5—
Floodplain Mapping

The National Flood Insurance Act of 1968 (Public Law 90-448, Title 13) made the adoption of local floodplain regulations a condition for the availability of flood insurance in any community. The National Flood Insurance Program (NFIP) is the main focus of the federal government's effort to mitigate flood problems through the use of nonstructural measures.

The NFIP had two major goals. First, it was expected to reduce flood losses by requiring local governments to implement land-use regulations that would result in safe building practices in flood hazard areas. Second, the economic burden of flood losses was to be shifted from the taxpayers in general, through disaster relief programs, to the occupants of flood-prone areas (Burby et al., 1985).

Communities, rather than individuals, must qualify for inclusion in the NFIP by adopting floodplain management techniques that are appropriate for the level of flood hazard that exists within their boundaries. This requires communities to begin to develop flood hazard maps to assess how to manage particularly vulnerable floodplain areas.

The NFIP was modified by the Flood Disaster Protection Act of 1973 (Public Law 93-234) that essentially converted participation in the program from being voluntary to mandatory. This act requires property owners or developers to purchase flood insurance for any property in a designated flood hazard area that is purchased with funds from any bank or savings and loan institution regulated or insured by the federal government (Platt, 1979). In communities that do not participate in the NFIP, property owners are penalized twofold—not only is this federally subsidized insurance not available to them, but federal postdisaster recovery assistance is prohibited.



