HomeMy WebLinkAboutCOM 0176.041 2006-2008 +`~,,°i Phone: (808) 326-5421
Fax: (808) 329- 4786
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BRENDA FORD, ,i E-Mail: bford@co.hawaii.hi.us
Council Member, District 7
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HAWAII COUNTY COUNCIL
County of Hawai `i = _
Kailua Trade Center
75-5706Hanama Place, Suite 109 rte.
Kailua-Kona, Hawaii 96740 _
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DATE: May 7, 2007
TO: Pete Hoffmann, Chair, and Council Members of the Hawaii County Counral
25 Aupuni Street
Hilo,Hawai`i 96720 ~j~~
FROM: Brenda Ford, Council Member District 7 ~"""~"'~~r-"
RE: Public Works Committee Agenda
Information for the Council on Bi1151
Dear Chair Hoffmann,
I am requesting that Hawaii County Council Members receive information on the Flood Hazard
Analyses for the South Kona Area. Growth in the South Kona area was causing problems when
the report was written in 1977, now 30 years later, many of these problems have still not been
addressed. This study is illustrative of the island-wide problems of flooding and on pages 11, 12,
and 13 has information regarding Floodplains with a diagram on page 12 that clearly illustrates
the 100 year floodplain. Please send this information to the Council members as soon as
possible.
BF/dh
Comm. fJo. ~ ~ ~ •
Ref. To: Prby>-~~-
Ref. Uate ~ s""if (1 9 2~6?
Hawat `i County Is An Equal Opportunity Provider And Employer
FLOOD HAZARD ANALYSES
SOUTH KONA AREA
Hawaii County, Hawaii
Prepared By:
United States Department of Agriculture
Soil Conservation Service
Prince Kuhio Federal Building
P. 0. Box 50004
Honolulu, Hawaii 96850
In Cooperation With:
Kona Soil and Water Conservation District
County of Hawaii
Department of Land and Natural Resources, State of Hawaii
July 1977
TABLE OF CONTENTS
P_~
FOREWQRD AND LOCATION MAP
INTRODUCTION 1
State And Local Needs for Study 1
Requesting and Participating Agencies 1
Study Authorities 1
. .
Limitations of the Study 2
DESCRIPTION OF THE STUDY AREA 3
FLOOD HISTORY 5
March 31, 1956 S
April 29, 1963 5
November 20, 1967 7
May 26, 1976 7
FLOOD POTENTTAL (PRESENT CONDITIONS) 9
FLOOD POTENTIAL (FUTURE CONDITIONS) 10
FLOOD PLAIN MANAGEMENT 11
RECOMMENDATIONS 13
APPENDICES
STUDY INVESTIGATION AND ANALYSES IS
Field Surveys 15
Hydrologic Studies 15
Hydraulic Studies 16
GLOSSARY OF TERMS 17
BIBLIOGRAPHY 18
TABLE 1. FLOOD DISCHARGE-ELEVATION-FREQUENCY DATA 19
TABLE 2. AREAS SUBJECT' TO INUNDATION 61
TABLE 3. WATERCOURSE LENGTHS 62
FIGURES
1 -April 29, 1963 Storm 6
2 -November 20, 1967 Storm 6
3 -May 26, 1976 Storm - 8
4 -May 26, 1976 Storm 8
5 - Floodway Schematic 12
FOREWORD
The purpose of this Flood Hazard Analyses Report is to provide
state and local units of government with basic technical data
- concerning the flooding problems and possible solutions for the South
Kona area. The report provides the following data:
1. Identifies 25 watercourses that have caused floodwater
damages.
2. Establishes elevation-discharge relationships for each
watercourse for the 10-year, 50-year, 100-year, and 500-year
frequency flood events.
3. Delineates the flood hazard area of each watercourse for the
100-year and S00-year flood events on topographic maps
(Flood Hazard Maps).
