6. TRANSPORT, ENTRAINMENT AND DEPOSITION OF COHESIVE SEDIMENTS 



6.1 Transport Modes of Cohesive Sediments 



The dispersion of cohesive sediments in a coastal environment is affected 

 by a variety of mechanisms, as shown in Figure 6.1. After entering into the 

 water column from rivers or bottoms, the movement of sediments is influenced 

 by the various transport modes including convection, turbulent mixing, and 

 gravitational settling. Exchange between the suspended sediments and the 

 bottom sediments is governed by the entrainment and deposition modes. 



The transport of cohesive sediments in the water column depends on the 

 properties of cohesive sediments, the physico-chemical properties of the 

 fluid, and the turbulence and the mean currents of the flow field. In 

 general, a particle size distribution exists at any given point in the water 

 column. Due to the dynamic nature of the flow field, this distribution is 

 usually a function of time. In the following, the general nature of this 

 particle size distribution will be first described. The various parameters 

 that may affect this distribution will then be discussed. 



6.2 Particle Size Distribution 



In fresh water, cohesive sediments possess a relatively flat particle 



size distribution. This is illustrated in Figure 6.2 based on measurement of 



settling speed of cohesive sediments from the Mississippi Sound. Sediment 



particles can be approximately divided into six groups, each with a different 



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 median settling speed ranging from about 10 cm/sec to about 1 cm/sec. 



Assuming Stokes flow, these correspond to a relatively even distribution of 



particle sizes, ranging from about 4.3 urn to about 135 Mm. It is apparent 



that, even in fresh water, due to coagulation and other causes, sediments 



often exist as floes which are bigger than individual clay particles. To 



adequately describe the sediment dynamics, a mass conservation equation would 



have to be solved for each of the major groups of particles. However, 



relatively weak interaction exists among the various particle groups. 



As the salinity of the water is increased, due to the increasing cohesion 

 of particles, coagulation of particles becomes increasingly important. The 

 particle size distribution in Figure 6.2 shows that, at 30 ppt salinity, about 



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