for each synthetic hurricane generated for the determination of the 100-year 

 surge level; however, the hydrographs are not saved. Since for northern 

 states the surge and duration associated with a northeaster may be higher and 

 longer, respectively, these quantities must also be determined. 



112. Once the model has been used to compute an estimate of the volume 

 loss above the surge level, this estimate is multiplied by the variability 

 factor (e.g., 2.0 from Part I, or another representative value). If inade- 

 quate dunes exist to survive this loss, the back-dune area is allowed to 

 flood. 



113. Because the flooding will be caused by localized dune breaching 

 and overwash (unless the surge level erodes or overtops the dune crest), the 

 depth of flooding is difficult to predict. If the surge is of short dura- 

 tion, the flooding will occur as weir flow, and there will be a drop in water 

 level across the eroded dune. However, once the dune overwashes inland, the 

 maximum surge level should be assumed. Since the overwash process is poorly 

 documented and unquantif ied , a conservative and recommended approach is to 

 use the maximum flood level if the dune is flooded. 



114. Finally, the results should be evaluated (i.e., given a final 

 engineering review) for reasonability relative to historic data and results 

 for adjacent or similar areas. 



Historic Data and Long-Term Erosion 



115. Considering our basic lack of understanding of the beach erosion 

 process, the best and most direct estimate of erosion during a possible 

 future storm is made through use of measurements collected during similar 

 storms. This procedure is seldom practical. Either the data are of insuf- 

 ficient quality or, more frequently, the design storm has never occurred or 

 been measured. The large variability discussed in Part I further complicates 

 the situation. However, historic shoreline position data (surveys, air 

 photos, high-water marks, etc.) should always be used as a check of numerical 

 predictions. A good example of the potential use of historic data is in the 

 determination of overwash zones. In many areas, poststorm aerial photographs 

 have documented overwash penetration limits. Since many storms are less 

 severe than a 100-year event, these photographs provide a direct measure of 

 at least minimum overwash penetration distances. Overwash features are 



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