Multi-Hazard Mitigation Plan: 05. Tropical Cyclones

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Chapter 5:Hazard Analysis—Tropical Cyclones The duration of storm surge is usually relatively short,being dependent upon the elevation of the tide, which rises and falls twice daily in most coastal places and the speed of a storm's onset. In Hawaii, the high velocities of hurricane winds often produce wave heights higher than the maximum level of the prevailing high tide or of the surge itself. In studying the aftermath of Hurricane Iniki it was discovered by researchers at the Army Corps of Engineers and the University of Hawaii (Fletcher and others, 1994)that the greatest threat related to hurricane overwash in the Hawaiian Islands is due to water-level rise from wave forces rather than wind forces. This differs from the mainland where the wind in a hurricane is known to drive water against the coast and cause flooding, called wind set-up. During Iniki, the strongest component of the overwash was the result of large waves, called wave set-up. Wind set-up appeared to be relatively less important. Other factors leading to coastal overwash are the low atmospheric pressure, the tide stage, coastal topography, and the location relative to the eye of the hurricane. Overwash mitigation must be enacted prior to the event. This would include adequate building setbacks so that development does not occur in high hazard areas of the coastal zone, elevation of existing structures to recommended levels, break-way ground floor walls that permit overwash flooding without compromising an entire structure, and other coastal construction techniques designed to reduce flood damage. FEMA has incorporated hurricane inundation from model scenarios into a flood insurance study for all islands including Hawaii, as discussed in the section below. 5.1.1.1 Hurricane Flood Insurance Study for the Hawaiian Islands The Hurricane Flood Insurance Study for the Hawaiian Islands was conducted under FEMA contract number EMW-2003-CO-0046, RMTC/URS Task Order 013. Under this contract, RMTC/URS, a joint venture consisting of R.M. Towill, URS, Dewberry, TerraPoint, Airborne 1, and Sea Engineering, was tasked to evaluate and map the magnitude and extent of coastal hazards due to hurricanes for six Hawaiian Islands, divided into four counties: Kauai (Kauai County), Oahu (City and County of Honolulu), Molokai, Maui, Lanai (Maui County), and Hawaii (Hawaii County). Although the effective (i.e., past) FIRMs for each county have accounted for tsunami hazards, the hurricane flood hazard had not been previously separately evaluated in a comprehensive study throughout the islands. In general, the hurricane coastal hazard analysis was limited to the southern coast of each island. This is due to several factors, including the predominance of tsunami hazards and limited low-lying areas susceptible to hurricane hazards along the north shore of the islands. Table 5-1 provides a summary of study limits. 5-2 Hawaii County Multi-Hazard Mitigation Plan