas of permeable soils, and swales in
newly redeveloped areas. While some site constraints may exist due to the status of already existing
infrastructure, opportunities should not be missed to maximize infiltration, slow runoff, reduce impervious
areas, and disconnect directly connected impervious areas.
O&M Recommendations
• Do not use pesticides and fertilizers during wet weather or when rain is forecast, and minimize
their use during dry weather.
• Do not blow or rake leaves, grass, or garden clippings into the street, gutter, or storm drain.
• Do not apply any chemicals (insecticide, herbicide, or fertilizer) directly to surface waters, unless
the application is approved and permitted by the state.
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Appendix A: Source Control BMP Fact Sheets
• Dispose of grass clippings, leaves, sticks, or other collected vegetation as garbage, or by
composting. Do not dispose of collected vegetation into waterways or storm drainage systems.
• Use mulch or other erosion control measures on exposed soils.
• Check irrigation schedules so pesticides will not be washed away and to minimize non-storm
water discharge.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County,
Department of Public Works, May 2002.
Stormwater Management Manual for Western Washington, Washington State Department of Ecology,
August 2001.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
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Description
Various roof runoff controls are available to address storm water that drains off rooftops. The objective
is to reduce the total volume and rate of runoff from individual lots, and retain the pollutants on site that
may be picked up from roofing materials and atmospheric deposition. Roof runoff controls consist of
directing the roof runoff away from paved areas and mitigating flow to the storm drain system through
one of several general approaches: cisterns or rain barrels; dry wells or infiltration trenches; green roofs
(LID, see TC-07), pop-up emitters, and foundation planting. The first three (3) approaches require the roof
runoff to be contained in a gutter and downspout system. Foundation planting provides a vegetated strip
under the drip line of the roof.
Approach
Design of individual lots for single-family homes as well as lots for higher density residential and
commercial structures should consider site design provisions for containing and infiltrating roof runoff
or directing roof runoff to vegetative swales or buffer areas. Retained water can be reused for watering
gardens, lawns, and trees. Benefits to the environment include reduced demand for potable water used for
irrigation, improved storm water quality, increased groundwater recharge, decreased runoff volume and
peak flows, and decreased flooding potential.
Suitable Applications
Appropriate applications include residential, commercial and industrial areas planned for development or
redevelopment.
SC-02: Roof Runoff Controls
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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Design Considerations
Designing New Installations
Cisterns or Rain Barrels
One (1) method of addressing roof runoff is to direct roof downspouts to cisterns or rain barrels. A cistern
is an above ground storage vessel with either a manually operated valve or a permanently open outlet.
Roof runoff is temporarily stored and then released for irrigation or infiltration between storms. The
number of rain barrels needed is a function of the rooftop area. Some low impact developers recommend
that every house have at least two (2) rain barrels, with a minimum storage capacity of 1,000 liters. Roof
barrels serve several purposes including mitigating the first flush from the roof which has a high volume,
amount of contaminants, and thermal load. Several types of rain barrels are commercially available.
Consideration must be given to selecting rain barrels that are vector proof and childproof. In addition,
some barrels are designed with a bypass valve that filters out grit and other contaminants and routes
overflow to a soak-away pit or rain garden.
If the cistern has an operable valve, the valve can be closed to store storm water for irrigation or
infiltration between storms. This system requires continual monitoring by the resident or grounds crews,
but provides greater flexibility in water storage and metering. If a cistern is provided with an operable
valve and water is stored inside for long periods, the cistern must be covered to prevent mosquitoes from
breeding.
A cistern system with a permanently open outlet can also provide for metering storm water runoff. If
the cistern outlet is significantly smaller than the size of the downspout inlet (say ¼ to ½ inch diameter),
runoff will build up inside the cistern during storms, and will empty out slowly after peak intensities
subside. This is a feasible way to mitigate the peak flow increases caused by rooftop impervious land
coverage, especially for the frequent, small storms.
Dry Wells and Infiltration Trenches
Roof downspouts can be directed to dry wells or infiltration trenches. A dry well is constructed by
excavating a hole in the ground and filling it with an open graded aggregate, and allowing the water to fill
the dry well and infiltrate after the storm event. An underground connection from the downspout conveys
water into the dry well, allowing it to be stored in the voids. To minimize sedimentation from lateral
soil movement, the sides and top of the stone storage matrix can be wrapped in a permeable filter fabric,
though the bottom may remain open. A perforated observation pipe can be inserted vertically into the dry
well to allow for inspection and maintenance.
In practice, dry wells receiving runoff from single roof downspouts have been successful over long
periods because they contain very little sediment. They should be sized according to the amount of
rooftop runoff received, but are typically 4 to 5 sq-ft, and 2 to 3 ft deep, with a minimum of 1-ft soil cover
over the top (maximum depth of 10 ft).
To protect the foundation, dry wells must be set away from the building at least 10 ft. The location of
drywells should be determined by a licensed engineer. They must be installed in solids that accommodate
infiltration. In poorly drained soils, dry wells have very limited feasibility. Overflow shall be directed
away from the structure and surrounding buildings.
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Infiltration trenches function in a similar manner and would be particularly effective for larger roof areas.
An infiltration trench is a long, narrow, rock-filled trench with no outlet that receives storm water runoff.
These are described under Treatment Controls.
Pop-up Drainage Emitter
Roof downspouts can be directed to an underground pipe that daylights some distance from the building
foundation, releasing the roof runoff through a pop-up emitter. Similar to a pop-up irrigation head, the
emitter only opens when there is flow from the roof. The emitter remains flush to the ground during dry
periods, for ease of lawn or landscape maintenance.
Foundation Planting
Landscape planting can be provided around the base to allow increased opportunities for storm water
infiltration and protect the soil from erosion caused by concentrated sheet flow coming off the roof.
Foundation plantings can reduce the physical impact of water on the soil and provide a subsurface matrix
of roots that encourage infiltration. These plantings must be sturdy enough to tolerate the heavy runoff
sheet flows, and periodic soil saturation.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Examples
• City of Ottawa’s Water Links Surface –Water Quality Protection Program
• City of Toronto Downspout Disconnection Program
• City of Boston, MA, Rain Barrel Demonstration Program
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
Hager, Marty Catherine, Stormwater, “Low-Impact Development,” January/February 2003. http://www.
stormh2o.com/SW/Articles/226.aspx
Low Impact Urban Design Tools, Low Impact Development Design Center, Beltsville, MD. www.lid-
stormwater.net.
Start at the Source, Bay Area Stormwater Management Agencies Association, 1999 Edition.
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Description
Irrigation water provided to landscaped areas may result in excess irrigation water being conveyed into
storm water drainage systems.
Approach
Project plan designs for development and redevelopment should include application methods of irrigation
water that minimize runoff of excess irrigation water into the storm water conveyance system.
Suitable Applications
Appropriate applications include residential, commercial and industrial areas planned for development or
redevelopment. Detached residential single-family homes are typically excluded from this requirement.
Design Considerations
Design Guidelines
• Design irrigation systems to each landscape area’s specific water requirements.
• Implement landscape plans consistent with City water conservation resolutions, which may
include provision of drip irrigation, water sensors, programmable irrigation times (for short
cycles), etc.
• Design timing and application methods of irrigation water to minimize the runoff of excess
irrigation water into the storm water drainage system.
• Group plants with similar water requirements in order to reduce excess irrigation runoff and
promote surface filtration.
SC-03: Automatic Irrigation System
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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Designing New Installations
The following methods to reduce excessive irrigation runoff should be considered, and incorporated and
implemented where determined applicable and feasible:
• Employ rain-triggered shutoff devices to prevent irrigation after precipitation.
• Design irrigation systems to each landscape area’s specific water requirements.
• Include design featuring flow reducers or shutoff valves triggered by a pressure drop to control
water loss in the event of broken sprinkler heads or lines.
• Implement landscape plans consistent with City water conservation resolutions, which may
include provision of water sensors, programmable irrigation times (for short cycles), etc.
• Design timing and application methods of irrigation water to minimize the runoff of excess
irrigation water into the storm water drainage system.
• Group plants with similar water requirements in order to reduce excess irrigation runoff and
promote surface filtration. Choose plants with low irrigation requirements (for example, native or
drought tolerant species). Consider design features such as:
▪Using mulches (such as wood chips or bar) in planter areas without ground cover to minimize
sediment in runoff.
▪Installing appropriate plant materials for the location, in accordance with amount of sunlight
and climate, and use native plant materials where possible and/or as recommended by the
landscape architect.
▪Leaving a vegetative barrier along the property boundary and interior watercourses, to act as
a pollutant filter, where appropriate and feasible.
▪Choosing plants that minimize or eliminate the use of fertilizer or pesticides to sustain
growth.
• Employ other comparable, equally effective methods to reduce irrigation water runoff.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
O&M Recommendations
• Inspect irrigation system periodically to ensure that the right amount of water is being applied and
that excessive runoff is not occurring.
• Minimize excess watering, and repair leaks in the irrigation system as soon as they are observed.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix A: Source Control BMP Fact Sheets
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
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Description
Waste materials dumped into storm drain inlets can have severe impacts on receiving and ground waters.
Posting notices regarding discharge prohibitions at storm drain inlets can prevent waste dumping. Storm
drain signs and stencils are highly visible source controls that are placed directly adjacent to storm drain
inlets.
Approach
The stencil or affixed sign contains a brief statement that prohibits dumping of improper materials into
the urban runoff conveyance system. Stencils and signs alert the public to the destination of pollutants
discharged to the storm drain.
Suitable Applications
Stencils and signs alert the public to the destination of pollutants discharged to the storm drain. Signs are
appropriate in residential, commercial, and industrial areas, as well as any other area where contributions
or dumping to storm drains is likely.
Design Considerations
Storm drain message markers or placards are recommended at all storm drain inlets within the boundary
of a development project. The marker should be placed in clear sight facing toward anyone approaching
the inlet from either side. All storm drain inlet locations should be identified on the development site map.
Design Guidelines
• Provide stenciling or labeling of all storm drain inlets and catch basins, constructed or modified,
within the project area with prohibitive language.
• Place the marker in clear sight facing toward anyone approaching the inlet from either side.
SC-04: Storm Drain Inlet
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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• Be aware that signage on face of curbs tends to be worn by contact with vehicle tires and sweeper
brooms.
• Post signs with prohibitive language and/or graphical icons, which prohibit illegal dumping at
public access points along channels and creeks within the project area.
Designing New Installations
The following methods should be considered for inclusion in the project design and shown on project
plans:
• Provide stenciling or labeling of all storm drain inlets and catch basins, constructed or modified,
within the project area with prohibitive language. Examples include “Dump No Waste” and/or
other graphical icons to discourage illegal dumping.
• Post signs with prohibitive language and/or graphical icons, which prohibit illegal dumping at
public access points along channels and creeks within the project area.
Note: DFM-SWQ has approved specific signage and/or storm drain message placards for use. Consult
local agency storm water staff to determine specific requirements for placard types and methods of
application.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious
surface area on an already developed site. If the project meets the definition of “redevelopment,” then the
requirements stated under “designing new installations” above should be included in all project design
plans.
Supplemental Information
Maintenance Considerations
Legibility of markers and signs should be maintained. If required by the agency with jurisdiction over
the project, the owner/operator or homeowner’s association should enter into a maintenance agreement
with the agency or record a deed restriction upon the property title to maintain the legibility of placards or
signs.
Placement
• Signage on top of curbs tends to weather and fade.
• Signage on face of curbs tends to be worn by contact with vehicle tires and sweeper brooms.
O&M Recommendations
• Inspect signage regularly and maintain as appropriate to ensure legibility.
• Inspect regularly, at least annually, for structural deterioration or significant build-up of debris or
sediment.
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References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
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Description
Alternative building materials are selected instead of conventional materials for new construction and
renovation. These materials reduce potential sources of pollutants in storm water runoff by eliminating
compounds that can leach into runoff, reducing the need for pesticide application, reducing the need for
painting and other maintenance, or by reducing the volume of runoff.
Approach
Alternative building materials are available for use as lumber for decking, roofing materials, home siding,
and paving for driveways, decks, and sidewalks.
Suitable Applications
Appropriate applications include residential, commercial and industrial areas planned for development or
redevelopment.
Design Considerations
Design New Installations
Decking
One of the most common materials for construction of decks and other outdoor construction has
traditionally been pressure treated wood, which is now being phased out. The standard treatment is called
CCA, for chromated copper arsenate. The key ingredients are arsenic (which kills termites, carpenter ants
and other insects), copper (which kills the fungi that cause wood to rot) and chromium (which reacts with
the other ingredients to bind them to the wood). The amount of arsenic is far from trivial. A deck just 8 ft
by 10 ft contains more than 1 1/3 pounds of this highly potent poison. Replacement materials include a
new type of pressure treated wood, plastic and composite lumber.
SC-05: Alternative Building Materials
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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There are currently over 20 products in the market consisting of plastic or plastic-wood composites.
Plastic lumber is made from 100% recycled plastic, # 2 high-density polyethylene (HDPE), and
polyethylene plastic milk jugs and soap bottles. Plastic-wood composites are a combination of plastic and
wood fibers or sawdust. These materials are a long lasting exterior weather, insect, and chemical resistant
wood lumber replacement for non-structural applications. Use it for decks, docks, raised garden beds and
planter boxes, pallets, hand railings, outdoor furniture, animal pens, boat decks, etc.
New pressure treated wood uses a much safer recipe, ACQ, which stands for ammoniacal copper
quartenary. It contains no arsenic and no chromium. Yet the American Wood Preservers Association has
found it to be just as effective as the standard formula. ACQ is common in Japan and Europe.
Roofing
Several studies have indicated that metal used as roofing material, flashing, or gutters can leach metals
into the environment. The leaching occurs because rainfall is slightly acidic and slowly dissolved the
exposed metals. Common traditional applications include copper sheathing and galvanized (zinc) gutters.
Coated metal products are available for both roofing and gutter applications. These products eliminate
contact of bare metal with rainfall, eliminating one source of metals in runoff. There are also roofing
materials made of recycled rubber and plastic that resemble traditional materials.
A less traditional approach is the use of green roofs (See TC-07). These roofs are not just green, they’re
alive. Planted with grasses and succulents, low- profile green roofs reduce the urban heat island effect,
storm water runoff, and cooling costs, while providing wildlife habitat and a connection to nature
for building occupants. These roofs are widely used on industrial facilities in Europe and have been
established as experimental installations in several locations in the US, including Portland, Oregon.
Paved Areas
Traditionally, concrete is used for construction of patios, sidewalks, and driveways. Although it is non-
toxic, these paved areas reduce storm water infiltration and increase the volume and rate of runoff. This
increase in the amount of runoff is the leading cause of stream channel degradation in urban areas.
There are a number of alternative materials that can be used in these applications, including porous
concrete and asphalt, modular blocks, and crushed granite. These materials, especially modular paving
blocks, are widely available and well established methods to reduce storm water runoff.
Building Siding
Wood siding is commonly used on the exterior of residential construction. This material weathers fairly
rapidly and requires repeated painting to prevent rotting. Alternative “new” products for this application
include cement-fiber and vinyl. Cement-fiber siding is a masonry product made from Portland cement,
sand, and cellulose and will not burn, cup, swell, or shrink.
Pesticide Reduction
A common use of powerful pesticides is for the control of termites. Chlordane was used for many years
for this purpose and is now found in urban streams and lakes nationwide. There are a number of physical
barriers that can be installed during construction to help reduce the use of pesticides.
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Sand barriers for subterranean termites are a physical deterrent because the termites cannot tunnel through
it. Sand barriers can be applied in crawl spaces under pier and beam foundations, under slab foundations,
and between the foundation and concrete porches, terraces, patios and steps. Other possible locations
include under fence posts, underground electrical cables, water and gas lines, telephone and electrical
poles, inside hollow tile cells and against retaining walls.
Metal termite shields are physical barriers to termites which prevent them from building invisible tunnels.
In reality, metal shields function as a helpful termite detection device, forcing them to build tunnels on
the outside of the shields which are easily seen. Metal termite shields also help prevent dampness from
wicking to adjoining wood members which can result in rot, thus making the material more attractive to
termites and other pests. Metal flashing and metal plates can also be used as a barrier between piers and
beams of structures such as decks, which are particularly vulnerable to termite attack.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Other Resources
There are no good, independent, comprehensive sources of information on alternative building materials
for use in minimizing the impacts of storm water runoff. Most websites or other references to “green” or
“alternative” building materials focus on indoor applications, such as formaldehyde free plywood and low
VOC paints, carpets, and pads. Some supplemental information on alternative materials is available from
the manufacturers.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
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Description
Fueling areas have the potential to discharge oil and grease, solvents, car battery acid, coolant and
gasoline to the storm drain. Spills can be a significant source of pollution because fuels contain toxic
materials and heavy metals that are not easily removed by storm water treatment devices.
Approach
Project plans must be developed for cleaning near fuel dispensers, emergency spill cleanup, containment,
and leak prevention.
Suitable Applications
Appropriate applications include commercial, industrial, and any other areas planned to have fuel
dispensing equipment, including retail gasoline outlets, automotive repair shops, and major non-retail
dispensing areas.
Design Considerations
Design requirements for fueling areas are governed by Building and Fire Codes and by current local
agency ordinances and zoning requirements. Design requirements described in this fact sheet are meant to
enhance and be consistent with these code and ordinance requirements.
Design Guidelines
• Covering. Include an overhanging roof structure or canopy over fuel dispensing areas. The
cover’s minimum dimensions must be equal to or greater than the area within the grade break.
The cover must not drain onto the fuel dispensing area and the downspouts must be routed to
prevent drainage across the fueling area. If fueling large equipment or vehicles that prohibit the
use of covers or roofs, the fueling island should be designed to accommodate the larger vehicles
and equipment and to prevent storm water run-on and runoff.
SC-06: Vehicle/Equipment Fueling
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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• Surfacing. Pave fuel dispensing areas with Portland cement concrete (or equivalent smooth
impervious surface). Extend the paved area a minimum of 6.5 ft from the corner of each fuel
dispenser, or the length at which the hose and nozzle assembly may be operated plus 1 ft,
whichever is less. The use of asphalt concrete is prohibited. Use asphalt sealant to protect asphalt
paved areas surrounding the fueling area.
• Grading/Contouring. Slope the dispensing areas to prevent ponding, and separate it from the
rest of the site by a grade break that prevents run-on. Grade the fueling areas to drain toward a
dead-end sump or vegetated/landscaped area. Direct runoff from downspouts/roofs away from
fueling areas towards vegetated/landscaped areas if possible.
• Drains. Label all drains within facility boundaries using paint or stencil, to indicate whether flow
is to the storm drain, sewer, or oil/water separator.
Designing New Installations
Covering
Fuel dispensing areas should provide an overhanging roof structure or canopy. The cover’s minimum
dimensions must be equal to or greater than the area within the grade break. The cover must not drain
onto the fuel dispensing area and the downspouts must be routed to prevent drainage across the fueling
area. The fueling area should drain to the project’s treatment control BMP(s) prior to discharging to the
storm water conveyance system. Note: If fueling large equipment or vehicles that would prohibit the use
of covers or roofs, the fueling island should be designed to sufficiently accommodate the larger vehicles
and equipment and to prevent storm water run-on and runoff. Grade to direct storm water to a dead-end
sump.
