(c) Slope of the foreshore tends to decrease with increasing wave 

 height, again with scatter. 



(d) For design of beach profiles on ocean or gulf beaches, use 

 Figure 4-32, keeping in mind the large scatter in the basic data on Fig- 

 ure 4-33, much of which is caused by the need to adjust the data to 

 account for differences in nearshore wave climate. 



4.53 LONGSHORE TRANSPORT RATE 



4.531 Definitions and Methods . Littoral drift is the sediment (usually 

 sand) moved in the littoral zone imder action of waves and currents. The 

 rate, Q, at which littoral drift is moved parallel to the shoreline is 

 the longshore transport rate. Since this movement is parallel to the 

 shoreline, there are two possible directions of motion, right and left, 

 relative to an observer standing on the shore looking out to sea. Move- 

 ment from the observer's right to his left is motion toward the left, in- 

 dicated by the subscript It. Movement toward the observer's right is 

 indicated by the subscript rt. 



Gross longshore transport rate, Qg, is the sum of the amounts of 

 littoral drift transported to the right and to the left, past a point on 

 the shoreline in a given time period. 



Qg = Qrt + Qfit ' (4-21) 



Similarly, net longshore transport rate, Q^, is defined as the 

 difference between the amounts of littoral drift transported to the right 

 and to the left past a point on the shoreline in a given time period. 



%=%t- Q£. • C4-22) 



The quantities 0^.^, Q£t> On ^^id Q^ have engineering uses: for 

 example, Qg is used to predict shoaling rates in uncontrolled inlets; 

 Qj^ is used for design of protected inlets and for predicting beach ero- 

 sion on an open coast; Q^,^ and Q^^ are used for design of jetties and 

 impoundment basins behind weir jetties. In addition, Q^ provides an 

 upper limit on other quantities. 



Occasionally, the ratio 



7 = — , (4-23) 



^1 



rt 



is known, rather than the separate values Q^^ and Qj,^. Then Q^ is 

 related to Q^ in terms of y by ^ 



(1 +7) 

 Q„ = Q„ ^^ — ■ (4-24) 



This equation is not very useful when y approaches 1. 



4-88 



