transport. In specific examples using hindcast and measured wave data, they 

 showed that in a certain case for the Atlantic coast of the United States as 

 much as 86 percent of the waves could be considered as effectively calm, 

 eliminating the necessity for performing the shoreline change calculation at 

 the particular time step in which they appear in the time series. 



202. The procedure is applied by scanning the wave time series and 

 propagating waves to breaking by assuming plane and parallel bottom contours. 

 A modified time series of deepwater wave conditions is then developed in which 

 waves not satisfying the threshold criterion described below are made to 

 indicate a calm condition, accomplished by either setting the value of the 

 wave height to zero or the wave period to -999. In reading such a value, 

 GENESIS will move to the next wave condition if there are multiple waves per 

 time step or to the next time step, not executing the transport rate calcul- 

 ation and, possibly, not performing the shoreline change calculation if there 

 are no effective waves in the given time step. 



203. The cutoff for effective longshore sand transport is given as 



H b X b V =3.9 (m 3 /sec) (40) 



where 



X b = width of the surf zone (distance between shoreline and breaker 

 line) 



V = mean speed of the longshore current 



For the purpose here, using X b =» D b /tan^ and Equation 14 (H b = 71^), the 



width of the surf zone can be expressed as X b = H b /(7tan^) . For V , Komar 



and Inman (1970) empirically found that V = 1. 35(H b /2) (7g/H b ) 1/2 sin20 bs for 



the situation of the longshore current generated by obliquely incident waves. 



Substitution of these expressions for X b and V into Equation 40 gives a 



formula that can be used with a simple wave transformation program to test for 



noneffective longshore transport conditions: 



H b ^sin2* bs - liM) JUI^HL (41) 



1.35 g 1 ' 2 



95 



