have currently almost disappeared from vertical sand accumulation. The 

 statistical significance of the long-term trends is given in Table 8. 



In summary, the locations with statistically significant long-term 

 trends that coincide with the 27-month trends of the VIMS-CERC study, 

 are the accretional trends at profile lines 14, 17, and 18. Profile 

 line 8 had a statistically significant long-term erosional trend, and 

 a statistically nonsignificant short-term accretional trend. 



4. Periodicity and Seasonality in Long-Term Trends . 



Shepard (1958) calls an erosional beach, a winter beach, and an 

 accretional beach, a summer beach because, in California, the damaging 

 waves are in the winter and the "accretional" waves in the summer. 

 Both the yearly beach cycles and long-term cycles (i.e., multiyear) 

 coincide with local climatic conditions. 



However, Shepard' s winter-summer concept of erosion and accretion 

 may not be directly applicable to southeast Virginia. Galvin and Hayes 

 (1969) state: 



"Development of winter profiles on beaches of the U.S. Atlantic 

 coast north of Delaware Bay, and on beaches of the California coast, 

 differs in a way that appears to depend on mean wave climates, and 

 seasonal changes in wave climates of the two regions. Eroded winter 

 profiles, typical of California, are less well developed and sometimes 

 absent on northern Atlantic beaches." 



Sonu (1966) also found "profiles resembling the accepted summer and 

 winter type barely several hundred meters apart on the same section of 

 beach," at Cape Hatteras, North Carolina. The seasonal (winter-summer) 

 differential in mean monthly wave heights are much greater for the west 

 coast of the United States than for the east coast. (SPM, Fig. 4-10, 

 U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1975). 



Frisch (1977) calculated the percent time of erosion and accretion 

 at each profile location from the slope of the profile volume change compu- 

 tations in Appendixes B and C (i.e., a time of erosion is defined as the 

 time interval when the profile volume curve has a negative slope, and 

 accretion as the time when the curve has a positive slope). The resulting 

 tables and graphs were then divided into calendar seasons, and the percent 

 of the total time per season that a profile was erosional was calculated. 



These data indicated that there is a seasonal cycle of beach changes 

 in southeast Virginia which is dominated by erosion in the fall (late 

 September through late December). This is followed by general accretion, 

 of widely varying amount and spatial distribution, throughout the rest 

 of the year. The percent time of erosion for the falls of 1969, 1970, 



63 



