Study On The Vertical Distribution Of Water Flow Velocity And Sediment Concentration In Water Permeable Barrier

The vegetation in river can effectively improve the ecological environment,maintain the ecological diversity,prevent soil erosion,protect the banks and beaches.The existence of vegetation changes the flow structure and sediment concentration characteristics,the river hydraulics problems become more complex.At present,some scholars have studied the influence of vegetation about flow and sediment.However,there is little research on vertical distribution of velocity and sediment concentration under submerged vegetation.This research is of great significance for river rehabilitation and environmental protection in the future.Based on the results of previous studies,this experiment selecting representative characteristics of rigid and flexible materials to simulating vegetation,choosing two types layout density and 15 kinds of submergence degree,using the method of isolating other factors and only highlighting single factor,analyzing the influence of submergence degree,vegetation density and vegetation characteristics and other factors one by one,and then carrying on the contrast analysis,studying on the influence of vegetation on the flow structure and sediment concentration.The results show that:(1)The presence of vegetation significantly affects the velocity and turbulence characteristics of the flow.The prevented action of vegetation effects on flow is greater than the bottom shear stress,which changing the vertical distribution structure of flow velocity.Velocity is no longer presenting a semi logarithmic distribution affected by the existence of rigid vegetation.Instead,the vertical is divided into three regions,namely:the bottom area of vegetation layer(h<0.8h_v),the top area of vegetation layer(0.8h_v<h<1.5h_v)and the upper flow area(h>1.5h_v).In the range of h<0.8h_v,vertical velocity increases slowly,until the range of 0.8h_v<h<1.5h_v,the flow velocity increases linearly quickly,and a logarithmic velocity distribution appears in the region of h>1.5h_v;the vertical velocity of submerged flexible vegetation flow is also divided into three zones,the difference is that the bottom area of vegetation layer ranged from 0 to h<0.6h_v;(2)The vegetation has the same characteristics is consistent in vertical velocity distribution between different planting density,but with the decrease of vegetation layout density,the flow velocity in the region of h>1.5h_v is getting stable earlier.(3)Under the condition of same submergence degree and different layout density,the roughness value is different.With the rigid vegetation layout density increasing,the roughness value become greater,the difference of roughness value decreases with the increasing of the submergence degree;under the condition of the same arrangement,with the increasing of submergence degree,rigid vegetation roughness value change from big to small than the flexible vegetation firstly,this is due to the physical characteristics of the flexible vegetation.As a result,the formula about roughness value and submergence degree in power function distribution:n(28)C(7)H h_v(8)~m.(4)When the sediment concentration is small in the sediment laden flow,the sediment concentration at the bottom is slightly larger.At the condition of??<3.0,the vertical sediment concentration decreases slowly with the increasing of the distance from the bottom of the sink,which is basically the same as that without the vegetation.When???3.0,the vertical sediment concentration decreases with the increasing of the distance from the bottom of the sink at the vegetation layer,and the vertical sediment concentration in the upper part of vegetation layer decreased slowly,which accords with the Rouse formula.