Study On The Effect Of Vegetation Composition On Hydrodynamic Characteristics Of Overland Flow

Overland flow is a main source of slope erosion,and its hydrodynamic characteristics are sensitive to surface features and rainfall intensity.Therefore,in-depth studies on hydraulic characteristics of slope flow resistance under different vegetation conditions are of great significance for investigating slope erosion on the Loess Plateau.Indoor bed erosion tests under artificial simulated vegetation conditions were carried out in this study.And different hydrodynamic characteristics of overland flow under rigid and flexible vegetation conditions were also analyzed on the basis of hydrodynamics and hydraulics theory.Then,hydrodynamic characteristics especially the resistance characteristics of overland flow under a combination of grass and stalk vegetation were further analyzed based on the comparative study.Finally,a formula for sheet flow resistance was proposed under combined vegetation conditions.These findings not only explore a new method when calculating the flow resistance,but also lay a theoretical foundation for an extension of the open-channel theories.There were some differences in the hydrodynamic characteristics of water flows under rigid and flexible vegetation conditions.The flow velocity coefficient under rigid vegetation conditions was larger and the mean flow velocity under flexible vegetation coverage was more susceptible to flow discharges;The Froude number under rigid vegetation coverage did not exceed 1,therefore the water flow belonged to subcritical flow zone.However,as the discharge increased,overland flow under flexible vegetation coverage developed towards supercritical flow.Under the condition of rigid vegetation coverage,the drag coefficient of the overland flow increased first and then decreased with increasing flow discharges,Reynolds number and water depth.The resistance coefficient under flexible conditions was negatively correlated with flow discharges,water depth and Reynolds number.In addition,under the flexible vegetation coverage,the drag coefficient decreased and the rigid vegetation varied as a single-peak curve with an increasing submergence degree.The velocity distribution in vertical directions under a combined vegetation condition was more uniform.And the numerical dispersion was smaller than that under shrub conditions.Under the combined vegetation condition,test points were distributed in the slow transitional zone,followed by the slow laminar flow zone,the torrent transitional zone,and the torrent laminar flow zone.When the flow discharge Q was less than 60L/min.the slow-flow effect of combined vegetation conditions is better than their sole influences.But when the flow discharge Q reached 60L/min,the ab ove effect was no longer significa nt.And the variation trend of slow flow coeff icien ts un der shrub vegetation condit’ions was quite diff e;rent.The grass tended to be inun dated with a larger flow discharge,.so the eff ect was we,ak ened.The Gombined vegetation bad the advantages of shrub vegetation and grass coverage.Comp,ared with shrub vegetation,the water l.evel difference between the ups tream and the backwater surface of water flow was re du ced.And the slope erosion caused by the tail water vortex was also mitigated.In oth er words,under comb ined vegetation c on ditions,wa ter depth was higher than the grass coverage and the resista nce was further enhanced.Under conditions of simulated grass or shrubs,relationships between the drag coef ficient and Reynolds number prominently depended on the resistan ce.When the coverage degree was less,the particle re sistance played a dominant role.and the relationship between the two was negatively correlated(when λg=0.07).As the coverage degree increased,the morph ological resaistance gradually exerted a dominant influence.The relationship between the two gradually turned into a positive correlation.There was an additional resistance under the action of combin ed vegetatin and the ratio of it to total resistan ce was larger overall.Its relationship with coverage was not significant.When the grass was inundated,the additional resistance deicreased with increasing water depth,and the ef fect of combin ed veg etation was reduced.