Experimental Study On Adjustment Law Of Sand-Gravel Bed Scouring In Downstream Reservoir

The construction of the reservoir makes the discharged water sediment concentration decreased significantly, with sediment carrying capacity of flow in deep saturation resulting in downstream river erosion has greatly changed. It has a impact on navigation, irrigation, ecology, environment and the development and utilization of shoreline etc. In spite of,many scholars have done a lot research about bed scouring of the reservoir Downstream. But erosion adjustment mechanism of sand pebble bed still insufficient. Therefore, this article analyzes the evolution of scouring characteristics about the reach of Yichang to Yangjianao of the Three Gorges reservoir downstream. Meanwhile, the change of bed material, bed morphology and flow resistance and so on researched under the condition of different sand pebble bed by flume experiment.Thus there search of riverbed coarsening and adjustment feedback to the carrying current mechanism will be researched.Obtain the following main results:(1) Since the use of the Three Gorges Project, the reach of Yichang to Yangjianao the overall performance scouring, the water level dropped, thalweg decline, bed coarsening, resistance increased. Early riverbed erosion mainly concentrated in Yidu reach; after 2008 year, with the main erosion reach obviously move down to Zhijiang reach; 2010 year later, erosion of Yichang river and Yidu river has been getting smaller and smaller, the erosion of Zhijiang river amplitude increases. Local river coarsened obviously, the water level dropped, thalweg incised; meanwhile, local river thalweg accretion. Lujiahe stretch experienced process of cholestasis expansion, erosion atrophy, siltation upstream expansion, reduced erosion, erosion division.(2) The flume experiment indicated that,the upper reaches near the reservoir first complete flushing, with water line down, bed elevation,0.6-10mm sand pebble bed loss and coarsening,. continuous gradation transition to discontinuous gradation; the sand bed composition after the final scouring and armoring directly affect level vertical slope, scouring depth, turbulence intensity, sand resistance. Sediment concentration 42% Group has the maximum vertical gradient, Sediment concentration 55% group scours most, Sediment concentration 30% group has maximum turbulence intensity and the largest resistance. Beach blocked inhibit is the upstream water level decline, changing the swing of the mainstream line and affecting the river erosion.(3)The velocity and flushing of beach head are the most.A narrow silting zone detected at the beach tail, what’s more, it have a backflow in the edge and tail of the beach. Due to the interaction of flow and bed material,the relation curve of the same vertical turbulence intensity along with the relative depth is approximate the shape of "L". Beach morphology affect turbulence intensity, the dynamic strength on the far side of the beach is greater. The turbulent dynamic strength average value of below beach is bigger than the upper beach.(4)The sand resistance is positively correlated with the mean velocity of the cross section and the number of beach, it is negatively related to the initial sediment concentration. The mass fractal dimension has the statistical fractal features, can better describe the sediment gradation; the mass fractal dimension is positively related to the initial sediment concentration; the mass fractal dimension is negatively related to average flow velocity, number of beach, median grain size. Ratio of sand resistance to median size is exponentially positive related to the mass fractal dimension. Through using the mass fractal dimension Dm, the Froude number Fr, median size d50,proposed a formula of sand resistance coefficient nb=0.034(Dm1+0.3Fr2d50)1/6。...