Study On Hydrodynamic Properties Of Overland Flow

Hydrodynamic properties of overland flow are the foundation to explore the mechanics of water erosion.However,due to the complexity of the flow regemie and the limitation of the measuring technique,the current understanding on the flow velocity and structure features of overland flow is very limited,and the dynamic characters of overland flow under the interaction of multiple factors has not reached a general consensus.Taking advantage of Particle Image Velocimetry(PIV),the 2D velocity profile features of overland flow are studied.The differences between overland flow and open channel flow are compared.Flow structures upstream of isolated vegetation are explored for overland flow encountering rigid vegetation.Conducting on flume scouring and artificial rainfall experiments,the hydrodynamic properties of overland flow are also discussed under the influence of surface roughness,rainfall intensity,and simulated vegetation.Simultaneously,the relationship between total resistance of overland flow and individual resistance is studied.The main results are as following:1.The friction velocity u*derived from Reynolds stress distribution can be effectively used to estimate the velocity profile and characteristic parameters of overland flow.Hereby,the velocity profile can be divided into the inner region affected by viscous stress and the outer region affected by Reynolds stress,and the boundary between the inner zone and the outer zone is variable.Four sub-regions are further recognized according to velocity features along flow depth:viscous sublayer region,buffer layer,log-law region and intermediate region.According to properties of the boundary of inner and outer region y/H,integration constant C and damping coefficient A,the critical depth of overland flow distinguished from open channel flow is 1.8 cm;2.Time-averaged and instantaneous velocity characteristics of overland flow upstream of simulated vegetation are extracted.Two preferred flow states have been found according to the topology of Probability Density Functions,i.e.back-flow mode and down-flow mode.The latter one is induced by a portion of fluid which is unable to penetrate the vortex nearby flume bed due to the extremely low momentum and then vertically impinged the bed.Therefore,it is considered as a special flow pattern existing in overland flow.Features of Horseshoe Vortex(HV)are observed and researched,because it is responsible for scouring at the base of vegetation.Then,with increasing cylinder Reynolds numbers ReD,three stages of HV are significantly recognized.The significant effect of surface roughness on HV is also verified;3.From artificial rainfall simulation experiments,overland flow resistance is positive correlated with surface roughness while is negative with rainfall intensity as well as Reynolds number Re.Rainfall would reduce the total resistance and overland flow resistance decreases as increasing rainfall intensity Relationship between overland flow resistance and Re is positive for vegetated slope while that is negative for non-vegetation slope.The discrepancy between total resistance and sum of rainfall resistance and grain resistance is also discussed and results show that total resistance is larger than the sum of the individual resistance through t-test,as well as vegetation resistance and grain resistance.The results indicate that it is unsuitable to apply linear superposition principle in the calculation of overland flow resistance;4.The effects of vegetation coverage and configuration are studied.The effects of vegetation coverage on overland flow are larger than that of vegetation configuration on overland flow.In addition,effects of vegetation configuration become weak as increasing slope gradient.The largest resistance to overland flow is vegetation with 0 uniform angle index for same vegetation coverageThe above research results can provide a theory foundation for the mechanism of soil erosion,simultaneously,providing some technical support for forest managers.