1.01 



7 p — I- 



3 4 



1 1 I I I I ■ ■ 1 1 



RETURN PERIOD, YEARS 

 5 10 20 



-i — i | i i i i| \— 



T I | I I I l| 



• • • • 



y 



j* 



1 U L 



I i 



_L_i_ 



0.3 0.5 0.7 0.8 0.9 0.95 0.98 0.99 0.995 



FREQUENCY OF OCCURRENCE 



Figure 20. Yearly storm surge return period for extratropical 

 storms at Hampton Roads, Va. (after Ebersole 1982) 



density function. Figure 20 illustrates the yearly return period for extra- 

 tropical storms which produce the given storm surge elevations. 



48. Hayden (1975) studied secular variations in storm occurrence. In a 

 hindcast study of extratropical storms (i.e., with waves greater than 1.6 m) , 

 he found that storm occurrences were at a maximum in March between 1942 and 

 1960, but that the maximum had moved to January by 1974. The mean annual 

 number of storms with waves over 1.6 m did not vary significantly (the annual 

 average was about 34); however, Hayden (1975, p 982) found the number of 

 events in which the waves exceeded 2.5 m had increased 1.9 times between the 

 1942-1965 period and the 1965-1974 period. Hayden notes that the increased 

 frequency of large stormwaves is consistent with observed trends in shoreline 

 erosion. 



49. Hurricanes generally move from southwest to northeast in the study 

 area (Ho and Tracey 1975), with an increase in the frequency of hurricanes 

 from Cape Henry to Cape Hatteras (Figure 21). Simpson and Riehl (1981, p 109, 

 292) show below-average frequencies predominated from 1895 to 1930; in 1931 



37 