Surfacing
Fuel dispensing areas should be paved with Portland cement concrete (or equivalent smooth impervious
surface). The use of asphalt concrete should be minimized. Use asphalt sealant to protect asphalt paved
areas surrounding the fueling area. This provision may be made to sites that have pre-existing asphalt
surfaces.
The concrete fuel dispensing area should be extended a minimum of 6.5 ft from the corner of each fuel
dispenser, or the length at which the hose and nozzle assembly may be operated plus 1 ft, whichever is
less.
Grading/Contouring
Dispensing areas should have an appropriate slope to prevent ponding, and be separated from the rest of
the site by a grade break that prevents run-on of urban runoff (slope is required to be 2 to 4% in some
jurisdictions’ storm water management and mitigation plans).
Fueling areas should be graded to drain toward a dead-end sump. Runoff from downspouts/roofs should
be directed away from fueling areas. Do not locate storm drains in the immediate vicinity of the fueling
area.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
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Appendix A: Source Control BMP Fact Sheets
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
In the case of an emergency, provide storm drain seals, such as isolation valves, drain plugs, or drain
covers, to prevent spills or contaminated storm water from entering the storm water conveyance system.
O&M Recommendations
• Maintain clean fuel-dispensing areas using dry cleanup methods such as sweeping, or use of rags
and absorbents for leaks and spills.
• If you clean by washing, place a temporary plug in the downstream drain and pump out the
accumulated water. Properly dispose the water.
• Install vapor recovery nozzles to help control drips as well as air pollution.
• Use secondary containment when transferring fuel from the tank truck to the fuel tank. Cover
storm drains in the vicinity during transfer.
• Post signs at the fuel dispenser or fuel island warning vehicle owners/operators against "topping
off" of vehicle fuel tanks.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
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Description/Approach
Several measures can be taken to prevent operations at maintenance bays from contributing a variety of
toxic compounds, oil and grease, heavy metals, nutrients, suspended solids, and other pollutants to the
storm water conveyance system. In designs for maintenance bays containment is encouraged. Preventive
measures include overflow containment structures and dead-end sumps.
Design Guidelines
Design requirements for vehicle maintenance and repair are governed by Building and Fire Codes, and by
current local agency ordinances, and zoning requirements. The design requirements described hereon are
meant to enhance and be consistent with these code requirements.
• Locate repair/maintenance bays indoors; or design them to preclude run-on and runoff.
• Pave repair/maintenance floor areas with Portland cement concrete (or equivalent smooth
impervious surface).
• Provide impermeable berms, drop inlets, trench catch basins, or overflow containment structures
around repair bays to prevent spilled materials and wash-down waters from entering the storm
drain system. Connect drains to a sump for collection and disposal. Direct connection of the
repair/maintenance bays to the storm drain system is prohibited.
• Label all drains within facility boundaries using paint or stencil, to indicate whether flow is to the
storm drain, sewer, or oil/water separator.
O&M Recommendations
• Avoid hosing down work areas. If work areas are washed, collect and direct wash water to
sanitary sewer.
• Do not pour liquid waste down floor drains, sinks, outdoor storm drain inlets, or other storm
drains or sewer connections.
SC-07: Vehicle/Equipment Repair
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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• Do not dispose of used or leftover cleaning solutions, solvents, and automotive fluids and oil in
the sanitary sewer.
• Keep drip pans or containers under vehicles or equipment that may drip during repairs.
• When steam cleaning or pressure washing parts, the wastewater must be discharged to an on-site
oil water separator that is connected to a sanitary sewer or blind sump.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
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Description
Vehicle washing, equipment washing, and steam cleaning may contribute high concentrations of
pollutants to wash waters that drain to storm water conveyance systems. Wash water may not be conveyed
to a sewer without a sewer connection permit.
Approach
Project plans should include appropriately designed area(s) for washing-steam cleaning of vehicles and
equipment. Depending on the size and other parameters of the wastewater facility, wash water may be
conveyed to a sewer, an infiltration system, recycling system or other alternative. Pretreatment may be
required for conveyance to a sanitary sewer.
Suitable Applications
Appropriate applications include commercial developments, restaurants, retail gasoline outlets,
automotive repair shops, condominiums, apartment buildings, and others.
Design Considerations
Design requirements for vehicle maintenance are governed by Building and Fire Codes, and by current
local agency ordinances, and zoning requirements. Design criteria described in this fact sheet are meant to
enhance and be consistent with these code requirements.
Design Guidelines
Commericial Applications
Incorporate at least one of the following features for equipment washing/steam cleaning:
• Be self-contained and/or covered with a roof or overhang.
• Be equipped with a clarifier or other pretreatment facility.
SC-08: Vehicle/Equipment Cleaning
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
A-30 Storm Water BMP Guide for New and Redevelopment
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• Have a proper connection to a sanitary sewer.
• Install sumps or drain lines to collect wash water. Divert wash water to the sanitary sewer, an
engineered infiltration system, or an equally effective alternative.
• Direct and divert surface water runoff away from the exposed area around the wash pad, and
wash pad itself to alternatives other than the sanitary sewer.
• Cover areas used for regular washing of vehicles, trucks, or equipment, surround them with a
perimeter berm, and clearly mark them as a designated washing area.
• Label all drains within facility boundaries using paint or stencil, to indicate whether flow is to the
storm drain, sewer, or oil/water separator.
Residential Applications
• Designate a car wash area and post signs for area.
• Divert wash water to a vegetated area where it may percolate into the ground, the sanitary sewer,
an engineered Infiltration system, or an equally effective alternative.
• Direct and divert surface water runoff away from the wash area to alternatives other than the
sanitary sewer.
• Approval for a sanitary connection must be obtained from the City Department of Environmental
Services.
Designing New Installations
Areas for washing/steam cleaning should incorporate one of the following features:
• Be self-contained and/or covered with a roof or overhang.
• Be equipped with a clarifier or other pretreatment facility.
• Have a proper connection to a sanitary sewer.
• Include other features which are comparable and equally effective.
Car Wash Areas
Wash water from the areas may be directed to the sanitary sewer, to an engineered infiltration system, or
to an equally effective alternative. Pre-treatment may also be required. For residential applications, it can
be appropriate to direct wash water to a planted area and allow it to percolate into the ground.
Developers are to direct and divert surface water runoff away from the exposed area around the wash pad
(parking lot, storage areas), and wash pad itself to alternatives other than the sanitary sewer. Roofing may
be required for exposed wash pads.
It is generally advisable to cover areas used for regular washing of vehicles, trucks, or equipment,
surround them with a perimeter berm, and clearly mark them as a designated washing area. Sumps or
drain lines can be installed to collect wash water, which may be treated for reuse or recycling, or for
discharge to the sanitary sewer. Some areas may require some form of pretreatment, such as a trap, for
these areas.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
Storm Water BMP Guide for New and Redevelopment
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Appendix A: Source Control BMP Fact Sheets
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Maintenance Considerations
Storm water and non-storm water will accumulate in containment areas and sumps with impervious
surfaces. Contaminated accumulated water must be disposed of in accordance with applicable laws and
cannot be discharged directly to the storm drain or sanitary sewer system without the appropriate permit.
O&M Recommendations
• Mark the area clearly as a wash area.
• Post signs to state that washing is only allowed in wash area.
• Provide trash container with lids in wash area.
• Recycle, collect or treat wash water effluent prior to discharge to the sanitary sewer system.
• Do not conduct oil changes and other engine maintenance in the designated washing area.
Perform these activities in a place designated for oil change and maintenance activities.
• Cover the wash area when not in use to prevent contact with rain water.
• Do not permit steam cleaning wash water to enter the storm drain.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Storm Water Quality Control Measures, July 2002.
A-32 Storm Water BMP Guide for New and Redevelopment
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Description
Several measures can be taken to prevent operations at loading docks from contributing a variety of toxic
compounds, oil and grease, heavy metals, nutrients, suspended solids, and other pollutants to the storm
water conveyance system.
Approach
In designs for loading docks, containment is encouraged. Preventive measures include overflow
containment structures and dead-end sumps. However, in the case of loading docks from grocery stores
and warehouse/distribution centers, engineered infiltration systems may be considered.
Suitable Applications
Appropriate applications include commercial and industrial areas planned for development or
redevelopment.
Design Considerations
Design requirements for vehicle maintenance and repair are governed by Building and Fire Codes, and by
current local agency ordinances, and zoning requirements. The design criteria described in this fact sheet
are meant to enhance and be consistent with these code requirements.
Design Guidelines
Design requirements for vehicle maintenance and repair are governed by Building and Fire Codes, and by
current local agency ordinances, and zoning requirements. The design requirements described hereon are
meant to enhance and be consistent with these code requirements.
• Cover all loading dock areas, or design them to preclude run-on and runoff.
• Do not allow runoff from depressed loading docks (truck wells) to discharge into storm drains.
SC-09: Loading Dock
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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• Drain below-grade loading docks from grocery stores and warehouse/distribution centers of fresh
food items through water quality inlets, an engineered infiltration system, or an equally effective
alternative.
• Grade and/or berm the loading/unloading area to a drain that is connected to a dead-end.
• Pave loading areas with concrete instead of asphalt.
Designing New Installations
Designs of maintenance bays should consider the following:
• Repair/maintenance bays and vehicle parts with fluids should be indoors; or designed to preclude
urban run-on and runoff.
• Repair/maintenance floor areas should be paved with Portland cement concrete (or equivalent
smooth impervious surface).
• Repair/maintenance bays should be designed to capture all wash water leaks and spills. Provide
impermeable berms, drop inlets, trench catch basins, or overflow containment structures
around repair bays to prevent spilled materials and wash-down waters from entering the storm
drain system. Connect drains to a sump for collection and disposal. Direct connection of the
repair/ maintenance bays to the storm drain system is prohibited. If required by local jurisdiction,
obtain an Industrial Waste Discharge Permit.
• Other features may be comparable and equally effective.
The following designs of loading/unloading dock areas should be considered:
• Loading dock areas should be covered, or drainage should be designed to preclude urban run-on
and runoff.
• Direct connections into storm drains from depressed loading docks (truck wells) are prohibited.
• Below-grade loading docks from grocery stores and warehouse/distribution centers of fresh food
items should drain through water quality inlets, or to an engineered infiltration system, or an
equally effective alternative. Pre-treatment may also be required.
• Other features may be comparable and equally effective.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Storm water and non-storm water will accumulate in containment areas and sumps with impervious
surfaces. Contaminated accumulated water must be disposed of in accordance with applicable laws and
cannot be discharged directly to the storm drain or sanitary sewer system without the appropriate permit.
O&M Recommendations
• Develop an operations plan that describes procedures for loading and/or unloading.
• Conduct loading and unloading in dry weather if possible.
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• Load and unload all materials and equipment in covered areas if feasible.
• Load/unload only at designated loading areas.
• Check loading and unloading equipment regularly for leaks.
• Look for dust or fumes during loading or unloading operations.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
A-36 Storm Water BMP Guide for New and Redevelopment
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Description
Storm water runoff from areas where trash is stored or disposed of can be polluted. In addition, loose
trash and debris can be easily transported by water or wind into nearby storm drain inlets, channels, and/
or streams.
Approach
Preventive measures including enclosures, containment structures, and impervious pavements to mitigate
spills, should be used to reduce the likelihood of contamination.
Suitable Applications
Appropriate applications include residential, commercial, and industrial areas planned for development or
redevelopment. Detached residential single-family homes are typically excluded from this requirement.
Design Considerations
Design requirements for waste handling areas are governed by Building and Fire Codes, and by current
local agency ordinances and zoning requirements. The design criteria described in this fact sheet are
meant to enhance and be consistent with these code and ordinance requirements. Hazardous waste should
be handled in accordance with legal requirements established in Hawaii Administrative Rules Title 11
Chapter 58.1 Solid Waste Management Control, and enforcement by the State of Hawaii Department of
Health Solid and Hazardous Waste Branch.
Wastes from commercial and industrial sites are typically hauled by either public or commercial carriers
that may have design or access requirements for waste storage areas. The design criteria in this fact
sheet are recommendations and are not intended to be in conflict with requirements established by the
waste hauler. The waste hauler should be contacted prior to the design of your site trash collection areas.
Conflicts or issues should be discussed with the local agency.
SC-10: Outdoor Trash Storage
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
A-38 Storm Water BMP Guide for New and Redevelopment
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Design Guidelines
• Hazardous waste must be handled in accordance with legal requirements established in Hawaii
Administrative Rules Title 11 Chapter 58.1 Solid Waste Management Control, and enforcement
by the State of Hawaii Department of Health solid and Hazardous Waste Branch.
• Berm trash storage areas to prevent run-on from adjoining roofs and pavement, or grade areas
towards vegetated/landscaped areas.
• Reduce/prevent leaking of liquid waste by incorporating at least one of the following:
▪Lined bins or dumpsters.
▪Low containment berm around the dumpster area.
▪Drip pans underneath dumpsters.
• Prevent rainfall from entering containers with roofs, awnings, or attached lids.
• Pave trash storage areas with an impervious surface to mitigate spills.
• Do not locate storm drains in immediate vicinity of the trash storage area.
• Post signs on dumpsters indicating that hazardous material are not to be disposed of therein.
Designing New Installations
Trash storage areas should be designed to consider the following structural or treatment control BMPs:
• Design trash container areas so that drainage from adjoining roofs and pavement is diverted
around the area(s) to avoid run-on. This might include berming or grading the waste handling
area to prevent run-on of storm water.
• Make sure trash container areas are screened or walled to prevent off-site transport of trash.
• Use lined bins or dumpsters to reduce leaking of liquid waste.
• Provide roofs, awnings, or attached lids on all trash containers to minimize direct precipitation
and prevent rainfall from entering containers.
• Pave trash storage areas with an impervious surface to mitigate spills.
• Do not locate storm drains in immediate vicinity of the trash storage area.
• Post signs on all dumpsters informing users that hazardous material are not to be disposed of
therein.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Maintenance Considerations
The integrity of structural elements that are subject to damage (i.e., screens, covers, and signs) must
be maintained by the owner/operator. Maintenance agreements between the local agency and the
owner/ operator may be required. Some agencies will require maintenance deed restrictions to be recorded
of the property title. If required by the local agency, maintenance agreements or deed restrictions must be
executed by the owner/operator before improvement plans are approved.
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O&M Recommendations
• Spot clean leaks and drips routinely to prevent runoff of spillage.
• Post “no littering” signs.
• Use only watertight waste receptacle(s) and keep the lid(s) closed.
• Do not overfill or fill with any liquid. Keep lid closed at all times.
• Periodically inspect for leaks. If found contact the leasing company immediately.
• Never wash down or rinse with a hose. Contact leasing company for cleaning.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
Hawaii Administrative Rules Title 11 Chapter 58.1 Solid Waste Management Control, Honolulu Hawaii
Department of Health, 2003: http://health.hawaii.gov/shwb/files/2013/06/11-5811.pdf
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
A-40 Storm Water BMP Guide for New and Redevelopment
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Description
Proper design of outdoor storage areas for materials reduces opportunity for pollutants to enter the storm
water conveyance system. Materials may be in the form of raw products, by-products, finished products,
and waste products.
Approach
Outdoor storage areas require a drainage approach different from the typical infiltration/detention strategy.
In outdoor storage areas, infiltration is discouraged and containment is encouraged. Preventative measures
include enclosures, secondary containment structures and impervious surfaces.
Suitable Applications
Appropriate applications include residential, commercial and industrial areas planned for development or
redevelopment.
Design Considerations
Some materials are more of a concern than others. Toxic and hazardous materials must be prevented from
coming in contact with storm water. Non-toxic or non-hazardous materials do not have to be prevented
from storm water contact. However, these materials may have toxic effects on receiving waters if allowed
to be discharged with storm water in significant quantities. Accumulated material on an impervious
surface could result in significant impact on the rivers or streams that receive the runoff.
Material may be stored in a variety of ways, including bulk piles, containers, shelving, stacking, and
tanks. Storm water contamination may be prevented by eliminating the possibility of storm water contact
with the material storage areas either through diversion, cover, or capture of the storm water. Control
measures may also include minimizing the storage area. Design requirements for material storage areas
are governed by Building and Fire Codes, and by current City ordinances and zoning requirements.
Control measures are site specific, and must meet local agency requirements.
SC-11: Outdoor Material Storage
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
A-42 Storm Water BMP Guide for New and Redevelopment
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Design Guidelines
Design requirements for material storage areas are governed by Building and Fire Codes, and by current
City ordinances and zoning requirements. Control measures are site specific, and must meet local agency
requirements.
• Materials with the potential to contaminate storm water must either be placed in an enclosure that
prevents contact with runoff or spillage to the storm water conveyance system, or protected by
secondary containment structures such as berms, dikes, or curbs.
• Pave the storage area with Portland cement concrete (or equivalent smooth impervious surface) to
contain leaks and spills.
• Slope the storage area towards a dead-end sump to contain spills.
• Direct runoff from downspouts/roofs away from storage areas.
• Cover the storage area with an awning that extends beyond the storage area to minimize
collection of storm water within the secondary containment area. A manufactured storage shed
may be used for small containers.
Designing New Installations
Where proposed project plans include outdoor areas for storage of materials that may contribute
pollutants to the storm water conveyance system, the following structural or treatment BMPs should be
considered:
• Materials with the potential to contaminate storm water should be:
▪Placed in an enclosure such as, but not limited to, a cabinet, shed, or similar structure that
prevents contact with runoff or spillage to the storm water conveyance system, or
▪Protected by secondary containment structures such as berms, dikes, or curbs.
• The storage area should be paved and sufficiently impervious to contain leaks and spills.
• The storage area should slope towards a dead-end sump to contain spills and direct runoff from
downspouts/roofs should be directed away from storage areas.
• The storage area should have a roof or awning that extends beyond the storage area to minimize
collection of storm water within the secondary containment area. A manufactured storage shed
may be used for small containers.
Note that the location(s) of installations of where these preventative measures will be employed must be
included on the map or plans identifying BMPs.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Storm water and non-storm water will accumulate in containment areas and sumps with impervious
surfaces. Contaminated accumulated water must be disposed of in accordance with applicable laws and
cannot be discharged directly to the storm drain or sanitary sewer system without the appropriate permits.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix A: Source Control BMP Fact Sheets
O&M Recommendations
• Protect materials from rainfall, run-on, runoff, and wind dispersal.
• Employ safeguards against accidental releases.
• Inspect storage areas regularly for leaks or spills.
• Keep storage areas clean and dry.
• Keep containers in good condition without corrosion or leaky seams.
• Cover and contain stockpiles of raw materials to prevent storm water run-on. If infeasible,
implement erosion control practices around site perimeter and catch basins.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
A-44 Storm Water BMP Guide for New and Redevelopment
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Description
Proper design of outdoor work areas (grinding, painting, coating, sanding, parts cleaning, etc.) reduces
opportunity for pollutants to enter the storm water conveyance system.
Approach
In outdoor work areas, infiltration and discharge to the storm drain are discouraged; collection and
conveyance to the sanitary sewer are encouraged.
Suitable Applications
Appropriate applications include residential, commercial, and industrial areas planned for development or
redevelopment.
Design Considerations
Design requirements for outdoor work areas are governed by Building and Fire Codes, and by current
City ordinances, and zoning requirements.
