OtherEffects on Bottom Shear Stress 



Bottom features and interaction between sediment particles and flow can 

 affect the bottom shear stress. The effect of relatively small bottom 

 features can be parameterized in terms of a roughness height Zq that appears 

 in the logarithmic relationship. When sizeable bottom features such as 

 vegetation canopy or dredged material mound formed at disposal site exist, the 

 total bottom stress includes contribution from the skin friction drag and, 

 more importantly, the profile drag, created by the pressure difference in the 

 flow direction on two sides of the bottom feature. Assuming the total shear 

 stress at a certain height above the bottom feature could be measured or 

 computed, only a fraction of this total stress contributes to the skin 

 friction drag at the bed to cause entrainment of sediment. The partitioning 

 between skin friction drag and profile drag depends on the detailed structure 

 of the bottom feature and its interaction with the bottom flow. This problem 

 has been investigated by us with a dynamic turbulence model (Appendix D). 



The presence of sediment particles within the bottom boundary layer can 

 also alter the bottom shear stress. At relatively high concentrations, the 

 vertical distribution of sediment particles generally leads to a stable 

 density distribution, thus lowering the drag coefficient in the quadratic 

 stress law. In addition, the sediment particles may lead to damping of 

 turbulence. Turbulent eddies within the bottom boundary layer have smaller 

 length and time scales. Hence, the sediment particles generally do not follow 

 the eddy motion, thus resulting in the damping of turbulence. Both of these 

 effects have not been well studied but can be investigated by further 

 extending the dynamic turbulence model presented in Appendix D. 



6.8 Laboratory Studies on Sediment Entrainment 



Our Study 



Due to the complexity of the problem, a comprehensive theoretical model 

 for the entrainment of cohesive sediment is not available at this time. 

 Instead, for a given type of sediment from a given site, laboratory studies 

 have to be performed to investigate the dependence of entrainment on various 

 parameters. We have performed entrainment studies in an annular flume, which 



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