Experimental Study On The Characteristics Of Erosion And Sediment Transport On The Downstream Channel Bed Caused By Overtopped Outburst Flow

In recent years,under the trend of global warming and increasing seismic activity,the frequency of overtopping failure of landslide dam has increased,increasing the risk of disasters caused by the outburst flood.In this study,the erosion and sediment-carrying characteristics of outburst flood on the downstream channel bed were explored through flume tests and numerical studies.Firstly,the hydraulic characteristics of overtopped flood of natural dams under different conditions were analyzed.The discharge curve of the coarse and fine mixtures was relatively fatter,and that of homogenous sandy soil was slimmer;the increase in dam downstream slope angle enlarged the peak discharge and shortened the outburst duration.The absolute value of the erosion rate of each section in the early stage of the failure was larger than that in the later stage;with the increase of channel bed slope,the breaching discharge increased firstly and then decreased,and the failure duration was shortened.When the channel bed slope was gentle(smaller than 7°),flow velocity gradually decreased with distance away from the dam,and the erosion ability was weak;as the channel bed slope became steeper,the maximum velocities at different sections were basically at the same level,and the effect of erosion was enhanced.Secondly,the erosion characteristics and sediment-carrying characteristics of overtopping outburst flood were discussed.The absolute value of the erosion rate of outburst flow of the homogenous sandy soil dam was relatively smaller.The character of“deposition-scouring-deposition” occurred on the downstream channel bed,and the farther away from the dam,the weaker the flow sediment-carrying intensity was on the channel bed.For coarse and fine mixtures,due to the coarsening of the bed surface and the existence of sandbars,a “braided river” in the downstream was formed after the failure process;while for homogeneous sandy soil,the downstream riverbed was relatively flat,and local sedimentation was not obvious,and small grooves were formed on the channel surface.The farther away from the dam,the weaker the erosion ability was on the channel bed.In addition,the increase of dam downstream slope enlarged the erosion rate,decreasing the final sediment height on the channel bed.The absolute value of the erosion rate was increased after the channel bed slope increased,and the carrying ability of the outburst flood was enhanced.Thirdly,the evolution characteristic sediment concentration of outburst flood was explored.The sediment concentration at the end of the flume for the two materials increased firstly and then decreased,but the volume concentration of the homogeneous sandy soil was much larger than the coarse and fine mixtures,which reached the critical concentration of debris flow.As the dam downstream slope angle increased,the peak volume concentration decreased firstly and then increased,and as the dam volume increased,the peak volume concentration firstly decreased and then increased;the peak volume concentration firstly decreased and then increased with increasing peak discharge.As for natural dams with gentle downstream slope,the peak volume concentration was primarily affected by dam volume;when downstream slope got steep,breaching discharge was dominant factor affecting the volume concentration.When the channel bed slope was gentle,the volume concentration decreased at first and then increased,after the peak concentration,it decreased;as the channel bed slope became steep,the sediment concentration increased firstly and then decreased.And that,the increase of channel bed slope increased the sediment concentration.For homogenous sandy soils,the farther away from the dam,the lower the suspended sediment concentration was;while for coarse and fine mixtures,the suspended sediment concentration was higher at the further section away from the dam.Lastly,based on the original calculation formula of scour rate of clear water,the variation of sandy flow concentration was considered in the scour rate of outburst flood,and the scour rate formula of outburst flood was established.The theoretical calculation value of the sediment-laden flow scour rate was compared with the actual measured value and it was found that the calculation formula of the scour rate of the sediment-laden flow considering the change of the volume concentration was suitable for the case where the ratio of the limit concentration to the volume concentration is greater than or equal to 1.35.