(Figure 6.8(b)). It should be pointed out that, even at the high 

 concentration of 27000 mg/l , only a small fraction of the bottom sediments are 

 eroded from the bottom. In fact, throughout our studies, we found that 

 entrainment of sediments is very much a surface phenomenon that takes place at 

 the sediment water interface. Consequently, unless exceedingly high bottom 

 stress takes place, bulk physical properties of even 1 cm below the initial 

 sediment-water interface may be largely irrelevant to most entrainment events. 



Effect of Bottom Shear Stress 



Similar experiments have been performed for sediments from other sites 

 with different settling time of sediments allowed. The effect of the bottom 

 shear stress on entrainment for all of our flume runs is summarized in Figures 

 6.9(a) and 6.9(b) for fresh water conditions and salt water conditions, 

 respectively. 



The critical shear stress for the Mississippi Sound sediment is on the 



2 



order of 0.8 dyne/cm . As the bottom shear stress is increased from 1 to 



2 



5 dyne/cm , more than an order of magnitude increase in the equilibrium 

 concentration can be expected. Under normal range of applied shear stress, 

 the entrainment of cohesive sediment is generally not dependent on the bulk 

 properties of the soil such as bulk shear strength or plastic strength. 

 Rather, the resistance to entrainment depends on the inter-floc strength of 

 sediments as modified by the physico-chemical and biological parameters within 

 the benthic boundary layer. At extremely high shear stress exceeding the bulk 

 shear strength of the sediment, the entire bed may fail and become entrained. 



Effect of Bed Properties 



One of the important parameters in characterizing the properties of the 

 surfacial sediments is the water content (or moisture content, or porosity) 

 which is directly related to the bulk density of the bed. Attempts by some 

 previous investigators to correlate the shear strength of the bed with the 

 bulk density have not been satisfactory (Parchure, 1980). 



In our studies, the bed is primarily prepared by allowing the sediment 

 slurry to settle within the flume for a time period between one to ten days. 

 Previous studies of Lake Erie sediments indicated that with longer settling 



151 



