value of the exact energy flux for sinusoidal wave heights with a Rayleigh 

 distribution. Since this means that P^g is proportional to energy flux 

 and not equal to it, P^s is referred to as the longshore energy flux 

 factor in Table 4-8 and the following sections. 



Longshore energy flux in this general case (P^) is given by equa- 

 tion 4-27. This is an exact equation for the longshore component of 

 energy flux in a single small -amplitude, periodic wave. This equation 

 is good for any specified depth, but because the wave refracts, P^ will 

 have different values as the wave moves into shallower water. The value 

 of Pj, in Equation 4-27 can be manipulated through use of small -an^Jlitude 

 wave theory to obtain the four equivalent formulas for Pjj, shown in Table 

 4-7. 



In order to use Pj, for longshore transport computations in the surf 

 zone, it is necessary to approximate P^ for conditions at the breaker 

 position. These approximations are shown as P^g in Table 4-8, evaluated 

 in foot-pound per second units. The bases for these approximations are 

 shown in Table 4-9. Measurements show that the longshore transport rate 

 depends on P^g. (See Figures 4-36 and 4-37.) 



As implied by the definition of Pusy the energy flux factors in 

 Figures 4-36 and 4-37 are based on significant wave heights. The plotted 

 P£g values were obtained in the following manner. For the field data of 

 Watts (1953) and Caldwell (1956) , the original references give energy flux 

 factors based on significant height, and these original data (after unit 

 conversion) are plotted as Pj^g in Figures 4-36 and 4-37. Similarly, the 

 one field point of Moore and Cole (1960), as adopted by Saville (1962), 

 is assumed to be based on significant height. (See Figure 4-37.) Finally, 

 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 4-36 and 4-37. 



For laboratory conditions (Fig. 4-36 only), waves of constant height 

 are assumed. When these heights are used in the equations of Table 4-8, 

 the result is an approximation of the exact longshore energy flux. In 

 order to plot the laboratory data in terms of an energy-flux factor con- 

 sistent with the plotted field data, this energy flux is multiplied by 2 

 before plotting in Figure 4-36. 



For the purpose of this section, it is assumed that the shoaling co- 

 efficient. Kg, for nearshore breaking waves is equivalent to the breaker 

 height index, H^^/Ho', found from observation. (See Figure 2-65.) 



The choice of equations to determine P^g depends on the data avail- 

 able. The right hand columns of Tables 4-7 and 4-8 tabulate the data 

 required to use each of the formulas. An example using the second P^g 

 formula is given in Section 4.533. 



Possible changes in wave height due to energy losses as waves travel 

 over the Continental Shelf are not considered in these equations. Such 



4-96 



