Studies On The Properties Of Hydraulic Dynamics Of The Overland Flow At The Different Surface Roughness

Water and soil are the most important natural resources on which human beings depending for survival and development. However, owing to the combined actions of natural factors and human factors such as precipitous topography, constant rock erosion by concentrated precipitation, rapid growth of world population, unreasonable production and operation by human beings, indiscriminate cutting and reclamation of steep slopes, water and soil resources sufferd from great loss and destruction, which leads to increasing deterioration of ecological environment. Hydraulic erosion, overland flow for source power, have the widest distribution among all the soil erosion types. Thus, study on properties of hydraulic dynamics on overland flow is necessary for a good mastery of hydraulic erosion rules. Precipitation, slope shape, slope geotechnical structure vegetation, human activities and so on were important factors that have effect on erosion of overland slow.Using the combined method of experimental research and theoretical analysis, this paper investigated effects on hydraulic dynamics on overland flow of gradient, flow and surface roughness based on primary principles of soil mechanicals and hydraulics and varying slope flume experiment on fixed-bed conducted indoor in which the underlying surfaces were made up of the same and different soil materials. Primary resources for soil erosion model building all over the world and scientific theoretical foundation for water and soil reservation are provided by interaction process analysis, regression analysis and prediction model building.(1)Variation rules of hydraulic dynamic properties on overland flow under the same underlying surfaceVelocity, Reynolds number and resistance coefficient were affected comparatively significant. The velocity could be described using power function in the form of V=KqaSb and Reynolds number in Re=1447.428q1.081. Resistance coefficient decreased with the increase of flow under the same gradient and the smaller the flow, the more significant the variation. They were negatively related. Resistance coefficient could be described by power function in the form of= 0.4488q-1.076. Yet Froude number was suffered from interaction of gradient and flow. The steeper the slope, the rapider the overland flow.This paper studied relationships among Froude number, resistance coefficient and Reynolds number respectively under the same gradient. The relationship between Froude number and Reynolds number was quite similar to that between Froude number and flow. They were positively related by favorable power function under the same gradient and the expression is Fr-0.0012 Re0.5427 while resistance coefficient declined with the increase of Reynolds number and they were described negatively related by favorable power function with the expression f=350.48 Re-0.912.In the discussion of relationship between velocity and roughness coefficient, we found out that a decline power function was showed between them under the same gradient. The velocity on each gradient varies little in the range of 5°-20°; When it increased to 25°, the decreasing trend of velocity aggravated and velocity decreased quite rapidly.(2)Variation rules of hydraulic dynamic properties on overland flow under different underlying surfacesEffect of surface roughness on hydraulic dynamic properties had much similarity to that of gradient and flow factors. Velocity, Reynolds number and Froude number were negatively related to surface roughness and they increased with the increase of roughness. Resistance coefficient was negatively related to surface roughness and declined as the roughness increases.Effects of surface roughness on hydraulic dynamics of runoff were influenced by soil properties. However, the relationship between Froude number and surface roughness could be described with the power function Fr-aDb whatever the underlying surface was.By the regression analysis to experiment results and a combination to statistic principles, we obtained the simple prediction models of hydraulic dynamics in condition of three underlying surfaces whose basic expression is Y= aDbqcSd.(3)Variation rules of surface roughnessShear strength of red soil, sand, mixture of soil and sand at percent of 30%(sand), at 50% and at 70% respectively could be achieved by direct shear test. Internal friction varied little while cohesion force varies with variation of the content of sand and soil in the sample with a range of 3.8-6.1Kpa.Factors of physicochemical properties of soil, precipitation or upslope runoff affected significantly on surface roughness thus making it a random value and it was related to shear strength negatively. Variation sections of surface roughness under different underlying surfaces were obtained by contacting probes as follows:red soil surface ranged from 0.3644 to 1.4320, sand surface from 0.2125 to 0.9046, surface of mixture of soil with sand at the percent of 30%(sand) from 0.3216 to 1.1385, at 50% from 0.2554 to 0.9424 and at 70% from 0.1643 to 0.8432.The variation of surface roughness with flow was positive relationship under the same gradient. Under the same flow, surface roughness varied with gradient positively below the gradient of 20 and negatively above 20, indicating that there was a critical gradient existing between 20°and 25°. The relationship between surface roughness and gradient as well as flow could be described by power function D= KqaSb.Effect of surface roughness on hydraulic roughness was a dynamic process and it did not present a single variation rule between which there exist a critical value and only after that value will the rule stay steady. This paper obtained the critical value of sand in 0.6315 and red soil in 0.9337 preliminarily.