this distance is approximately equal to the width of the surf zone, which 

 depends on the incident waves, principally the breaking wave height. 



118. In GENESIS, the sand bypassing algorithm requires a depth of 

 active longshore transport, which is directly related to the width of the surf 

 under the assumption that the profile is a monotonically increasing function 

 of distance offshore, as discussed in the next section. In Version 2 of 

 GENESIS, a quantity called "the depth of active longshore transport," D LT is 

 defined and set equal to the depth of breaking of the highest one -tenth waves 

 at the updrift side of the structure. Under standard assumptions, this depth 

 is related to the significant wave height H 1/3 used throughout GENESIS, by 



Dlt - -^Y- (H 1/3 ) b (4) 



where 



1.27 = conversion factor between one -tenth highest wave height and 

 significant wave height 



7 = breaker index, ratio of wave height to water depth at breaking 

 (H 1/3 ) b = significant wave height at breaking 

 If 7 = 0.78 is used in Equation 4, then D LT * 1.6(H 1/3 ) b . The depth defin- 

 ing the seaward extent of the zone of active longshore transport D LT is much 

 less than the depth of closure D c , except under extremely high waves. 



119. GENESIS uses another characteristic depth, termed the "maximum 

 depth of longshore transport" D LTo to calculate the average beach slope 

 tan/3 appearing in Equation 2. The quantity D LTo is calculated as 



H 

 = (2.3 - 10.9H o ) — (5) 



where 



H /L = wave steepness in deep water 



H — significant wave height in deep water 



L = wavelength in deep water 

 The deepwater wavelength is calculated from linear wave theory as 

 L = gT z /2f , in which g is the acceleration due to gravity, and T is the 



55 