4. Presents recommendations for alleviating current and future
floodwater damages.
The above data will provide a basis for the enactment of sound flood
plain management and land use programs.
This report was prepared by the Soil Conservation Service, U.S.
Department of Agriculture, in cooperation with the Kona Soil and
Water Conservation District, County of Hawaii, and the State
Department of Land and Natural Resources.
SOUTH KONA AREA FLOOD HAZARD ANALYSES
HAWAII COUNTY, HAWAII
INTRODUCTION
State and Local Needs for Study
Residents of the urban and agricultural lands in the area are
subject to increasing hazards from floodwater damages as they
continue to move into flood-prone azeas. Local units of government
have an immediate need for flood hazard information so that they may
initiate sound flood plain management and land use programs in the
study azea. The programs should encourage future land use decisions
appropriate to the degree of flood hazard involved, thus reducing
future problems.
Requesting and Participating Agencies
In June 1974, the County of Hawaii submitted a request to the
USDA, Soil Conservation Service (SCS) for assistance in making a
detailed flood hazard analyses of the South Kona area. In response
to the county's request, a Plan of Study for the South Kona area was
prepared. The Plan of Study, completed in June 1975, outlined the
objectives and procedures to be followed in completion of the Flood
Hazard Analyses Study. A Joint Coordination Agreement was signed
between the State of Hawaii Department of Land and Natural Resources
(DLNR) and SCS for flood hazard analyses and flood plain studies for
the state in August 1974.
Study Authorities
The Soil Conservation Service is authorized to provide technical
assistance to state, federal and local governing bodies in carrying
out flood hazard studies under Section 6 of the Watershed Protection
and Flood Prevention Act of 1954 (Public Law 83-566, as amended).
This is in accordance with: Recommendation 9(c), of House Document
No. 465, 89th Congress, 2nd Session; Executive Order 11296, dated
August 10, 1966; and USDA Secretary's Memorandums 1606 and 1607.
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Chapter 179, Hawaii Revised Statutes, as amended, designates the
Department of Land and Natural Resources (DLNR) as the state agency
responsible for coordinating the flood control and related activities
of the various government agencies in Hawaii. DLNR, through its
Division of Water and Land Development, is the agency responsible for
implementing the statewide flood control program and for providing
technical and financial assistance to the political subdivisions of
the state (counties and Soil and Water Conservation Districts).
Limitations of the Study
The entire South Kona area is characterized by many under-
developed and poorly defined watercourses, all subject to potential
flooding. Special efforts were made to locate probable problem areas
by interviewing local residents and studying storm damage reports
made by the personnel of Bishop Estate and the SCS. These efforts
resulted in the identification of 25 watercourses most likely to
carry excessive storm runoff. It must be emphasized that this does
not preclude the possibility of other unidentified watercourses also
carrying damaging runoff. This is caused by several unpredictable
circumstances: (1) development and clearing in both agricultural and
urban areas will alter existing topography, (2) accumulation of
sediment and debris in any of the 25 identified watercourses can
divert flow to form new watercourses not identified in this study,
and (3) a storm may center over areas of undefined and unidentified
watercourses.
There also exists in the South Kona area the possibility of
shallow, alluvial-type flooding resulting from overbank flows that
remain unconfined. Such shallow flooding, with average depths lass
than one foot, are not usually associated with channel flooding and
flood profiles. Reliable determination of depths, extent of
flooding, and direction of flow by normal open-channel hydraulic
methods would be extremely difficult if not impossible. Because of
the steep slopes of the existing terrain, any attempt to confine such
flows would result in extremely hazardous velocities. Any one storm
mhy not cover the entire South Kona area with alluvial flooding but,
• because .of the possibility of sediment and debris altering tfie
direction of flow, all areas may experience this type of flooding.
