where Cj^ was effectively equation (5) and C was equation (3) . Since 

 equation (5) probably overestimates wave speed (by about 7 percent) and 

 equation [3) underestimates it (by about 10 percent), the resulting breaker 

 angle probably should be multiplied by (1 - 0.10)/(1 + 0.07) or 0.84 to be 

 theoretically correct. 



The net result of the variable estimates of wave speed and breaker angle 

 is to suggest that equation (4) is a logical compromise, and this is what is 

 used in the SPM equations . 



IV. SUMMARY 



This report describes the energy flux method of estimating longshore 

 transport rate and provides detailed explanations of the three most frequently 

 asked questions about this method (see Apps . A, B, and C) . The following gen- 

 eral conclusions result from this study. 



1. Energy flux may be estimated by four separate methods, depending on 

 the available field data. The results show that the energy flux factor, P^s ' 

 is proportional to any one of the following groups of wave variables (App. A): 



(a) H^^^^ sin laj^ 



(c) H^ T sin ajy cos a^ 



(d) yj-j sin a^ 



2. The wave height used in these equations is the significant wave height 

 (App. B). 



3. Longshore transport rate, Q, is directly proportional to the energy 

 flux factor, P£g • The indicated proportionality constant is 7,500 for tradi- 

 tional units (Q in cubic yards per year and P^g in foot-pounds per second 

 per foot). Appendix C describes the data used to derive this constant. 



4. There is uncertainty in the proportionality constant because of varia- 

 tions in field data, in the equation for breaker speed, and in measurement of 

 breaker angle. An intuitive estimate of the uncertainty in the constant is 



± 40 percent. 



5. The energy flux method is expected to be improved with further 

 knowledge. 



12 



