Multi-Hazard Mitigation Plan: 12. High Surf

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Chapter 12:Hazard Analysis—High Surf by wave forces (wave set-up). If the surge occurs at high tide, water height is even greater. The water rise enables the storm waves to reach further inland with the associated scouring and erosion caused by the wave forces. In studying the aftermath of Hurricane Iniki, the Corps of Engineers determined that wave set-up was the stronger component than wind set- up,unlike the mainland coastal areas.66 12.2 Significant Historic Events Hurricane Estelle illustrates an example of Hawaii County's susceptibility to wind and wave setup, as well as storm events. On July 22, 1986, the eye of Estelle passed over 100 miles south of Hawaii County. In addition to the high spring tide, high waves generated from Estelle, crashed on the shores of the Big Island. From available reports regarding Hurricane Estelle, major damage in Hawaii occurred at the Vacationland area. The high waves washed away 5 beachfront homes and severely damaged dozens of others. According to records at the Hawaii County Planning Department, 18 houses suffered minor damage that totaled $42,500. In addition, 12 houses had major damage that totaled $194,000 and 7 houses were completely destroyed with an estimated property damage of $160,883. 12.3 Probability of Occurrence High surf events occur quite frequently on all coasts of Hawaii County; however events which actually cause damage to property are far less common. The FIRM coastal zone flood classifications give probabilities of coastal flooding in zones that may be impacted by episodes of high surf as shown in Table 12-1. Although the coastal flood zones were not developed exclusively to address the impacts high surf (development of these zones is discussed in Chapters 9 and 10) they do provide a conservative delineation of areas that may be at risk. Table 12-1. FIRM Coastal flood zone classifications ZONE DESCRIPTION Coastal areas with a 1%or greater chance of flooding and an additional hazard associated with storm F V waves.These areas have a 26%chance of flooding over the life of a 30-year mortgage.No base flood elevations are shown within these zones. Coastal areas with a 1%or greater chance of flooding and an additional hazard associated with storm VE,VI -30 waves.These areas have a 26%chance of flooding over the life of a 30-year mortgage.Base flood elevations derived from detailed analyses are shown at selected intervals within these zones. 66 Fletcher,C,B.Richmond,E.Grossman,A.Gibbs,Atlas of Natural Hazards in the Hawaiian Coastal Zone, Prepared in cooperation with University of Hawaii,State of Hawaii Office of Planning,and National Oceanic and Atmospheric Administration,U.S.Geological Service Geologic Investigations Series I-2761, 2002.,p. 17. 12-2 Hawaii County Multi-Hazard Mitigation Plan