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DESCRIPTION OF STUDY AREA
The island of Hawaii, designated by the U.S. Water Resources
Council as Subregion I of the Hawaii Region, lies at the southeastern
end of the Hawaiian archipelago, near the northern limit of the
tropics. The study area encompasses approximately 137,600 acres in
the South Kona district of the island of Hawaii (see map).
Located on the western slopes of Mauna Loa, the study area
consists of a series of narrow drainage areas with underdeveloped
watercourses that drain into the Pacific Ocean. Twenty-five such
watercourses were identified as having flood hazard potential. As
development and clearing occur in both agriculture and urban lands,
other watercourses may also become problem areas. Because the
topography is steep, existing undefined watercourses could easily be
diverted to form new watercourses creating hazard areas not
identified in this study.
All watercourses within the study area are intermittent. Kiilae
Stream, which has the longest sustained flow, goes dry during the
winter months. Much of the flood flows percolate into lava
formations and reach the ocean as surface flow only after periods of
intense and sustained rainfall.
The drainage azea above 6,000 feet elevation appears not to
contribute to surface runoff. This area includes nearly barren lava,
volcanic ash, pumice, and cinders.
The drainage azea is blocked from the tradewinds by the mountain
masses of Mauna Loa, Mauna Kea, and Hualalai. As a result the pre-
v~iling wind circulation is an alternating land-sea system resulting
from the differential heating of the land and water masses. During
the day, the land heats faster than the ocean resulting in a sea
breeze; during the night, the land cools faster than the ocean
resulting in a land breeze.
Rainfall is mostly produced by the daytime sea breeze that push
moist air over the land mass. This moist air yields regular, and
sometimes heavy, showers as it ascends the steep mountain Slopes.
These showers are usually of short duration and high intensity.
Seasonal distribution of the rainfall is unique in this area.
Summer months are wet while the winter months are dry. This is con-
trary to distribution in other parts of the state. This u~hique
pattern is largely due to greater intensity of the onshore breeze
during the summer months.
The mean annual rainfall ranges from approximately 40 inches at
sea level to about 80 inches at 3,000 feet elevation. Above the
3,000 feet level, rainfall gradually decreases from 80 inches to
about 15 inches near the summit of Mauna Loa.
Soils in the drainage area are dominated by well-drained, very
shallow soils formed from organic matter over pahoehoe lava or
fragmental as lava. The thickness of the organic layer increases
with increasing rainfall but is generally less than 10 inches. The
vegetatation consists of ohia, tree fern, koa, guava, Christmas
berry, pasture grasses, coffee and macadamia nuts.
The upper portion of the drainage area consists of nearly barren
lava rocks, volcanic ash, pumice and cinders.' The natural vegetation
consists of lichen, moss, ohia, amaumau fern, naio, Kentucky
bluegrass, and sweet vernal.
The middle portion of the watershed has small areas of volcanic
ash soils interspersed among the thin organic soils. These volcanic
ash soils vary in depth from 10 to 40 inches over pahoehoe or as
lava. These are generally used for truck crops, pasture, coffee, and
macadamia nut orchards.
Development within the study area is concentrated along a strip
of the Mamalahoa Highway connecting the urban centers of Kealakekua,
Captain Cook, Honaunan and Kealia. These areas are oriented toward
agriculture and commercial developments consisting primarily of
general stores, service stations and supermarkets.
Agriculture is the major industry within the area with cattle
ranching, macadamia nuts, and coffee being the major enterprises.
The climate is favorable for agriculture but soils and topography
limit the amount of mechanization. Marketing problems, shortage of
water, and shortage of labor have combined to limit the realization
o~-the potential for agricultural production.
Tha 1970 census estimated a population of 4,004 for South Kona.
This population is expected to increase only slightly during the next
10 to 15 years.
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FLOOD HISTORY
Flooding problems have been largely due to localized high
intensity rainfall from about 1,000 feet elevation to 5,000 feet
elevation. Such storms can occur anywhere along the mountain slopes
of South Kona. In addition to these localized storms, a few general
storms have occurred covering the whole width of the study area.
