Longshore transport rates are usually given in units of volume per 

 time (cubic yards per year in the U.S.). Typical rates for oceanfront 

 beaches range from 10^ to 10^ cubic yards per year. (See Table 4-6.) 

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

 cent solids. 



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 modifictions 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, sur- 

 veys, and dredging records are primary sources). 



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. (Das, 1972.) 



4. A recently developed empirical method (Galvin, 1972) is avail- 

 able to estimate gross longshore transport rate from mean annual near- 

 shore 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 judgement 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, I-tethod 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. Section 4.532 

 utilizes Methods 3 and 4. Methods 1 and 2 are discussed in Section 4.8. 



4.532 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 flux in the surf zone is approxi- 

 mated by assuming conservation of energy flux in shoaling waves, using 

 small -amplitude theory, and then evaluating the energy flux relation at 

 the breaker position. 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 perpendicular to the direction of wave advance is (from 

 Section 2.238, combining Equations 2-39 and 2-40): 



P = EC = ^ H2 C . (4-25) 



4-89 



