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

 4-35, keeping in mind the large scatter in the basic data in Figure 4-36, 

 much of which is caused by the need to adjust the data to account for 

 differences in nearshore wave climate. 



3. Longshore Transport Rate . 



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

 sand) moved in the littoral zone under 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 to left, relative to an 

 observer standing on the shore looking out to sea. Movement from the 

 observer's right to his left is motion toward the left, indicated by the 

 subscript £t ; movement toward the observer's right is indicated by the 

 subscript rt . 



Gross longshore transport rate, q^ , 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. 



%- 



<rt 



+ Q 



Jit 



(4-29) 



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: 



Qn = Qrt - Q 



It 



(4-30) 



The quantities Q^^ , Q^ , Q^ , and Qg have engineering uses: for 

 example, Q^ is used to predict shoaling rates in uncontrolled inlets. (^ 

 is used for design of protected inlets and for predicting beach erosion on an 

 open coast; Q^^ and Q . are used for design of jetties and impoundment 

 basins behind weir jetties. In addition Qg provides an upper limit on other 

 quantities. 



Occasionally, the ratio 



Y = 



^ 

 U 



(4-31) 



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

 related to Q^ in terms of y by 



Then 



"k 



IS 



^g ^n (1 - Y) 

 This equation is not very useful when y approaches 1. 



(4-32) 



Longshore transport rates are usually given in units of volume per time 

 (cubic meters per year in the United States). Typical rates for oceanfront 



4-89 



