beaches range from 100,000 to 250,000 cubic meters per year (see Table 4-7). 

 These volume rates typically include about 40 percent voids and 60 percent 

 solids. 



Another representation of longshore transport rate is the immersed v«ight 

 rate I which is given in units of force per unit time (such as pounds per 

 second or newtons per second) . The conversion from Q to I is 



where 



1^ = (Pg - P) ga'Q (4-35) 



p = mass density of sand 

 s 



p = mass density of water 



g = acceleration of gravity 



a' = volume solids/total volume (accounts for the sand porosity) 



This equation is valid for any consistent set of units. Table 4-8 lists 

 commonly assumed values for the parameters in equation (4-35) . If better 

 estimates of p , p , and a' are known for a specific site, they should be 

 used in equation (4-35). Further discussion of equation (4-35) is provided by 

 Galvin (1972b). 



At present, there are four basic methods to use for the prediction of 

 longshore transport rate 



(1). The best way to predict longshore transport at a site is to 

 adopt the best known rate from a nearby site, with modifications based on 

 local conditions. 



(2). If rates from nearby sites are unknown, the next best way to 

 predict transport rates at a site is to compute them from data showing 

 historical changes in the topography of the littoral zone (charts, 

 surveys, and dredging records are primary sources). 



Some indicators of the transport rate are the growth of a spit, 

 shoaling patterns and deposition rates at an inlet, and the growth of a 

 fillet adjacent to a jetty or groin. As an example, the longshore trans- 

 port rate across Cold Spring Inlet, New Jersey, was estimated based on 

 fillet growth next to the updrift jetty and surveys of the surrounding 

 area to account for the sand that was not impounded by the jetty (U.S. 

 Congress, 1953b). The rates of growth for Sandy Hook, New Jersey (U.S. 

 Army Engineer District, New York, 1954), and for Sheshalik Spit, Alaska 

 (Moore and Cole, 1960), were used to estimate longshore transport rate. 

 Bruno and Gable (1976) measured the deposition behind the offshore break- 

 water and adjacent to the updrift jetty at Channel Island Harbor, 

 California, to find the longshore transport rate. 



(3). If neither method 1 nor method 2 is practical, then it is 

 accepted practice to use either measured or calculated wave conditions to 

 compute a longshore component of "wave energy flux" which is related 

 through an empirical curve to longshore transport rate (Galvin and 

 Schweppe, 1980). 



4-90 



