As shown in the figure, the beach is stable when the yearly range is about 

 12 cubic yards per foot or less. There may be a yearly net loss of about 0.2 

 cubic yards per foot for each cubic yard per foot of yearly range above 12 

 cubic yards per foot. Thus, where the seasonal onshore -offshore exchange 

 shown in Figure 36 exceeds 12 cubic yards per foot, there is a net loss of 

 about 20 percent of the sand. This sand loss apparently results from movement 

 off the profile above MSL in the fall through spring, with a lower replacement 

 volume returning during the summer. 



The volume of sand in storage above MSL and the shape of the beach profile 

 appear to be significant factors in the amount of erosion or accretion occiiring 

 on a beach. For example, after a beach has been subject to low and moderate 

 wave conditions for a considerable period, such as in late summer, a berm forms, 

 the foreshore steepens, and the sand volume increases (Fig. 33). This volume 

 then serves as a source for the sand eroded during fall and winter storms. 

 The steeper storm-produced waves plane the berm off and create a very gradual 

 foreshore. The resulting profile shape is then closer to equilibrium with the 

 steep waves than the summer profile is. 



b. Submarine Bars . Submarine bars are important because they are sources 

 and later sinks in the seasonal movement of sand off and on the subaerial beach. 

 Submarine bars are also an important longshore transport path. In designing 

 structures to intercept alongshore sand movement, the presence of submarine 

 bars, their position relative to shore, and the volume moved along them must 

 be considered. Figures 47 to 51 show the following submarine bar conditions, 

 with respect to coastal processes : 



(a) Bars frequently began at the shore in the north and extended 

 do\vmcoast at a slight angle seaward of the coast. The cause of this 

 nonparallelism may have been a more rapid movement of the bars at their 

 northern ends as they migrated landward, or possibly the initial forma- 

 tion of the bars was closer to shore in the north. 



(b) Bars appeared to intercept the coast in specified regions 

 which include areas just doundrift of groin systems where ridge-and- 

 runnel systems are most common. 



(c) Bars are less pronounced off groin fields. 



(d) Bars tend to angle in a greater seaward direction near groin 

 fields than elsewhere. 



VI. IMPLICATIONS FOR COASTAL ENGINEERING 

 1. Beach Fill . 



Should a beach fill be planned on Ludlam Beach, the results of this study 

 would provide a useful background on the historical behavior of the beach. 

 Data are available on where erosion has occurred in the past and where it might 

 be expected in the future (Figs. 27, 29, and 39). The regions at the Corson 

 Inlet end of the island and in the indentations north and south of the Sea Isle 

 City groins are unstable. Under present conditions, artifically placed fill 

 material or possibly protective structures will be required to halt shoreline 

 retreat in these unstable areas (net -S.2 feet per year). 



63 



