Erosion Process Of Straw-Mulched Cultivated Loess Slope Under Extreme Rainfall Events And Its Hydrodynamics Mechanism

The soil and water losses affect social economy,natural environments,and people’s living conditions.Slope loess farmlands are the most serious soil erosion places and one of primary sediment sources of river systems.This research aims at the study of slope loess farmland affected by plow pan,to study spatial distribution of flow velocity along slope,and infiltration,runoff and erosion processes over loess slope under extreme rainfall events.Sampling method for measuring sediment concentration of loess sediment-laden water is presented in this dissertation.The main results and conclusions are showed as follows.A new method was proposed to measure sediment concentration of big number of samples with massive mass.After placed in a bucket for a period of time,the top water was poured out carefully,to get the wet sediment samples,as mixture of sediment and water.A small number of the wet sediment samples were taken from the total samples,to dry out by oven drying method.The proportional coefficient between the dry sediment mass and that of the wet sample was determined.This proportional coefficient was used to predict the dry sediment mass of the remaining samples.The sediment concentration was computed from the dry sediment mass and the original runoff volume.This method could be used to measure sediment concentration of the massive runoff samples accurately and energy-efficiently.A method was suggested to measure and compute overland runoff flow produced by rainfall.Solute centroid method of electrolyte tracing was used to measure flow velocity along the slope,under slope gradients of 5°,10° and 20° and rainfall intensities of 80,120 and 160 mm/h.The results indicated that the flow velocity ranged from 0.050～0.352 m/s under different rainfall intensities and slope gradients.Velocity increased with slope gradient.Runoff rate increased with rainfall intensity and slope length to cause increase in flow velocity.The increase magnitude of velocity gradually decreased with increases in slope gradient,rainfall intensity and slope length.An empirical mathematic model was suggested to quantify the flow velocity under different slope gradients and rainfall intensities.Solute centroid method of electrolyte tracing method was used to measure flow velocity under straw-mulched slopes.Under different slope gradients and rainfall intensities,the flow velocities under straw mulch rates of 0,2 and 8 t/hm2 were measured.Measured flow velocities under the experimental conditions ranged 0.016 to 0.117 m/s for both 2 and 8 t/hm2 mulch rates.The flow velocity was 0.278 times of that of soil surface without mulch.Experiments were made to study the mechanism of straw mulch to increase infiltration and to cause fast saturation of straw-mulched cultivated soil layer with low permeability under soil layer.Different mulch rates of 0,2 and 8 t/hm2 were designed to study infiltration and runoff processes under different slope gradients and rainfall intensities.Results indicated that straw mulch significantly increased infiltration rat to lead to postponed runoff initiation,to maintain much higher infiltration rate than slope without mulch.This way,the cultivated layer under mulch reached to saturation rapidly.Under the impact of plough bottom layer with higher bulk density,the infiltration rate of the cultivated layer decreased rapidly to fast reach to an infiltration rate controlled by hard pan layer.This caused much high runoff rate than the slope under no mulch.The increase in runoff from mulched slopes caused intense flush to the cultivated slope,to lead to more intense erosion of the mulch slope than a slope without mulch.Experimental data set were used to present the soil erosion process.The results indicated that under steep slopes,runoff rate and sediment concentration generated from mulched slopes were all significantly higher than those from slopes without mulch.Total soil loss from mulched slope during a rainfall event was also greater than that from slope without mulch.Under extreme rainfall conditions,straw mulch could increase soil and water losses from steep slope.Under gentle slope,mulched slopes generated low flow velocity,which produced randomized erosion behavior,with lower sediment transport capability.Under such circumstance,straw mulch can increase rainwater conservation.