7.2 Sediment Dispersion Due to Tidal Currents 



To examine the effectiveness of tidal currents in causing entrainment and 

 transport of sediments in the Mississippi Sound, we have performed simulations 

 based on the computed tidal currents during 4 Sept. to 5 Sept. 1980 as 

 presented in Section 5.1. 



Initially, (72 hours after 1 Sept. 1980) the suspended sediment 

 concentration is assumed to be 500 mg/1 within two 3 Km square areas, one in 

 the middle of the Biloxi Channel and one in the middle of the Pascagoula 

 Channel, and zero everywhere else. Sediment concentrations are then computed 

 throughout a complete tidal cycle until the 96th hour. Four simulations are 

 performed and described in the following. 



The first simulation is concerned with the transport of a dissolved 

 species by tidal currents. The settling velocity w^ is zero and the bottom 

 boundary condition is that of zero net flux. The results at 24 hours later 

 are shown in Figure 7.4. While the peak concentrations (~ 440 mg/1) are still 

 located near their original positions, only a very small amount of the species 

 have been transported out of the Sound. Due to vertical mixing, little 

 difference exists between the species concentrations near the surface and near 

 the bottom. 



The second simulation is concerned with the transport of sediment 



particles with a settling velocity w^ of -0.05 cm/sec and a bottom boundary 



condition of zero flux. This idealized condition implies that there is a 



balance between entrainment and deposition of sediments at all locations and 



all times. Starting with the same initial condition as the first simulation, 



the results at 24 hours later are shown in Figure 7.5. Near the surface, the 



peak concentration has dropped to a little under 200 mg/1. Near the bottom, 



however, the peak concentration has increased to about 800 mg/1. 



Concentration distribution along the west-east transect in the Sound is shown 



in Figure 7.6. Appreciable variation in concentration exists in the vertical 



direction. A simple calculation indicates that the vertical settling time 



D/w,, where D is the depth, is on the order of 1.5 hours and is comparable to 



2 

 the vertical diffusion time D /Av. 



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