6.6 Entrainment of Cohesive Sediments 



Entrainment of cohesive sediments occurs when flow-induced shear stress 

 at the sediment-water interface exceeds the inter-particle or inter-floc 

 cohesive force of the surfacial sediments. Suspended sediment particles in 

 the water column exist as floes of various sizes depending on the 

 hydrodynamic, chemical, and biological conditions during the entrainment 

 process. Entrainment occurs primarily as a surface phenomenon. The 

 hydrodynamic process within the relatively thin bottom boundary layer plays a 

 predominant role in causing entrainment of sediments. Of course, the 

 entrainment process also depends on properties of sediment and properties of 

 interstitial and overlying water. Various sediment properties such as water 

 content, organic content, and mineralogical composition have been found to 

 appreciably affect the rate of entrainment. 



To obtain quantitatively accurate estimate on entrainment of cohesive 

 sediments, the detailed flow dynamics within the bottom boundary layer needs 

 to be analyzed first. Next, the entrainment rates for a variety of sediments 

 under various hydrodynamic, chemical, and biological conditions need to be 

 determined. Although these two problems are dynamically coupled, a 

 comprehensive quantitative analysis is extremely difficult to carry out. As a 

 first step, they have to be treated separately. Over the shallow coastal 

 waters, interaction of the wave-induced orbital current and the slowly-varying 

 tide- or wind-driven current generally takes place within the bottom boundary 

 layer. Following a discussion on the bottom boundary layer dynamics, 

 laboratory studies on determining the entrainment rate as a function of 

 several important parameters will be presented. 



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