III. DETERMINATION OF LONGSHORE ENERGY FLUX FACTOR 



The following equation is equivalent to equation (4-28) in the SPM when 

 calculating the longshore energy flux factor, 



_ Pg H2,W V^gQ Cf 



V 2 J \VoJlh 



where 



p = fluid density 



g = acceleration of gravity 



H^j = breaking wave height 



W = width of surf zone 



^LEO ~ average longshore current due to breaking waves 



Cf = friction factor (assume 0.01) 

 and 



where X is the distance to dye patch from shoreline and (V/Vo)/^/^ is the 

 Longuet-Higgins dimensionless longshore current velocity for an assumed mixing 

 coefficient, P = 0.4, which agrees reasonably well with laboratory data (see 

 Longuet-Higgins, 1970). The derivation of equation (1) is presented in the 

 Appendix, as well as reference to equation (2). 



It should be noted that as previous calculation equations for P^g are 

 based on significant wave heights (e.g., Ch. 4 in the SPM) equation (1) should 

 also use significant wave height for breaking wave height. The recorded value 

 of Hj, in the LEO observation program is a reasonable approximation to signif- 

 icant breaking wave height. It should also be noted that as the LEO current 

 observations are time-averaged, computing P£g by the present method may pro- 

 vide a lower value of the longshore energy flux factor than given by equations 

 based on significant breaking wave height to higher powers such as those in 

 Chapter 4 of the SPM. 



IV. EXAMPLE PROBLEM 



GIVEN : A LEO observation with the following measured values of wave height, 

 longshore current velocity, width of surf zone, and distance of dye patch 

 from the shoreline 



Hj, = 3.0 feet (0.91 meter) 



^LEO - 0-5 foot (0.15 meter) per second 



W = 150 feet (45.7 meters) 



X =50 feet (15.2 meters) 



