The net rate is a vector sum of transport rates and is the quantity needed to 

 determine whether a section of coast will erode or accrete. The rates Q 

 used by GENESIS to compute shoreline change through differences in transport 

 rates alongshore are net rates. 



Effective threshold for transport 



199. Inspection of Equation 2 for the longshore sand transport rate 

 shows that the first and dominant term has a dependence on breaking wave 

 height and direction as 



Q <v (H b ) 5/2 sin20 b8 (39) 



since the wave group speed at breaking is C gb ^ (H b ) 1/2 . Consider two break- 

 ing waves, one with height of 1 m and the other of 0.1 m, which have the same 

 angle at breaking. By Equation 39, the 1-m wave will have a transport rate 

 300 times greater than the 0.1-m high wave. For the same wave period and 

 deepwater direction, a higher deepwater wave will break at a larger angle, 

 also increasing the disparity in magnitudes of transport rates associated with 

 high/low waves and large/small deepwater wave angles. 



200. A coast open to the ocean will experience a range of wave condi- 

 tions from completely calm to stormy. Because of the great amplification of 

 the longshore transport rate through the wave height and, to a lesser extent, 

 wave angle, it is reasonable to apply a cutoff or threshold to eliminate from 

 the times series wave conditions that have negligible transport rates and are 

 not significant factors contributing to shoreline change. 



201. Empirical evidence for an effective threshold of longshore sand 

 transport was found by Kraus and Dean (1987), later revised by Kraus , 



Ginger ich, and Rosati (1988), based on sand trap measurements in the field for 

 a sand of nominal grain diameter of 0.2 mm. Komar (1988) made a comprehensive 

 study on the physical controls on the longshore sediment transport rate and 

 concluded there is no empirical evidence that the rate depends on the grain 

 size for typical beach sands. This result implies that the criterion found by 

 Kraus, Gingerich, and Rosati should apply to any sandy coast. Kraus, Hanson, 

 and Larson (1988) developed a method for applying this threshold to eliminate 

 in an objective manner wave events that would produce negligible longshore 



94 



