study can be obtained by measuring wave conditions at the shoreline locality 

 for at least 1 year. In many cases a visual observation program can also 

 provide useful data. A study of year-to-year variation in vave height 

 statistics collected at CERC wave gages (Thompson and Harris, 1972) indicates 

 that six observations per day for 1 year gives a reliable vave height 

 distribution function to the 1 percent level of occurrence. Even one observa- 

 tion a day for 1 year appears to provide a useful height-distribution function 

 for exposed ocean sites. 



5. Effect of Extreme Events. 



Infrequent events of great magnitude, such as hurricanes, cause signifi- 

 cant modification of the littoral zone, particularly to the profile of a 

 beach. An extreme event could be defined as an event, great in terms of total 

 energy expended or work done, that is not expected to occur at a particular 

 location more than once every 50 to 100 years on the average. Hurricane 

 Camille in 1969 and the Great East Coast Storm of March 1962 can be considered 

 extreme events. Because large storms are infrequent, and because it does not 

 necessarily follow that the magnitude of a storm determines the amount of 

 geomorphic change, the relative importance of extreme events is difficult to 

 establish. 



Wolman and Miller (1960) suggested that the equilibrium profile of a beach 

 is more related to moderately strong winds that generate moderate storm waves, 

 rather than to winds that accompany infrequent catastrophic events. Saville 

 (1950) shoved that for laboratory tests with constant wave energy and angle of 

 attack there is a particular critical wave steepness at which littoral 

 transport is a maximum. Under field conditions, there is probably a similar 

 critical value that produces transport out of proportion to its frequency of 

 occurrence. The winds associated with this critical wave steepness may be 

 winds generated by smaller storms, rather than the winds associated with 

 extreme events. 



The effect of an extreme event is determined by a complex combination of 

 many variables. Table 4-5, after Kana (1977), identifies 13 variables which 

 are qualitatively evaluated according to significance. Included in the table 

 are storm, beach, and water level factors. 



Most storms move large amounts of sand from the beach to offshore; but 



after the storm, the lower waves that follow tend to restore this sand to the 



face of the beach. Depending on the extent of restoration, the storm may 

 produce little permanent change. 



While rapid recovery has been documented (Birkemeier, 1979; Kana, 1977; 

 Sonu, 1972), extreme storms may result in a net movement and loss of material 

 to the offshore as the profile rapidly adjusts to a slow rise in sea level 

 following a period of few major storms (Dean, 1976). Severe storms may also 

 drive sand either far offshore, into depths deeper than can be recovered by 

 normal wave action, or landward, overwashing the beach and moving sand 

 inland. Both processes can result in a net loss of material from the littoral 

 zone. 



4-43 