Design Guidelines
Design requirements for outdoor work areas are governed by Building and Fire Codes, and by current
City ordinances, and zoning requirements.
• Create an impermeable surface such as concrete or asphalt, or a prefabricated metal drip pan,
depending on the use.
• Cover the area with a roof to prevent rain from falling on the work area and becoming polluted
runoff.
• Berm or perform mounding around the perimeter of the area to prevent water from adjacent areas
from flowing on to the surface of the work area.
SC-12: Outdoor Work Areas
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
A-46 Storm Water BMP Guide for New and Redevelopment
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• Directly connect runoff to the sanitary sewer or other specialized containment system(s). This
allows the more highly concentrated pollutants from these areas to receive special treatment that
removes particular constituents. Approval for this connection must be obtained from the City.
• Locate the work area away from storm drains or catch basins.
Designing New Installations
Outdoor work areas can be designed in particular ways to reduce impacts on both storm water quality and
sewage treatment plants.
• Create an impermeable surface such as concrete or sealed asphalt, or a prefabricated metal drip
pan, depending on the use.
• Cover the area with a roof. This prevents rain from falling on the work area and becoming
polluted runoff.
• Berm or perform mounding around the perimeter of the area to prevent water from adjacent areas
from flowing on to the surface of the work area.
• Directly connect runoff. Unlike other areas, runoff from work areas is directly connected to
the sanitary sewer or other specialized containment system(s). This allows the more highly
concentrated pollutants from these areas to receive special treatment that removes particular
constituents. Approval for this connection must be obtained from the appropriate sanitary sewer
agency.
• Locate the work area away from storm drains or catch basins.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
O&M Recommendations
• Dry clean the work area regularly.
• Inspect storage areas regularly for leaks or spills.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
A-47
Appendix A: Source Control BMP Fact Sheets
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
A-48 Storm Water BMP Guide for New and Redevelopment
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Description
Outdoor process equipment operations such as rock grinding or crushing, painting or coating, grinding
or sanding, degreasing or parts cleaning, may contribute a variety of pollutants to the storm conveyance
system.
Approach
In outdoor process equipment areas, infiltration is discouraged and containment is encouraged,
accompanied by collection and conveyance.
Suitable Applications
Appropriate applications include commercial and industrial areas planned for development or
redevelopment.
Design Considerations
Design requirements for outdoor processing areas are governed by Building and Fire codes, and by
current local agency ordinances, and zoning requirements.
Design Guidelines
Design requirements for outdoor processing areas are governed by Building and Fire codes, and by
current local agency ordinances, and zoning requirements.
• Cover or enclose areas that would be the most significant source of pollutants; or slope the
area toward a dead-end sump; or, discharge to the sanitary sewer system following appropriate
treatment in accordance with conditions established by the applicable sewer agency.
• Grade or berm area to prevent run-on from surrounding areas.
• Do not install storm drains in areas of equipment repair.
SC-13: Outdoor Process Equipment
Operations
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
A-50 Storm Water BMP Guide for New and Redevelopment
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• Provide secondary containment structures (not double wall containers) where wet material
processing occurs (e.g., electroplating), to hold spills resulting from accidents, leaking tanks, or
equipment, or any other unplanned releases. Note: if these are plumbed to the sanitary sewer, they
must be with the prior approval of the City.
Designing New Installations
Operations determined to be a potential threat to water quality should consider to the following
recommendations:
• Cover or enclose areas that would be the most significant source of pollutants; or slope the
area toward a dead-end sump; or, discharge to the sanitary sewer system following appropriate
treatment in accordance with conditions established by the applicable sewer agency.
• Grade or berm area to prevent run-on from surrounding areas.
• Do not install storm drains in areas of equipment repair.
• Consider other features that are comparable or equally effective.
• Provide secondary containment structures (not double wall containers) where wet material
processing occurs (i.e., electroplating), to hold spills resulting from accidents, leaking tanks, or
equipment, or any other unplanned releases. Note: if these are plumbed to the sanitary sewer, they
must be with the prior approval of the City or other applicable sanitary sewer agency.
Redeveloping Existing Installations
The City’s SWMPP defines “redevelopment” as development that would create or add impervious surface
area on an already developed site. The definition of “redevelopment” must be consulted to determine
whether or not the requirements for new development apply to areas intended for redevelopment. If the
definition applies, the steps outlined under “designing new installations” above should be followed.
Supplemental Information
Storm water and non-storm water will accumulate in containment areas and sumps with impervious
surfaces. Contaminated accumulated water must be disposed of in accordance with applicable laws and
cannot be discharged directly to the storm drain or sanitary sewer system without the appropriate permit.
O&M Recommendations
• Dry clean the work area regularly.
• Inspect storage areas regularly for leaks or spills.
• Develop and implement a Spill Prevention Control and Countermeasure (SPCC) Plan.
References
California Stormwater Quality Association (CASQA) Best Management Practices Handbook New
Development and Redevelopment, 2003.
A Manual for the Standard Urban Storm Water Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of San Diego,
and Cities in San Diego County, February 14, 2002.
Storm Water BMP Guide for New and Redevelopment
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Appendix A: Source Control BMP Fact Sheets
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood Control
District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures, July 2002.
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Description/Approach
Parking lots and storage areas can contribute a number of substances, such as trash, suspended solids,
hydrocarbons, oil and grease, and heavy metals that can enter receiving waters through storm water runoff
or non-storm water discharges. The protocols in this fact sheet are intended to prevent or reduce the
discharge of pollutants from parking/storage areas.
Design Guidelines
Direct pavement runoff towards vegetated/landscaped areas if possible.
O&M Recommendations
• Clean leaves, trash, sand, and other debris regularly.
• Routinely sweep, shovel, and dispose of litter in the trash. Sweep entire parking lot at least once
before the onset of the wet season.
• Provide an adequate number of covered trash receptacles. Clean out frequently.
• Re-seal or pave only on dry days, and stop immediately before rainfall.
• Pre-heat, transfer or load hot bituminous material away from storm drain inlets.
• Do not allow any solids, liquids, or slurries to enter storm drains.
• Use dry clean-up methods (absorbents) on auto spills and/or drips.
• Do not hose down unless absolutely necessary. If you must pressure wash, discharge wash water
to the sanitary sewer or a vegetated area. Do not allow wash water to enter storm drains.
SC-14: Parking Areas
Design Objectives
Maximum Infiltration
Provide On-Site Retention
Slow Runoff
Minimize Impervious Land Coverage
Implement LID
Prohibit Dumping of Improper Materials
Contain Pollutants
Collect and Convey
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Appendix B: Treatment Control BMP Fact
Sheets
On the following pages are fact sheets for each Treatment Control BMP specified in the Water Quality
Rules. The following information is provided for each BMP:
• Brief description
• BMP category
• Expected pollutant removals
• Minimum design criteria
• Feasibility criteria
• Step-by-stem sizing procedure
• Pretreatment considerations
• Area requirements
• Sizing example
• Other design considerations
• Typical schematic
• General maintenance rank and requirements
The sizing procedures are based on simple dynamic and static principles and therefore may result in larger
BMPs than are necessary. More rigorous sizing methods (such as detailed routing methods or continuous
simulation models) may be used with City approval. Sizing worksheets are available on the DPP’s
website.
BMPs not included herein, such as Stormwater wetlands, wet ponds, and proprietary devices, may be
used with written City approval.
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Appendix B: Treatment Control BMP Fact Sheets
Description
An infiltration basin is a shallow impoundment with no outlet, where storm water runoff is stored and
infiltrates through the basin invert and into the soil matrix.
Minimum Design Criteria
Design Parameter Units Value
Invert Slope percent 0
Maximum Interior Side Slope (length per unit height)3:1
Drawdown (drain) Time hours 48
Minimum Soil Infiltration Rate inches/hour 0.5
Minimum Freeboard feet 1.0
Minimum Depth from Basin Invert to Groundwater Table feet 3
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-01: Infiltration Basin
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
Halawa District Park
B-4 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 2. Calculate the maximum allowable water storage depth (dmax) using the underlying soil
infiltration rate (k) and the required drawdown time (t):
dmax = kt/(Fs × 12)
Where dmax =Maximum Storage Depth (ft)
k =Soil Infiltration Rate (in/hr)
t =Drawdown (drain) Time (hrs)
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 3. Select a design ponding depth no greater than the maximum allowable depth calculated in
Step 2.
dp ≤ dmax
Where dp =Design Ponding Depth (ft)
dmax =Maximum Storage Depth from Step 2 (ft)
k =Soil Infiltration Rate (in/hr)
Step 4. Calculate the basin bottom surface area (Ab):
Ab = WQV/(dp + kT/12Fs )
Where Ab =Bottom Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dp =Design Ponding Depth from Step 3 (ft)
k =Soil Infiltration Rate (in/hr)
T =Fill Time (time for the BMP to fill with water [hrs])
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 5. Select a basin bottom width (wb), and calculate the basin bottom length (lb):
lb = Ab/wb
Where lb =Bottom Length (ft)
Ab =Bottom Surface Area from Step 4 (sq-ft)
wb =Bottom Width (ft)
Storm Water BMP Guide for New and Redevelopment
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B-5
Appendix B: Treatment Control BMP Fact Sheets
Step 6. Calculate the total area occupied by the BMP excluding pretreatment (ABMP) using the basin
bottom dimensions, embankment side slopes, and freeboard:
ABMP = [Wb + 2z(dp + f)] × [lb + 2z(dp + f)]
Where ABMP =Area Occupied by BMP Excluding Pretreatment (sq-ft)
wb =Bottom Width from Step 5 (ft)
z =Basin Interior Side Slope (length per unit height)
dp =Design Ponding Depth from Step 3 (ft)
f =Freeboard (ft)
lb =Bottom Length from Step 5 (ft)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the ponding depth (if it’s not already set to the maximum depth), and/or reduce the
Infiltration Rate Factor of Safety (if minimum number of test pits and permeability tests have
not been performed) and repeat the calculations.
Pretreatment Considerations
Infiltration facilities are highly susceptible to clogging and premature failure from sediment, trash,
and other materials. Suitable pretreatment systems maintain the infiltrate rate of the device without
frequent and intensive maintenance. For measured soil infiltration rates below 3 in/hr, pretreatment is
strongly recommended, and the pretreatment device should be sized for at least 25% of the WQV. For
measured soil infiltration rates greater than 3 in/hr, pretreatment is mandatory to minimize groundwater
contamination risks, and the pretreatment device must be sized for at least 50% of the WQV if the
measured soil infiltration rate is below 5 in/hr and 100% of the WQV if the measured soil infiltration
rate is greater than 5 in/hr. Pretreatment may be achieved with vegetated swales, vegetated filter strips,
sedimentation basins or forebays, sedimentation manholes, and manufactured treatment devices.
Area Requirements
An infiltration basin requires a footprint equivalent to 7% - 20% of its contributing impervious drainage
area, excluding pretreatment. The lower value reflects the maximum allowable infiltration rate and
minimum allowable factor of safety, while the upper value reflects the minimum allowable infiltration rate
and maximum allowable factor of safety.
Sizing Example
Calculate the size of an infiltration basin serving a 1-acre residential development. Assume the following
design parameters:
Design Parameter Units Value
Percent Impervious Cover, I %70
Design Storm Depth, P inches 1
Basin Fill Time, T hours 2
Drawdown (drain) Time, t hours 48
Basin Interior Side Slope (length per unit height), z 3
B-6 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Design Parameter Units Value
Soil Infiltration Rate, k inches/hour 1.0
Infiltration Rate Factor of Safety, Fs 2
Freeboard, f feet 1
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 70
C =0.68
WQV =PCA G 3630
WQV =1 G 0.68 G 1 G 3630
WQV =2,468 cu-ft
2. Calculate the maximum allowable water storage depth of the infiltration trench (dmax):
dmax =kt/12Fsdmax=1.0 G 48/(12 G 2)
dmax =2.0 ft
3. Select a ponding depth (dp) is no greater than the maximum allowable depth:
dp =2.0 ft
4. Calculate the basin bottom surface area (Ab):
Ab =WQV/(dp + kT/12Fs)
Ab =2,468/[2.0 + 1.0 G 2.0/(12 G 2)]
Ab =1,185 sq-ft
5. Set the basin bottom width (wb) to 25 ft, and calculate the basin bottom length (lb):
lb =Ab/Wblb=1,185/25
lb =47.4 ft
6. Calculate the total area excluding pretreatment (ABMP):
ABMP =[wb + 2z(dp + f)] G [lb + 2z(dp + f)]
ABMP =[25 + 2 G 3(2 + 1)] G [47.4 + 2 G 3(2 + 1)]
ABMP =2,812 sq-ft
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Appendix B: Treatment Control BMP Fact Sheets
Other Design Considerations
• If a temporarily-filled pond creates a potential public safety issue, perimeter fencing may be
considered. A vegetative screen around the basin to restrict direct view from adjacent properties
may improve the aesthetics of the site and public acceptance of the facility.
• If feasible, include vehicle access to the basin invert for maintenance.
• If the area around the basin has a recreational use, a safety shelf around the perimeter of the basin
can be included for times when the basin is flooded.
• The infiltration basin should be designed with an outlet structure to pass peak flows during a
range of storm events, as well as with an emergency spillway to pass peak flows around the
embankment during extreme storm events that exceed the combined infiltration capacity and
outlet structure capacity of the facility.
• To help ensure maintenance of the design permeability rate over time, a 6-inch layer of sand may
be placed on the bottom of an infiltration basin. This sand layer can intercept silt, sediment, and
debris that could otherwise clog the top layer of the soil below the basin. The sand layer will also
facilitate silt, sediment, and debris removal from the basin and can be readily restored following
removal operations.
• Observation wells are recommended. They will indicate how quickly the basin dewaters
following a storm and it will provide a method of observing how quickly the basin fills up with
sediments.
Construction/Inspection Considerations
• Before construction begins, stabilize the entire area draining to the facility. If impossible, place
a diversion berm around the perimeter of the infiltration site to prevent sediment entrance during
construction or remove the top two (2) inches of soil after the site is stabilized. Stabilize the entire
contributing drainage area, including the side slopes, before allowing any runoff to enter once
construction is complete.
• Place excavated material such that it cannot be washed back into the basin if a storm occurs
during construction of the facility.
• Build the basin without driving heavy equipment over the infiltration surface. Any equipment
driven on the surface should have extra-wide (“low pressure”) tires. Prior to any construction,
rope off the infiltration area to stop entrance by unwanted equipment.
• After final grading, till the infiltration surface deeply.
• Use appropriate erosion control seed mix for the specific project and location.
B-8 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of an Infiltration Basin
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Appendix B: Treatment Control BMP Fact Sheets
Description
An infiltration trench is a rock-filled trench with no outlet, where storm water runoff is stored in the void
space between the rocks and infiltrates through the bottom and into the soil matrix.
Minimum Design Criteria
Design Parameter Units Value
Maximum Trench Depth feet 8
Maximum Trench Width feet 25
Maximum Top Backfill Layer Thickness inches 6
Maximum Bottom Sand Layer Thickness inches 12
Drawdown (drain) Time hours 48
Minimum Soil Infiltration Rate inches/hour 0.5
Trench Rock Size inches 1.5 – 3.0
Minimum Depth from trench invert to groundwater table feet 3
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-02: Infiltration Trench
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
City of Bellingham, WA (source: cob.org/services/environment/lake-whatcom)
B-10 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 2. Calculate the maximum allowable water storage depth (dmax) using the underlying soil
infiltration rate (k) and the required drawdown time (t):
dmax = kt/(Fs × 12)
Where dmax =Maximum Storage Depth (ft)
k =Soil Infiltration Rate (in/hr)
t =Drawdown (drain) Time (hrs)
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 3. Select a ponding depth (optional), trench rock (or alternative material) depth, and sand layer
depth (optional) such that the total effective storage depth is no greater than the maximum
allowable depth calculated in Step 2:
dt = dp + lbnb + lsns ≤ dmax
Where dt =Total Effective Water Storage Depth (ft)
dp =Ponding Depth (ft)
lb =Backfill Material Thickness Depth (ft)
nb =Backfill Material Porosity
ls =Sand Layer Thickness Depth (ft)
ns =Sand Porosity
dmax =Maximum Storage Depth from Step 2 (ft)
Step 4. Calculate the trench surface area (ABMP):
ABMP = WQV/(dt + kT/12Fs)
Where ABMP =BMP Surface Area excluding Pretreatment (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (ft)
k =Soil Infiltration Rate (in/hr)
T =Fill Time (time for the BMP to fill with water [hrs])
Fs =Infiltration Rate Factor of Safety (see Section 5)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the ponding depth or trench rock depth or sand layer depth (if the total effective
depth is not already equal to the maximum depth), and/or reduce the Infiltration rate factor of
safety (if minimum number of test pits and permeability tests have not been performed) and
repeat the calculations.
Pretreatment Considerations
Infiltration facilities are highly susceptible to clogging and premature failure from sediment, trash,
and other materials. Suitable pretreatment systems maintain the infiltrate rate of the device without
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
frequent and intensive maintenance. For measured soil infiltration rates below 3 in/hr, pretreatment is
strongly recommended, and the pretreatment device should be sized for at least 25% of the WQV. For
measured soil infiltration rates greater than 3 in/hr, pretreatment is mandatory to minimize groundwater
contamination risks, and the pretreatment device must be sized for at least 50% of the WQV if the
measured soil infiltration rate is below 5 in/hr and 100% of the WQV if the measured soil infiltration
rate is greater than 5 in/hr. Pretreatment may be achieved with vegetated swales, vegetated filter strips,
sedimentation basins or forebays, sedimentation manholes, and manufactured treatment devices.
Area Requirements
An infiltration trench requires a footprint equivalent to 2% - 20% of its contributing impervious drainage
area, excluding pretreatment. The lower value reflects the maximum allowable infiltration rate, minimum
allowable factor of safety, and minimal ponding, while the upper value reflects the minimum allowable
infiltration rate, maximum allowable factor of safety, and no ponding.
Sizing Example
Calculate the size of an infiltration basin serving a 1-acre residential development. Assume the following
design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 70
Design Storm Depth, P inches 1
Trench Fill Time, T hours 2
Drawdown (drain) Time, t hours 48
Backfill porosity, nb 0.35
Sand porosity, ns 0.40
Soil Infiltration Rate, k inches/hour 1
Infiltration Rate Factor of Safety, Fs 2
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 70
C =0.68
WQV =PCA G 3630
WQV =1 G 0.68 G 1 G 3630
WQV =2,468 cu-ft
2. Calculate the maximum allowable water storage depth in the basin (dmax):
dmax =kT/12Fsdmax=1.0 G 48/(12 G 2)
dmax =2.0 ft
B-12 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
3. Select a design ponding depth (dp), trench rock depth (dr), and optional sand layer depth (ds) such
that the total effective storage depth (dt) is no greater than the maximum allowable depth:
dp =0.0 ft
lb =5.0 ft
ls =0.5 ft
dt =dp + lbnb + lsnsdt=0.0 + 5.0 G 0.35 + 0.5 G 0.40
dt =1.95 ft
4. Calculate the BMP surface area excluding pretreatment (ABMP):
ABMP =WQV/(dt + kT/12Fs)
ABMP =2,468/[1.95 + 1.0 G 2.0/(12 G 2)]
ABMP =1,214 sq-ft
Other Design Considerations
• Observation wells are recommended at 50 ft intervals over the length of the infiltration trench.
