time step. On the basis of this information, GENESIS calculates Q L at each 

 time step, automatically accounting for the placement of beach fills, skipping 

 over wave data for calm events, and performing other "book-keeping" tasks that 

 depend on the time step in combination with the number of wave sources. Each 

 wave source increases computation time of the modeling system. 



Derived transport rates 



196. In shoreline change modeling, it is convenient to analyze long- 

 shore sand transport rates and shoreline change from the perspective of an 

 observer standing on the beach looking toward the water. Two directions of 

 transport can then be defined (SPM 1984, Chapter 4) as left moving, denoted by 

 the subscripts it , and right moving, denoted by the subscripts rt . The 

 corresponding rates Q^ t and Q rt do not have a sign associated with them, 

 i.e., they are intrinsically positive; information on transport direction or 

 sign is contained in the subscripts. Use of these two rates is convenient for 

 two reasons: first, the terminology is independent of the orientation of the 

 coast and, therefore, provides uniformity and ready understanding independent 

 of the coast; second, the awkwardness of dealing with the sign is eliminated. 

 Two other very useful rates entering in engineering applications can be 

 defined in terms of these basic quantities, the gross transport rate and the 

 net transport rate. 



197. The gross transport rate Q g is defined as the sum of the trans- 

 port to the right and to the left past a point (for example, grid cell i) on 

 the shoreline in a given time period. 



Qg - Qrt + Qit (37) 



A navigation channel at a harbor or inlet and a catch basin adjacent to a 

 jetty will trap sand arriving from either the left or the right. This 

 quantity is estimated by computing the gross transport rate. 



198. The net transport rate Q n is the difference between the right- 

 and left-moving transport past a point on the shoreline in a given time 

 period. It is defined as 



Qn = Qrt " Qit (38) 



93 