According to storm damage reports by SCS, there have been 17 da-
maging floods since 1956. Other floods have occurred before this
date but no detailed accounts are available.
Most of the flood damages were due to poorly defined
watercourses, the presence of developments in these watercourses, and
the absence of properly constructed road crossings. During large
storms, floodwaters overflow onto and erode adjacent lands. Further
downstream, sediment and debris accumulate and compound the problem
in roads and in homes that have been built on some of these poorly
defined watercourses.
Accurate data on rainfall and flood flows are nonexistent but
general accounts are available from storm damage reports.
Recent Notable Floods
March 31, 1956
Rainfall was estimated at about 8 to 12 inches with the highest
intensity occurring from 6 to 9 p.m. This rainfall amount was
greater than the 100-year, 24-hour frequency storm event as defined
in the U.S. Weather Bureau Technical Paper No. 43. Damage occurred
throughout the South Kona area along the Mamalahoa Highway. Examples
of damages are flooding of the Honaunau Post Office building and
several segments of Mamalahoa Highway.
April 29, 1963
Approximately 4 inches of rainfall fell within 2 hours. Total
rainfall amount was equivalent to the 100-year, 24-hour frequency
storm. Damages resulted in losses of 0.25 to 1.5 inches of soil from
bare truck crop lands. Several macadamia nut and coffee orchards
ware severely eroded and trees were lost. Damage occurred to Hookena
Road.
The watercourse crossing the Mamalahoa Highway near the
Greenweli property (see Fig. 1) resulted in considerable erosion
damage below the road.
Many of the watercourses experienced flows but little damage
resulted. The town of Kainaliu, near the study area, was flooded.
t -5-
Figure Z Water overtopping euZvert at Mcanalahoa HZghmcry
on Watercourse No. Z. The flooding ~uas a reauZt
of a high intensitg rainfaZZ, 4 inches in
2 hours, an April 29, 2963.
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Figure 2 Picture of Honaunau Post Office after storm of
November 20, 2967. OverfZaw from Watercourse
Po. Z3.
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November 20, 1967
Heavy rains fell at elevations of approximately 4,000 feet in
the Honaunau area which generated stream flow causing serious damage
to the Honaunau Post Office building, farm roads, and buildings
surrounding the post office, The rain began at 2 p.m. and subsided
at 4:35 p.m. During this period, 4 inches of rainfall was measured.
This rainfall amount was equivalent to the 100-year, 24-hour
frequency storm. (See Fig. 2.)
May 26, 1976
Rainfall began at about 3 p.m. and continued to about 7 p.m.
The only recording rain gage (USGS) in the area was not functioning
properly but estimated rainfall at the Honaunau Post Office for the
4-hour duration was about 5 inches. This rainfall amount had a
frequency greater than once in 100 years. Ali defined watercourses
between Watson and McCandless Ranch office flowed past the Mamalahoa
Highway but no runoff reached the ocean due to seepage and storage
losses.
Damages consisted of water damage to the Honaunau Post Office (8
inches of water above the post office floor), erosion to adjacent
lands, and sediment deposits on the highway (see Fig. 3); erosion and
sediment damage around Morihara Store and damage to City of Refuge
Road (see Fig. 4); flooding and erosion of the cemetery adjacent to
the Painted Church; and damage to road culverts on City of Refuge
Road.
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4 _ . _ - - - -
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Figure 3 High water mark on Honaunau Post Office as a
result of a storm on May 26, 2976. Eight
inches of matex covered the poet office
floor. OverfZau from Watercourse Po. Z3.
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Figure 4 Road damage adjacent to City of Refuge Road at
Xeohea. Storm on May 26, 2976. Overflora from
Watercourse Po. Z9:
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FLOOD POTENTIAL (PRESENT CONDITIONS)
Each of the 25 watercourses within the study area has the
potential to produce sufficient runoff to result in floodwater
damages to urban and agricultural Lands. These damages have occurred
almost annually from one or more of the watercourses. Flooding
generally lasts only a few hours, but is long enough to cause water
damages.