They will indicate how quickly the trench dewaters following a storm and it will provide a
method of observing how quickly the trench fills up with sediments.
• Infiltration trenches should not be deeper than the longest surface area dimension. Otherwise, they
meet the USEPA definition of Class V Injection Wells under the federal Underground Injection
Control (UIC) Program, and are subject to applicable federal and state requirements.
• Vegetation may be planted over the infiltration trench provided that adequate soil media is
provided above the trench.
• There must be an overflow route for storm water flows that overtop the facility or in case the
infiltration facility becomes clogged.
Construction/Inspection Considerations
Stabilize the entire area draining to the infiltration before construction begins. If impossible, place
a diversion berm around the perimeter of the infiltration site to prevent sediment entrance during
construction.
Stabilize the entire contributing drainage area before allowing any runoff to enter once construction is
complete.
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of an Infiltration Trench
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Appendix B: Treatment Control BMP Fact Sheets
Description
An subsurface infiltration system is a rock storage (or alternative pre-manufactured material) bed below
other surfaces such as parking lots, lawns, and playfields for temporary storage and infiltration of runoff.
Minimum Design Criteria
Design Parameter Units Value
Drawdown (drain) Time hours 48
Minimum Soil Infiltration Rate inches/hour 0.5
Minimum Depth from system invert to groundwater table feet 3
Any applicable manufacturer's criteria
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Follow the manufacturer’s guidelines for appropriate sizing calculations and selection of appropriate
device/model.
Pretreatment Considerations
Infiltration facilities are highly susceptible to clogging and premature failure from sediment, trash,
and other materials. Suitable pretreatment systems maintain the infiltrate rate of the device without
frequent and intensive maintenance. For measured soil infiltration rates below 3 in/hr, pretreatment is
strongly recommended, and the pretreatment device should be sized for at least 25% of the WQV. For
TC-03: Subsurface Infiltration
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
Kroc Community Center, Kapolei
B-16 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
measured soil infiltration rates greater than 3 in/hr, pretreatment is mandatory to minimize groundwater
contamination risks, and the pretreatment device must be sized for at least 50% of the WQV if the
measured soil infiltration rate is below 5 in/hr and 100% of the WQV if the measured soil infiltration
rate is greater than 5 in/hr. Pretreatment may be achieved with vegetated swales, vegetated filter strips,
sedimentation basins or forebays, sedimentation manholes, and manufactured treatment devices.
Area Requirements
The below-grade footprint requirements for commercially-available infiltration chambers vary by
manufacturer. However, similarly to above-grade non-proprietary systems, the space will be minimized
for sites with higher infiltration rates and lower infiltration rate factors of safety.
Sizing Example
Follow the manufacturer’s guidelines for appropriate sizing calculations and selection of appropriate
configuration.
Other Design Considerations
Refer to manufacturer guidelines.
Construction/Inspection Considerations
Refer to manufacturer guidelines.
Schematic of a Subsurface Infiltration
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Appendix B: Treatment Control BMP Fact Sheets
Description
A dry well is a subsurface aggregate-filled or prefabricated perforated storage facility, where roof runoff is
stored and infiltrates into the soil matrix.
Minimum Design Criteria
Design Parameter Units Value
Drawdown (drain) Time hours 48
Minimum Soil Infiltration Rate inches/hour 0.5
Aggregate Size (if used)inches 1.0 - 3.0
Minimum Depth from well invert to groundwater table feet 3
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-04: Dry Well
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
Courtesy www.brickstoremuseum.org
B-18 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 2. Calculate the maximum allowable water storage depth (dmax) using the underlying soil
infiltration rate (k) and the required drawdown time (t):
dmax = kt/(Fs × 12)
Where dmax =Maximum Storage Depth (ft)
k =Soil Infiltration Rate (in/hr)
t =Drawdown (drain) Time (hrs)
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 3. Select a ponding depth (optional) and dry well backfill material depth such that the total
effective storage depth is no greater than the maximum allowable depth calculated in Step 2:
dt = dp + lbnb ≤ dmax
Where dt =Total Effective Water Storage Depth (ft)
dp =Ponding Depth (ft)
lb =Backfill Material Thickness Depth (ft)
nb =Backfill Material Porosity
dmax =Maximum Storage Depth from Step 2 (ft)
Step 4. Calculate the BMP surface area (ABMP):
ABMP = WQV/(dt + kT/12Fs)
Where ABMP =BMP Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (ft)
k =Soil Infiltration Rate (in/hr)
T =Fill Time (time for the BMP to fill with water [hrs])
Fs =Infiltration Rate Factor of Safety (see Section 5)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the ponding depth or rock depth (if the total effective depth is not already equal to
the maximum depth), and/or reduce the infiltration rate factor of safety (if minimum number
of test pits and permeability tests have not been performed) and repeat the calculations.
Pretreatment Considerations
Roof gutter guards or leaf gutter screens are required for roof runoff to reduce dry well clogging from
sediment, leaves, and other organic material. If the dry well receives non-roof runoff, pretreatment must
be provided by vegetated swales, vegetated filter strips, or manufactured treatment devices.
Area Requirements
A dry well requires a footprint equivalent to 2% - 20% of its contributing impervious drainage area.
The lower value reflects the maximum allowable infiltration rate, minimum allowable factor of safety,
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Appendix B: Treatment Control BMP Fact Sheets
and minimal ponding, while the upper value reflects the minimum allowable infiltration rate, maximum
allowable factor of safety, and no ponding.
Sizing Example
Calculate the size of a dry well serving the roof runoff from a 3,000 sq-ft commercial building. Assume
the following design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 100
Design Storm Depth, P inches 1.0
Dry Well Fill Time, T hours 2
Drawdown (drain) Time, t hours 48
Backfill material porosity, nb 0.35
Soil Infiltration Rate, k inches/hour 1.0
Infiltration Rate Factor of Safety, Fs 2
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 100
C =0.95
WQV =PCA G 3630
WQV =1 G 0.95 G (3,000/43,560) G 3630
WQV =238 cu-ft
2. Calculate the maximum allowable water storage depth in the dry well (dmax):
dmax =kt/12Fsdmax=1.0 G 48/(12 G 2)
dmax =2.0 ft
3. Select a design ponding depth (dp) and backfill material depth (lb) such that the total effective
storage depth (dt) is no greater than the maximum allowable depth:
dp =0.0 ft
lb =5.5 ft
dt =dp + lbnbdt=0.0 + 5.5 G 0.35
dt =1.925 ft
B-20 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
4. Calculate the BMP surface area:
ABMP =WQV/(dt + kT/12Fs)
ABMP =238/[1.925 + 1.0 G 2.0/(12 G 2)]
ABMP =118 sq-ft
Other Design Considerations
• Dry wells are typically deeper than they are wide or long, and therefore meet the USEPA
definition of Class V Injection Wells under the federal Underground Injection Control (UIC)
Program, and are subject to applicable federal and state requirements.
• The dry well must be able to safely convey overflows to either vegetated areas or the storm drain
system.
• The design may include an intermediate box with an outflow higher to allow sediments to settle
out. Water would then flow through a mesh screen and into the dry well.
• Trees and other large vegetation should be planted away from drywells such that drip lines do not
overhang infiltration beds.
Construction/Inspection Considerations
Refer to manufacturer guidelines.
Schematic of a Dry Well
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a rain garden, a bioretention basin is an engineered shallow depression that
collects and filters storm water runoff using conditioned planting soil beds and vegetation. The filtered
runoff infiltrates through the basin invert and into the soil matrix.
Minimum Design Criteria
Design Parameter Units Value
Mulch Thickness inches 2 - 4
Planting Soil Depth feet 2 - 4
Drawdown (drain) Time hours 48
Maximum Interior Side Slope (length per unit height)3:1
Maximum Ponding Depth inches 12
Minimum Depth from basin invert to groundwater table feet 3
Minimum Freeboard feet 1.0
Minimum Soil Infiltration Rate inches/hour 0.5
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-05: Bioretention Basin
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
Heeia State Park (www.huihawaii.org)
B-22 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Step 2. Calculate the maximum allowable water storage depth (dmax) using the underlying soil
infiltration rate (k) and the required drawdown time (t):
dmax = kt/(Fs × 12)
Where dmax =Maximum Storage Depth (ft)
k =Soil Infiltration Rate (in/hr)
t =Drawdown (drain) Time (hrs)
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 3. Select a ponding depth, planting media thickness (depth), and reservoir layer thickness
(depth, optional) such that the total effective storage depth is no greater than the maximum
allowable depth calculated in Step 2:
dt = dp + lmnm + lrnr ≤ dmax
Where dt =Total Effective Water Storage Depth (ft)
dm =Ponding Depth (ft)
lm =Planting Media Thickness Depth (ft)
nm =Planting Media Porosity
lr =Reservoir Layer Thickness Depth (ft)
nr =Reservoir Layer Porosity
dmax =Maximum Storage Depth from Step 2 (ft)
Step 4. Calculate the basin bottom surface area (Ab):
Ab = WQV/(dt + kT/12Fs)
Where ABMP =Bottom Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (ft)
k =Soil Infiltration Rate (in/hr)
T =Fill Time (time for the BMP to fill with water [hrs])
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 5. Select a basin bottom width (wb), and calculate the basin bottom length (lb):
Ab = WQV/(dt + kT/12Fs)
Where lb =Bottom Length (ft)
Ab =Bottom Surface Area from Step 4 (sq-ft)
wb =Bottom Width (ft)
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Step 6. Calculate the total area occupied by the BMP excluding pretreatment (ABMP) using the basin
bottom dimensions, embankment side slopes, and freeboard:
ABMP = [wb + 2z(dp + f)] × [lb + 2z(dp + f)]
Where ABMP =Area Occupied by BMP Excluding Pretreatment (sq-ft)
wb =Bottom Width from Step 5 (ft)
z =Basin Interior Side Slope (length per unit height)
db =Design Ponding Depth from Step 3 (ft)
f =Freeboard (ft)
lb =Bottom Length from Step 5 (ft)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the ponding depth or planting soil depth or gravel depth (if the total effective depth
is not already equal to the maximum depth), and/or reduce the Infiltration rate factor of safety
(if minimum number of test pits and permeability tests have not been performed) and repeat
the calculations.
Pretreatment Considerations
Infiltration facilities are highly susceptible to clogging and premature failure from sediment, trash,
and other materials. Suitable pretreatment systems maintain the infiltrate rate of the device without
frequent and intensive maintenance. For measured soil infiltration rates below 3 in/hr, pretreatment is
strongly recommended, and the pretreatment device should be sized for at least 25% of the WQV. For
measured soil infiltration rates greater than 3 in/hr, pretreatment is mandatory to minimize groundwater
contamination risks, and the pretreatment device must be sized for at least 50% of the WQV if the
measured soil infiltration rate is below 5 in/hr and 100% of the WQV if the measured soil infiltration
rate is greater than 5 in/hr. Pretreatment may be achieved with vegetated swales, vegetated filter strips,
sedimentation basins or forebays, sedimentation manholes, and manufactured treatment devices.
Area Requirements
A bioretention basin requires a footprint equivalent to 4% - 13% of its contributing impervious drainage
area, excluding pretreatment. The lower value reflects the maximum allowable infiltration rate and
minimum allowable factor of safety, while the upper value reflects the minimum allowable infiltration rate
and maximum allowable factor of safety.
Sizing Example
Calculate the size of a bioretention basin serving a 1-acre residential development. Assume the following
design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 70
Design Storm Depth, P inches 1.0
Basin Fill Time, T hours 2
Drawdown (drain) Time, t hours 48
B-24 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Design Parameter Units Value
Basin Interior Side Slope (length per unit height), z 3
Planting Media Porosity, nm 0.25
Reservoir Layer Porosity, nr 0.30
Soil Infiltration Rate, k inches/hour 1.0
Freeboard, f feet 1.0
Infiltration Rate Factor of Safety, Fs 2
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 70
C =0.68
WQV =PCA G 3630
WQV =1 G 0.68 G 1 G 3630
WQV =2,468 cu-ft
2. Calculate the maximum allowable water storage depth in the dry well (dmax):
dmax =kt/12Fsdmax=1.0 G 48/(12 G 2)
dmax =2.0 ft
3. Select a ponding depth (dp), planting media depth (lm), and optional reservoir layer depth (lr) such
that the total effective storage depth (dt) is no greater than the maximum allowable depth:
dp =0.67 ft
lm =4.0 ft
lr =1.0 ft
dt =dp + lmnm + lrnrdt=0.67 + 4.0 G 0.25 + 1.0 G 0.30
dt =1.97 ft
4. Calculate the basin bottom surface area (Ab):
Ab =WQV/(dt + kT/12Fs)
Ab =2,468/[1.97 + 1.0 G 2.0/(12 G 2)]
Ab =1,204 sq-ft
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
5. Set the basin bottom width (wb) to 25 ft, and calculate the basin bottom length (lb):
lb =Ab/wblb=1,204/25
lb =48.2 sq-ft
6. Calculate the total area excluding pretreatment (ABMP):
ABMP =[wb + 2z(dp + f)] G [lb + 2z(dp + f)]
ABMP =[25 + 2 G 3(0.67 + 1)] G [48.2 + 2 G 3(0.67 + 1)]
ABMP =2,037 sq-ft
Other Design Considerations
• The plantings should emulate a terrestrial forest community ecosystem. Native species should be
selected, taking into account the local climate, expected water depth in the basin, and expected
tolerances to pollutant loads and varying soil moistures. The trees should be smaller ones similar
to those found in the forest understory, since it is more difficult to perform maintenance with the
tall trees that are normally part of the forest canopy. Ground cover, such as grasses or legumes,
should be planted after the trees and shrubs are in place.
• An overflow device (e.g., domed riser, spillway) must be included to safely convey runoff from
large storm events when the surface/subsurface capacity is exceeded.
• If a mulch layer is used on the surface of the planting bed, consideration should be given to
problems caused by flotation during storm events.
• Observation wells are recommended. They will indicate how quickly the basin dewaters
following a storm and it will provide a method of observing how quickly the basin fills up with
sediments.
Construction/Inspection Considerations
Bioretention basins should not be established until contributing watershed is stabilized.
B-26 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Bioretention Basin
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as pervious pavement or porous pavement, permeable pavement refers to any
porous, load-bearing surface that allows for temporary rainwater storage in an underlying aggregate layer
until it infiltrates into the soil matrix. It includes pervious concrete, porous asphalt, interlocking paver
blocks, and reinforced turf and gravel filled grids.
Minimum Design Criteria
Design Parameter Units Value
Maximum Depth of Reservoir Layer feet 3
Drawdown (drain) Time hours 48
Minimum Depth from Reservoir Invert to Groundwater Table feet 3
Minimum Soil Infiltration Rate inches/hour 0.5
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-06: Permeable Pavement
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients High Pesticides High
Sediment High Oil & Grease High
Trash High Metals High
Pathogens High Organic Compounds High
UH Hale Halawai Overflow Parking
B-28 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 2. Calculate the maximum allowable water storage depth (dmax) using the underlying soil
infiltration rate (k) and the required drawdown time (t):
dmax = kt/(Fs × 12)
Where dmax =Maximum Storage Depth (ft)
k =Soil Infiltration Rate (in/hr)
t =Drawdown (drain) Time (hrs)
Fs =Infiltration Rate Factor of Safety (see Section 5)
Step 3. Select a pavement course thickness (lp) and reservoir course thickness (lr) such that the total
effective storage depth (dt) is no greater than the maximum allowable depth:
dt = (lpnp + lrnr)/12 ≤ dmax
Where dt =Total Effective Water Storage Depth (ft)
lp =Pavement Course Thickness (in)
np =Pavement Course Porosity
lr =Reservoir Layer Thickness Depth (in)
nr =Reservoir Layer Porosity
dmax =Maximum Storage Depth from Step 2 (ft)
Step 4. Calculate the BMP surface area (ABMP):
ABMP = WQV/(dt + kT/12Fs)
Where ABMP =BMP Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (ft)
k =Soil Infiltration Rate (in/hr)
T =Fill Time (time for the BMP to fill with water [hrs])
Fs =Infiltration Rate Factor of Safety (see Section 5)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the pavement course depth or reservoir course depth or gravel depth (if the total
effective depth is not already equal to the maximum depth), and/or reduce the Infiltration
rate factor of safety (if minimum number of test pits and permeability tests have not been
performed) and repeat the calculations.
Pretreatment Considerations
Pretreatment is not required as long as the permeable pavement does not receive run-on from other
surfaces. If it does, pretreatment is necessary to prevent premature failure due to clogging with fine
sediment, and may be achieved with gravel filter strips, vegetated buffer strips, or vegetated swales.
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Area Requirements
Permeable pavement requires a footprint equivalent to 5% - 18% of its contributing impervious drainage
area. The lower value reflects the maximum allowable infiltration rate and minimum allowable factor of
safety, while the upper value reflects the minimum allowable infiltration rate and maximum allowable
factor of safety.
Sizing Example
Calculate the size of a section of permeable pavement serving the runoff from a 1-acre parking lot.
Assume the following design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 100
Design Storm Depth, P inches 1.0
Reservoir Layer Time, T hours 2
Drawdown (drain) Time, t hours 48
Pavement Course Porosity, np 0.15
Reservoir Course Porosity, nr 0.35
Soil Infiltration Rate, k inches/hour 1.0
Infiltration Rate Factor of Safety, Fs 2
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 100
C =0.95
WQV =PCA G 3630
WQV =1 G 0.95 G 1 G 3630
WQV =3,449 cu-ft
2. Calculate the maximum allowable water storage depth in the dry well (dmax):
dmax =kT/12Fsdmax=1.0 G 48/(12 G 2)
dmax =2.0 ft
B-30 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
3. Select a pavement course thickness (lp) and reservoir course thickness (lr) such that the total
effective storage depth (dt) is no greater than the maximum allowable depth:
lp =12.0 ft
lr =24.0 ft
dt =(lpnp + lrnr)/12
dt =(12 G 0.15 + 24.0 G 0.35)/12
dt =0.85 ft
4. Calculate the pavement surface area:
AIMP =WQV/[dt + (kT/12Fs)]
AIMP =3,449/[0.85 + (1.0 G 2.0/(12 G 2)]
AIMP =3,695 sq-ft
Other Design Considerations
• All porous paving and permeable paver with storage bed systems must include measures that
will allow runoff from the design storm to enter the storage bed in the event that the porous
or permeable paver surface course becomes clogged or otherwise incapable of conveying the
maximum design storm runoff to the bed.
• Additional design details on specific pavement systems are provided by the National Asphalt
Pavement Association, the National Ready Mix Concrete Association, the Interlocking Concrete
Pavement Institute, and the American Association of State Highway and Transportation Officials.
• Perforated pipes along the bottom of the bed may be used to evenly distribute runoff over the
entire bed bottom. Pipes should lay flat along the bed bottom and provide for uniform distribution
of water. Depending on size, these pipes may provide additional storage volume.
• Flows in excess of the design capacity of the permeable pavement system will require an
overflow system connected to a downstream conveyance or other storm water runoff BMP.
