The energy flux of computing longshore transport rate is based on the 

 empirical relationship between the longshore component of wave energy flux 

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

 of force per unit time, thus 



h = ^^is 



(4-48) 



is the immersed weight transport rate (force/time), K a dimen- 



where I 



sionless 'coefficient, and P 



Q can be substituted for ^ 



K 



I. 



the longshore energy flux factor (force/time), 

 by using equation (4-33) to produce 



Q = 



(Pg - P) ga' 



Field measurements of 



and 



is 



Cs 



(4-49) 



are plotted in Figure 4-37. The data 

 For Watts (1953b) and Caldwell (1956), 



were obtained in the following manner, 

 the original references give energy flux factors based on significant height, 

 and these original data (after unit conversion) are plotted as P in Figure 

 4-37. The field data of Komar (1969) are given in terms of root-mean-square 

 energy flux. This energy flux is multiplied by a factor of 2 (Das, 1972), 

 converted to consistent units, and then plotted in Figure 4-37. 



A similar conversion was done for the Bruno et al. (1981) data. The equation 

 of the line drawn through the data points in Figure 4-37 defines the design 

 relation: 



Q^ - 1290 



3 ^ 



m -s 



is \ m-s 



(4-50a) 



Q P^ 

 dimensions of 



= 7500 



yd -s 



Ib-yr / Is \f t-s 



ft-lb 



(4-50b) 



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

 constants (1290 and 7500) are dimensional. Using these dimensional constants 

 and the values in Table 4-9, K in equation (4-49) is found to be 0.39. 

 Therefore equation (4-48) becomes 



h = °-3^ ^is 



where 0.39 is dimensionless. This equation is essentially the same as Komar 

 and Inman's (1970) design equation I = 0.77 P , with the factor of approx- 

 imately 2 difference due to Komar and Inman's use of H ^^ in the energy flux 



term instead of H_ as used herein. 



G 



rms 



Judgment is required in applying equation (4-49). Although the data 

 follow a definite trend, the scatter is obvious, even on the log-log plot. 

 The dotted lines on Figure 4-37 are drawn at Q ± 50 percent and envelope most 

 of the data points. Therefore, the accuracy of Q found using the energy 

 flux method can be estimated to be ± 50 percent. 



As an aid to computation. Figures 4-38 and 4-39 gives lines of constant 

 Q based on equation (4-49) and equations (4-43) and (4-44) for P given in 

 Table 4-10. To use Figures 4-38 and 4-39 to obtain the longshore transport 



4-96 



