Hydrodynamic Characteristics Of Overland Flow And Its Increasing Resistance Mechanisms

At present,most researches on overland flow study the hydrodynamic characteristics,mechanisms of surface phenomena and the actual process of erosion and sediment yield based on the theoretical formula of open-channels.Indoor artificial experiments affected by surface roughness,simulated rainfall,and vegetation cover were systematically carried out to study how rainfall intensity,surface roughness,as well as flow discharges influenced the hydrodynamic characteristics of overland flow.And then,the variation laws of roll waves and flow resistance are concluded.The following results are:（1）Overland flow over rough bed surfaces can be considered as laminar and turbulent flow when using Re.Similarly,the average value of flow index m is 0.543,indicating that overland flow is a mixed flow such that transitional flow plays the primary role while laminar flow plays the secondary role.The viscosity-depth ratio is a better method than the traditional Reynolds number discriminant method to a certain extent.Hence,overland flow exists in rolling-wave flow zone.Considering the influence of gradient,a critical water depth formula suitable for overland flow was re-derived,and it is concluded that this test water flow consists of supercritical flow in most cases,and subcritical flow in only a few cases.（2）When the particle resistance plays an important role,the resistance coefficient and Reynolds number shows a power function relationship,and the power index is approximately equal to1.Considering the"increasing resistance"mechanism of roughness,it is mainly due to four aspects including frictional resistance,thickness of the viscous sublayer,pressure drag and roll waves.The rolling waves of the continuous water flow cause the cross-section of the water to increase suddenly,resulting in a larger coefficient.Moreover,water depth increases as the roughness increases such that the contact area between water surface and the side walls increases.This indicates that more energy tends to be consumed in the action of water flow against resistance,which directly causes the“increasing flow resistance”.Meanwhile,the viscous sublayer is not thick enough to overlay the roughness of the bed as it increases.The rough elements will emerge and extend into the mainstream such that the flow path increases,which is similar to water flow on uneven surfaces.The water flow is more chaotic and causes energy loss such that f increases.Finally,R_ed increases with increasing bed roughness,whereas the viscous resistance decreases.Therefore,the turbulence intensity increases such that the pressure differences between the upstream and downstream particles become large.Hence,pressure drag（a component of total flow resistance）rises,resulting in an increase in f.（3）The rainfall intensity has a"reducing resistance"effect.And it is found that rainfall had a"reducing-resistance"effect,but compared with smooth surface conditions,this effect is not obvious on artificially roughened bed surfaces.This is believed to result from the larger raindrop momentum in the downslope direction.According to the momentum theorem,when the velocity of raindrops falling on the bed surface is larger than the surface velocity,the surface velocity of water will increase and the velocity of raindrop will decrease until the speed of the two drops is consistent.Also,rainfall increases the thickness of the viscous sublayer corresponds to the effect of reducing the bed roughness,so that rainfall has a"reducing-resistance"effect.Based on the notion of segmentation,resistance to overland flow can be partitioned into several categories of grain resistance,form resistance,wave resistance,and rain resistance.Because all points are obtained on fix beds and there are no changes in bed shape during the tests,wave resistance is ignored.Finally,a formula for sheet flow resistance is proposed under simulated rainfall based on resistance segmentation and multi-element linear regression.（4）Grass coverage is an effective measure to prevent water flow.Under grass cover,the flow velocity and water depth of slope flow increases with the increasing coverage degree,and the slope flow is still a mixed flow with laminar flow and transitional flow,which is the same as the test results without grass cover.The resistance coefficient and roughness coefficient all increases with the increasing coverage degree,and when the coverage degree is less than 47.20%,this increasing trend is not obvious.As for the relationship between the resistance coefficient and Reynolds number,when there is no vegetation cover on the bed,the resistance coefficient is inversely proportional to the Reynolds number.Under vegetation cover,there is a critical coverage degree between the resistance coefficient and the Reynolds number.When it is larger than this value,the resistance coefficient first decreases and then increases with the Reynolds number,which still exists in the relationship between the resistance coefficient and the submergence degree.As for the proportion of form resistance,when the coverage degree is small,it increases first and then decreases with flow discharge.On the contrary,it increases with the increasing flow discharge,and the growth decreases with the increasing coverage degree.Finally,the formula is derived based onπtheorem,which can simulate the resistance coefficient of grass cover accurately.It helps to build soil erosion models and clarify inherent mechanisms of sediment transport process.