Construction/Inspection Considerations
• Permeable surfaces can be laid without cross-falls or longitudinal gradients.
• The blocks should be lain level.
• They should not be used for storage of site materials, unless the surface is well protected from
deposition of silt and other spillages.
• The pavement should be constructed in a single operation, as one of the last items to be built, on a
development site. Landscape development should be completed before pavement construction to
avoid contamination by silt or soil from this source.
• Surfaces draining to the pavement should be stabilized before construction of the pavement.
• Inappropriate construction equipment should be kept away from the pavement to prevent damage
to the surface, sub-base or sub-grade.
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Permeable Pavement
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a vegetated roof or eco-roof, a green roof is a roof that is entirely or partially
covered with vegetation and soils for the purpose of filtering, absorbing, evapotranspirating, and
retaining/detaining the rain that falls upon it.
Minimum Design Criteria
Design Parameter Units Value
Minimum Depth of Soil Media inches 2
Minimum Depth of Drainage Layer inches 2
Maximum Slope on Roof percent 20
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
Step 2. Select initial values for the soil media thickness (lm), drainage layer thickness (ld), and
allowable ponding depth (dp).
TC-07: Green Roof
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients Medium Pesticides Medium
Sediment High Oil & Grease High
Trash High Metals Medium
Pathogens Medium Organic Compounds Medium
University of Hawaii C-MORE Hale (hahana.soest.hawaii.edu)
B-34 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 3. Calculate the total effective storage depth based on instantaneous storage capacity using the
void space in the soil media and drainage layer, and the allowable ponding:
dt = (dp + lmnm + ldnd)/12
Where dt =Total Effective Water Storage Depth (ft)
dp =Ponding Depth (in)
lm =Planting Media Thickness Depth (in)
nm =Planting Media Porosity
ld =Drainage Layer Thickness (in)
nd =Drainage Layer Porosity
Step 4. Calculate area required (ABMP) based on the instantaneous storage capacity:
ABMP = WQV/dt
Where ABMP =BMP Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (ft)
If the calculated area does not fit in the available space, either reduce the tributary area and/
or increase one or more of the design depths (ponding, soil media, drainage layer), and repeat
the calculations.
Pretreatment Considerations
Green roofs do not require pretreatment.
Area Requirements
A green roof requires a footprint equivalent to 11% - 100% of the contributing roof drainage area. The
lower value corresponds to 4 inches of ponding and maximum depths for both the planting media and
drainage layer depths, while the higher value corresponds to no ponding and minimum planting media
and drainage layer depths.
Sizing Example
Calculate the size of a green roof serving the roof runoff from a 1,500 sq-ft fast food restaurant. Assume
the following design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 100
Design Storm Depth, P inches 1.0
Soil Media Porosity, nm 0.20
Drainage Layer Porosity, nd 0.25
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 G 100
C =0.95
WQV =PCA G 3,630
WQV =1 G 0.95 G (1,500/43,560) G 3,630
WQV =119 cu-ft
2. Select initial values for the soil media depth (dm), drainage layer depth (dd), and ponding
depth (dp):
dm =3 in
dd =2 in
dp =0.5 in
3. Calculate the total effective storage depth:
dt =(dp + lmnm + ldnd)/12
dt =(0.5 + 3 G 0.20 + 2 G 0.25)/12
dt =0.133 ft
4. Calculate the area (ABMP):
ABMP =WQV/dtABMP=119/0.133
ABMP =891 sq-ft
891 sq-ft is available, so the design is okay.
Other Design Considerations
• Safety measures against wind uplift must be taken into account during design, especially for areas
susceptible to high winds during the summer trade-wind period.
• The maximum load bearing capacity of the roof construction must be considered when installing
vegetated roofs. The water saturated weight of the green roof system, including vegetation must
be calculated as permanent load. Generally, vegetated roofs weigh between 15 and 30 lb/ sq-
ft depending on the thickness of the vegetated roof system. In addition, construction elements
such as pergolas and walkways cause high point loads and, therefore, have to be calculated
accordingly.
• The design must include adequate roof access for delivery of construction materials and for
routine maintenance.
• The drainage layer below the growth media should be designed to convey the flood design storm
without backing water up to into the growing media. The drainage layer should convey flow to
an outlet or overflow system such as a traditional rooftop drainage system with inlets set slightly
above the elevation of the vegetated roof surface.
B-36 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Green Roof
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Description
This category of BMPs may also be referred to as a bioretention filter, storm water curb extension, tree
box filter, or planter box. A vegetated bio-filter is an engineered shallow depression or above ground
system that collects and filters storm water runoff using conditioned planting soil beds and vegetation.
The filtered runoff discharges through an underdrain system.
Minimum Design Criteria
Design Parameter Units Value
Planting Soil Coefficient of Permeability feet/day 1.0
Mulch Thickness inches 2 – 4
Planting Soil Depth feet 2 – 4
Drawdown (drain) Time hours 48
Maximum Ponding Depth inches 12
Minimum Underdrain Diameter inches 6
For proprietary systems, follow the manufacturer’s guidelines for appropriate sizing calculations and
selection of appropriate device/model.
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-08: Vegetated Bio-Filter
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients Medium Pesticides Unknown
Sediment High Oil & Grease High
Trash High Metals High
Pathogens Medium Organic Compounds High
Waikiki
B-38 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
Step 2. Select initial values for the soil media thickness (lm), drainage layer thickness (ld), and
allowable ponding depth (dp).
Step 3. Use Darcy’s Law to calculate the required Filter Bed Surface Area:
Ab = (WQV × lm)/k (lm + dp/24) (t/24)
Where Ab =Filter Bed Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
lm =Planting Media Depth from Step 2 (ft)
k =Planting Media Permeability Coefficient (ft/day)
dp =Maximum Ponding Depth from Step 2 (in)
t =Filter Bed Drain Time (hr)
Step 4. Select a filter bed width (wb), and calculate the filter bed length (lb):
lb = Ab/wb
Where lb =Filter Bed Length (ft)
Ab =Filter Bed Surface Area from Step 3 (sq-ft)
wb =Filter Bed Width (ft)
Step 5. Calculate the total area occupied by the BMP excluding pretreatment (ABMP) using the filter
bed dimensions, embankment side slopes, and freeboard:
ABMP = [ wb + 2z(dp+f) ] × [ lb + 2z(dp +f) ]
Where ABMP =Area occupied by BMP excluding Pretreatment (sq-ft)
wb =Filter Bed Width from Step 4
z =Filter Bed Interior Slope (length per unit height)
dp =Maximum Ponding Depth from Step 2 (ft)
f =Freeboard (ft)
lp =Filter Bed Length from Step 4 (ft)
If the calculated area does not fit in the available space, either reduce the drainage area,
reduce the planting soil depth (if it’s not already set to the minimum), and/or increase the
ponding depth (if it’s not already set to the maximum depth), and repeat the calculations.
Pretreatment Considerations
Pretreatment should be provided where sediments or trash may cause a concern or decreased BMP
functionality, and when space permits. Pretreatment may be achieved with vegetated swales, vegetated
buffer strips with pea gravel or stone diaphragm, or manufactured treatment device.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Area Requirements
A vegetated bio-filter requires a footprint equivalent to 3.3% - 3.8% of its contributing impervious
drainage area, excluding pretreatment. The lower value reflects the minimum planting media depth
and maximum ponding depth, while the upper value reflects the maximum planting media depth and
minimum ponding depth.
Sizing Example
Calculate the size of a vegetated bio-filter serving a 1-acre residential development. Assume the following
design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 70
Design Storm Depth, P inches 1.0
Planting Soil Coefficient of Permeability, k feet/day 1.0
Drawdown (drain) Time, t hours 48
Interior Side Slope (length per unit height), z 0
Freeboard, f feet 0.5
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.0039 G 70
C =0.68
WQV =PCA G 3,630
WQV =1 G 0.68 G 1 G 3,630
WQV =2,468 cu-ft
2. Select a planting soil depth (ds) and ponding depth (dp):
ds =2 ft
dp =6 in
3. Calculate the Filter Bed Surface Area (ABMP):
ABMP =WQV x ds/[ k(ds + (dp/24))(t/24) ]
ABMP =2,468 x 2/[ 1(2 + (6/24))(48/24) ]
ABMP =1,097 sq-ft
4. Set the bottom width (wb) to 6 ft, and calculate the bottom length (lb):
lb =Ab/wblb=1,097/6
lb =182.8 ft
B-40 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix B: Treatment Control BMP Fact Sheets
5. Calculate the total area excluding pretreatment (ABMP):
ABMP =[ wb + 2z(dp + f) ] x [ lb + 2z(dp+ f) ]
ABMP =[ 6 + 2 × 0 × (0.5 + 0.5) ] × [ 182.8 + 2 × 0 × (0.5 + 0.5) ]
ABMP =1,097 sq-ft
Other Design Considerations
• All bio-filters must be able to safely overflow or bypass flows in excess of the storm water quality
design storm to downstream drainage systems when the surface/subsurface capacity is exceeded.
• If a mulch layer is used on the surface of the planting bed, consideration should be given to
problems caused by flotation during storm events.
• A cleanout pipe should be tied into the end of all underdrain pipe runs.
Construction/Inspection Considerations
Vegetated biofilters should not be established until contributing watershed is stabilized.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Vegetated Bio-Filter
B-42 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Tree Box Filter
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a bioretention swale or dry swale, an enhanced swale is a shallow linear channel
with a planting bed and covered with turf or other surface material (other than mulch or plants). Runoff
filters through a planting bed, is collected in an underdrain system, and discharged at the downstream end
of the swale.
Minimum Design Criteria
Design Parameter Units Value
Maximum Interior Side Slope (length per unit height)3:1
Bottom width feet 2 - 8
Maximum Longitudinal Slope without check dams percent 2
Maximum Longitudinal Slope with check dams percent 5
Maximum check dam height inches 12
Maximum Ponding Depth at downstream end inches 18
Media depth inches 18 - 36
Minimum Freeboard feet 0.5
Minimum Underdrain Diameter inches 6
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-09: Enhanced Swale
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients Medium Pesticides Unknown
Sediment High Oil & Grease Medium
Trash High Metals Medium
Pathogens Unknown Organic Compounds Unknown
Georgia Stormwater Management Manual, 2001.
B-44 Storm Water BMP Guide for New and Redevelopment
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Appendix B: Treatment Control BMP Fact Sheets
Step 2. Select values for the planting media thickness, drainage layer thickness, planting media
porosity, drainage layer porosity, maximum surface ponding depth (if check dams are used),
bottom width, and interior side slope (length per unit height).
Step 3. Calculate the total effective storage depth based on the instantaneous storage capacity using
the void space in the planting media and drainage layer, and the average ponding depth
(assumed to be one-half the maximum ponding depth):
dt = [(dp/2) + lmnm + ldnd)]/12
Where dt =Total Effective Water Storage Depth (ft)
dp =Maximum Ponding Depth from Step 2 (in)
lm =Planting Media Thickness from Step 2 (in)
nm =Planting Media Porosity, typically around 0.25
ld =Drainage Layer Thickness from Step 2 (in)
nd =Drainage Layer Porosity, typically around 0.40
Step 4. Calculate the swale invert area required (Ab) based on the instantaneous storage capacity
(neglecting the additional ponding capacity due to the shape of the swale sides):
Ab = WQV/dt
Where Ab =Bottom Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
dt =Total Effective Water Storage Depth from Step 3 (cu-ft)
Step 5. Calculate the total area required (ABMP) taking into account the side slopes along the length of
the swale:
ABMP = [ b + 2z(f + dp/12) ] × ( Ab/b )
Where ABMP =Total Surface Area (sq-ft)
b =Swale Bottom Width from Step 2 (ft)
z =Interior Swale Side Slope (length per unit height) from Step 2
dp =Ponding Depth from Step 2 (in)
f =Freeboard (ft)
Ab =Bottom Surface Area from Step 4 (sq-ft)
If the minimum surface area is larger than the available space, reduce the tributary area and/or
increase one or more design depths (media, gravel, ponding), and repeat the calculations.
Pretreatment Considerations
Pretreatment for enhanced swales is provided by a shallow sediment forebay at the initial point of the
channel. The volume of this forebay should be equal to at least 0.05 in. per impervious acre of drainage.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
A pea gravel diaphragm can be used along the top of the channel to provide pretreatment for lateral flows
entering the swale.
Area Requirements
An enhanced swale requires a footprint equivalent to 8% - 40% of its contributing impervious drainage
area. The lower value corresponds to the maximum allowable values for the mentioned dependent
variables, while the upper value reflects the minimum allowable values for all specified parameters.
Sizing Example
Calculate the size of an enhanced swale serving a 1-acre residential development. Assume the following
design parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 70
Design Storm Depth, P inches 1.0
Media porosity, nm 0.25
Drainage layer porosity, nd 0.40
Freeboard, f feet 0.5
Drawdown (drain) Time, t hours 48
1. Calculate the volumetric runoff coefficient (C) and WQV:
C =0.05 + 0.009I
C =0.05 + 0.0039 G 70
C =0.68
WQV =PCA G 3,630
WQV =1 G 0.68 G 1 G 3,630
WQV =2,468 cu-ft
2. Select a media thickness (lm), drainage layer thickness (ld), ponding depth (dp), bottom width (b),
and interior side slope (z):
lm =18 in
ld =6 in
dp =12 in
b =8 ft
z =3
3. Calculate the total effective storage depth:
dt =[ (dp/2) + lmnm + ldnd]/12
dt =( 6 + 18 + 0.25 + 6 × 0.40 )/12
dt =1,075 sq-ft
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4. Calculate the minimum invert area (Ab) needed for the WQV and depths:
Ab =WQV/dtAb=2,468/1.075
Ab =2,296 sq-ft
5. Calculate the total area required (ABMP):
ABMP =[ b + 2z(f + dp/12) ] × ( Ab/b)
ABMP =[ 8 + 2 × 3 × (0.5 + 12/12) ] × ( 2,296/8)
ABMP =4,879 sq-ft
Other Design Considerations
• Landscape design should specify proper grass species based on specific site, soils and hydric
conditions present along the channel. Vegetation should be designed for regular mowing, like a
typical lawn, or less frequently (annually or semi-annually).
• Enhanced swales must be adequately designed to safely pass flows that exceed the design storm
flows.
Construction/Inspection Considerations
• Include directions in the specifications for use of appropriate fertilizer and soil amendments
based on soil properties determined through testing and compared to the needs of the vegetation
requirements.
• Install swales at the time of the year when there is a reasonable chance of successful
establishment without irrigation; however, it is recognized that rainfall in a given year may not be
sufficient and temporary irrigation may be used.
• If sod tiles must be used, they should be placed so that there are no gaps between the tiles; stagger
the ends of the tiles to prevent the formation of channels along the swale or strip.
• Use a roller on the sod to ensure that no air pockets form between the sod and the soil.
• Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days after
the first rainfall of the season.
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Schematic of an Enhanced Swale
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a grass swale, grass channel, or biofiltration swale, a vegetated swale is a broad
shallow earthen channel vegetated with erosion resistant and flood tolerant grasses. Runoff typically
enters the swale at one end and exits at the other end.
Minimum Design Criteria
Design Parameter Units Value
Maximum Interior Side Slope (length per unit height)3:1
Maximum Flow Velocity feet/second 1
Maximum Water Depth inches 4
Minimum Hydraulic Residence Time minutes 7
Maximum Bottom Width feet 10
Minimum Freeboard feet 0.5
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the WQF Rate.
Step 2. Select initial values for swale bottom width (b), depth of flow (y), swale side slope (z), swale
longitudinal slope (s), and hydraulic residence time (T).
TC-10: Vegetated Swale
Kauai Federal Credit Union (courtesy of Group 70)
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients Low Pesticides Unknown
Sediment Medium Oil & Grease Medium
Trash Low Metals Medium
Pathogens Low Organic Compounds Unknown
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Appendix B: Treatment Control BMP Fact Sheets
Step 3. Calculate the cross-sectional area (A), wetted perimeter (WP), and hydraulic radius (R) using
the dimensions established in Step 2:
A = (by/12) + (zy2/144)
WP = b (2y/12)√(1+z2 )
R = A/WP
Where A =Cross Sectional Area (sq-ft)
WP =Wetter Perimeter (ft)
R =Hydraulic Radius (ft)
b =Swale Bottom Width from Step 2 (ft)
y =Depth of Flow for WQV from Step 2 (in)
z =Swale Side Slope (length per unit height) from Step 2
Step 4. Calculate the design flow rate in the swale using the selected dimensions and Manning’s
Equation:
Q = (1.49AR2/3s1/2)/n
Where Q =Design Flow Rate (cu-ft/sec)
A =Cross Sectional Area from Step 3 (sq-ft)
R =Hydraulic Radius from Step 3 (ft)
s =Longitudinal Slope from Step 2 (percent)
R =Manning's n value
Unless another value can be justified, use a Manning’s n value of 0.20 for water quality
calculations (lower values, such as 0.035, are only applicable for flood control calculations).
If the calculated flow rate is not equal to or greater than the WQF from Step 1, decrease the
tributary area and/or increase one or more swale dimensions (bottom width, depth of flow,
side slope, or longitudinal slope) and repeat the calculations.
Step 5. Once an appropriate design flow rate is achieved, calculate the design flow velocity using the
flow continuity equation:
V = Q/A
Where V =Design Flow Velocity (ft/sec)
Q =Design Flow Rate from Step 4 (cu-ft/sec)
A =Cross Sectional Area from Step 3 (sq-ft)
If the design flow velocity is greater than the maximum allowed velocity, either include check
dams with vertical drops of no more than 12 inches, or revise one or more swale dimensions
and repeat the calculations.
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Appendix B: Treatment Control BMP Fact Sheets
Step 6. Multiply the velocity by the hydraulic residence time to determine the length:
L = 60VT
Where L =Swale Length (ft)
T =Hydraulic Resistance Time from Step 2 (min)
V =Design Flow Velocity from Step 5 (ft/sec)
Step 7. Calculate the total area required (ABMP) taking into account the side slopes along the length of
the swale and the freeboard:
ABMP = [ b + 2z(f + y/12)] × L
Where ABMP =Total Surface Area (sq-ft)
b =Swale Bottom Width from Step 2 (ft)
z =Interior Swale Side Slope (length per unit height) from Step 2
y =Depth of Flow from WQV from Step 2 (in)
f =Freeboard (ft)
L =Swale Length from Step 6 (ft)
If the calculated are does not fit in the available area, reduce the drainage area, reduce the
hydraulic residence time (if it is longer than the minimum), and/or revise one or more swale
dimensions, and repeat the calculations.
Pretreatment Considerations
Vegetated swales do not require pretreatment.
Area Requirements
A vegetated swale requires a footprint equivalent to 2% - 4% of its contributing impervious drainage area.
The lower value corresponds to maximizing the flow depth and slope, while the upper value corresponds
to maximizing the bottom width and slope.
