Determination of Rate of Entrainment and Deposition Velocity 



In our study, the values of E and v^j are determined from the laboratory 

 flume studies. During the initial period of a laboratory experiment, only 

 entrainment is present and little deposition takes place. Hence the rate of 

 entrainment can be determined from the rate of increase of suspended sediment 

 in the water column as: 



t-0 ^^ 





E = lim ;^ I Cdz (7.4) 



where -h is the water depth of the flume. As time increases, deposition also 

 increases until eventually an equilibrium is reached between the entrainment 

 and the deposition at the bottom of the water column, i.e., 



E = Vd C,q (7.5) 



Since E and Cg„ are both known, v^ can thus be computed. 



During most of our fresh water experiments, v^j was formed to be generally 

 on the order of 0.005 cm/sec with no distinct trend of variation, v^j for 

 salt-water experiments has been found to be on the order of 0.01 cm/sec. 

 These values appear to be smaller than the median gravitational settling 

 velocity as shown in Figure 6.2. Several factors could contribute to this 

 discrepancy. First of all, the gravitational settling velocities in 

 Figure 6.2 are measured in a settling cylinder with little turbulent mixing. 

 The actual particle size distribution during the flume experiment could be 

 quite different from that in the settling velocity measurement. However, no 

 measurements on particle size or settling velocity distribution were made 

 during the flume experiment. In addition, our analysis assumes the rate of 

 entrainment does not vary with time during the entrainment process. In 

 reality, however, the initial stage of entrainment usually leads to a large 

 gradient in sediment concentration near the bottom. This causes an unstable 

 density gradient and hence a smaller bottom shear stress and a smaller rate of 

 entrainment. Towards the end of the experiment, when entrainment balances 



169 



