Equations 95 and 96 were used in the present numerical study for calculating 

 longshore transport beyond the surf zone. 

 Erosion and deposition 



68. In this report we are considering only the transport of noncohesive 

 sand sediments. These sediments have relatively large fall velocities and 

 thus remain in suspension (as suspended or bed load) for only short periods of 

 time unless a forcing function is continually in effect. Thus these sediments 

 act differently from cohesive sediments that may move considerable distances 

 before returning to the ocean floor. For cohesive sediments, the concentra- 

 tion of sediment at a particular location may depend upon more than just the 

 local forcing function. It may also depend upon the concentrations at other 

 locations and flow patterns. For example, large wave activity at one location 

 can produce a large concentration of cohesive sediments. These sediments can 

 then be carried to an area of low wave activity and still remain in suspension 

 for a long period of time so that the concentration is high in an area of low 

 wave activity. 



69. In the case of noncohesive sediments, if a parcel of water with a 

 high concentration of noncohesive sediment (e.g., as a result of large wave 

 activity) enters a region of low wave activity, the material drops rapidly out 

 of suspension, and a much smaller quantity of sediment is resuspended. Thus 

 in any given grid .cell (with dimensions larger than the typical horizontal 

 distances that noncohesive sediments travel when they drop out of suspension) , 

 the quantity of sediment being transported is a function of the wave and 

 current activity within the cell. If adjacent cells supply sediment to the 

 cell in greater quantities than the wave and current activity in the cell can 

 support, then part of the sediment deposits in the cell and only the sediment 

 that can be transported by the local waves and currents within the cell are 

 transported through the cell. Conversely, if the cell can transport greater 

 quantities of material than is supplied by adjacent cells, then erosion must 

 occur within the cell as bottom material supplies the additional required 

 material. For example, if an offshore breakwater is built, sediment will 

 deposit in its lee where wave activity is lower than in adjacent areas. If 

 something increases wave convergence in an area, erosion will occur. 



70. Since the rate of noncohesive sediment transport within a cell is 

 dependent only upon wave and current conditions within the cell, transport can 

 be modeled using a "box model." In this model, the transport in and out of 



53 



