bottom sediments. The sediment becomes harder to erode at higher 

 salinity, with most variation occurring between and 15 °/oo. The 

 sediment also becomes harder to erode as more time is allowed for the 

 preparation of the bed. From the laboratory flume data, proper bottom 

 boundary conditions for the sediment transport model can be derived in 

 terms of the rate of resuspension (E) and deposition (V^jC) 

 (Sheng, 1981). The bottom boundary condition for the sediment 

 concentration (C) equation can be written as: 



3C 



Net Upward Flux = w^C - K^ -^ = E - V^jC 



dz 



(22) 



where the deposition velocity Vh > while the settling velocity 

 Ws < 0. 



Sediment Movement in the Mississippi Sound Due to a Westerly Wind 



As an example to illustrate the important role of resuspension and 

 deposition, we performed a 1-day simulation of sediment movement due to 

 a Westerly wind. 



Initially, the background concentration is assumed to be zero 

 everywhere except within a square area (shown in Figure 5) where the 

 concentration is 500 mg/1 (newly introduced sediment). The sediment 

 concentration is then computed with three different bottom boundary 

 conditions: (1) zero net flux and zero settling speed, (2) with 

 deposition and resuspension, but no resuspension of old sediment (vs. the 

 newly introduced sediment) is allowed, and (3) deposition and 

 resuspension allowed at all locations. For (2) and (3), a settling 



C0NCENTRflTI0N RT TIME 0F 24.0 H0URS AND DEPTH 0F 0.5 M 



Figure 12. Suspended Sediment Concentration at 0.5 m Depth at the 

 End of 1-day Simulation. Westerly Wind; No Settling; 

 Zero Net Flux at Bottom. 



284 



Sheng 



