observations of the increase in suspended sediment concentration are then 

 made. To minimize the spin-up effect, the constant shear stress is achieved 

 by slowly increasing the rotation rate of the ring. When the applied shear 

 stress is greater than the critical shear stress, entrainment of the sediments 

 occurs. Suspended sediment concentration is measured at 5 min intervals until 

 it reaches a steady-state or equilibrium value. The applied shear stress is 

 then gradually increased to a higher value, and the increase in suspended 

 sediment concentration is monitored as before. Ideally, this procedure could 

 continue to higher and higher shear stresses until all the sediments are 

 resuspended. In this study we have generally started with an applied shear 



2 



stress of 1 dyne/cm until a steady state is reached. The shear stress is 



2 



then increased to 3 dyne/cm until another equilibrium concentration is 



2 



arrived. Finally, the shear stress is increased to 5 dyne/cm . These shear 

 stresses are within the range of realistic shear stresses in the Mississippi 

 Sound, as is evidenced by the results of our hydrodynamic model computations 

 in previous sections. 



In previous study by us and most other investigators, only one shear 

 stress is applied in a single flume experiment. The present procedure of 

 incrementing the shear stress in a single flume run results in significant 

 saving of time and more realistic representation of the erosional events in 

 nature. In a real estuarine or coastal environment, it is rare that the 

 bottom shear stress would stay at the same value for an extended time period. 

 Recently, a similar procedure was used by Parchure (1980) in studying the 

 critical shear stress of kaolinite as a function of bed depth. 



Typical Results 



A typical variation of the suspended sediment concentration with time 

 after the shear stress is applied is shown in Figure 6.8(a). Sediments from 

 Site 1 of the Mississippi Sound were allowed to settle in the flume for one 



2 



day before the shear stress was applied. At 1 dyne/cm , sufficient sediments 



are resuspended and reached an equilibrium concentration (€„„) in less than 



two hours. As the shear stress was increased to 3 dyne/cm , C^^ increased 



from 530 mg/1 to 14000 mg/l . CpQ reached 27000 mg/1 at 5 dyne/cm . The 



^ 2 



equilibrium concentration at 1 dyne/cm is smaller than that (3000 mg/1) for 



the shale based sediments in Table 6.2 which has a higher clay content 



149 



