Comparison of measured dam-break flood waves in triangular and rectangular channels

Abstract

Dam-break flows in both triangular and rectangular channels are investigated experimentally in the present study. Laboratory measurements are carried out in a prismatic and smooth flume with a horizontal and wet bed for two different reservoir heads and four tailwater levels. During the experiments, digital image processing is applied as a flow measurement technique. Both water surface profiles and stage hydrographs are obtained effectively by means of video images with eight adjacent cameras. The dam-break flows in triangular and rectangular channels show common features: jet formation in the downstream of the dam during the initial dam-break stage; similarity of the water surface profiles behind the dam for different depth ratios at a specific time; insignificant effects of the depth ratio on the negative wave front; and strong dependence of the shock wave front on the depth ratio. In a triangular channel, the negative wave front moves slower than that in a rectangular channel; and extra negative waves propagating towards upstream direction does not form for all depth ratios considered in this study while they have been observed in a rectangular channel for large values of α representing the ratio of initial tailwater depth and reservoir depth (i.e., α = 0.3 and 0.4). Meantime, these extra negative waves result in an undulation of the water surface profile in the reservoir and an apparent increase of water depth at the dam. The water depth at the dam section for different depth ratios is 60–70% of the initial reservoir head; and the shock wave front propagates slower as the depth ratio increases for the triangular channel. For water depth ratios α = 0.1 and 0.2, the shock wave front moves faster than that in a rectangular channel, and the contrary is true for α = 0.4. For α = 0.3, the propagation speed of the shock wave front approximates that in a rectangular channel. Present study contributes to the understanding of the effect of wet downstream conditions on dam-break flows in triangular channels and provides detailed experimental data for validating both the analytical and the numerical solutions.