Experimental Study On Critical Starting Condition And Scour Stability Of Congeries

Earthquake is often accompanied by geological disasters such as landslide,collapse,debris flow and so on,Causing the river sediment to flow continuously into the channel and form congeries along the banks.The existence of the accumulation body will lead to the change of the boundary conditions of the river channel,which will have a great influence on the law of sediment erosion and deposition,the flood flow of the river channel and the safety of the downstream power station.At present,cascade hydropower stations on mountain rivers have been developed one after another,and the water depth in the upper reaches of the hydropower stations has been deepened after the hydropower stations began to operate.In the flood season,the increase in rainfall has led to rising the water level of the river,in order to ensure the river flood,the safe operation of hydropower station and the safety of people’s lives and property downstream,the hydropower station will discharge the flood,at this time the upstream water level receding makes the flow conditions become more complicated,increasing the possibility of sediment starting in the river.It is of great significance for engineering disposal to study the starting conditions of congeries and find out the starting rules of congeries,which can lay a foundation for the subsequent experimental study of sediment transport.River courses in mountainous areas are susceptible to the influence of congeries after earthquakes,and this study can also provide theoretical support for optimizing the treatment scheme of river courses in mountainous areas after earthquakes.On the basis of previous studies,this paper uses modern measuring means to conduct experimental research on sediment starting of the congeries.According to the flow field data near the sediment starting of the congeries measured by PIV,the law of sediment starting on the slope of the congeries is analyzed.The main conclusions are as follows:(1)The variation of the water surface line near the slope of the congeries when the sediment starts is analyzed.Whether the congeries is permeable has a certain influence on the variation of the water surface line along the slope.The variation trend of water surface line is basically the same under different starting state and different discharge.The variation amplitude of water surface line has nothing to do with the starting state and discharge of sediment on the slope of the congeries.(2)The distribution shape of vertical average velocity is similar under different influencing factors,and the vertical velocity is roughly close to logarithmic distribution.The greater the distance from the bed,the greater the velocity.The starting velocity is proportional to the starting amount,discharge and particle size of the sediment.The shear strain rate around the flow of the congeries is larger.Near the bottom,the shear strain rate on the surface of the congeries is more concentrated.As the flow layer rises,the shear strain rate distribution begins to spread outwards.The distribution shape of shear strain rate is similar under different sediment starting states.The shear strain rate is proportional to the amount of sediment starting,discharge and grain size,and the inflection point location is affected by it.The beam narrowing effect of the congeries causes a large velocity gradient around the flow,and the strong shear effect of the flow makes the positive vorticity value on the surface of the congeries larger,so it is easy to cause relatively serious scour in this area.The vorticity value of water flow near the bottom layer is large and its distribution is concentrated.With the increase of the flow layer,the vorticity value gradually decreases and the distribution begins to spread outwards.The vertical vorticity distribution shape is similar under different influencing factors,and the vorticity size is proportional to the sediment starting amount,discharge and sediment particle size,and the inflection point location is affected by it.(3)According to the experimental observation and field measurement,the sediment starting area is mainly distributed on the bank slope.By using the sliding starting model,combined with the knowledge of probability theory and mathematical statistics,the calculation formula of sediment starting probability on the slope of the congeries is obtained.According to the starting probability standard proposed by Dou Guoren,combined with the relevant knowledge of probability theory and mathematical statistics,the particle size correction coefficient f(d)was further introduced,and eventually,a formula flow rate formula for the stacked shore slopes of the slope of the shore,the method of quantitative analysis and theoretical analysis was adopted,and the starting stress was inferior to the pile of the pile of the slopes of the slopes of the shore.The formula of this article has a high calculation accuracy of the formula for starting flow velocity and starting stress.The Reynolds number of sand grains on both sides of Shields curve was calculated by the Shields starting shear stress formula.This test also verified that the formula of starting condition of sediment of collapsed body and landslide body after earthquake was accurate to a certain extent.The fitting relationship between critical single-width dimensionless flow q* and water surface specific drop J is not ideal,which indicates that dimensionless starting power is not suitable for starting sediment on slope of congeries.(4)According to the three-dimensional topographic map drawn by the elevation data,the erosion on the slope of the congeries is mainly distributed on the bank slope and the negative slope,which is consistent with the phenomenon observed in the experiment that the erosion gradually develops from the bank slope to the negative slope.With the increase of the flow rate,the erosion area will be slightly higher.The scour location of the congeries is exactly where the shear strain and vorticity are large.The strong shear and vorticity of the current in this area promote the starting and transport of sediment.After the water flows around the congeries,its energy will dissipate rapidly.Therefore,after the erosion,the sediment started on the slope of the congeries slides to the riverbed and accumulates at the foot of the congeries to form two ridges.The scour degree of the deposit body under general motion is greater than that under moderate motion.The scour degree of the deposit body is not significantly affected by the flow rate,but is significantly affected by the sediment particle size.The area around the flow with large vorticity is just the area with large scour degree.