The probability that a given section of coast will experience storm waves 

 depends on its ocean exposure, its location in relation to storm tracks, and 

 the shelf bathymetry. Using the Atlantic coast (characterized by Atlantic 

 City, New Jersey) as an example, the frequency of storm occurences (both 

 northeasters and hurricanes) can be studied. Though the effect of a storm 

 depends on the complex combination of variables, storm occurrence can be 

 examined simply by studying the frequency of periods of high waves. Figure 

 4-12 illustrates the variation in storm occurrence over a 20-year period, and 

 Figure 4-13 shows the seasonal variation, both for Atlantic City, New Jersey. 



The data used in Figure 4-12 and Figure 4-13 are hindcast significant wave 

 heights obtained from the Waterways Experiment Station Wave Information 

 Study. Note that surge and tide effects have not been included in the 

 hindcast. For the purpose of the two figures, a "storm" is defined as a 

 period during which the wave height exceeded a critical value equal to the sum 

 of the long-term average wave height plus one standard deviation (1.1 meters 

 or 3.6 feet) for Atlantic City. Though six different wave height groups are 

 shown, probably those producing peak wave heights less than 2.0 meters (6.6 

 feet) can be considered as insignificant. 



According to Figure 4-12, there is an average of 35 storms per year, 

 though the number varied from 22 to 42. Storms with waves greater than 4.0 

 meters (13.1 feet) occurred in only 9 of the 20 years of record (45 percent), 

 while those with waves greater than 4.5 meters (14.8 feet) occurred in only 3 

 years. 



Figure 4-13 dramatically shows the seasonal variation in storm occurrence 

 from a summer low of 5.5 percent of all storms in July to 10.7 percent in 

 November; 82 percent of all storms with wave heights greater than 2.5 meters 

 (8.2 feet) occur within 6 months of the year (November to April). Storm 

 frequencies for other east coast areas should be generally similar to those 

 shown for Atlantic City, but more frequent and more intense to the north and 

 less frequent to the south. 



Neumann et al . (1978) discuss the frequency of occurrence of tropical 

 storms and hurricanes along the Atlantic and gulf coasts. Figure 4-14 

 illustrates the annual variation in the number of hurricanes, which averages 

 4.9 per year. Figure 4-15 shows the seasonal variation in hurricane occur- 

 rence, with most of the storms occurring between August and October (note that 

 this is out of phase with the occurrence of winter northeasters as shown in 

 Figure 4-13). The probability of a hurricane reaching land varies widely 

 along the coast, as shown in Figure 4-16. 



3. Nearshore Wave Climate . 



Desirable wave climate data for the predition of littoral processes 

 include summaries of wave height, period, and direction just prior to breaking 

 for all major wave trains at a site of interest. Such data are rarely 

 available. Summaries of significant wave height and dominant wave period from 

 gage measurements with no identification of separate wave trains are becoming 

 increasingly available (e.g. Thompson, 1977; California Coastal Data 

 Collection Program, 1977-1981), but even this information is still lacking for 

 most localities. Wave direction measurements, which are especially difficult 

 to collect, are very rare. When data are available at one locality they may 

 not be applicable to nearby localities because of localized effects of bottom 

 topography. 



4-31 



