This equation is valid for any consistent set of lonits. Table 1 lists commonly 

 assumed values for the parameters in equation (3). If better estimates of Pg, 

 p, and a' are knovvn for a specific site, they should be used in the equation. 

 A discussion of equation C3) is provided by Galvin (1979). 



Table 1. Values of parameters in equation (3). 



Term 



U.S. Customary^ 



Metric^ 



Ps 



5.14 slugs/ft 3 



2,650 kg/m3 



p (saltwater) 



1.99 slugs/ft3 



1,025 kg/m3 



p (freshwater) 



1.94 slugs/ft3 



1,000 kg/m^ 



a' 



0.6 



0.6 



g 



32.2 ft/s2 



9.81 m/s2 



^Q in cubic feet per second; Ij^ in pounds per second. 

 2q in cubic meters per second; I^ in newtons per second. 



III. fvETHODS FOR COMPUTING LONGSHORE TRANSPORT RATE 



The four methods described in the SPM for computing longshore transport are 

 listed below in order of decreasing preference: 



Method 1 . The best way to predict longshore transport at a site 

 is to adopt the best known rate from a nearby site, with necessary 

 modifications for local conditions (assuming the rate from a nearby 

 site is well established). 



Method 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 nearshore zone. 



Method 3 . If neither method 1 nor method 2 is practical, the 

 accepted practice is to use 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. 



Method 4 . Another empirical method is available which estimates 

 gross longshore transport rate from mean annual nearshore breaker 

 height. The gross rate, so obtained, can be used as an upper limit 

 on net longshore transport rate. 



The following steps should be taken in selecting a method, depending on the 

 data available. Figure 1 is a graphical representation of the decision process. 



Step 1 . Given a particular location where the longshore transport 

 rate is needed, determine if there already exists an estimate of the 

 rate for a nearby site (e.g., see SPM Table 4-6). If so, go to Section 

 111, 1 of this report and use method 1; if not, proceed to step 2. 



