the bigger floes. However, the bigger floes are influenced by the weaker 

 shearing rate associated with the larger eddies. The maximum attainable floe 

 size depends on the type of cohesive sediments, the turbulence field, and the 

 physico-chemical properties of fluid. 



We have laid out the steps for the development of a comprehensive model 

 describing the coagulation dynamics of cohesive sediments. However, the 

 complete development of such a model requires further studies and is beyond 

 the scope of the present work. 



6.5 Settling Speed of Sediments 



In general, the settling speeds of a group of sediment particles depend 



on (1) particle Reynolds number, (2) distance of particles above the bottom, 



(3) turbulence, (4) salinity, and (5) particle concentration. Effects of 



these parameters are described in the following. 



For an individual particle settling in water, the terminal settling speed 

 can be determined from a balance between the drag force and the gravitational 

 force acting on the particle. At low particle Reynolds number, the drag force 

 can be described by the Stokes law and the terminal settling speed is 

 proportional to the square of particle radius as shown in Table 6.1. High 

 Reynolds number effect needs to be included for non-Stokesian sediment 

 particles. At very high Reynolds number, the drag coefficient asymptotes to a 

 constant value and the terminal settling speed becomes proportional to the 

 square root of the particle radius. 



As a sediment particle settles through the water column and approaches 

 within a few particle radii from the bottom, the drag force acting on the 

 particle increases and the terminal settling speed is gradually reduced from 

 its Stokesian settling speed. For smaller particles a few microns or less in 

 radius, impaction becomes the major mechanism bringing particles to the 

 surface. This will be discussed later in the section on deposition. 



As indicated in Figure 6.2, it is not always meaningful to assign a 

 single settling speed to a group of cohesive sediment particles collected from 

 the natural environment. Results for sediments from Site-2 and Site-4 are 

 shown in Figure 6.3. To obtain sediment speed distribution resembling that in 



133 



