deposition, suspended sediment concentration does not vary significantly over 

 the water column. Hence the bottom shear stress and entrainment rate should 

 be higher than those in the initial stage. 



In a meso-scale sediment transport model, the deposition velocity v^j 

 should depend on the location of the first grid point above the bottom. The 

 inverse of the deposition velocity represents the sum of resistance through 

 the various layers (constant flux layer and sublayer) between that point and 

 the bottom. Laboratory experiments should be carried out to verify the 

 theoretical formulation on deposition velocity as described in Section 6.9. 



The rate of entrainment, E, however, exhibited distinct functional 

 dependence on several parameters. For example, in a fresh-water study, Sheng 

 and Lick (1979) found a bi-linear functional relationship between E and t^^ 

 (Figure 7.1). Using this relationship for their sediment transport model, 

 they were able to achieve reasonable agreement between model prediction and 

 synoptic data obtained during an episodic event in Lake Erie. 



The dependence of equilibrium concentration of the Mississippi Sound 

 sediments on various parameters, as shown in the last chapter, can be 

 transformed into relationships for the entrainment rates versus the various 

 parameters. For example, for the site-3 sediment with a 1-day settling time, 

 the dependence of E on t^ is shown in Figure 7.2(a). For site-1 sediment with 

 3-day settling time, E versus tj^ is plotted in Figure 7.2(b). Although the 

 critical shear stress is of similar magnitude in both cases, E is typically an 

 order of magnitude smaller for the longer settling experiment. The effect of 

 settling time on the entrainment rate is summarized in Figure 7.3(a). Thus, 

 to allow accurate estimation of the entrainment rate in a coastal environment, 

 it is extremely important to have a knowledge of the time history of the 

 bottom sediments. Another parameter which can cause more than an order of 

 magnitude variation in E is the salinity. This is illustrated in 

 Figure 7.3(b). 



170 