Sizing Example
Calculate the size of a grass swale serving the runoff from a one acre parking lot. Assume the following
design parameters:
Design Parameter Units Value
Weighted Runoff Coefficient, C 0.95
Rainfall Intensity, i inches/hour 0.4
Interior Side Slope (length per unit height)3
Manning’s n value -0.20
Longitudinal Slope, s 0.016
Hydraulic Residence Time, T minutes 7
Freeboard, f feet 0.5
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1. Calculate the WQF Rate:
WQF =CiA
WQF =0.95 G 0.4 G 1.0
WQF =0.38 cu-ft/sec
2. Select initial values for swale bottom width (b), depth of flow (y), swale side slope length per unit
height (z), swale longitudinal slope (s), and hydraulic residence time (T):
b =2.75 ft
y =3.5 in
z =3 in
s =0.017
T =7 mins
3. Calculate the cross-sectional area (A), wetted perimeter (WP), and hydraulic radius (R):
A =( by/12) + ( zy2/144 )
A =( 2.75 × 3.5/12) + ( 3 × 3.52/144 )
A =1.06 sq-ft
WP =b + ( 2y/12 )√(1+z2)
WP =2.75 + ( 2 × 3.5/12 )√(1+32)
WP =4.59 ft
R =A/WP
R =1.06/4.59
R =0.23 ft
4. Calculate the design flow rate (Q):
Q =1.49 AR2/3s1/2/n
Q =1.49 × 1.06 × 0.232/3 × 0.0171/2/0.20
Q =0.39 cu-ft/sec (≥ WQF, OK)
5. Calculate the velocity in the swale (V):
V =Q/A
V =0.39/1.06
V =0.36 ft/sec ( < 1 ft/sec, OK)
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Appendix B: Treatment Control BMP Fact Sheets
6. Calculate the minimum length of the swale (L):
L =60 × VT
L =60 × 0.36 × 7
L =153 ft
7. Calculate the total area required (ABMP):
ABMP =[ b + 2z(f + y/12)] × L
ABMP =[ 2.75 + 2 × 3(0.5 + 3.5/12)] × 153
ABMP =1,148 sq-ft
Other Design Considerations
• The calculated WQF may be reduced by 25% if the soil beneath the BMP is classified as
Hydrologic Soils Group (HSG) “A” or “B”, as reported by the USDA Natural Resources
Conservation Service (http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm), or if the soil
beneath the BMP is amended by incorporating 6 inches of compost/amendments and tilled up to
8 inches.
• In cases where a vegetated swale is located on-line, it should be sized as a treatment facility and
as a conveyance system per the CCH’s standards for flood control.
• Vegetate the swale with dense turf grass to promote sedimentation, filtration, and nutrient uptake,
and to limit erosion through maintenance of low flow velocities.
• Check dams may be used to achieve flow velocity requirements. They are often employed to
enhance infiltration capacity, decrease runoff volume, rate, and velocity, and promote additional
filtering and settling of nutrients and other pollutants.
Construction/Inspection Considerations
• Include directions in the specifications for use of appropriate fertilizer and soil amendments
based on soil properties determined through testing and compared to the needs of the vegetation
requirements.
• Install swales at the time of the year when there is a reasonable chance of successful
establishment without irrigation; however, it is recognized that rainfall in a given year may not be
sufficient and temporary irrigation may be used.
• If sod tiles must be used, they should be placed so that there are no gaps between the tiles; stagger
the ends of the tiles to prevent the formation of channels along the swale or strip.
• Use a roller on the sod to ensure that no air pockets form between the sod and the soil.
• Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days after
the first rainfall of the season.
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Vegetated Swale
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a vegetated filter strip or biofiltration strip, a vegetated buffer strip is a grassy
slope vegetated with turf grass that is designed to accommodate sheet flow. They may resemble natural
ecological communities and remove pollutants by vegetative filtration.
Minimum Design Criteria
Design Parameter Units Value
Maximum Flow Velocity feet/second 1
Maximum Upstream Area Flow Length feet 75
Minimum Length feet 15
Maximum Flow Depth inches 1
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the WQF Rate.
Step 2. Select values for the buffer strip width (w) and buffer strip longitudinal slope (s). Note that
if a strip width is selected that is not the same as the width of the upstream flow path, a
transition structure will be necessary to capture all the runoff and/ or establish uniform sheet
flow across the entire strip width.
TC-11: Vegetated Buffer Strip
Virginia DCR Stormwater Design Specification No. 2. 2001.
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients Low Pesticides Unknown
Sediment Medium Oil & Grease Medium
Trash Medium Metals Medium
Pathogens Low Organic Compounds Medium
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Appendix B: Treatment Control BMP Fact Sheets
Step 3. Compute the design flow depth for the WQF using a simplified form of Manning’s Equation
assuming a shallow flow depth:
y = 12 × [nQ/1.49√(s/100)]0.6
Where y =Design Flow Depth for WQF (in)
n =Manning's n Value
Q =WQF Rate from Step one (cu-ft/sec)
w =Design Width from Step 2 (ft)
s =Longitudinal Slope from Step 2 (percent)
Unless another value can be justified, use a Manning’s n value of 0.25 for water quality
calculations (lower values, such as 0.035, are only applicable for flood control calculations).
If the calculated depth is greater than the maximum allowed depth, reduce the tributary area,
increase the design width, or increase the longitudinal slope, and repeat the calculation.
Step 4. Calculate the Design flow velocity across the strip using the flow continuity equation:
V = 12Q/wy
Where V =Design Flow Velocity (ft/sec)
Q =WQF Rate from Step 1 (cu-ft/sec)
w =Design Width from Step 2 (ft)
d =Design Flow Depth from Step 3 (in)
If the design flow velocity is greater than the maximum allowed velocity, revise one or more
design parameters and repeat the calculations.
Step 5. Select a design buffer strip length (L) equal to or greater than the minimum length, and
calculate the total BMP area:
L = 20.0 ft
ABMP = L × w
Where ABMP =Vegetated Buffer Strip Area (sq-ft)
L =Design Length (ft)
w =Design Width from Step 2 (ft)
Pretreatment Considerations
Vegetated Buffer Strips do not require pretreatment.
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Appendix B: Treatment Control BMP Fact Sheets
Area Requirements
A vegetated buffer strip requires a footprint equivalent to no less than 0.4% of its contributing impervious
drainage area. While there is no upper value because there is no maximum design width or design length,
the minimum footprint corresponds to the minimum length and the maximum slope and minimum width
combination that provide the maximum allowable design depth.
Sizing Example
Calculate the size of a vegetated buffer strip serving the runoff from a one acre parking lot. Assume the
following design parameters:
Design Parameter Units Value
Weighted Runoff Coefficient, C 0.95
Rainfall Intensity, i inches/hour 0.4
Manning’s n value -0.25
Longitudinal Slope 0.06
1. Calculate the WQF Rate:
WQF =CiA
WQF =0.95 G 0.4 G 1.0
WQF =0.38 cu-ft/sec
2. Select a design buffer strip width (w) and longitudinal slope (s):
w =20.0 ft
s =0.06
3. Calculate the depth of flow for the WQF (y):
y =12 × ( WQF × n/1.49w√s)0.6
y =12 × [0.38 × 0.25/1.49 × 20√(0.06])0.6
y =0.89 in (≤ 1 in, OK)
4. Calculate the velocity across the buffer strip (V):
V =12 × WQF/(yw)
V =12 × 0.38/(0.89 × 20.0)
V =0.26 cu-ft/sec (≤ 1 cu-ft/sec, OK)
5. Select a design buffer strip length (L) at least equal to the minimum required length, and calculate
the total BMP area (ABMP):
ABMP =L × w
ABMP =20 × 20
ABMP =400 sq-ft
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Other Design Considerations
• The calculated WQF may be reduced by 25% if the soil beneath the BMP is classified as
Hydrologic Soils Group (HSG) “A” or “B,” as reported by the USDA Natural Resources
Conservation Service (http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm), or if the soil
beneath the BMP is amended by incorporating 6 inches of compost/ amendments and tilled up to
8 inches.
• A pea gravel diaphragm or engineered level spreader should be provided at the upper edge of
the BMP when the width of the contributing drainage area is greater than that of the filter. Level
spreader options include porous pavement strips, stabilized turf strips, slotted curbing, rock-filled
trench, or concrete sills.
• The selection of plants should be based on their compatibility with climate conditions, soils and
topography, and their ability to tolerate urban stresses from pollutants, variable soil moisture
conditions and ponding fluctuations.
Construction/Inspection Considerations
Vegetated filter strips should protected with temporary sediment and erosion control BMPs until
contributing areas are stabilized.
Schematic of a Vegetated Buffer Strip
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as capture/reuse or rainwater harvesting, is the collection and temporary storage of
roof runoff in rain barrels or cisterns for subsequent non-potable outdoor use (landscape irrigation, vehicle
washing).
Minimum Design Criteria
One of two equivalent performance standards shall be met:
1. Harvest and use BMP is designed to capture at least 80% of average annual (long term) runoff
volume and meet 80% of the annual overall demand.
2. Harvest and use BMPs are sized to drain the tank in 48 hrs following the end of rainfall. The size
of the BMP is dependent on the demand at the site.
It is rare cisterns can be sized to capture the full WQV and use this volume in 48 hrs. So when using
Infeasibility worksheets in the Water Quality Rules Appendix F, if it is determined that harvest and use
BMP is feasible then the BMP should be sized to the estimated 48-hr demand. The remaining WQV not
captured needs to be either retained onsite or, if infeasible, treated with biofiltration per the requirements
in Section 1.3.
Feasibility Criteria
See Section 5.5: Feasibility Criteria.
Sizing Procedure
Step 1. Define the irrigation demand by selecting values for the irrigation area (Ai), pan evaporation
coefficient (Kp), landscape coefficient (Kl), irrigation system efficiency (e). Unless specific
data is available, use a value of 0.80 for Kp (Guidelines for the Reuse of Gray Water), 0.60 for
TC-12: Harvest/Reuse
Hawaii Baptist Academy (Courtesy of Group 70)
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients Low Pesticides Unknown
Sediment Medium Oil & Grease Medium
Trash Medium Metals Medium
Pathogens Low Organic Compounds Medium
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Kl (warm season turfgrass, A Guide to Estimating Irrigation Water Needs of Landscape
Plantings in California), and 0.90 for e (well-designed system, Estimating Irrigation Water
Needs of Landscape Plantings in California).
Step 2. Define the non-irrigation demand (D0) which may include other non-potable use.
Step 3. Define the runoff available for reuse by selecting values for the drainage (i.e., roof) area (Ad),
percent of impervious cover (I), and cistern size (C).
Step 4. Identify the project’s nearest reference point (Makakilo City, Waimanalo, Waialua, Village
Park, Waianae, UH Mauka, Mililani, Opaeula, Maunawili, and Kalihi Valley) and use the
corresponding monthly rainfall rates and monthly pan evaporation rates (Epan).
Step 5. Perform a month-to-month analysis, starting with January and ending with December. Set the
beginning cistern volume in January to 0.
Step 5a. Calculate the reference evapotranspiration rate for the month using the pan
evaporation rate and the pan evaporation coefficient:
ET0 = Epan × Kp
Where ET0 =Reference Evapotranspiration Rate for the Month (in)
Epan =Pan Evaporation Rate for the Month (in) from Step 4
Kp =Pan Evaporation Coefficient from Step 1
Step 5b. Calculate the actual evapotranspiration rate for the month using the reference
evapotranspiration rate and the landscape coefficient:
ETa = ETo × Kl
Where ETa =Actual Evapotranspiration Rate for the Month (in)
ET0 =Reference Evapotranspiration Rate from Step 5a
Kl =Landscape Coefficient from Step 1
Step 5c. Calculate the total demand for the month by multiplying the irrigation area by
the difference between the actual evapotranspiration rate and the rainfall, and
adding the non-irrigation demand:
Dt = 7.48 × A × (ETa - r)/(12 × e) + D0
Where Dt =Total Demand for the Month (gallons)
A =Irrigation Area from Step 1 (sq-ft)
ETa =Actual Evapotranspiration Rate from Step 5b
r =Total Rainfall for the Month (in) from Step 4
e =Irrigation System Efficiency from Step 1
D0 =Other Non-Irrigation Demand for the Month (gallons)
from Step 2
If the total demand for the month is negative (because the rainfall amount
exceeds the evapotranspiration rate), set the total demand to 0.
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Step 5d. Calculate the amount of runoff generated for the month by multiplying the
drainage area by the rainfall by the volumetric runoff coefficient:
Rg = 7.48 × Ad × r × (0.05 + 0.009 × I)/12
Where Rg =Runoff Generated for the Month (gallons)
Ad =Drainage Area from Step 2 (sq-ft)
r =Total Rainfall for the Month (in) from Step 4
I =Percent of Impervious Cover from Step 3 (percent)
Step 5e. Compare the total demand (Dt) to the amount of runoff in the cistern at the
beginning of the month (Cb) plus the runoff generated during the month (R). If
the monthly demand is greater, set the amount of runoff reused (Ru) to the sum
of Cb and R. If the monthly demand is less, set the amount of runoff reused to
Dt.
Step 5f. Compare the Cistern capacity (C) to the amount in the cistern at the beginning
of the month (Cb) plus the Runoff generated during the month (Rg) minus the
amount of runoff used (Ru). Set the amount of runoff in the cistern at the end of
the month (Ce) to the lower of the two values.
Step 5g. Calculate the amount of cistern overflow by the following:
O = Cb + Rg - Dt - Ce
Where O =Total Cistern Overflow for the Month (gallons)
Cb =Amount of Runoff in Cistern at the beginning of the
Month (gallons)
Rg =Runoff Generated for the Month (gallons)
Dt =Total Demand for the Month (gallons)
Ce =Amount of Runoff in Cistern at the end of the Month
(gallons)
If the overflow is negative (because the amount of runoff in the cistern at the end
of the month is less than the cistern capacity), set the overflow to 0.
Step 5h. Calculate the amount of runoff captured in the cistern by subtracting the
Overflow from the amount of runoff generated:
Rc = Rg - O
Where Rc =Runoff Capture in the Cistern for the Month (gallons)
Rg =Runoff Generated for the Month (gallons)
O =Total Cistern Overflow for the Month (gallons)
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Step 5i. Set the beginning cistern amount for the next month equal to the ending cistern
amount for the current month. Repeat Steps 5 through 13 for each subsequent
month. Continue on to Step 5 after Steps 4a through 4i have been performed for
all 12 months.
Step 6. Calculate the overall runoff capture efficiency by dividing the cumulative runoff captured by
the cumulative runoff generated:
12 12Ec = 100 × ∑Rc/∑Rg 1 1
Where Ec =Overall Runoff Capture Efficiency (percent)
Rc =Runoff Capture from each Month (gallons)
Rg =Runoff Generated from each Month (gallons)
If the calculated efficiency is below the minimum design criteria value, revise one or more of
the following parameters and return to Step 3: drainage area (Ad), cistern size (C), irrigation
area (Ai), and other non-irrigation demand (D0).
Step 7. Calculate the overall demand met efficiency by dividing the cumulative runoff used by the
cumulative demand:
12 12Ed = 100 × ∑Ru/∑Dt 1 1
Where Ed =Overall Demand Met Efficiency (percent)
Ru =Runoff Used from each Month (gallons)
Dt =Total Demand from each Month (gallons)
If the calculated efficiency is below the minimum design criteria value, revise one or more of
the following parameters and return to Step 3: drainage area (Ad), cistern size (C), irrigation
area (Ai), and other non-irrigation demand (D0).
Pretreatment Considerations
Roof gutter guards or leaf gutter screens are required for roof runoff to reduce dry well clogging from
sediment, leaves, and other organic material.
Area Requirements
Rain barrel/cistern sizes can vary greatly depending on the project area, roof size, and irrigation area. The
size can be anywhere from less than 1,000 gallons to more than 10,000 gallons per 1,000 sq-ft of roof
area.
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Sizing Example
Calculate the size of a cistern serving the roof runoff from an 800 sq-ft auto repair shop in Kapolei.
Assume the following design parameters:
Design Parameter Units Value
Minimum overall runoff capture efficiency, Ec percent 80
Minimum overall demand met efficiency, Ed percent 80
1. Select initial demand values for the Irrigation Area (Ai), pan evaporation coefficient (Kp),
landscape coefficient (Kl), irrigation system efficiency (e), and non-irrigation demand (D0):
Ai =115 sq-ft
Kp =0.80
Kl =0.60
e =0.90
D0 =0
2. Select initial values for the drainage area (Ad), percent of impervious cover (I), and cistern size
(C):
Ad =800 sq-ft
I =100%
C =5,000 gallons
3. The nearest reference point to Kapolei is Makakilo City.
4a. Calculate the monthly reference evapotranspiration rates (ET0). The calculation for January is as
follows, and the results for the entire year are provided in the table in #4c.
ET0 =Epan × KlET0=5.46 × 0.8
ET0 =4.37 in
4b. Calculate the actual evapotranspiration rates (ETa). The calculation for January is as follows, and
the results for the entire year are provided in the table in #4c.
ETa =ET0 × KlETa=4.37 × 0.6
ETa =2.62 in
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4c. Calculate the total demand (Dt). The calculation for January is as follows, and the results for the
entire year are provided in the table below.
Dt =7.48Ai (ETi − r)/(12e) + D0Dt=7.48 × 115 x (2.62 − 2.58)/(12 × 0.9) + 0 Dt =3 gallons
Month Rainfall (inches)P (inches)ET0 (inches)ETa (inches)Dt (gallons)
January 2.58 5.46 4.37 2.62 3
February 3.05 5.75 4.60 2.76 0
March 1.87 7.12 5.70 3.42 123
May 1.12 7.75 6.20 3.72 207
May 0.86 8.41 6.73 4.04 253
June 0.55 8.99 7.19 4.32 300
July 0.58 9.74 7.79 4.68 326
August 0.48 9.65 7.72 4.63 331
September 0.74 8.48 6.78 4.07 265
October 2.00 7.54 6.03 3.62 129
November 2.06 6.29 5.03 3.02 76
December 2.69 5.59 4.47 2.68 0
4d. Calculate the generated roof runoff (Rg). The calculation for January is as follows, and the results
for the entire year are provided in the table below:
Rg =7.48Adr (0.05 + 0.009I)/12
Rg =7.48 × 800 × 2.58 (0.05 + 0.009 × 100)/12
Rg =1,222 gallons
4e. Calculate the runoff used (Ru) by comparing the total demand (Dt) to the amount of runoff in the
cistern at the beginning of the month (Cb) plus the runoff generated during the month (R). The
calculation for January is as follows, and the results for the entire year are provided in the table
below:
Cb =0 gallon
Ru =Dt = 3 gallons [ since Rg + Cb > Dt ]
4f. Calculate the amount of runoff in the Cistern at the end of the month (Ce) by setting it to the
lower value of the amount of runoff in the Cistern at the beginning of the month (Cb) plus the
runoff generated (Rg) minus the runoff used (Ru), and the cistern capacity (C). The calculation for
January is as follows, and the results for the entire year are provided in the table below:
Ce =min(Cb + Rg − Ru, C)
Ce =min(0 + 1,222 − 3, 500)
Ce =1,219 gallons
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
4g. Calculate the Cistern overflow (O). The calculation for January is as follows, and the results for
the entire year are provided in the table below:
O =Cb + Rg − Dt − CeO=0 + 1,222 − 3 − 1,219
O =0 gallons
4h. Calculate the runoff captured in Cistern (Rc). The calculation for January is as follows, and the
results for the entire year are provided in the table below:
Rc =Rg − O
Rc =1,222 − 0
Rc =1,222 gallons
4i. Set Cb for the next month equal to Ce of the previous month and repeat the calculations.
