of longshore transport (Fig. 63). This created the phase difference in cut 

 and fill above MSL near the groins (Fig. 35). In the northern part of the 

 groin system and north of the groins the beaches accreted most rapidly from 

 August to December, the period of greatest net transport south. This is also 

 the period when sediment was not available to the southern part of the groin 

 system and the downdrift beaches. From January to May the northern beaches 

 lost sand less rapidly than other areas of Ludlam Beach because longshore 

 transport was also to the south. Although net shore-normal transport was off- 

 shore, the material moving south compensated for the offshore losses. From 

 May through August, longshore transport was to the north. As shown in Figure 

 35, this resulted in net loss in the region immediately updrift of the groin 

 field and gains in the south. May through August is a period of net onshore 

 movement for the island (Fig. 33). < 



Storm losses above MSL were reduced within the groins when compared to the 

 rest of Ludlam Beach (Fig. 32). This probably occurred because the updrift 

 groin acted as a barrier to waves approaching shore at an acute angle while 

 the downdrift groin trapped sand above MSL. Seaward of the MSL shoreline, 

 however, significant erosion could occur, especially in the area where water 

 is deflected seaward. 



4. Sea Level Rise. 



Sea level rise, which may or may not continue in the future, is rapid 

 enough to influence the effectiveness of a shore structure during its project 

 life. The retreat of the shoreline caused by a rise in sea level is an apparent 

 one because no actual sand volume is lost. However, since structure effective- 

 ness and the magnitude of shore processes are water-depth dependent, the rise 

 is very important. Fir most practical purposes it should be considered in 

 coastal engineering. 



It is possible to determine the apparent loss of sediment from the active 

 profile as caused by sea level rise. According to Hicks (1972) the rise of 

 the water surface with respect to the adjacent land at Atlantic City is 0.015 

 foot per year (1920-70 period). A similar rate probably holds for Ludlam Beach. 

 Assume the shore-normal profile shape remains in equilibrium with wave- and 

 current -carried sediment out to some specified depth; i.e., the profile shape 

 will not vary, but will be translated landward 0.5 foot per year for a foreshore 

 slope of 0.03 and upward 0.015 foot per year, as shown in Figure 72. When the 

 effective seaward limit of the active profile remains at a constant depth, the 

 apparent sediment loss is approximately equal to one-half the depth of the 

 seaward limit times the change in shoreline position. 



An important difficulty in calculating the apparent sediment loss is in 

 determining the "effective" seaward limit of sand movement to and from the 

 beach. It has been suggested the limit exists at the boundary between the 

 shore-parallel bathymetric contours and the seaward contours that do not follow 

 the trend of the shore (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1977) . Although the Ludlam Beach region is irregular due to 

 linear shoals directed northeasterly, it appears that shore parallelism termi- 

 nates at or landward of the 40-foot contour. 



An additional, but complementary, method of finding the limit is to obtain 

 cross sections of the coast and check them for significant changes in shape 



93 



