Effect Of Grass Distribution On Hydrodynamic Characteristics Of Overland Flow And Runoff And Sediment Yield

The hydrodynamic characteristics of overland flow are the hydrodynamic basis of the study of water erosion.However,due to the complex slope surface features and incomplete observation methods for overland flow,we have limited understanding of the dynamic characteristics and resistance mechanism of overland flow,and have not yet obtained a conclusion on the variation law of the dynamic characteristics of overland flow affected by the distribution of the grass cover.In this study,a flume experiment was carried out under the condition of grass cover.We studied the variation of dynamic parameters of overland flow under the influence of grass density and grass distribution pattern,and analyzed the formation mechanism of thin layer flow resistance and the relationship between each resistance component under grass cover quantitatively.The simulated rainfall experiment was carried out under the condition of grass cover.We studied the effects of grass cover distribution pattern on slope hydrodynamic parameters,and explored the regulatory effects of different grass cover patterns on slope runoff and sediment yield under rainfall conditions.The main research results of this study are as follows:(1)The response of slope water dynamics to grass cover density was be investigated.With the increase of stem density,water depth increases by 115% ～ 403% and flow velocity decreases by 401% ～ 438% at different flow discharge.Although the flow pattern not reach turbulence under most conditions,the flow pattern develops from transition to turbulence with the increases of stem density.When the slope increases,the flow pattern develops from slow flow to rapid flow.On the contrary,when the grass stem density increases,the flow regime changes from rapid flow to slow flow.The influence of slope and grass stem density on the flow pattern is mutually restricted.(2)The resistance formula of overland flow under grass cover condition was be established and the variation law of overland flow resistance under grass cover condition was be investigated.Under the condition of no rainfall,the total flow resistance can be divided into the form resistance caused by grass and the grain resistance caused by rough bed surface.The contribution of form resistance to the total resistance is much greater than that of grain resistance.When the stem density is greater than 2.88%,the proportion of form resistance to the total resistance increased with the increase of flow rate,and the increase gradually decreased with the increase of stem density.With the increase of stem density,the proportion of form resistance in the total resistance is increases,but the contribution of form resistance to the total resistance is decreases when the stem density is between 1.73% and 2.3%.The proportion of the grain resistance in the total resistance will increase accordingly,which is easy to cause the erosion and soil transport.(3)The variation of relative hydrodynamic parameters of overland flow with different grass distribution patterns was be investigated.When the discharge and slope are constant,the slow-flow effect of the grassland from high to low were as follows: transverse strip,patch,checkerboard,random distribution and vertical strip pattern.The variation of the relative hydrodynamic parameters of the overland flow is affected by the distribution of grass cover.With the increase of flow discharge,the relative resistance coefficient of horizontal strip distribution is increases most obviously,and the relative resistance coefficient of patchy,checkerboard and random distribution pattern is increases have same trend.(4)The variation of hydrodynamic parameters of overland flow under different grass distribution and simulated rainfall conditions were be investigated,and its regulation effect on slope runoff yield and sediment loss were be studied.Compared with bare slope,there are significant differences in flow resistance,flow shear force and flow power under different grass distribution patterns.Except for the bare slope condition,the chessboard pattern,longitudinal strip pattern and staggered strip pattern have the shortest start time of runoff production,while the diamond pattern and transverse strip pattern have the latest start time of runoff production.The transverse strip distribution has the best water reduction effect,and the average water reduction effect is 77.9%.The water reduction effect of random patch pattern is slightly smaller,and the average water reduction effect is 61.78%.The water reduction effect of staggered bands and rhombus distribution pattern is the worst,and the average water reduction effect is 19.9% and 32.15%,respectively.Strip distribution and random patch distribution show stronger water reducing and weaker sediment transport capacity than checkerboard distribution and longitudinal strip distribution.In addition,there is a good correlation between water flow power and soil erosion rate under different grass distribution.