Month r
(inches)
D
(gallons)
Rg
(gallons)
Cb
(gallons)
Ce
(gallons)
Ru
(gallons)
O
(gallons)
R
(gallons)
January 2.58 3 1,222 0 1,219 3 0 1,222
February 3.05 0 1,445 1,219 2,664 0 0 1,445
March 1.87 123 886 2,664 3,426 123 0 886
May 1.12 207 531 3,426 3,750 207 0 531
May 0.86 253 407 3,750 3,904 253 0 407
June 0.55 300 261 3,904 3,865 300 0 261
July 0.58 326 275 3,865 3,814 326 0 275
August 0.48 331 227 3,814 3,710 331 0 227
September 0.74 265 351 3,710 3,796 265 0 351
October 2.00 129 947 3,796 4,614 129 0 947
November 2.06 76 976 4,614 5,000 76 514 462
December 2.69 0 1,274 5,000 5,000 0 1,274 0
Total 18.58 2,014 8,802 2,014 1,788 7,014
5. Calculate the overall runoff capture efficiency (Ec) and overall demand efficiency (Ed):
12 12Ec = 100 × ∑Rc/∑Rg 1 1
Ec = 100 × 7,014/8,802
Ec = 80%
12 12Ed = 100 × ∑Ru/∑Dt 1 1
Ed = 100 × 2,014/2,014
Ed = 100%
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Appendix B: Treatment Control BMP Fact Sheets
6. Calculate the WQV for which credit is received:
WQV =PCA × 3,360
WQV =1 × (0.05 + 0.009 × 100) × (800/43,560) × 3,360
WQV =63 cu-ft
Other Design Considerations
• Local pan evaporation and rainfall data may be used if available.
• Tanks should have tight fitting covers to exclude contaminants and animals, and above ground
tanks should not allow penetration of sunlight to limit algae growth
• In areas where the tank is to be buried partially below the water table, special design features
must be employed to keep it from “floating.”
Schematic of a Harvesting/Reuse System
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Description
Sometimes referred to as a dry extended detention basin, a detention basin is a shallow man-made
impoundment intended to provide for the temporary storage of storm water runoff to allow particles to
settle. It does not have a permanent pool and is designed to drain between storm events.
Minimum Design Criteria
Design Parameter Units Value
Maximum Interior Side Slope (length per unit height)3:1
Minimum length to width ratio 2:1
Maximum depth feet 8
Drawdown (drain) time for WQV hours 48
Drawdown (drain) time for 50% of WQV hours 24-36
Basin invert slope percent 1-2
Minimum outlet size inches 4
Minimum freeboard feet 1
Feasibility Criteria
Detention Basins are considered infeasible for any of the following conditions:
• Basin invert would be below seasonally high groundwater table.
• Unable to operate off-line and unable to operate in-line with safe overflow mechanism.
• Excavation would disturb iwi kupuna or other archaeological resources.
• Unable to meet minimum length to width ratio design criteria naturally or artificially.
TC-13: Detention Basin
Miliani Mauka
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low
Expected Pollutant Removals
Nutrients Low Pesticides Unknown
Sediment Medium Oil & Grease Medium
Trash High Metals Low/Med
Pathogens Low Organic Compounds Unknown
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Appendix B: Treatment Control BMP Fact Sheets
Sizing Procedure
Detention Basins are sized using detailed routing calculations to demonstrate that the storage volume
is adequate. However, a reasonable first estimate can be determined using the following simple routing
method which assumes triangular hydrographs for the inflow and outflow.
Step 1. Use the procedure presented previously to compute the pre-project (i.e., undeveloped) and
post-project (i.e., developed) weighted runoff coefficients.
Step 2. Compute the peak inflow rate using the Rational Method:
qi = CaiA
Where qi =Peak Inflow Rate into Basin (cu-ft)
Ca =Post-Project Weighted Runoff Coefficient
i =Peak Rainfall Intensity (in/hr)
A =Drainage Area (acres)
Step 3. Compute the peak outflow rate using the pre-project runoff coefficient, which effectively
forces the detention basin to maintain pre-project discharge rates:
qo = CbiA
Where qo =Peak Inflow Rate leaving Basin (cu-ft)
Cb =Pre-Project Weighted Runoff Coefficient
i =Peak Rainfall Intensity (in/hr)
A =Drainage Area (acres)
Step 4. Calculate the estimated basin storage volume:
s = 3,630 × PA [1 − (qo/qi)]
Where s =Storage Volume in the Basin (cu-ft)
P =Design Storm Runoff Depth (in)
A =Drainage Area (acres)
qo =Peak Outflow Rate from Step 3 (cu-ft)
qi =Peak Inflow Rate from Step 2 (cu-ft)
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Appendix B: Treatment Control BMP Fact Sheets
Step 5. Select initial values for the detention basin total width (wt), total length (lt), and depth (d)
based on space availability, topography and existing drainage facilities. Also select values for
the interior side slopes (z) and required freeboard (f). Calculate the basin invert width and
invert length:
wb = wt − 2z( d+f )
lb = lt − 2z( d+f )
Where wb =Basin Bottom Width (ft)
lb =Basin Bottom Length (ft)
wt =Basin Total Width (ft)
lt =Basin Total Length (ft)
z =Basin Interior Side Slope (length per unit height)
d =Depth of Flow for Storage Volume (ft)
f =Freeboard (ft)
Step 6. Calculate the resulting storage volume using the prismoidal formula for trapezoidal basins:
V = wblbd + (wb + lb)zd2 + 4z2d3/3
Where V =Volume of Trapezoidal Basin (cu-ft)
wb =Basin Bottom Width from Step 5 (ft)
lb =Basin Bottom Length from Step 5 (ft)
d =Depth of Flow for Storage Volume from Step 5 (ft)
z =Basin Interior Side Slope from Step 5
Compare the calculated volume (V) to the required volume (s) from Step 4. If the calculated
volume is greater than or equal to the required volume, the selected dimensions (wt and lt)
and depth (d) are adequate for preliminary design. If the calculated volume is less than the
required volume, increase one or both of the dimensions and/or the depth (d) and repeat Steps
5 and 6. If the footprint area and depth are set to maximum allowable values based on site
characteristics and the calculated volume is still less than the required volume, reduce the
drainage area (A) and repeat Steps 2 through 6.
Pretreatment Considerations
If significant amounts of sediment or sand are anticipated at the site, sediment forebays should be located
at each major inlet to provide pretreatment, preserve the capacity of the basin, and reduce maintenance
requirements in the basin. The forebay consists of a separate cell that drains into the main basin, formed
by an acceptable barrier, such as an earthen berm or gabion baskets, etc.). If used, the total volume of all
forebays should be at least 5% of the total WQV.
Area Requirements
A detention basin requires a footprint equivalent to 1% - 9% of its contributing impervious drainage area.
The actual value is dependent on a number of variables, including the drainage area, pre-project and post-
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Appendix B: Treatment Control BMP Fact Sheets
project runoff coefficients, and basin depth. Footprints at the lower range reflect deep basins (e.g., 8 ft)
serving large drainage areas (e.g., 50 acres), while footprints at the upper range reflect shallow basins
(e.g., 1 ft) serving small drainage areas (e.g., 1 acre).
Sizing Example
Calculate the preliminary size of a detention basin serving the runoff from a one acre parking lot. Assume
the following design parameters:
Design Parameter Units Value
Rainfall Intensity, i inches/hour 0.4
Runoff Volume, Q inches 1
Basin Interior Side Slope (length per unit height), z 3
Freeboard, f feet 1
1. Compute the pre-project (i.e., undeveloped) and post-project (i.e., developed) weighted runoff
coefficients:
Cb =0.20
Ca =0.95
2. Compute the peak inflow rate:
qi =CaiA
qi =0.95 × 0.40 × 1
qi =0.38 cu-ft
3. Compute the peak outflow rate:
qo =CbiA
qo =0.20 × 0.40 × 1
qo =0.08 cu-ft
4. Calculate the estimated basin storage volume:
s =3,630 × PA[1 − (qo/qi)]
s =3,630 × 1 × 1 × [1 − (0.08/0.38)]
s =2,866 cu-ft
5. Select initial values for the detention basin total width (wt), total length (lt), depth (d), interior side
slopes (z) and required freeboard (f):
wt =38 ft
lt =53 ft
d =3.5 ft
z =3 ft
f =1 ft
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Appendix B: Treatment Control BMP Fact Sheets
Calculate the basin bottom width and length:
wb =wt − 2z(d + f)
wb =38 − 2 × 3 × (3.5 + 1)
wb =11 ft
lb =lt − 2z(d + f)
lb =53 − 2 × 3 × (3.5 + 1)
lb =26 ft
6. Calculate the resulting storage volume using the prismoidal formula for trapezoidal basins:
V =wblbd + (wb + lb)zd2 + 4z2d3/3
lt =11 × 26 × 3.5 + (11 + 26) × 3 × 3.52 + 4 × 32 × 3.53/3
d =2,875 cu-ft
The calculated volume is greater than the required volume, so the preliminary design is
acceptable.
Other Design Considerations
• Credit for infiltration may be given if the soils beneath the detention basin invert have a measured
infiltration rate of at least 0.5 inches per hour and none of the infeasibility criteria for infiltration
basins are applicable. However, low flow channels should not be included if infiltration is
expected.
• If a temporarily-filled pond creates a potential public safety issue, perimeter fencing may be
considered. Warning signs should be used wherever appropriate
• In order to meet designs storm requirements, detention basins should have a multistage outlet
structure. Three elements are typically included in this design:
1. A low-flow outlet that controls the extended detention and functions to slowly release the
water quality design storm.
2. A primary outlet that functions to attenuate the peak of larger design storms.
3. An emergency overflow outlet/spillway
• Design methodology options are provided in manuals included in the References, including the
Georgia Stormwater Management Manual, the Urban Storm Drainage Criteria Manual, and the
USEPA Stormwater Best Management Practice Design Guide.
Construction/Inspection Considerations
• Inspect facility after first large storm to determine whether the desired residence time has been
achieved.
• When constructed with small tributary area, orifice sizing is critical and inspection should verify
that flow through additional openings such as bolt holes does not occur.
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Appendix B: Treatment Control BMP Fact Sheets
Schematic of a Detention Basin
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Appendix B: Treatment Control BMP Fact Sheets
Description
A manufactured treatment device is a proprietary water quality structure utilizing settling, filtration,
adsorptive/absorptive materials, vortex separation, vegetative components, or other appropriate
technology to remove pollutants from storm water runoff.
Minimum Design Criteria
Design Parameter Units Value
Minimum Total Suspended Solids water (TSS) Removal Percent 80
Feasibility Criteria
Manufactured treatment devices are considered infeasible for any of the following conditions:
• Bottom of BMP is below seasonally high groundwater table.
• Unable to operate off-line and unable to operate in-line with safe overflow mechanism.
• Excavation would disturb iwi kupuna or other archaeological resources.
Sizing Procedure
Follow the manufacturer’s guidelines for appropriate sizing calculations and selection of appropriate
device/model.
Pretreatment Considerations
No pretreatment is required.
TC-14: Manufactured Treatment Device
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Low to Medium
Expected Pollutant Removals
Varies based on System
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Appendix B: Treatment Control BMP Fact Sheets
Area Requirements
The footprint requirements for proprietary manufactured treatment devices vary by manufacturer.
Sizing Example
No example is provided as sizing procedures vary by manufacturer, and presenting any specific product
might be interpreted as an endorsement.
Other Design Considerations
• The device must provide a TSS removal rate of 80%, certified for general use by the Washington
State Department of Ecology Technology Assessment Protocol (TAPE) or certified by the New
Jersey Department of Environmental Planning (NJDEP).
• Systems not meeting the required TSS removal criteria can be used as pre-treatment for other
BMPs.
• Although the device must meet TSS requirements, many manufactured treatment devices also
treat other pollutants. The City recommends selecting devices capable of treating the potential
pollutants at your site.
• All manufactured treatment devices must be able to safely overflow or bypass flows in excess of
the storm water quality design storm to downstream drainage systems.
Construction/Inspection Considerations
Follow manufacturer's recommendations.
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Revised: July 2017
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Appendix B: Treatment Control BMP Fact Sheets
Description
A sand filter is an open chambered structure that captures, temporarily stores, and treats storm water
runoff by passing it through sand.
Minimum Design Criteria
Design Parameter Units Value
Sand Coefficient of Permeability feet/day 3.5
Filter media depth inches 18
Drawdown (drain) Time hours 48
Maximum Interior Side Slope if earthen (length per unit height)3:1
Minimum Underdrain Diameter inches 6
Feasibility Criteria
Sand filters are considered infeasible for any of the following conditions:
• Bottom of BMP is below seasonally high groundwater table.
• Unable to operate off-line and unable to operate in-line with safe overflow mechanism.
• Excavation would disturb iwi kupuna or other archaeological resources.
• Site lacks sufficient hydraulic head to support BMP operation by gravity.
Sizing Procedure
Step 1. Use the procedure presented previously to compute the Volumetric Runoff Coefficient and
WQV.
TC-15: Sand Filter
Portland Stormwater Management Manual, 2004.
BMP Category
Retention
Biofiltration
Other
O&M Requirements
Medium
Expected Pollutant Removals
Nutrients Low/Med Pesticides Unknown
Sediment High Oil & Grease High
Trash High Metals Med/High
Pathogens Medium Organic Compounds Med/High
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Appendix B: Treatment Control BMP Fact Sheets
Step 2. Select values for the filter media depth (lm) and maximum ponding depth (dp).
Step 3. Use Darcy’s Law to calculate the required Filter Bed Surface Area:
Afb = (WQV × lm)/[ k(lm + dp/24)(t/24)]
Where Afb =Filter Bed Surface Area (sq-ft)
WQV =WQV from Step 1 (cu-ft)
lm =Filter Media Depth from Step 2 (ft)
k =Filter Media Permeability Coefficient (ft/day)
dp =Maximum Ponding Depth from Step 2 (in)
t =Filter Bed Drain Time (hr)
Step 4. Calculate the total area occupied by the BMP (ABMP) using the embankment side slopes and
assuming a square basin:
ABMP = [ √(Afb) + 2z(dp/12 +f) ]2
Where ABMP =Area Occupied by BMP (sq-ft)
Afb =Filter Bed Surface Area from Step 3 (sq-ft)
z =Filter Bed Interior Side Slope (length per unit height)
dp =Maximum Ponding Depth from Step 2 (in)
f =Freeboard (ft)
If the calculated area does not fit in the available space, either reduce the drainage area,
increase the ponding depth, and/or increase the interior side slope (if it’s not already set to the
maximum) and repeat the calculations.
Pretreatment Considerations
Pretreatment is required for sand filters in order to reduce the sediment load entering the sand bed, prevent
premature clogging, and ensure filter longevity. The pretreatment device must be sized for at least 25%
of the WQV, and may be achieved with vegetated swales, vegetated filter strips, sedimentation basins
or forebays, sedimentation manholes, and manufactured treatment devices. The typical method is a
sedimentation basin that has a length to width ratio of 2:1, and is sized using the Camp-Hazen equation.
Area Requirements
A sand filter requires a footprint equivalent to 1.5% - 3% of its contributing impervious drainage area,
excluding pretreatment. The lower value reflects minimum filter media and ponding depths, while the
upper value reflects higher filter media and ponding depths.
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Appendix B: Treatment Control BMP Fact Sheets
Sizing Example
Calculate the size of a sand filter serving a 1-acre residential development. Assume the following design
parameters:
Design Parameter Units Value
Percent Impervious Cover, I percent 70
Design Storm Depth, P inches 1.0
Sand Coefficient of Permeability, k feet/day 3.5
Interior Side Slope (length per unit height)3:1
Freeboard feet 0.5
Drawdown (drain) Time, t hours 48
1. Calculate the volumetric runoff coefficient and WQV:
C =0.05 + 0.009I
C =0.05 + 0.009 × 70
C =0.68
WQV =PCA × 3,630
WQV =1 × 0.68 x 1 × 3,630
WQV =2,468 cu-ft
2. Select a filter media depth (lm) and maximum ponding depth (dp):
lm =1.5 ft
dp =24 in
3. Calculate the Filter Bed Surface Area (Afb):
Afb =( WQV × lm)/[ k(lm + dp/24)(t/24)]
Afb =(2,468 × 1.5)/[3.5(1.5 + 24/24)(48/24)]
Afb =212 sq-ft
4. Calculate the total area occupied by the BMP (ABMP):
ABMP =[ √(Afb) + 2z(dp/12 +f) ]2
ABMP =[ √(212) + 2 × 3(24/12 +0.5 ]2
ABMP =873 sq-ft
Other Design Considerations
• A flow spreader should be installed at the inlet along one side of the filter to evenly distribute
incoming runoff across the filter and to prevent erosion of the filter device.
• A cleanout pipe should be tied into the end of all underdrain pipe runs.
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Appendix B: Treatment Control BMP Fact Sheets
Construction/Inspection Considerations
Tributary area should be completely stabilized before media is installed to prevent premature clogging.
Schematic of a Sand Filter
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
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Appendix C: O&M Fact Sheets
This section describes minimum inspection and maintenance requirements for post-construction BMPs.
Actually inspection and maintenance may be more frequent to ensure long-term performance of post-
construction BMPs then what is suggested. Maintenance should be performed whenever needed, based on
maintenance indicators presented in the fact sheets below.
The fact sheets have been grouped into the following categories based on common maintenance
requirements:
• Bio-Retention Basin
• Detention Basin
• Green Roof
• Infiltration Trench/Basin
• Manufactured Treatment Device
• Pervious Pavement
• Rainwater Harvesting
• Sand Filter
• Vegetated Biofilter
• Vegetated Swale/Strip
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Appendix C: Operations and Maintenance Fact Sheets
OM-01: Bio-Retention Basin
This category includes BMPs that treat storm water by infiltrating it through vegetation and/or soil.
Regular inspection and maintenance is needed to ensure flow is unobstructed, erosion is prevented,
and soils are biologically active. General conditions when maintenance is needed and the associated
maintenance action triggered by those conditions are provided below.
Monthly or as Needed After Storm Event
• Remove obstructions, debris and trash from bio-retention area and dispose of properly.
• Inspection bio-retention area for ponded water.
• Inspect inlets for channeling, soil exposure or other evidence of erosion. Clear obstructions and
remove sediment.
• Inspect depth of mulch and replenish as necessary (2 inches per soil specifications).
Bi-Annually
• Maintain vegetation and irrigation system.
• Prune, weed and remove/replace any dead plants.
Annually
• Inspect energy dissipation at the inlet.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands in the bio-retention area between
storms and does not drain within 24 hrs after
rainfall.
Any of the following results could apply:
sediment or trash blockages removed, improved
grade from head to foot of bio-retention area, and
drains per design specification.
Trash and Debris
Trash and debris accumulated in the bio-retention
area and around the inlet and outlet.
Trash and debris removed from the bio-retention
area and disposed of properly.
Sediment
Evidence of accumulated sediment in the basin.
Visual evidence of dumping.
Material removed so that there is no clogging or
blockage. Material is disposed of properly.
Erosion
Channels have formed around inlets, there are
areas of bare soil, or there is other evidence of
erosion.
