| The study of overland flow under vegetation cover is the foundation link for soil erosion research,and although many achievements have been made,the influence mechanisms of vegetation community and its spatial configuration on water and sediment transport are remain poorly defined.Meanwhile,due to the complex flow conditions under vegetation cover,the precise prediction of overland flow resistance is still a difficult point for current research.This study investigated the pattern of changing in the hydrodynamic and relative hydro-dynamic properties of vegetation conditions through a combination of clear water tests and sand-bearing flow tests for different vegetation community and spatial configurations,and constructed a calculation model for the overland flow resistance coefficient under vegetation community.The variable characteristics of indicators of the intensity of sand containing currents,the barrage volume and the barrage effect,were analysed,and based on this,the vegetation community was probed for the mechanism by which it functions to block water.The main conclusions:(1)to explore the changing pattern of hydrodynamic characteristics under vegetation cover.The water depth covered by vegetation was not only significantly positively correlated with the flow rate,but also profoundly influenced by vegetation type,coverage,and spatial configuration.At the same coverage and the flow rate was less than 20 L/min,the water depth was greater and the flow rate was smaller with shrubs cover than other cover.When vegetation type was fixed,grass cover increased from 1.50%to 7.60%with 74.76%water depth increased,while 34.4%,17.14%,41.50%decreased in flow rate,Reynolds number,and Froude number.And greater change values could be achieved under vegetation community cover.Therefore,to pursue larger water depths and smaller flow rates,Froude number,it is advisable to adopt a single row of alternating pattern cover at 2°and an upstream shrubs downstream grass cover pattern at other slopes.(2)An integrated resistance coefficient calculation model under vegetation cover was developed,and the important effects of vegetation type,coverage,and spatial configuration on the resistance coefficient were also investigated.It was found that at the same cover,the resistance coefficient decreased from 2.694 to 0.820 in the shrubs group and to 5.796 in the grass cover group first from 2.932 to 1.859 as the flow rate increased.When grass cover increased to 7.60%under the same type of vegetation,the mean resistance coefficient increased from 1.065,3.403 to 6.875,12.586 under vegetation community cover,respectively,and the visible increase of cover and vegetation community cover increased the resistance coefficient.The spatial configurations were ranked from large to small according to the resistance coefficient as follows:Grass integument pattern downstream of the upstream shrubs,single row alternating pattern,double row alternating pattern,and uniform pattern.And the resistance coefficients all decrease with increasing slope.Analysis of the test data found that,under vegetation community,there was a trend of community added resistance and pattern added resistance,and the increase of grass cover would lead to a trend of first increase and then decrease in community additional resistance.At all spatial configurations,the average percentage of grain resistance,grass passive resistance,shrubs resistance,combined vegetation added resistance,and pattern added resistance were about 12.63%,6.03%,24.33%,18.31%,38.74%,respectively,and the greatest percentage of pattern added resistance was found at the grass passive resistance downstream of the upstream shrubs.(3)The pattern of response of relative hydro-dynamic properties to vegetation type was explored.The flow rate before and after 20 L/min was covered by shrubs and grass,respectively,which can obtain the optimal backdraft slow flow and increase resistance effect.For vegetation under the same type,an increase in grass cover contributes to an increase in hydrodynamic property parameters other than relative Reynolds number.Whereas the relative Reynolds number increased with increasing cover only under single grass cover because of the influence of shrubs,it increased firstly and then decreased under vegetation community.At 2°slope,a single row of alternating pattern had the best enhancement effect of slow flow,while grass downstream of upstream shrubs were found to be the best pattern at other slopes.Regarding the flow pattern,its optimality was assigned to a uniform pattern and an upstream shrubs downstream grass cover pattern,respectively.(4)The changing patterns of the kinetic properties of sandy water were investigated.Hydrodynamic property parameters were influenced by both sand content and vegetation spatial configuration,with an increasing trend of water depth and resistance coefficient with increasing sand content,whereas the flow rate,Reynolds number,and Froude number showed a decreasing trend.The optimals for water depth,flow velocity versus Reynolds number were assigned to a double row alternating pattern,while Froude number versus resistance coefficient were assigned to a single row alternating pattern.The barrage volume decreases with slope and was significantly influenced by the amount of sand from water and the spatial configuration.The barrage volume was greatest at 200 kg/m~3,and neither an increase nor a decrease in the amount of sand resulted in a significant reduction in the barrage volume.Vegetation had a significant effect on the barrage volume,with the largest single row alternating pattern,and the smallest barrage capacity on bare slopes. |
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