Rvt

Upper deep groove ball bearing Outer hollow drive shaft Upper main tapered roller bearing Inner rotating solid shaft Lower main tapered roller bearing Slip ring assembly Lower deep groove

^ LDj ^ ball bearing

FIGURE 8.2 A sectional view of the rotating circular flume used in the experimental study.

and the instrument was operated in its flow-through mode. The size distribution of the sediment flocs were monitored at regular intervals of time. The complete details of the flume and the instruments can be found in Krishnappan.17

8.3.2 Deposition Tests

Deposition tests were carried out by placing the pond water and a known amount of sediment in the flume and operating the flume at the maximum speed to mix the sediment thoroughly. The amount of sediment added was enough to produce a fully mixed concentration of about 200 mg/l. The flume and the top cover were operated at the maximum speed for about 20 min, and then the speed was lowered to the desired shear stress level. The flume was then operated at this level for about 5 h. During this time, both suspended sediment concentration and the size distribution of sediment in suspension were monitored at regular intervals of time.

Figure 8.3 shows the variation of suspended sediment concentration as a function of time for five different bed shear stress conditions. From this figure, we can see that after the initial 20-min mixing period, the concentration decreases gradually and tends to reach a steady state value for all the shear stresses tested. The steady state concentration is a function of the bed shear stress. From such data, it is possible to calculate the amount of sediment that would deposit under a particular bed shear stress under a steady flow condition.

The size distribution data measured during the deposition experiments are summarized in Figure 8.4. In this figure, the median sizes of the distributions are plotted as a function of time for three of the five deposition tests. For the lowest bed shear stress

Time (min)

FIGURE 8.3 Concentration vs. time curves for different shear stresses during deposition.

Time (min)

FIGURE 8.3 Concentration vs. time curves for different shear stresses during deposition.

Time (min)

FIGURE 8.4 Median-size variations as a function of time for different bed shear stresses.

Time (min)

FIGURE 8.4 Median-size variations as a function of time for different bed shear stresses.

test (0.056 N/m2) the median size of the sediment decreases gradually suggesting that larger particles are settling out leaving the finer fractions in suspension, in a manner analogous to the settling of discrete particles. When the bed shear stress is low such as in this case, the particles were undergoing settling without particle interaction and flocculation. On the other hand, when the bed shear stress was increased to 0.121 N/m2, there was a clear evidence of flocculation as can be inferred from the median size variation shown in Figure 8.4 for this shear stress. From this curve, we can see that the distributions were becoming progressively coarser starting from a median size of 30 ixm to a final steady state size of about 55 ^m. As the bed shear stress was further increased, the floc sizes decreased as shown by the curve corresponding to the bed shear stress of 0.213 N/m2. At this shear stress, the increased turbulence has limited the floc growth and hence the maximum size of the floc formed was only about 45 ixm.

The size distribution data shown in Figure 8.4 have demonstrated that the sediment from the stormwater detention pond undergoes flocculation when subjected to the flow field in the rotating circular flume. For formulating the flocculation model, Krishnappan and Marsalek15 selected three tests among the five deposition tests and the concentration and the size distribution data collected from these three tests were used to calibrate and verify the model.

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