In the third simulation, the sediment particles are assumed to have a 



settling velocity of -0.05 cm/sec and are allowed to be deposited and 



entrained at the bottom. However, entrainment is only allowed for the newly 



deposited sediments but not for the sediments which are at the bottom 



initially. This condition represents the limiting case when climate over the 



coastal environment has been rather calm over an extended time period such 



that the bottom sediments become highly resistable to entrainment. For the 



newly deposited sediments, the entrainment relationship as described in 



Figure 7.2 applies. A deposition velocity of 0.01 cm/sec is used. Starting 



with the same initial condition as the previous two simulations, most of the 



initial suspended sediments have settled out of the water column at 24 hours 



later. The peak concentration near the bottom is on the order of 10 mg/1 



(Figure 7.7) and is even smaller near the surface. Contours of thickness of 



deposited sediments at this time is shown in Figure 7.8. Maximum deposition 



of 0.15 cm has occurred. Little entrainment has occurred due to the 



relatively small bottom shear stresses generated by the tidal currents within 



the dumping areas. Bottom shear stresses at three locations in the Sound are 



shown in Figure 7.9. Although the stress at (I ,J)=(24,50) in the vicinity of 



2 

 the Biloxi Channel is on the order of 0.8 dyne/cm , it is not sufficient to 



cause entrainment of the newly deposited sediments. 



For the fourth simulation, entrainment and deposition are allowed for 

 both the newly deposited and the old sediments. The near-surface suspended 

 sediment concentration at the end of 24 hours of simulation is shown in 

 Figure 7.10. Maximum concentration is on the order of 1000 mg/1. The 

 near-bottom sediment concentration is even higher. Net entrainment up to 1 cm 

 has occurred in most of the tidal inlets (Figure 7.11) due to the high bottom 

 shear stresses in these areas. This explains the fact why sediments found 

 within the tidal inlets are generally composed of the coarser sand particles. 



The difference between the results of this simulation and those of the 

 previous simulation illustrates the significance of the time history of bottom 

 sediments in affecting the entrainment process, and hence the ultimate 

 suspended sediment concentration. After being left alone for an extended time 

 period, sediments dumped in the low bottom stress area can form a thick mound, 

 become extremely hard to entrain, and hence affect the local circulation 



178 



