Effect of consecutive obstacles on density current frontal control in dam reservoirs

Dams reservoirs are among the most important sources of fresh water. In the event of a loss in the useful volume of the reservoirs, their capacity for water containment and storage decreases. The most important factor in reducing the useful volume of the reservoirs is sedimentation, and the most important factor of sediment transfer in reservoirs is density current. The density current is a stream that created by the effect of the difference in density between two fluids on the gravitational acceleration. Figure 1 demonstrates the density current movement. Fig1_Shematic of dense current (Altinakara et al. 1996)

Experiments were carried out at the Hydraulic Research Laboratory of Faculty of Water Science Engineering, Shahid Chamran University of Ahvaz, in a 780-mm-long, 70-cm-long flume with a slope of minus 1 to3.86%. The flume was divided into two sections by a sluice gate. The right side of this gate was filled with the clear water. The clear water level was maintained by the gate at the end of flume. On the left side, the solvent of water and salt is entered. This gate was able to be rapidly opened and discharge the water and salt solvent with a specific flow rate into the clear water. In the present study, four different modes of application of three successive obstacles were investigated for density current controlling. The height of the obstacles was considered by three ratios of density current body height as hr =0.5, 0.75, 1(hr = hm/h, where hm is the obstacle height and h is the average height of the body of the density current). In 3 modes of obstacles configuration, the height of the obstacles was identical and in the fourth state, three obstacles with similar dimensions were placed ascending. Experiments were performed at 3 slopes of 0, 1.5 and 2.5%, with two concentrations of 10 and 20 gr /lit and a constant flow rate of 1 L/s.

The results showed that the density flow frontal at all slopes and both concentrations of 10 and 20 gr/lit, had almost the same velocity before reaching to the obstacles. As the forehead encounters the first obstacle, the speed of the forehead decreases significantly and continues to decrease until the last obstacle. The highest percentage of frontal discharge control at two concentrations of 10 and 20 gr/lit was for h_r(1) S_0 and was calculated to be 84 and 87 percent, respectively, and the lowest percentage of frontal discharge control at same concentrations was for h_r(0.5) S_2.5, and calculated to be 25 and 31 percent , respectively. For the ascending configuration of obstacles, the density current frontal discharge control percentage at the slopes of 0, 1.5 and 2.5% for the concentration of 10 gr/lit is 74, 52, 44 and for the 20 gr/ lit was measured to be 78, 58 and 48 percent.