Experimental Study On Hydraulic Characteristics Of Overflow Runoff Under Combined Vegetation

Vegetation is an important factor affecting the hydraulic characteristics of overland runoff.The existence of vegetation in the slope will change the flow structure,increase the loss of kinetic energy,delay the flow velocity,and then affect the flow regime and flow resistance of overland runoff,as well as the ability of flow scouring and sediment transport.Vegetation on slopes of river basins,whether natural or planted,will show obvious differences in stem diameter due to different vegetation types and growth conditions.That is to say,vegetation on slopes is a combination of vegetation with different stem thickness.Therefore,it is necessary to study the effects of combined vegetation with different stem thickness on hydraulic characteristics of slopes so as to accurately simulate slope yield.The confluence process and the establishment of slope soil erosion prediction model provide theoretical support.In this paper,plastic cylindrical straws were used as simulated vegetation,and indoor flume tests were carried out under 9 vegetation combinations(2mm&3mm,2mm&4mm,3mm&4mm,4mm&5mm,2mm&6mm,3mm&6mm,4mm&6mm,5mm&6mm,3mm&5mm),5 slope gradients(0.0%,0.5%,1.0%,2.0%,3.0%)and 3 vegetation arrangement modes(transverse arrangement,longitudinal arrangement,staggered arrangement).Through the processing and analysis of experimental data,the effects of stem diameter,arrangement of combined vegetation and slope on hydraulic characteristics of overland runoff were studied.The results show that:(1)When the combined vegetation is staggered and arranged at the same water depth,keeping the diameter of one vegetation stem unchanged,the flow velocity v,Reynolds number Re and Froude number Fr decrease gradually,the Darcy-Weisbach resistance coefficient f increases gradually,the energy loss hf has no significant change when the water depth h is less than 0.02 m,but increases gradually when the water depth h is more than 0.02 m.(2)When the combined vegetation is staggered,the Reynolds number Re,Froude number Fr and Darcy-Weisbach resistance coefficient f are all power functions with the stem diameter(D,d)of any combination vegetation at the same water depth,and the Reynolds number Re,Froude number Fr and Darcy-Weisbach resistance coefficient f are power functions with the water depth h under the same vegetation combination.(3)Under the same water depth and slope,the flow velocity and Reynolds number corresponding to the three arrangement modes of combined vegetation are as follows:vlongitudinal arrangement>νtransverse arrangement>νstaggered arrangement,Relongitudinal arrangement>Retransverse arrangement>Restaggered arrangement;When the water depth is less than 0.05 m,the Froude values corresponding to the three arrangement modes of the combined vegetation range from large to small as follows:Frlongitudinal arrangement>Frtransverse arrangement>Frstaggered arrangement.When the water depth is more than 0.05 m,there is no significant difference in Froude number of the three arrangement modes of the combined vegetation.When the water depth is less than 0.03m,the energy loss along the three arrangement modes of the combined vegetation ranges from large to small as follows:hflongitudinal arrangement>hftransverse arrangement>hfstaggered arrangement,and when the water depth is not less than 0.03m,there is no significant difference in the energy loss along the overland runoff corresponding to the arrangement of the combined vegetation.The Darcy-Weisbach resistance coefficient corresponding to the three arrangement modes of combined vegetation are as follows:fstaggered arrangement>ftransverse arrangement>flongitudinal arrangement,which is more significant when 0.02 m≤h≤0.05 m.(4)Under the same water depth,the flow velocity v,energy loss hf,Reynolds number Re and Froude number Fr increase with the increase of slope.When the slope i is less than 1.0%,the slope change has no significant effect on the Darcy-Weisbach resistance coefficient f;when the slope i is more than 1.0%,the slope change has a significant effect on the Darcy-Weisbach resistance coefficient f in a certain depth range,and the depth range of the effect is proportional to the slope.In addition,under the same water depth,the Froude number Fr and Reynolds number Re are exponentially related to the slope i of the slope.