where 



P = wave power in foot-pounds per foot of beach per tg 



t = time interval equal to 1 month 



Y = water density at 64 pounds per cubic foot in saltwater 



C = wave group velocity in feet per second in shallow water, where 



the acceleration of gravity is 32.2 feet per second squared, and 

 water depth is 18 feet 



H = wave height in feet 



To compute wave approach angle, using the sector method (Fig. 17), the 

 observed approach angles were assumed to be normally distributed within each 

 of five sectors. Thus, the frequency distribution of wave approach angle in 

 sector 2 was assumed to be identical to that in sector 4 even though the total 

 number of observations in sector 2 was larger. If a skewed distribution were 

 used, based on the number of observations, the net longshore transport rate to 

 the south would be larger than that calculated. About 58 percent of all obser- 

 vations were in sector 3, within 5° of shore normal. 



The longshore component of wave power, P^, was computed using equation 

 (4-27) in the Shore Protection Manual (SPM) (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1977, p. 4-91): 



P^ = P cos a sin a (3) 



in which a = angle between a line normal to the shore and the wave orthogonal 

 at the breakpoint. They were obtained using the relation (U.S. Army, Corps of 

 Engineers, Coastal Engineering Research Center, 1977, p. 4-100) 



Q = (7.5 X 103) P^ (4) 



in which Q = longshore transport rate. The resulting north and south longshore 

 transport rates (the monthly gross and net rates) are shown in Figure 63. 

 Absolute values at Sea Isle City (profile line 14, where wave angle measurements 

 were made) in thousands of cubic yards per year, are: 



North South Gross Net 



357 786 1,143 429 south 



These longshore transport rates are considerably different from those 

 previously reported for Ludlam Beach. Caldwell (1966) noted a rate of 200,000 

 cubic yards per year to the south at the Cold Spring Inlet jetty (20 miles south 

 of Ludlam Beach) and a rate of 100,000 cubic yards per year at the Absecon Inlet 

 jetty (20 miles north of Ludlam Beach). He indicated that the inlets act as 

 traps for sandy material moving along the coast and the amount of material 

 stored in the inlets becomes a constant quantity as the inlet reaches a stable 

 cross-sectional area. He believed that "excessive" sand trapping was occurring 

 as evidenced by the large floodtide shoals and by the fact that losses from the 

 shore were considerably in excess of the measured longshore transport rates. 



75 



