Experimental investigation of the effect of bottom vanse on sediment diversion at a bifurcation

Abstract

Bottom vanes are small aspect ratio submerged /low-training structures mounted vertically on the river-bed and oriented at an angle to the primary now direction. Their application is related to bank protection, navigation and/or re-distribution of sediment and discharge. It is still undefined, how the sediment diversion can be controlled in small rivers. and how much sediment is diverted at a bifurcation under different vane arrangements. though sediment management. In particular the control of sediment movement, scour and deposition is of utmost importance in river engineering. Regarding this, the present study is performed under fixed bed conditions with the aim of making the estimates of sediment diversion in two channels under different velocity and discharge conditions using single or multi vanes, with fixcd vane height and variable angles of attack. Finally, the velocity and the sediment distribution data are analyzed for the optimum combination of vane arrangements. For this purpose, a total number of 16 test runs are performed with 40 cm vane length and two different angles of attack (20", and 30°). Vane height of 12 cm was selected for the experimentation. The vane height was selected by considering the aspect ratios of tested vanes in previous researches. Considering transport equation of Engelund and Hansen, non grinded light weight polystyrene ball is chosen as the sediment. For the total experiment. the discharge and water level are kept constant to a value 200 liter/sec and 30 cm respectively. Single, two and three vanes are used at different spacing and offsets. The base condition is established by conducting test run with no vane and observing the diversion of sediment in the two channels and taking water level, and velocity for the condition. Sediment distribution data are analyzed and comparisons are made among different vane arrangements to find out the vane arrangement that diverts more amount of sediment. Flow velocities in both stream wise and transverse direction are analyzed to observe the variations for different vane arrangements and angles of attack. Moreover, an attempt has been made to explain the morphological changes induced by vane by analyzing the variations in flow field. Velocity contour maps and vector plotting arc also prepared 10 observe the change in flow field due to multi-vane arrangements. On completion of these experiments and analysis, it appeared that, most of the sediment is diverted at the intended (left) channel for all vane arrangements. For test run 12, maximum amount of sediment (80.82%) is diverted at the lelt channel and the diversion at right channel in this case is much lower (19.18%). The second highest sediment diversion (76.94%) occurs at left channel for test run 16, and the diversion at right channel in this case is 23.06%. The study showed that due to installation of multi-vane towards right wall, obstructed area is increased in this region and high velocity generates at left channel. This high velocity diverts more sediment at the left channel. It also appeared from the test results that 20 degree angle of attack diverts more sediment than that of 30 degree angle of attack. The study further showed that vane has no significant impact on the upstream velocity. At left channel stream wise velocity is always much higher than that of the right channel. Most of the transverse velocity at suction side of vanes is directed towards pressure side. This is the positive impact of multi-vane arrangements on sediment diversion.