Effect Of Gravel Particle Size And Coverage On Slope Runoff And Sediment Yield In Alpine Region

Qinghai-Tibet Plateau,known as the ’ Asian Water Tower ’,is an important ecological defense in China with special geographical environment,obvious elevation difference,unique climatic conditions,rich topography and geomorphology,and serious natural erosion.As the increasingly prominent contradiction between people and land,soil erosion continues to intensify..The rate of soil development in this area is relatively slow,and the gravel content is rich,and the soil resources are extremely precious.Therefore,it is particularly important to prevent soil erosion,protect and rationally develop and utilize soil resources for the management of land resources and ecological environment construction in this area.To do this,this study takes the typical slope soil collected from Lhasa River Basin in Tibet as the research object,and under the indoor artificial rainfall conditions(rainfall intensity: 60mm/h,slope gradient: 25°),from the aspects of runoff and sediment production process,hydrodynamic characteristics and rill erosion morphology.The effects of gravel particle sizes(0.2-2 cm,2-6 cm,6-10 cm)and coverages(0,20%,40%,60%)on runoff and sediment yield on pure soil slope(soil accounted for 100%)and soil-rock mixed slope(soil accounted for 60%,gravel particle size of 2-6 cm accounted for 40%)were studied.The following conclusions were obtained:(1)When the gravel particle size of pure soil slope is 2-6 cm and the coverage is 40%,and when the gravel particle size of soil-rock mixed slope is 2-6 cm and the coverage is 60%,the infiltration is the largest and the flow reduction effect is the most obvious.When there is no gravel cover,the initial runoff time and infiltration rate of pure soil slope are smaller than those of soil-rock mixed slope,and the runoff rate is opposite.When the gravel particle size are 0.2-2 cm and 2-6 cm and the coverage both are 60%,the infiltration rate of pure soil slope is smaller than that of soil-rock mixed slope,and the runoff rate is larger than that of soil-rock mixed slope.(2)When the gravel particle size of pure soil slope is 0.2-2 cm and the coverage is 20%,and when the gravel particle size of soil-rock mixed slope is 0.2-2 cm and the coverage is60%,the sediment yield is the least.When there is no gravel cover,the sediment yield of pure soil slope is larger than that of soil-rock mixed slope;when the gravel particle size is6-10 cm or the gravel coverage is 60%,the sediment yield of pure soil slope is higher than that of soil-rock mixed slope,and it is opposite under other conditions.The sediment yield of pure soil slope increases with the increase of gravel particle size and gravel coverage;the sediment yield of soil-rock mixed slope only increases with the increase of gravel coverage or particle size when the gravel particle size is the largest or the gravel coverage is the highest.(3)The flow regime of pure soil slope under all simulated test conditions are laminar and tranquil flow;when the gravel particle sizes are 2-6 cm and 6-10 cm,and the gravel coverage is 40%,the flow regime is laminar flow and rapid flow,and the gravel coverage condition is laminar flow and tranquil flow.When there is no gravel cover,the velocity,Froude number and flow power of pure soil slope are larger than those of soil-rock mixed slope,and other hydrodynamic parameters are smaller than those of soil-rock mixed slope.Runoff rate is significantly positively correlated with Reynolds number and flow power(P<0.01).Sediment content and soil detachment rate are significantly positively correlated with runoff velocity and Froude number(P<0.05).Soil detachment rate is significantly negatively correlated with Manning roughness coefficient(P<0.05).(4)When the gravel particle size of pure soil slope is 0.2-2 cm,no rills are found on the three kinds of covered slopes;when the gravel particle size is 0.2-2 cm and the coverage is60 %,no rills appeared on the slope surface.When there is no gravel cover,the rill density,rill fragmentation and width-depth ratio of pure soil slope are larger than those of soil-rock mixed slope.When the gravel cover is 2-6 cm and the coverage is 20% and 40%,the rill density of pure soil slope is smaller than that of soil-rock mixed slope,and the rill fragmentation and width-depth ratio are opposite under other conditions.Soil detachment rate,sediment yield and sediment concentration are significantly positively correlated with rill fragmentation and rill width-depth ratio(P<0.05),and runoff rate is significantly positively correlated with rill density(P<0.05).