Storm changes within the groin system were lower than the island average 

 (Fig. 32), In general, storm losses in the indentations north and south of 

 Sea Isle City were two to three times greater than those in the groins, sug- 

 gesting the groins were effective in reducing storm loss. In the southern 40 

 percent of the groin system, however, the net yearly losses were large, probably 

 because much of the sand lost from the beach was not replaced by sand from 

 either offshore or updrift. 



b. Coastal Processes . The response of the beach to various coastal 

 processes is fairly well documented for the region above MSL. The actual 

 mechanics of initial sediment movement, sediment transport, and deposition 

 which causes the changes, however, are poorly understood. Longshore and onshore- 

 offshore sediment transport data obtained in this study provide some information 

 to assist in inferring where, when, and how much sediment moves. 



Sediment movement is predominantly from north to south along Ludlam Beach. 

 However, the sediment moving in the longshore transport system appeared to be 

 deflected seaward by the groins, and returned to the subaerial beach consider- 

 ably downcoast , causing a deficiency in available sediment supply south of Sea 

 Isle City. In the northern section of the groins and updrift of the groins 

 an accretional fillet formed which slightly changed the configuration of the 

 coast . 



The slight accretion north of the groins was partially caused by sediment 

 moving south from the eroding indentation toward Strathmere. However, most of 

 it was probably associated with the large volume of sediment moving south in 

 the longshore transport system of the southern coast of New Jersey (about 

 400,000 cubic yards per year at Sea Isle City, Fig. 63). The material was 

 trapped updrift of the groins which caused the configuration of the coast to 

 prograde seaward in a very subtle fillet shape. 



Groins at Sea Isle City may have their greatest effect on the downdrift 

 coast by deflecting seaward the sand which is moving in an essentially parallel- 

 to-shore direction. With the predominant wave-induced and south-directed long- 

 shore current on Ludlam Beach, the sediment is carried some distance downdrift 

 before it is returned to the beach. Also, because it is carried to deeper water 

 at a greater distance from shore, it will require a longer period to be trans- 

 ported to the subaerial beach than will the sediment moved offshore elsewhere 

 along the island. 



It appears that the centroid of sediment deflected seaward by the groins 

 (Fig. 39) returned 7,000 feet south of profile line 13. The groins, thus, 

 produced a downdrift "shadow zone" where less than the normal amount of sediment 

 moved offshore was returned. As the net longshore component of sediment trans- 

 port decreases, the shadow zone is expected to become shorter. The amount of 

 sediment deflected seaward could probably be minimized if the seaward ends of 

 the groins were submerged (Vallianos, U.S. Army Engineer District, Wilmington, 

 N.C., personal communication, 1974). This would decrease the channeling effect 

 and still trap sand moving parallel to shore. Currents channeled seaward 

 apparently disrupted the submarine bar system off the groins. They also 

 appeared to deflect it seaward (Figs. 47 and 48). 



Seasonal accretion and erosion near the groins varied from the island 

 average as a direct result of seasonal changes in the direction and magnitude 



92 



