Experimental Study On The Recovery Process Of Suspended Sediment Concentration Under The Condition Of Clear Water With Sandy Riverbed

The construction and application of a reservoir make most of the sediment from the upstream deposited in the reservoir area.The operation of the reservoir will leads the suspended sediment concentration decreased sharply in the downstream river.In the process of sediment transport,the suspended sediment concentration will be transformed from sub-saturation state to saturation state accompanied with the flow to do work,to overcome the resistance work etc.This process has a negative impact on the downstream riverbed,it will cause severe adjustment on river regime that may have adverse influences on flood control,shipping,water resources utilization,water ecological environment.The existing researches are mostly based on ideal flow,saturated suspended sediment condition to find water movement characteristics and suspended sediment concentration distribution law but the comprehensive research based on sub-saturated suspended sediment condition which to find the law of flow velocity,turbulence intensity distribution law and the law of suspended sediment concentration changed by distance is relatively few.Especially,there are still some deficiencies in the understanding of the law of the change of suspended sediment concentration from sub-saturation to equilibrium.Therefore,the assumed silt-discharge condition of current research is difficult to meet the actual silt-discharge conditions,and the flume experiment under sub-saturated suspended sediment condition are less.All of these depicted that the existed matters are still worth to be studied deeply.In this study,a flume test system with advanced measuring instruments and automatic regulating functions applied carry out suspended sediment transport experiments under the condition of constant,uniform flow and uniform bed material.Eighteen flume experiments with different discharge,water depth,sediment grain size were carried out.We set up six velocity profiles and twenty-four suspended sediment sampling cross sections and obtained the flow velocity data,turbulence intensity data,water level data and sediment concentration data.Using these data,we studied how the vertical velocity distributed,vertical turbulence intensity distributed with time variation and suspended sediment concentration distributed changes by distance if the bed exist sand wave movement and explored the different hydraulic factors on the effects of suspended sediment concentration variation by the channel distance.The main research achievements are as follows:(1)It is expand that the discharge,water depth,sediment grain size all have effects on the vertical mean velocity with time variation.The change of the test conditions such as the increase of discharge,the decrease of the water depth and the smaller grain size will lead to the suspended sediment concentration recovery distance growth and the vertical average velocity of every section in the process of sediment concentration recovery decreased with time variation while these changes will not affect the vertical average velocity of the section which the sediment concentration in saturated state.(2)It is illustrated the vertical turbulence intensity distribution law with sand wave movement on the bed.It is found that the sand ripples strongly influenced the turbulence intensity distribution near the bed and affected the vertical turbulence intensity distribution in silt-laden flow.We can find that different bed form will lead different vertical turbulence intensity distribution: For the continuous scoured section,the smallest vertical turbulence intensity appears on the bed and gradually increase to the water surface(we define this situation as model one)and the value of turbulence intensity gets smaller with time variation.For the cross section which suspended sediment concentration transit from sub-saturated state to saturated state need to be divided into two situation: When the bed surface is continuously scoured but still fed by upstream suspended sediment,the vertical turbulence intensity distribution gradually increased from the water surface and reached the maximum value near the bed then decreases(we define this situation as model two);When the bed first silted by sediment and then scoured by water,the vertical turbulence distribution would transfer from model one to model two as described above;For the erosion-sedimentation cross section whom appeared at downstream of the flume,the vertical turbulence intensity will decrease from the bed to the water surface.Due to the sand wave movement,the turbulence intensity near the bed fluctuates greatly and the value of turbulence intensity gets smaller with the time variation.(3)It is discussed that the recovery process of suspended sediment concentration is affected by the water depth,discharge and sediment grain size.When the other influencing factors are constant,the greater the discharge,the longer the sediment concentration recovery distance and the greater the value of the sediment concentration in the saturated state.Moreover,with the increase of time,the sediment concentration recovery distance has also increased,but the value of suspended sediment concentration has decreased;the smaller the water depth,the larger sediment concentration recovery distance,the value of suspended sediment concentration in the saturated state also increases;the finer the sediment grain size,the longer the sediment concentration recovery distance and the larger value of suspended sediment concentration in the saturated state.At last,the variation law of suspended sediment concentration at the same relative water depth and the variation law of average suspended sediment concentration of the cross section are approximately the same by time variation: the suspended sediment concentration gradually increases and reaches to the saturated state at the end of the flume.