卵石沙洲发育与冲刷试验

Experimental study of development and erosion of gravel mid-channel bar

沙洲是塑造分汊型河道最重要的形态因子,其发育与蚀退由于上游来水来沙变化呈现冲淤交替,从而影响分汊河道输水输沙平衡。通过单个卵石沙洲的淤积和冲刷试验,揭示不同加沙速率、粒径和来流量条件下,沙洲淤积和冲刷规律,并建立简化理论模型分析沙洲淤积速率。结果表明,4组加沙试验中,分流点后出现明显淤积下延至洲头,左汊和右汊成为输沙通道,洲尾中心线两侧的左右汊道有泥沙淤积,洲尾未出现淤积。7组清水冲刷试验中,洲头最先承受冲刷和蚀退,并沿洲体冲刷延伸,洲头冲刷的泥沙沿左右汊水流带到下游,洲尾未出现明显冲刷。卵石沙洲以洲头淤积为主导发育模式,泥沙粒径、洲头坡角和分流角是决定淤积速率的关键因子。

Mid-channel bars occur in anabranching rivers. Fluvial processes of a mid-channel bar following variation of flow and sediment transportation are studied experimentally, which influences the equilibrium of flow and sediment transport in anabranching channel. The processes of development and erosion of a mid-channel bar were observed and measured under different flow discharges, sediment feeding rates, and with different grain sizes. A simplified model of bar head growth is established to analyze the rate of bar development. Results show that gravel deposition occurred along bar head from the branching point, and the left and right channels became sediment transport channels. There was little sediment deposition at the bar tail. In high flow discharge and low or no incoming sediment load experiments the bar head was firstly scoured and thence, extending scour occurred along the bar body. The sediment removed from the bar head was transported to downstream along the left and right channels, not deposited on the bar tail. Bar head deposition is a dominant process for gravel mid-channel bar growth. The growth rate is a function of sediment load, grain size, bed slope and branching angle.