A large storm such as the 100-year event would result in
substantial floodwater damage to developed areas adjacent to the
Mamalahoa Highway. The 100-year event was used as a reference flood
for analyses of the flood hazards of the area and should be the basis
for implementing flood plain and land use zoning ordinances.
The results from the hydrologic and hydraulic studies provide
elevation-discharge data for the 10-year, 50-year, 100-year, and
500-year flood events at each valley cross section (see Table 1).
The elevation-discharge data for the 100-year and 500-year flood
events were plotted on the topographic map at each section and the
flood plain delineation was interpolated between cross sections. The
flood pisin for each watercourse is shown on the flood hazard maps,
plates 1 through 25. The flood hazard maps are located on the Index
` Map of the study area.
In the reaches studied the flood plains ranged in width from 20
feet to 1,500 feet, depending on the topography (see maps). Table 2
lists the area flooded by the 100-year and 500-year flood event for
each watercourse. This variation in flood plain width resulted in
depth of flow variations for a given discharge. As shown on the
maps, little difference in flood plain width is evident between the
100-year and 500-year flood events in many of the valley cross
sections. Also, as a result of wide flood plains, at some valley
cross sections small variations in depth occurred between different
flood events. This small depth variation, combined with the steep
profiles, made plotting profiles of more than one flood event
impracticable. Therefore, only the 100-year flood event was selected
to describe the water surface profiles. (See Appendices.)
Typical valley cross sections are included in the appendices.
The elevation-discharge data in Table 1, the water surface
profiles in the Exhibits and the flood hazard maps (Plates 1 to 25)
may be used to determine flood hazard potential at a specific
location. The water surface elevation (Table I) determined at each
specific valley section may be used directly in field evaluations of
current flood potentials at that location. At points between cross
sections, the water surface elevations can be estimated by examining
the depth of flow at the upstream and downstream section and
interpolating the depth that can be added to the channel bottom
elevation (from flood hazard maps) at the point in question.
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4
FLOOD POTENTIAL (FUTURE CONDITIONS)
Future floodwater damages will be directly related to the amount
and gwality of growth within the study area. Approximately 1,000
acres in the study area have been zoned as urban or rural districts
by We•State Laad Use Commission. This zoning would allow only a
modest increase above present urban and rural land use and will
result in a low growth rate. The remaining lands in the study area
hare been zoned as agriculture or conservation district. If the
(load plafn management recommendations contained in this report are
foLlowbd, future flooding problems will be minimized.
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FLOOD PLAIN MANAGEMENT
This report is intended to be used as a technical guide by state
and local planning groups to control future flood plain land use
appropriate to the degree of flood hazard involved. Flood damages
will be reduced by implementation of nonstructural measures such as
zoning, land acquisition, and flood insurance or by the installation
of structural measures such as floodproofing.
The County of Hawaii has recently approved several ordinances
concerning Iand use within flood-prone areas. Ordinance No. 77,
effective December 31, 1974, established a "Safety Flood Hazard
District - SF." The Safety Flood Hazard District - SF was described
as: "This subdistrict shall apply to areas designated by the Federal
Insurance Administration as being subject to special flood and
tsunami hazards and indicated on the zone maps as SF districts." The
U.S. Department of Housing and Urban Development (HUD) has designated
the island of Hawaii for a future Flood Insurance Study under the
Regular Program. The island is participating in HUD's Emergency
Program. This permits the owners and occupiers of all walled and
roofed buildings and mobile homes to purchase subsidized insurance on
their structures and contents.
