usually detectable long after they are formed. V-zone limits should ignore 

 the existing dune (use the maximum surge level) in areas of historic^ overwash 

 unless the shoreline has been significantly altered through the construction 

 of a substantial duneline or high seawall. 



116. Although long-term erosion is not of primary concern during a 

 100-year event, its importance to storm-related losses cannot be discounted. 

 Quite simply, areas prone to long-term erosion will sustain greater damage 

 during the 100-year event and during every lesser event that precedes it. 

 Moreover, the protection offered by a substantial duneline may not survive 

 until the 100-year storm. 



117. Long-term erosion rates are usually based on measurements of the 

 shoreline position inferred from aerial photographs (or occasionally based on 

 actual surveys) taken 10 to 40 years apart. Usually, only a few measurements 

 through time are available, so the rates do not reflect rapid seasonal or 

 storm changes. They do provide an indication of how one section of shoreline 

 changes relative to another. Unfortunately, inclusion of a long-term erosion 

 rate requires specification of a time span. Predicted shoreline positions 

 might be computed, for example, based on a 30-year mortgage life or a 70-year 

 structure life. Whereas arguments can be made for both, use of a particular 

 time span may be unrealistic in determining V-zones. Yet, some provision for 

 recognizing high erosion areas must be included. 



118. Although this study does not specifically address this problem, 

 it may be possible to indirectly include some measure of long-term erosion by 

 adjusting the value of the variability factor for known high erosion areas. 

 Because the variability factor is based on longshore variability during 

 storms, it is reasonable to assume that it also reflects long-term longshore 

 variations since they result from the integrated effect of many storms. 

 Therefore, use of a variability factor based on the extreme value (instead of 

 the 75 percent) could be used in high erosion areas which could be classified 

 into general erosion categories of low, medium, and high rates based on 

 long-term measurements. Dunes in high erosion zones would have to be more 

 substantial than those in stable or low eroding areas. Further research on 

 this or an alternative method and a thorough evaluation using field data are 

 required. 



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