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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 5— Floodplain Mapping The National Flood Insurance Act of 1968 (Public Law 90-448, Title 13) made the adoption of local floodplain regulations a condition for the availability of flood insurance in any community. The National Flood Insurance Program (NFIP) is the main focus of the federal government's effort to mitigate flood problems through the use of nonstructural measures. The NFIP had two major goals. First, it was expected to reduce flood losses by requiring local governments to implement land-use regulations that would result in safe building practices in flood hazard areas. Second, the economic burden of flood losses was to be shifted from the taxpayers in general, through disaster relief programs, to the occupants of flood-prone areas (Burby et al., 1985). Communities, rather than individuals, must qualify for inclusion in the NFIP by adopting floodplain management techniques that are appropriate for the level of flood hazard that exists within their boundaries. This requires communities to begin to develop flood hazard maps to assess how to manage particularly vulnerable floodplain areas. The NFIP was modified by the Flood Disaster Protection Act of 1973 (Public Law 93-234) that essentially converted participation in the program from being voluntary to mandatory. This act requires property owners or developers to purchase flood insurance for any property in a designated flood hazard area that is purchased with funds from any bank or savings and loan institution regulated or insured by the federal government (Platt, 1979). In communities that do not participate in the NFIP, property owners are penalized twofold—not only is this federally subsidized insurance not available to them, but federal postdisaster recovery assistance is prohibited.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 FLOODPLAIN MANAGEMENT ON OAHU In June 1970 the city and county of Honolulu began to map flood-prone areas on the island of Oahu. Revisions to flood hazard boundary maps were made in 1971. Early efforts in flood hazard mapping concentrated on low-lying coastal areas that were at risk from hurricanes, tsunamis, and high tides. During the next decade, additional studies were conducted of flood-prone areas, and in September 1980 a Flood Insurance Rate Map (FIRM) became effective. This map was revised again in September 1987, only 3 months prior to the New Year's Eve floods. The FIRM specifies the recurrence intervals and inundation areas for various types of flood events (e.g., 500- or 100-year events). For properties within each of these hazard zones, nationally standardized insurance rates are applied. The rates differ by hazard zone but are the same for that particular zone anywhere in the United States. Rates are determined on the basis of each $100,000 of assessed value for a structure and each $10,000 for its contents. DAMAGE FROM THE NEW YEAR'S EVE FLOOD EVENT Flood-related residential and business damage due to Oahu's New Year's Eve floods occurred in FIRM zones that were identified as follows: Zone A 100-year flood hazard area; no base flood elevation determined. Zone AH 100-year flood hazard area; flood depths of 1 to 3 feet (usually areas of ponding). Zone B Areas of 500-year flood; areas of 100-year flood with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 100-year floods. (Actual zone X, shaded on FIRMs). Zone C Areas determined to be outside the 500-year floodplain. (Actual zone X, unshaded on FIRMs.) Zone D Areas in which flood hazards are undetermined. To determine where damage occurred in these zones, a two-step process was used. The actual neighborhood damage assessment sheets, completed under the supervision of the Red Cross during the first week after the event, were used to identify specific addresses that sustained damage. Damage was classified on these sheets as "none," "minor," ''major," or "destroyed." A total of 1,058 addresses were identified as having at least minor damage. With assistance from the Honolulu Department of Planning and the Department of Land Utilization, each of the addresses was categorized according to the FIRM zone in which it was located. Table 12 presents an overview of the type of damage that occurred by zones within geographic areas.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 TABLE 12 Residential Flood Damage by Extent, Area, and Flood Zone   Extent of Damage (%) Area Minor Major Destroyed Total Honolulu Zone D 39 4 1 44 Hahaione Valley Zone A 4 0 0 4 Zone D 24 1 0 25 Subtotal 28 1 0 29 Niu Valley Zone A 7 0 0 7 Zone D 74 13 3 90 Subtotal 81 13 3 97 Hawaii Kai Zone D 1 0 0 1 Kailua Zone A 165 157 1 323 Zone Ba 273 105 1 379 Zone Cb 147 13 0 160 Subtotal 585 275 2 862 Waimanalo Zone A 8 0 0 8 Zone AH 0 0 1 1 Zone Ba 11 0 0 11 Zone Cb 2 2 0 4 Zone D 1 0 0 1 Subtotal 22 2 1 25 Total Structures Damaged (all categories of damage): 1,058 a Zone B corresponds to Zone X (shaded) on FIRMs. b Zone C corresponds to Zone X (unshaded) on FIRMs. Source: Information on damage taken from actual damage assessment sheets provided by the Red Cross. As can be seen in Table 12, the Hawaii Kai areas affected by flooding (Honolulu, Hahaione Valley, Niu Valley, and Hawaii Kai) were limited to Zones A and D. It appears that the Zone A designation in Hahaione and Niu valleys was appropriate. Only minor damage was found in these two Zone A areas. The return period (recurrence interval) for this storm in these areas (see Chapter 3) was determined to be 100 years.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 The majority of damage on the leeward side of the island, however, occurred in Zone D areas, indicating that no assessment of their flood hazard had been made. This designation seems especially unfortunate since the only damage classified as major or destroyed on the leeward side of the island occurred in Zone D areas. Because no flood hazard had been identified for these areas, property owners were not required to purchase flood insurance. Clearly, it was the areas for which no hazard assessment had been made that were most negatively affected by this storm on the leeward side of Oahu. On the windward side of the island, in Kailua and Waimanalo, the picture was different. In Waimanalo most damage was classified as minor and occurred in areas that had a return rate of 100 years, which appears to reflect the characteristics of this storm (see Chapter 4). However, some consideration should be given to the evaluation of Zone C, which is based on a return period of 500 years, since some damage to structures in this zone occurred. The flooding in Kailua appears to be the most problematic in terms of adequacy of the FIRM zonal designations. Certainly, the damage that occurred in Zone AH appears reasonable given the designation; however, there were a substantial number of reports that water levels were higher than 1 to 3 feet. The Zone B designation may be misleading for the Kailua area. The return period for this area of flooding was estimated at 200 years (not 500 years as the zone designation could imply); the flood depth was greater than 1 foot; and the levee that restrained the marsh floodwaters did not function as expected (see Chapter 4). As in Waimanalo, the description of Zone C should be reviewed, since a substantial amount of damage occurred in this area. In Table 13 the extent of damage for the entire event is summarized by FIRM zone. Minor damage is distributed relatively uniformly across the zones, regardless of the degree of hazard associated with each zone. While most major damage occurred in the correctly evaluated zone (AH), it may have exceeded the designations for Zones B and C. The relatively low number of homes destroyed made it impossible to identify any useful patterns. TABLE 13 Summary of Damage by Extent and Flood Zone   Extent of Damage (%) Zone Minor Major Destroyed Total (%) A 2 0 0 2   AH 22 53 29 31 B 38 36 14 37   C 20 5 0 15   D 18 6 57 15   Total % 100 100 100 100 Total Number 756 295 7 1,058