Development in the SF District is regulated by County of Hawaii
Ordinance No. 76, effective December 31, 1974, which amended
Ordinance No. 501, Building Code Ordinance. Ordinance No. 76 is
intended to reduce future flood damages by controlling development
within floodprone areas. Concerning construction of residential
structures, paragraph 3(d) was added to Section III, paragraph 3,
Ordinance No. 501 stating: "(d) All new construction or substantial
improvements of residential structures in the SF District shall have
the lowest floor, including basement, elevated to or above the level
of the 100-year flood,"
The flood boundary lines shown on the flood hazard maps may be
used as the basic data for delineation of "Safety Flood Hazard
District - SF" and also as the basis for implementation of
appropriate county ordinances pertaining to development within flood
plains.
Floodways
pr; Encroachment on flood plains, including roads, buildings, and
artificial fill, reduces the flood-carrying capacity and increases
flood heights, thus increasing flood hazazds in areas above and below
the encroachment itself. One aspect of flood plain management
involves balancing the economic gain from flood plain development
against the resulting increase in flood hazard. For purposes of
HUD's National Flood Insurance Program, a floodway is used as a tool
to assist local communities in this aspect of flood plain management.
~11.
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Under this concept, the area of the 100-year flood is divided into a
floodway and a floodway fringe. The floodway is the channel of a
stream, plus any adjacent flood plain areas that must be kept free of
encroachment in order that the 100-year flood be carried without
substantial increases in flood heights. As minimum standards, the
Federal Insurance Administration limits such increases in flood 'v~, -
heights to 1.0 foot, provided that hazardous velocities' are not
produced.
The area between the floodway and the original boundary of the
100-year flood is termed the floodway fringe. The floodway fringe
thus encompasses the portion of the flood plain that could be
completely obstructed without increasing the water surface elevation
of the 100-year flood more than one foot at any point. Typical
relationships between the floodway and the floodway fringe and their
significance to flood plain development are shown in Figure 5.
100 Year Flood Plain
Floodwa Floodway Floodwa
Fringe Fringe
Stream _
Channel
Flood Elevation When
Confined Wi hi Floodway Encroach
Encroachment meat
C D
Surchar e*
A ~ -__s
Area of flood plain that could Flood elevation before:=~
be used for development by encroachment on flood
raising ground plain
Line A-B is the flood elevation before encroachment
Line C-D is the flood elevation after encroachment
*Surcharge not to exceed 1.0 foot (FIA requirement) or lesser amount
if s ifi
Figure 5 Floodway Schematic
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RECOI~NDATIONS
To reduce the present and potential flared hazards within the
study area, the following recommendations are presented:
Nonstructural
1. Preserve the conservation and agriculture land use districts
above Mamalahoa Highway.
2. Establish and maintain appropriate vegetative cover on sediment
and debris producing areas that are subject to heavy runoff damages from
sediment and rubble on lower areas.
3. Enforce county grading ordinance to reduce erosion and sedimen-
tation.
4. Implement land use zoning to restrict future development within
identified flood plains or require proper structural design to prevent
floodwater damages from the 100-year event.
5. Purchase flood insurance on all buildings and mobile homes,
especially those within the flood hazard areas delineated on the maps
and those in the areas subject to shallow alluvial-type flooding.
Structural
1. Relocate or floodproof buildings within flood-prone area.
2. Improve road culverts and bridges to carry a larger discharge and
provide additional ones where needed. Improve entrance design of culverts
to prevent clogging by rocks, sediment, and debris.
3. Develop a system of diversions, using lava tubes and natural catch-
ments to reduce the peak discharges at the highway.
4. Require aII structural or'Iand improvements to compensate for
increased runoff and prevent this increased runoff from affecting adja-
cent lands.
The above nonstructural and structural measures should be used
in combination to reduce the rate and volume of runoff, reduce water
velocities, and lessen the peak discharges in the watershed area
contributing to each watercourse. The objective of the measures
would be to encourage development that is compatible with nature and
let the watercourses develop naturally without interference from man,
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