Study On Hydrodynamic Characteristics Of Runoff Of Rill Erosion On Loess

Loss of land resources has become a global resource and environmental problem. And the situation faced by China and other developing countries is particularly serious. Sediment particles are kept and carried by water flow in rill channel, the increasing sediment concentration leads to serious soil and water loss and rill erosion. Erosion energy and transport capacity of rill flow are far greater than the erosion energy and transport capacity of raindrops strike and overland sheet flow. Slope erosion and rill development bring influence each other and interact, and was influenced by the flow, slope and slope length and other external conditions at the same time.Rill flow velocity experiments were conducted on a flume of 12 m length using scouring method to explore the dynamic process of rill flow velocity of loess hill slope. The experiments were designed under five slope gradient condition (5°,10°,15°,20°,25°) and three inflow rates(2,4,8 L/min). Study the relationship of rill erosion changing from process and flow velocity with rill runoff flow and slope, the relationship between sediment yield and rill runoff velocity, and explore the critical shear stress and maximum erosion rate of soil as soon as loess rill erosion occurs. There, is a very important theoretical significance and practical application value for revealing the slope soil erosion process and its inherent rule, establishing the physical model of soil erosion and improving the level of soil erosion prediction and forecast.Result are as follows:1. Eroded rill depth shows a trend that increase with rill length at the upper parts of eroded rill and then decrease with rill length. Max value of rill depth occurs at 0-2 m section, varies with flow rate. Indicating that the most serious rill erosion occurs at this section. And it also said that the falling cave is found randomly, and the rill depth changes fluctuate.2. Slope gradients and flow rates are the most important factors that affect loess rill erosion. Flow velocity increase with slope gradient and flow rate. The experimental results shows that average rill flow velocity is a power function of slope gradient and flow rate, the empirical formula in this study is v=1.488·q0.342 S0.644. Also a power function is found between rill velocity and flow rate:v=a·Qb.3. At the section 0～8 m of the eroded rill, rill flow velocity increases with rill length very rapidly, and slow down at the following section, and the rill flow velocity trend to a constant value indicating that rill erosion get to a dynamic equilibrium. Rill flow velocity shows a power function with rill length, the fitting function is v=c·Ld Parameter c varies ranging from 0.1772～0.8836,and parameter d varies ranging from 0.0747-0.3265.4. A power function relationship can be used to fit rill flow shear force and rill length:τ=k·Ln. Where parameter n is always minus indicating the negative relationship between shear force and rill length; parameter k shows that k increases with flow rate at a certain slope gradient and k increases with slope gradient at the same flow rate. Parameter k can be used as a indicator to study rill flow shear force.5. Experimental results show that both flow rate and sediment concentration have the positive influence on rill flow shear force. But the influence is not the same, that is to say, The coupling effect of sediment concentration is more significant than shear force. Rill flow shear force decreases with sediment concentration, the relationship between shear force and sediment concentration can be fitted by a linear function: τ=m+p·c.6. Rill flow shear force and detachment rate are the important factors to studying rill erosion. Rill detachment rate decrease with rill length, and eventually tend to 0. Shear force and detachment rate are the factors to study rill erosion. And shear force shows a linear relationship with detachment rate, and increase with detachment rate, the fitting function is Drmax=a+b·x. The value of parameter K(erodibility parameter) in this experiment is 0.1361 kg/(m2·s), τ0 (critical shear force) is 0.2367 N/m2.