3. Effects of the Sea Isle City Groins . 



Groins at Sea Isle City affect the coast north (updrift) and south (down- 

 drift), as well as within the groin system. This occurs because the groins 

 modify longshore and onshore -offshore movements of sand. 



a. Beach Behavior . . Groins at Sea Isle City are sited within and adjacent 

 to a bulge in the coastline (Fig. 21). The bulge, which is not centered 

 symmetrically over the groin system, appears to be positioned slightly north 

 of the central groin. Beaches north and south of the bulge were erosional 

 between 1962 and 1972 (Fig. 39). The southern one-third of the bulge was 

 erosional while its northern two-thirds was stable or accretional. 



Survey data on beach conditions before groin construction are not available. 

 However, more than 100 years of shoreline position data from charts (Fig. 27) 

 indicate the coastal reach where the groins exist today has fluctuated in 

 position with an intermittent bulge. Historically, the region north of the 

 groins was erosional (Fig. 39), although probably not as highly erosional as 

 it was from 1962 to 1972. South o-f the groin region the coast was only slightly 

 erosional before groin construction. From 1962 to 1972 downdrift beaches expe- 

 rienced intense erosion, perhaps the result of the groin system. 



Between 1962 and 1972 the bulge had an alongcoast length of 9,000 feet (Fig. 

 39) . Its accretional part was asymmetrical with an accretion maximum at the 

 northernmost groin. From there a slight net yearly accretion occurred 4,500 

 feet to the north. Significant accretion was measured 4,500 feet to the south 

 within the groin system. Ninety percent by volume of the accretion above MSL 

 occurred within the groin system. In the southern 40 percent of the groin sys- 

 tem the beach was highly erosional. In total, there was a 10,000-foot erosional 

 reach south of the accretional bulge (Fig. 39) . The erosional indentation was 

 asymmetrical with the highest net yearly loss measured at the south end of the 

 groins . 



Within the groin system the total yearly accretion in the northern 3,700 

 feet was twice the total yearly loss in the southern 2,100 feet. In the 9,000- 

 foot-long accretional bulge the net yearly gain of sand above MSL was 10,500 

 cubic yards while 13,500 cubic yards was lost in the 10,000-foot-long erosional 

 indentation to the south. The net loss to the beach in the northern bulge and 

 southern indentation was, therefore, 3,000 cubic yards per year or -0.16 cubic 

 yard per foot-year. This loss is 14 percent of the average sand loss for the 

 entire island. The actual section of beach affected by the groins is unknown. 

 It seems reasonable,' though, that the bulge and indentation were, at least in 

 part, the result of changes in coastal processes and sediment availability at 

 and adjacent to the groin system. 



Seasonal changes in sediment volume above MSL in the coastal bulge were not 

 in phase with those in the indentation or on the rest of the island (Fig. 35). 

 From 6,000 feet north of the groins (profile line 8) to profile line 13 in the 

 groin system, the yearly volume minimum occurred later in the year than the 

 island average in May (see Fig. 33). The volume maximum also occurred later 

 in the year (October) . Seasonal voliime changes in the indentation south of 

 the bulge were similar to the island average. 



The sand volume change from the yearly maximum to the yearly minimum was 

 least within the groin system (Fig. 36), averaging 12 versus 18 cubic yards 



90 



