Multi-Hazard Mitigation Plan: 13. Coastal Erosion

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Chapter 13:Hazard Analysis—Coastal Erosion likewise is linked to the magnitude and frequency of storms that impact on the coast. For example, during a 100 year high magnitude storm, waves may erode several meters into the foredune that sits well behind the normally active zone of accretion and erosion. Also, several lower-magnitude storms that occur in quick succession can produce a similar degree of erosion because the intervening periods are too short for constructive swell waves to push a significant amount of sediment back to the shoreline. The accretion of sand on beaches occurs during the more quiescent seasons when average swell waves deliver sediment back to the shoreline. Beach accretion, however, is generally a much slower process than beach erosion. For example, it may take several years for a beach to return to its pre-storm condition after one major storm or several smaller storms in quick succession. 13.4 Risk Assessment Since this is not a major hazard in Hawaii County,no method of assessing the economic risk due to coastal erosion has been utilized for this plan. Of greater concern than coastal erosion in Hawaii County is the potential for sea-cliff erosion. Homes perched upon these cliffs may be at risk if significant erosion does occur. There is no established recurrence interval for such events and early warning signs would allow ample time to mitigate any catastrophic failures. 13.5 Mitigation Strategies Below are several potential coastal mitigation measures that have been applied with varying degrees of success in other coastal regions. In the event that mitigation becomes necessary in Hawaii County the validity of each of these measures should be assessed in greater detail. 13.5.1 Revise the Shoreline Setbacks The intent of shoreline setbacks is to establish a coastal-hazard buffer zone to protect beach- front development from coastal erosion. Adequate setbacks allow the natural erosion and accretion cycles to occur and help maintain lateral beach access. Furthermore, setbacks provide open space for the enjoyment of the natural shoreline environment. A 40-foot shoreline setback has been often inadequate because they are not determined by historical site-specific rates of coastal erosion. Coastal-erosion hazard maps could be used to establish rate-based building setbacks. Setbacks would be site-specific in order to reflect the nature of coastal erosion. These setbacks would also incorporate the proposed style of development. Construction-style considerations would include the size and expected lifetime of the planned structure. Larger, immovable buildings and those with lifetimes of more than 50 years would have deeper setbacks than small,movable structures. 13-3 Hawaii County Multi-Hazard Mitigation Plan