surf zone and the immersed weight of sand moved. Both have units of force per 

 unit time 



KP 



(4) 



where Ij^ is the immersed weight transport rate (force/time) , K a dimension- 

 less coefficient, and ?£ the longshore component of wave energy flux (force/ 

 time). ?£ is defined as 



Pp = ^ h2c^ sin 2a 



16 



g 



(5) 



where H is the wave height, C„ the wave group velocity, and a the angle 



the wave crest makes with the shoreline. If the breaker values of the wave 



characteristics (Hj, C^^, o.j^') and the significant wave height (H^) are put 



into equation (5), the energy flux factor results 



2.S 



^ \\iuC„v, sin 2a 



16 



^sb 



■b 



(6) 



The significant wave height is the average height of the one-third highest waves 

 in a given wave condition, (See Section 3.21 of the SPM.) 



The enpirical relationship between longshore transport rate and P„ is 

 based on field measurements. Since the immersed weight of sand moved in the 

 field cannot be measured directly, the volume of sand moved is usually deter- 



mined. Therefore, Q 

 tion (3) to produce 



is substituted for 1^ in equation (4) , by using equa- 



Cp, 



K 



P) ga' 



Is 



(7) 



The SPM plots field data points of 

 produce the empirical relation 



Is 



as shown in Figure 2 to 



yd^ 

 yr 



= 7500 



ydVs 



Ib/yr 



ft-lb 



ft/s 



(8) 



where the dimensions of the factors are given in brackets. Note that the con- 

 stant (7500) is dimensional. Using this dimensional constant and the values in 

 Table 1, K in equation (7) is found to be 0.39. The scatter of the data points 

 in Figure 2 shows that the value of Q estimated from equation (8) is accurate 

 to only ±50 percent. This can be seen by drawing a line 50 percent higher and 

 a line 50 percent lower than the design curve in Figure 2. These two lines form 

 an envelope of the data points. 



Table 2 presents equations (9) to (12), which are alternate forms of Pjj,s. 

 The choice of one of these equations depends on the data available. For example, 

 if breaker wave height and direction are known, equation (9) should be used; if 

 deepwater wave height and direction are known, equation (10) should be used. 

 As a general rule, the closer the data have been collected to the surf zone, 

 the better the data are for estimating P^g- Therefore, if both deepwater and 

 breaker values of wave height and direction are available, with comparable accu- 

 racy, the latter should be used. Figures 3 and 4 plot values of Q for differ- 

 ent input data combinations based on equations (9) and (10). 



