Table 4-8. Values of parameters in equation 4-35 



Q in cubic meters per second; I in newtons per second. 

 Q in cubic feet per second; I in pounds per second. 



(4). An empirical method (Galvin, 1972b) is available to estimate 

 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. 



Method 1 depends largely on engineering judgment and local data. 

 Method 2 is an application of historical data, which gives usable answers 

 if the basic data are reliable and available at reasonable cost and the 

 interpretation is based on a thorough knowledge of the locality. By 

 choosing only a few representative wave conditions, method 3 can usually 

 supply an answer with less work than method 2, but with correspondingly 

 less certainty. Because calculation of wave statistics in method 3 

 follows an established routine, it is often easier to use than researching 

 the hydrographic records and computing the changes necessary for method 2. 

 Method 4 requires mean nearshore breaker height data. Sections V,3,b 

 through V,3,d utilize methods 3 and 4; methods 1 and 2 are discussed in 

 Section VIII. 



b. Energy Flux Method . Method 3 is based on the assumption that long- 

 shore transport rate Q depends on the longshore component of energy flux in 

 the surf zone. The longshore energy fluz in the surf zone is approximated by 

 assuming conservation of energy flux in shoaling waves, using small-amplitude 

 theory, and then evaluating the energy flux relation at the breaker posi- 

 tion. The energy flux per unit length of wave crest, or, equivalently, the 

 rate at which wave energy is transmitted across a plane of unit width perpen- 

 dicular to the direction of wave advance is (from Ch. 2, Sec. 11,3, combining 

 eqs. (2-39) and (2-40)): 



P = E C„ = -^ H^ C 



9 



8 



9 



If the wave crests make an angle, a with the shoreline, the energy flux in 

 the direction of wave advance per unit length of beaah is 



PgH^ 



P cos a = 



8 



C cos a 



4-92 



