HomeMy WebLinkAboutCOM 0176.093 2006-2008
May 31, 2007
Honorable Pete Hoffman, Chairperson
Members of the Hawaii County Council
333 Kilauea Avenue
Hilo, Hawaii 96720
Testimony of Bill 51 • Proposed Amendment to Chanter 27
If'~ }fly unc}~}~staCtdipg that the initial intent of Bi1151 was to amend the County flood
plain maPggB}hp}}1 R{c~inance so that it conforms to the National Flood Insurance Program
administele~ by F~IwIA. However, Bill 51 as written goes beyond the initial intent and
adds elel{}ggtnS that etlFeed tttR requirements of the NFIP
I this We pt1R ggree that flooding issues need to and should be addressed. The
perceived inability of the county to control growth along the floodways and flood hazard
zones ip W~St {-I~W~1i has resulted in a draft ordinance that is unreasonable and biased. It
forces the agricultural community to comply with rules and regulations that aze
impossible to achieve and further defeats the goal of self sufficiency.
Article 4, section 27-18 D (6) states that grading and filling of lands exceeding 10 percent
slope is restricted to 20 percent of the lot azea to reduce concentration of runoff and
increased flooding and erosion. I have been told that this was aimed at housing
developments, yet there is no mention of a grading exemption for farmers or ranchers
who have a conservation plan that has been approved by the respective Soil and Water
District. A large portion of Ka'u, Kona, Hilo and Hamakua will fall into the 10% slope
restriction. Even lands in Puna will be affected by this. Has the Council determined how
much land azea will be affected and what the economic impact will be to farmers,
ranchers and other land owners?
Article 4, section 27-20 (g) states that new Agricultural subdivisions may exist above
urban azeas, however, these pazcels will be required to have flood control structures to
designed to contain all rainfall from a 100 yeaz storm event. I have attached a case study
that was done by NRCS to illustrate the impact this requirement would have on a 20 acre
farm lot in Kau. The case study clearly shows that it is not economically feasible for any
rancher or farmer to comply with this requirement.
I support the amendments to Chapter 27 that brings this chapter into compliance with the
NIFP; however, I strongly oppose changing the intent and scope of Chapter 27 beyond
compliance with the NIFP.
Respectively submitted, Comm. No. ~ ~ ~ o
Ref. To: k~eMS~ ~
o
Rand~abral Ref. Date, 2001
Director - Ka'u S W CD
Bill 51 Case Study
(May 31, 2007)
Determine installation costs and land requirements for a new 20-acre farm in
Ka'u to comply with the proposed Bill 51 requirements for agricultural
developments to retain on their property the stormwater generated by a 100-year
storm.
Assumed:
1) Farm location is 5 miles north of Pahala.
2) Soil is Alapai silty clay loam, 10%-20% slope, Hydrologic Soil Group-A
(well-draining)
3) Drainage from 20 acres of farm site only. No upper drainage area to
the farm. If there is a drainage area above the farm, it will be
conveyed through the farm with a newly enlarged drainageway with a
100-year stormwater capacity.
4) Farm is 10 acres of row crop on contour and 10 acres fallow with good
cover,
Computed average Runoff Curve Number, 70.
5) Rainfall intensities are obtained from National Weather Bureau,
Technical Paper No. 43. The 100-year, 24-hour rainfall is 21 inches
6) The farm is laid out as a square with a 1,000-ft flow path to calculate
time of concentration. Time of concentration is 0.1 hours.
Methodology:
1) WinTR-55 computer program used to calculate total runoff. WinTR-55
output in separate hydrology file.
2) Calculate total volume of runoff by multiplying inches of runoff by the
20 acre area.
3) Determine the volume and total dimensions of a basin to retain the
storm runoff.
4) Determine basin costs and land requirement.
Results:
1) Runoff from 100-year storm = 16.6 inches = 1.38 ft (4.4 inches lost to
infiltration).
2) Runoff volume from 20-acre farm = 1.38 ft. X 20 ac. = 27.7 ac-ft. or 9.0
million gallons.
3) With a 12-ft deep pool requiring a 15-ft high embankment along lower
boundary of property, a basin to contain the runoff will be
approximately 750 ft. by 130 ft at the design pool level. Additional
storage volume will be needed if sediment storage is required.
4) The embankment and design pool will require 4.7 acres of land. An
18-ft high cut slope behind the pool will require an additional 1.1 acres.
5) The volume of excavation is approximately 130,000 cy. At $10/cy the
excavation cost is $1.3 million.
6) The embankment volume is approximately 35,000 cy. At $20/cy the
embankment cost is $0.7 million.
7) The emergency and principal spillway structures are estimated to cost
$0.5 million.
8) The total initial cost to provide the retention of the 100-yr storm runoff
on the 20-acre farm is the lost use of nearly 6 acres and approximately
$2.5 million to install the stormwater retention basin. Other long-term
costs will be operation and maintenance of the basin, including
sediment removal and periodic inspections. Due to the hazards
presented by the storage volume, liability insurance premium costs for
the farm will likely rise.
9) If the embankment rises above 25 feet or the storage volume exceeds
50 ac-ft (16.3 million gallons), the dam structure will fall under the
regulation of the State of Hawaii. Smaller dams with appreciable
downstream hazards such as highways or development will also need
to meet the state's dam safety requirements, which include a more
stringent application process, increased design and construction
requirements, preparation and implementation of an emergency action
plan, and periodic government inspection of the structure.
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D. Kubo Bill 51
100-year runoff volume in Ka'u
Hawaii County, Hawaii
Watershed Peak Tab4e
Sub-Area Peak Flow by Rainfall Return Period
or Reach 10-Yr 100-Yr
Identifier (cfs) (cfs)
SUBAREAS _-A--
Ka'u Farm 156.58 261.01
REACHES
OUTLET 156.58 261.01
iJ. Kubo Bill 51
100-year runoff volume in Ka'u
Hawaii County, Hawaii
Hydrograph Peak/Peak Time Table
Sub-Area Peak Flow and Peak Time (hr) by Rainfall Return Period
or Reach 10-Yr 100-Yr
Identifier (cfs) (cfs)
(hr) (hr)
SUBAREAS
Ka'u Farm 156.58 261.01
9.93 9.93
REACHES
OUTLET 156.58 261.01
(added from TR-20 Report)
TR-20 RUNOFF 10-Yr 100-Yr
(Inches) 9.90 16.60
D. Kubo Bill 51
100-year runoff volume in Ka'u
Hawaii County, Hawaii
Sub-Area Summary Table
Sub-Area Drainage Time of Curve Receiving Sub-Area
Ident~er Area Concentration Number Reach Description
(ac) (hr)
Ka'u Farm 20.00 0.100 70 Outlet 20-acre drainage area
Total Area: 20 (ac)
D. Kubo Bill 51
100-year runoff volume in Ka'u
Hawaii County, Hawaii
Sub-Area Time of Concentration Details
Sub-Area Flow Mannings's End Wetted Travel
Identifier/ Length Slope n Area Perimeter Velocity Time
(ft) (ftfft) (sq ft) (ft) (ft/sec) (hr)
Ka'u Farm
SHEET 100 0.1500 0.240 0.064
SHALLOW 600 0.1500 0.050 0.027
CHANNEL 300 0.1500 0.030 5.00 15.00 9.259 0.009
Time of Concentration 0.100