Obstructions and sediment removed so that
water flows freely and disperses over a wide
area. Obstructions and sediment are disposed of
properly.
Vegetation
Vegetation is dead, diseased or overgrown.Vegetation is healthy and attractive. Grass is
maintained at least 3 inches in height.
● Bio-Retention Basin ● Rain Garden
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Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Mulch
Mulch is missing or patchy. Areas of bare earth
are exposed or mulch layer is less than 3 inches
deep.
All bare earth is covered, except mulch is kept
6 inches away from trunks of trees and shrubs.
Mulch is even at a depth of 3 inches.
Inlet/Outlet
Sediment accumulations.Inlet/outlet is clear of sediment and debris and
allows water to flow freely.
Miscellaneous
Any condition not covered above that needs
attention for the bio-retention area to function as
designed.
The design specifications are met.
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Appendix C: Operations and Maintenance Fact Sheets
OM-02: Detention Basin
This category includes BMPs that are designed to treat storm water by allows pollutants to settle out
and gradually release detained storm water through an orifice. Maintenance primarily consists of
vegetation management, sediment removal, and mosquito abatement when there is standing water.
General conditions when maintenance is needed and the associated maintenance action triggered by those
conditions are provided below.
Monthly or as Needed After Storm Event
• Inspect for clogging and that it drains between storms per design specifications.
• Trim vegetation and inspect for woody vegetation.
• Inspect the level of sediment in the forebay.
Bi-Annually
• Evaluate the health of vegetation.
• Remove sediment, litter and debris.
• Inspect the outlet, embankment, dikes, berms, and side slopes for structural integrity and signs of
erosion or rodent burrows.
• Inspect outlets and overflow structures for plugging and signs of erosion.
• Check inlets to ensure the piping is intact and not plugged.
• Inspect security measures around the facility.
Annually
• Harvest vegetation during dry periods.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands in the basin between storms and
does not drain per design specifications.
Corrected any circumstances that restrict the flow
of water from the system and drainage is restored
to design condition.
Tree/Brush Growth, Woody Vegetation
Growth does not allow maintenance access
or interferes with maintenance activity, dead,
diseased, or dying trees.
Trees do not hinder maintenance activities.
Vegetation harvested annually, during dry periods.
Sediment
Accumulated sediment >10% of design basin
depth.
Sediment cleaned out to design shape and depth.
Basin reseeded to control erosion, if needed.
Sediment disposed of properly.
● Detention Basin ● Constructed Wetlands ● Wet Ponds ● Wetlands
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Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Erosion
Erosion on compacted berm embankment. Rodent
burrows on slope.
Cause of erosion is managed appropriately. Side
slopes or berm restored to design specifications,
as needed. Rodent burrows filled.
Inlet/Outlet
Piping broken or inlet/outlet blocked.Piping fixed. Debris/sediment removed and
disposed of properly.
Fences/Security Measures
Fences/security measures broken or missing.Fences/security measures around facility are
secure.
Miscellaneous
Any condition not covered above that needs
attention for the detention basin to function as
designed.
The design specifications are met.
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Appendix C: Operations and Maintenance Fact Sheets
OM-03: Green Roof
Green roofs are designed to reduce storm water runoff volume and improve water quality by intercepting,
filtering, absorbing, retaining or detaining storm water. Green roofs consist of the following components:
waterproof membrane, drainage layer, vegetated media and vegetation. Regular inspection and
maintenance is needed to ensure water flows unimpeded through the green roof and vegetation is healthy.
General conditions when maintenance is needed and the associated maintenance action triggered by those
conditions are provided below.
Monthly or as Needed After Storm Event
• Inspect media for runoff or wind scouring/exposed underlayment components.
• Inspect for standing water.
Bi-Annually
• Inspect inlet outlet for litter and debris accumulation.
• Inspect vegetation. Prune, weed and remove/replace any dead plants.
• Replenish mulch.
Annually
• Inspect for exposure of liner.
• Inspect for roof leaks.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Roof drainage system is clogged.There should be no areas of standing water on the
green roof. The drainage system is inspected for
clogging conditions and repaired or replaced as
needed.
Erosion
Areas of scoured media or bare roof.Green roof media stays in place and does not
migrate across or erode from roof surface. Eroded
media replaced and re-vegetated. If problem is
recurrent, consider media more resistant to wind
erosion or installing media retention components.
Leaky Roof
Roof liner has failed.Evaluate liner for cause of leaks. Repair or replace
as necessary.
Miscellaneous
Any condition not covered above that needs
attention for the green roof to function as
designed.
The design specifications are met.
● Green Roof
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OM-04: Infiltration Trench/Basin
This category includes BMPs that are designed to detain storm water by storing it in the void spaces of the
aggregate and then infiltrated into the underlying soil. Infiltration trenches/basins are prone to clogging
and maintenance consists primarily of preventing sediment buildup and clogging. General conditions
when maintenance is needed and the associated maintenance action triggered by those conditions are
provided below.
Monthly or as Needed After Storm Event
• Remove obstructions, debris and trash from treatment device and perimeter.
• Check observation well 2 to 3 days after storm to confirm drainage.
• Mow and trim surrounding vegetation.
• Inspect inflow and outflow structures for erosion. Repair as needed.
Annually
• Monitor observation well to confirm that trench is draining properly.
• Inspect the trench for clogging and restore to design conditions, if needed.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands in the infiltration trench between
storms and does not drain within 24 hrs after
rainfall.
There should be no areas of standing water once
inflow has ceased. Any of the following can
apply: sediment or trash blockages removed,
grade improved, or removed clogging at check
dams.
Trash and Debris
Trash and debris accumulated in the infiltration
trench and around the inlet and outlet.
Trash and debris removed and disposed of
properly.
Sediment
Evidence of accumulated sediment in the
infiltration trench.
Material removed so that there is no clogging or
blockage. Material is disposed of properly.
Erosion
Channels have formed around inlets, there are
areas of bare soil, or there is other evidence of
erosion.
Obstructions and sediment removed so that
water flows freely and disperses over a wide
area. Obstructions and sediment are disposed of
properly.
Inlet/Outlet
Sediment accumulations.Inlet/outlet is clear of sediment and debris and
allows water to flow freely.
● Dry Well ● Infiltration Basin ● Infiltration Trench
C-10 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Surface Materials
Material is missing or patchy; areas of bare earth
are exposed.
All bare earth is covered, except mulch is kept
6 inches away from trunks of trees and shrubs.
Mulch is even at a depth of 3 inches.
Miscellaneous
Any condition not covered above that needs
attention for the infiltration trench to function as
designed.
The design specifications are met.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-11
Appendix C: Operations and Maintenance Fact Sheets
OM-05: Manufactured Treatment Device
This category includes manufactured treatment devices. Each BMP is designed differently to treatment
storm water and has different maintenance needs. Please refer to the manufacture’s requirements for
maintenance for more detailed information on inspection and maintenance activities and frequencies.
General conditions when maintenance is needed and the associated maintenance action triggered by those
conditions are provided below.
Follow manufacture's requirement. Typical maintenance requirements may include:
• Inspect for clogging and standing water.
• Remove accumulated sediment, trash and debris.
• Replace media material, if applicable.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
When water stands over the manufactured
treatment device between storms and does not
drain per design specifications.
There should be no areas of standing water
after inflow has ceased Filer drains per design
specification. Any of the following could apply:
sediment or trash blockage removed, mulch media
replaced, and/or overflow pipe flushed/repaired in
manner that does not cause an illegal discharge.
Sediment, Trash, and Debris Accumulation
Sediment, trash and debris accumulated in the
media material, vault, or piping.
Sediment, trash and debris removed and disposed
of properly.
Mosquitoes
Evidence of mosquito larvae in treatment unit.No evidence of mosquito larvae.
Miscellaneous
Any condition not covered above that needs
attend in order for the manufactured treatment
device to function as designed.
The design specifications are met.
● Hydrodynamic/Vortex Separator
● Other Devices Approved by TAP or NJCAT with a TSS Removal Rate of 80 or more
C-12 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
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Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-13
Appendix C: Operations and Maintenance Fact Sheets
OM-06: Pervious Pavement
This category includes BMPs that are designed to treat storm water by detaining storm water in the
void spaces and then infiltrated into the underlying soil. Pervious pavement is prone to clogging and
maintenance consists primarily of preventing sediment buildup and clogging. General conditions when
maintenance is needed and the associated maintenance action triggered by those conditions are provided
below.
Quarterly or as Needed After Storm Event
• Check for sediment and debris accumulation. Prevent soil from washing or flowing onto the
pavement. Do not store sand, soil, mulch or other landscaping materials on pervious pavement
surfaces.
• Use commercially available regenerative air or vacuum sweeper to remove sediment and debris
from the surface.
• Perform vacuum sweeping, power washing, and/or reconstruction to restore surface permeability
as needed.
• Inspect for signs of pavement failure. Repair any surface deformations or broken pavers.
• Check for standing water on pavement within 30 minutes following a storm event.
• Inspect underdrain outlets. Remove trash and debris.
• Remove weeds. Mow vegetation in grid pavement (i.e., turf block) as needed.
• Replenish aggregate in joints or grids as needed.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands on the surface of the permeable
pavement and 48 hrs has passed since the last
rainfall.
There should be no areas of ponded/standing
water more than 48 hrs after a rain event. Any
of the following can apply: surface swept or
vacuumed, underdrains added, underdrains
flushed in manner that does not cause an illegal
discharge.
Trash and Debris
Leaves, grass clippings, trash, etc., are preventing
water from draining into the permeable pavement
and are unsightly.
Area is free of all debris and the permeable
pavement is draining properly.
Vegetation
Vegetation around the perimeter of the permeable
pavement is dead, diseased, or overgrown. Weeds
are growing on the surface of the permeable
pavement.
Area adjacent to pavement is well-maintained and
no bare/exposed areas exist; grass is maintained at
a height of 3–6 inches.
No weeds present in the pavement area.
● Pervious Concrete ● Porous Asphalt ● Interlocking Paver Blocks
● Reinforced Turf Grassing/Gravel Filled Grids
C-14 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Deteriorating Surface
The pavement is cracked; paver blocks are
misaligned or have settled.
The surface area is stabilized, exhibiting no signs
of cracks or uneven areas in the pavement area.
Miscellaneous
Any condition not covered above that needs
attention for the pervious pavement to function as
designed.
The design specifications are met.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-15
Appendix C: Operations and Maintenance Fact Sheets
OM-07: Rainwater Harvesting
This category includes BMPs that are designed to store water a specific volume of water to use later for
irrigation, non-potable water plumbing, vehicle washing, or other non-potable water uses. Maintenance
is primarily focused on preventing sediment buildup and clogging, which reduces the capacity of the
system. General conditions when maintenance is needed and the associated maintenance action triggered
by those conditions are provided below.
Quarterly or as Needed After Storm Event
• Inspect, clean and replace filters and screens, as needed.
• Inspect and clean debris from roof, gutters, downspouts, first flush devices, roof washers, or other
collection surfaces.
• Inspect for and repair leaks.
• If rainwater is used for indoor use, inspect and verify that treatment systems are operational and
maintaining minimum water quality requirements as determined by DPP or DOH.
Condition When Maintenance is Needed Results When Maintenance is Performed
Sediment and Debris Accumulation
Sediment or debris accumulated in filter, screens,
gutters, downspouts, first flush device, or roof
washers or on roof or other collection surfaces.
Sediment accumulated in cisterns(s).
Sediment removed. Collection surfaces do not
contribute sediment and debris.
Leaks
Water leaking from system.No leakage.
Water Quality
Treatment system is not working properly.Treatment system is operational and maintaining
minimum water quality requirements.
First Flush Diverter
First flush filter is full or clogged causing
permanent flow to the cistern.
First flush is diverted away from the cistern
when the first flush diverter valve is removed and
cleaned.
Cistern does not Drain within 48 hours
Outlet is clogged.Cistern completely drains in less than 48 hrs.
Cistern Drain under 24 hours
Cistern leaks or outlet allows excessive flows.Cistern drains in 24 to 48 hrs.
Miscellaneous
Any condition not covered above that needs
attention for the pervious pavement to function as
designed.
The design specifications are met.
● Cisterns ● Rain Barrels ● Vehicle Washing
● Other Non-Potable Water uses Approved by the Building Code
C-16 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
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Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-17
Appendix C: Operations and Maintenance Fact Sheets
OM-08: Sand Filter
Sand Filters are designed to treat storm water by removing pollutants as the storm water flows through the
media. Regular inspection and maintenance is needed to prevent sediment buildup and clogging, which
reduces pollutant removal efficiency. General conditions when maintenance is needed and the associated
maintenance action triggered by those conditions are provided below.
Monthly or as Needed After Storm Event
• Inspect for standing water, sediment, trash and debris.
• Remove accumulated trash and debris in the unit during routine inspections.
Annually
• Inspect to ensure that the facility is draining completely within five days and per manufacturer’s
specifications.
Per Manufacturer's Specification
• Replace the media per manufacturer’s inspection or as indicated by the condition of the unit.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
When water stands over the sand filter media
between storms and does not drain within 24
hours after rainfall.
There should be no areas of standing water after
inflow has ceased. Any of the following could
apply: sediment or trash blockages removed,
filter media surface scarified, media replaced
underdrains flushed in manner that does not cause
an illegal discharge.
Mosquitoes
Evidence of mosquito larvae in treatment unit.No evidence of mosquito larvae.
Sediment, Trash, and Debris Accumulation
Sediment, trash and debris accumulated in the
sand filter unit and around the inlet and outlet.
Sediment, trash, and debris removed so there is no
clogging.
Erosion
Channels have formed around inlets, there are
areas of bare soil, or there is other evidence of
erosion.
Obstructions and sediment removed so that
water flows freely and disperses throughout the
sand filter media. Obstructions and sediment are
disposed of properly.
Inlet/Outlet
Sediment accumulations.Inlet/outlet is clear of sediment and debris and
allows water to flow freely.
● Sand Filter
C-18 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Miscellaneous
Any condition not covered above that needs
attention for the sand filter to function as
designed.
The design specifications are met.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-19
Appendix C: Operations and Maintenance Fact Sheets
OM-09: Vegetated Biofilter
This category includes BMPs that are designed to detain and treat runoff without draining into the
underlying soil. Maintenance is primarily focused on maintaining healthy vegetation, avoiding clogging
and proper functioning of inlets, outlet and high-low bypass. General conditions when maintenance is
needed and the associated maintenance action triggered by those conditions are provided below.
Monthly or as Needed After Storm Event
• Inspect bio-filter system, overflow pipe, inlets, and sheet flow areas for clogging and impediments
to flow. Repair damaged pipes.
• Remove accumulated sediment, debris, sediment and repair damaged pipes.
• Inspect and repair/replace or replenish splash blocks or rocks, as needed.
Bi-Annually
• Inspect vegetation. Prune, weed and remove/replace any dead plants. Replenish mulch.
• Inspection irrigation system.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands in the vegetated biofilter between
storms and does not drain within 24 hrs after
rainfall.
There should be no areas of standing water after
inflow has ceased. Any of the following could
apply: sediment or trash blockages removed,
mulch replaced, underdrains flushed in manner
that does not cause an illegal discharge.
Trash and Debris
Trash and debris accumulated in and around the
inlet and outlet.
Trash and debris removed and disposed of
properly.
Sediment
Evidence of accumulated sediment.Material removed so that there is no clogging or
blockage. Material is disposed of properly.
Erosion
Channels have formed around inlets, there are
areas of bare soil, or there is other evidence of
erosion.
Obstructions and sediment removed so that
water flows freely and disperses over a wide
area. Obstructions and sediment are disposed of
properly.
Vegetation
Vegetation is dead, diseased, or overgrown.Vegetation is healthy and attractive. Grass is
maintained at least 3 inches in height.
● Stormwater Curb Extension ● Tree Box Filter ● Planter Box
C-20 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Mulch
Mulch is missing or patchy. Areas of bare earth
are exposed or mulch layer is less than 3 inches
deep.
All bare earth is covered, except mulch is kept
6 inches away from trunks of trees and shrubs.
Mulch is even at a depth of 3 inches.
Inlet/Outlet
Sediment accumulations.Inlet/outlet is clear of sediment and debris and
allows water to flow freely.
Affected Impervious Areas of Structures
Obvious effects on surrounding impervious areas
or structures.
Hydraulic restriction layers prevent impacts from
infiltration to surrounding structures.
Miscellaneous
Any condition not covered above that needs
attention for the vegetated bio-filter to function as
designed.
The design specifications are met.
Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
C-21
Appendix C: Operations and Maintenance Fact Sheets
OM-10: Vegetated Swale/Strip
This category includes BMPs that are designed to provide to remove pollutants by physically straining
and filtering water through vegetation or cobble within the swale/strip. Maintenance is primarily focused
on maintaining healthy vegetation and avoiding clogging. General conditions when maintenance is
needed and the associated maintenance action triggered by those conditions are provided below.
Monthly or as Needed After Storm Event
• Inspect swale/strip for ponding.
• Inspect inlets and sheet flow areas for impediments.
• Remove accumulated sediment, litter and debris.
Bi-Annually
• Inspect vegetation. Prune, mow, weed and remove/replace any dead plants.
• Replenish mulch.
• Inspection irrigation system.
Condition When Maintenance is Needed Results When Maintenance is Performed
Standing Water
Water stands in the vegetated swale/strip between
storms and does not drain within 24 hrs after
rainfall.
There should be no areas of standing water once
inflow has ceased. Any of the following could
apply: improved grading or underdrains flushed in
manner that does not cause an illegal discharge.
Sediment, Trash, and Debris Accumulation
Sediment, trash and debris accumulated in the
swale and around the inlet and outlet.
Material removed so that there is no clogging or
blockage. Sediment deposits removed without
significant disturbance of the vegetation.
Erosion
Channels have formed around inlets, there are
areas of bare soil, or there is other evidence of
erosion.
No erosion or scouring evident. Damaged areas
repaired with crushed gravel. Over time grass will
start to cover over rock.
Vegetation
Vegetation is dead, diseased, or overgrown.Vegetation is healthy and attractive. Grass is
maintained at least 3 inches in height.
Mulch (if used)
Mulch is missing or patchy. Areas of bare earth
are exposed or mulch layer is less than 3 inches
deep.
All bare earth is covered, except mulch is kept
6 inches away from trunks of trees and shrubs.
Mulch is even at a depth of 3 inches.
Inlet/Outlet
Sediment or debris accumulations.Inlet/outlet is clear of sediment and debris and
allows water to flow freely.
● Dry Swale ● Vegetated Buffer Strip ● Vegetated Swale
C-22 Storm Water BMP Guide for New and Redevelopment
Revised: July 2017
Appendix C: Operations and Maintenance Fact Sheets
Condition When Maintenance is Needed Results When Maintenance is Performed
Flow Spreader (if applicable)
Flow spreader uneven or clogged so that flows
are not uniformly distributed through entire swale
width.
Spreader leveled and cleaned such that flows are
distributed evenly over the entire swale width.
Visual Contaminants and Pollution
Visual evidence of oil, gasoline, contaminants, or
other pollutants.
No visual evidence of contaminants or pollutants
present.
Miscellaneous
Any condition not covered above that needs
attention for the vegetated swale/strip to function
as designed.
The design specifications are met.