was computed based on the 20-year hindcast and the maximum wave height per 

 storm event. Wave duration was defined as the period in hours that the wave 

 height exceeded a height equal to the 20-year average height plus one 

 standard deviation. 



Storm Surge 



23. A number of studies of storm effects have identified the storm 

 surge (defined as the increase in water level caused by the storm) as the 

 primary force causing beach erosion and coastal flooding (Richardson 1977; 

 Vellinga 1983, 1986; Dean 1976; Hughes and Chiu 1981). The height of the 

 storm surge results from a combination of variables including the 

 astronomical tide, wind stress, atmospheric pressure effects, earth's 

 rotation, wave setup, and bottom hydrography ( Shore Protection Manual (SPM) 

 1984). In addition to causing severe flooding, the storm surge allows waves 

 to attack upper portions of the beach which are not usually subject to wave 

 attack. It is also an important factor in dune breaching and overwash, 

 especially on low-lying beaches. 



24. This report presents actual water level measurements from gages at 

 Boston, Montauk Point, Sandy Hook, and Atlantic City (Figure 1). These gages 

 provide hourly measurements of the water level through each of the storms 

 studied. Although these gages provide an accurate measure of the water level 

 at each gage site, they provide only a relative measure of the actual water 

 level occurring on the study beaches. This results from different gage 

 locations and depths, sheltering, wave setup, and wave runup. Besides 

 quantifying the storm surge height, the water level time-history also allows 

 a comparison of the surge level and the storm duration between sites 

 (allowing for variation between gage locations). A sample water level time- 

 history plot is shown in Figure 6. Unlike the wave plots, the water level 

 time-histories show only a few days before and after the storm. 



25. For each storm and tide gage the surge height and the peak water 

 level were computed. The peak water level is the highest water level, 

 relative to msl, recorded during the storm. The surge height is the 

 difference between the actual and predicted water level at the time of peak 

 water level. The surge-return period of each storm was also estimated using 

 curves given by Ebersole (1982) for each tide gage. 



16 



