Study On Hydrodynamic Characteristics Of Overland Under Vegetation Coverage

Based on the hydraulics and sediment movement mechanics,by means of fixed bed scouring resistance experiment,we draw into flow slowing coefficient and relative hydrodynamic parameters,compare grass to stem vegetation on hydrodynamic characteristic and derive formula of vegetation resistance coefficient,The study provides reference for the study of the soil and water conservation vegetation types and resistance,which provides reference for the study of vegetation types and resistance in the soil and water conservation and(1)The slope causes a great influence on average flow velocity when the slope degree is less than 6 under the vegetation stem;however,the effect of the overland flow on the slope can be ignored when slope is steeper than 6.Depth of the water before vegetation and average depth will increase with the flow discharge;Depth in the back of vegetation fluctuated.Resistance coefficients increase as Reynolds number increase,but decrease as the Froude Number increase;(2)Under the same slope,velocity and Reynolds number increase as submergence degree increases under the grass coverage;Flow regime with submergence degree transforms from slow flow to the rapids under the flexible vegetation coverage,however,it stays slow flow under the rigid vegetation coverage;Relative shear velocity increases with submergence degree under the flexible vegetation coverage,but it changes inconspicuously under the rigid vegetation coverage;Resistance coefficient decreases with increasing submergence degree under the flexible vegetation coverage,but it appears single hump as submergence degree increases under the rigid vegetation coverage;both relative flow velocity and relative flow depth are increased and the vegetation effect in reducing flow velocity is enhanced as vegetation coverage or flow rate increases,And the variation trends in both manifest a cut-off coverage of 4.650%.The vegetation effect on relative flow velocity depends much on flow rate but little on land slope,and flow rate are also independent of land slope.An increase in vegetation coverage or flow rate causes a higher relative Froude number.Relative resistance coefficient grows with an increasing relative flow depth.(3)Vegetation resistance increased with equivalent coverages and equivalent hydraulic radius increasing,the NSE value of model of resistance coefficient was 0.84.the contribution rate of equivalent coverage was less than those in the open channel flow,however the contribution rate of equivalent hydraulic radius and the drag force coefficient were approximately identical with those in the open channel flow(4)Relative depth under the grass covrage is higher than the that under the vegetation stem coverage,relative depth under superposition of two vegetation coverages is higher than the sum of that under each vegetation coverage;flow slowing coefficient under vegetation stem coverage is greater than the grass vegetation;which increases with flow increasing,but be less affected by slope.Compared to stem vegetation,additional vegetation stem resistance and grass vegetation resistance play a main role in total resistance;Added vegetation resistance and slope and depth have positive correlation,but has nothing to do with flow.When grass is submerged,combination of vegetation resistance remains the same,however,additional resistance showed a trend of decline.