survey and the storm. Note that only those profiles not affected by a 

 "seawall" were reported. 



Although the data in Table 4-6 are not exactly comparable, they suggest 

 that the average volumes of sand eroded from above MSL for beaches 8 or more 

 kilometers (5 miles) long have a limited range of values. A moderate storm 

 may remove 10 to 25 cubic meters per meter of beach front above MSL (4 to 10 

 cubic yards per foot); an extreme storm (or a moderate storm that persists for 

 a long time) may remove 25 to 50 cubic meters per meter (10 to 20 cubic yards 

 per foot); rare storms that are most erosive due to a combination of 

 intensity, duration, and orientation may remove 50 to 125 cubic meters per 

 meter (20 to 50 cubic yards per foot). For comparison, a berm 30 meters (100 

 feet wide), 3 meters (10 feet) above MSL contains 90 cubic meters per meter of 

 beach front (37 cubic yards per foot), a quantity that would be adequate 

 except for extreme storms. 



In terms of horizontal changes a moderate storm can erode a typical beach 

 20 to 30 meters (75 to 100 feet) or more (Table 4-6) and leave it exposed to 

 greater erosion if a second storm follows before the beach has recovered. 

 This possibility should be considered in design and placement of beach fills 

 and other protective measures. 



Extreme values of erosion may be more useful for design than mean 

 values. Column 13 of Table 4-6 suggests that the ratio of the most eroded 

 profile (above MSL) to the median profile for each coast beaches ranges from 

 about 1.5 to 6.6. 



Although the dominant result of storms on the portion of a beach above MSL 

 is erosion, most poststorm surveys show that storms produce local accretion as 

 well. Of the 90 profiles from Cape Cod, Massachusetts, to Cape May, New 

 Jersey, surveyed immediately after the December 1970 storm, 18 showed net 

 accretion above mean sea level. Accretion can also result during over wash 

 when waves transport sand inland from the beach (Leatherman et al., 1977). 

 Survey data from a number of storms also indicate that the shoreline may move 

 seaward during a storm. This suggests movement of sand from higher to lower 

 elevations, but not necessarily offshore. DeWall et al. (1977) reported that 

 of the 89 profiles surveyed after the 17 December 1970 storm (Table 4-6) 52 

 percent shov«d seaward movement of the shoreline. Similar findings have been 

 shown by Birkemeier (1979) and Chiu (1977). 



Though above MSL changes are of greatest interest to the engineer, they 

 occur over only a small part of the active profile. Figure 4-32 illustrates 

 the types of offshore changes that can occur. The figure shows the response 

 of a profile line located 500 meters (1700 feet) south of CERC's Field 

 Research Facility in Duck, North Carolina. The four storms which occurred 

 during the period caused the bar to move offshore a total distance of 172 

 meters (564 feet). Though the first three storms had negligible effect on the 

 above MSL beach while causing considerable nearshore movement, only the fourth 

 storm, vAiich coincided with a high spring tide and which produced the highest 

 waves, caused the beach to erode. 



(2) Beach Recovery . The typical beach profile left by a severe storm 

 is a simple, concave-upward curve extending seaward to low tide level or 

 below. The sand that has been eroded from the beach is deposited mostly as a 



4-80 



