studies. 



Our study confirmed that different clay minerals behave rather 

 differently to changing environments. Alizadeh (1974) claimed that, under 

 flocculated conditions, smectite shows higher critical shear stress than 

 illite and kaolinite. Under dispersed conditions, however, kaolinite and 

 illite are more resistant to entrainment than is smectite. 



6.9 Deposition of Sediments 



Deposition is the process by which suspended sediment particles leave the 

 water column, temporarily or permanently, to become part of the bottom 

 sediments. Similar process occurs in the atmosphere where atmospheric 

 pollutants are deposited to the ground. Lewellen and Sheng (1980) performea a 

 theoretical study on dry deposition of gaseous and particulate species in the 

 atmosphere. Lewellen, Varma and Sheng (1972) analyzed the deposition model 

 sensitivity. Although deposition of sediment particles is an important 

 process affecting the ultimate fate of many pollutants, a comprehensive 

 theoretical study has not yet been performed due to the complexity of the 

 problem. 



Deposition can be described in terms of a deposition velocity, v^j, 

 defined as the flux of sediment particles divided by the suspended sediment 

 concentration within the water column. Physically it is equivalent to the 

 covariance of the turbulent fluctuations of the vertical velocity and the 

 fluctuations of the particle concentration divided by the mean concentration. 

 Since a concentration gradient is generally necessary to deliver a flux to the 

 bottom, Vj is necessarily a function of the height at which the normalizing 

 concentration is measured. Being a turbulent transport quantity, the 

 deposition velocity depends on all of the hydrodynamic parameters affecting 

 flow within the bottom boundary layer. It also depends on the particle 

 coagulation process as affected by turbulence and cohesion. In addition, it 

 depends on parameters such as detailed bottom roughness features, bottom 

 chemistry, and benthic community which affect the bottom boundary condition. 



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