Effects Of Raindrop Impact On The Resistance Characteristics Of Sheet Flow

Rainfall and sheet flow are the major driving forces for water erosion.The resistance coefficient of sheet flow is one of the important parameters reflecting the resistance characteristics of sheet flow.However,effects of rainfall on the resistance characteristics of sheet flow are still not clear.In this study,the methods of simulated rainfall,water scouring,and theoretical derivation were applied to investigate the flow resistance characteristics of rainfall-generated sheet flow,to quantify effects of raindrop impact on resistance characteristics of sheet flow under different rainfall intensity conditions,and to reveal the effect of rainfall on the resistance characteristics of sheet flow under different water depth and state conditions.Finally,the flow resistance equations were established based on the analysis of experiment data.In this study,the major results were as following:(1)Water depth and flow velocity of sheet flow generated by rainfall were zero at the flume upstream and they increased along the flume.The flow depth and velocity at the downslope segment were obviously greater than those at the upslope segment.By calculating the Darcy-Weisbach resistance coefficient under each slope case,it was found that the Darcy-Weisbach resistance coefficient were in the following order:the upslope segment(4-6 m)> the middle slope segment(2-4 m)> the downslope segment(4-6 m).However,when slope gradient was greater than 12%,the difference between the resistance coefficient for each slope segment decreased obviously.Moreover,raindrop impact can significantly increase the Darcy-Weisbach resistance coefficient of sheet flow in each segment,and the effect of raindrop impact on the resistance of sheet flow in the upstream slope segment was greater than that of sheet flow in the downstream segment.(2)Raindrop impact increased the flow resistance and Manning coefficient of sheet flow.The flow velocity reduced and the water depth increased by raindrop impact.Both the Darcy-Weisbach resistance and Manning coefficient decreased with the increasing slope,and decreased significantly in the slope range of 2-12%,and kept almost constant in higher slope range.When Reynolds number was less than 800,Darcy-Weisbach resistance coefficient decreased with the increasing Reynolds number in a good correlation.Reynolds number equal to 800 appeared to be a critical value laminar flow regime under rainfall conditions.When Reynolds number was greater than 800 approximately,Darcy-Weisbach resistance coefficient had no obvious relationship with Reynolds number,while raindrop impact increased the flow resistance of sheet flow.(3)In the cases of rainfall,both Darcy-Weisbach resistance coefficient and Manning coefficient decreased with increasing flow depth with power function.When the Reynolds number was less than 1070,the Darcy-Weisbach resistance coefficient decreased with increasing Reynolds number in a power function.The Reynolds number equal to 1070 was approximately the critical value of the laminar of sheet flow under no rainfall condition.For the combination of rainfall and inflow cases,rainfall can increase significantly the resistance of sheet flow under low flow depth and energy conditions.The effect of rainfall on resistance of sheet flow was obvious in the low Reynolds number(<1070).Both Darcy-Weisbach resistance coefficient and Manning coefficient of the sheet flow increased with increasing rainfall intensity.The resistance coefficients decreased with the increase of the ratio of flow depth to median diameter of raindrop.(4)Empirical equations relating Darcy-Weisbach resistance coefficient and Manning coefficient to Reynolds number and rainfall intensity were obtained based on experiment data.Empirical equations of the incremental resistance,due to raindrop impact were established for laminar flows of Reynolds number less than 800.In addition,the fitted equations among Reynolds number and the ratio between the flow depth and raindrop median diameter were established to predict resistance coefficients of sheet flow for upstream inflow with rainfall cases.This study can provide understanding for hydrological processes of sheet flow generated by rainfall and provide insight into effects of rainfall on resistance characteristics of sheet flow for different water depths and flow state conditions.The above results can provide a scientific basis for the modification and establishment of the flow resistance parameters of sheet flow due to rainfall in hydrological and soil erosion models.