Slopeland Erosive Rainfall Runoff Hydraulics And Its Effect On Sediment Transport

Research on slopeland erosive rainfall runoff hydraulics and erosion mechanism is thebasis of hydraulic erosion dynamic discipline, and it is also important for the study of energysources. Aim at the problems on overland flow such as velocity distribution, flow patterndetermination, resistance calculation and erosion kinetic mechanism, the hydraulics ofoverland flow and its effect on mechanisms of soil erosion and sediment transport weresystematically studied through the experimental simulation combined with the theoreticalanalysis, basing on theories of hydrodynamics, sediment transport mechanics, soil erosion andhydrology. The underlying surfaces included roughened fixed bed, mobile bed and differentgrassland slopes. The study had great theoretical significance and application prospects forerosion processes prediction and soil and water loss control on slope. The main results are asfollows:(1) The influences of surface roughness and sediment load on velocity distributionof overland flow were analyzed, and the difference of Karman constant betweenslopeland erosive runoff and open channel flow as well as the vertical velocitydistribution were revealed. The Karman constant could reflect the flow energy dissipation.Through the experimental simulation on the roughened fixed bed and mobile bed, it wasfound that significant particle exchange existed among different layers of overland flow, sothe flow was turbulent flow. Then the vertical velocity distribution formula of overland flowwas given through generalizing the runoff erosion model and introducing Prandtl’s mixinglength theory. The verification results showed that the distribution of the vertical velocity wasthe logarithm distribution, and the Karman constant value for overland flow was smaller thanthat of open channel flow. Velocity distribution formula deduced from surface velocity couldmeet the demand for accuracy, and had a better theory and application value compared withthe methods that using flow discharge to calculate the average velocity.Surface roughness and sediment load S had significant influence on velocity distributionof overland flow. Karman constant k increased with surface roughness increasing, implyingthat roughness affected the turbulence mixing length of overland flow, and then resulting inthe homogenize of vertical distribution due to increasing turbulence. The rougher the slopewas, the more homogenize of the transverse velocity distribution was. As the S increased, the k value firstly decreased and then increased, the critical sediment load was about300kg/m3.The k values for sediment-laden overland flow were lower than that of clear water, showingthat the vertical distribution of sediment-laden flow was less uniform than clear water.(2) The effect of surface roughness and sediment load on resistance of overland flowwas discussed, and the roll wave evolvement of sediment-laden overland flow wasinvestigated. The total resistance f on rough slope was partitioned into grain resistance andcircumfluent resistance according to resistance division method. Circumfluent resistanceaccounted for almost78%of the total f, and the relative depth was a key parameter tocalculate the total resistance and circumfluent resistance. The resistance coefficient ofsediment-laden flow was larger than that of clear water when S<300kg/m3and smaller thanthat when S>300kg/m3. As S increased, roll wave frequency and velocity decreased, and rollwave length and the critical slope length for roll wave formation increased, indicating that thesediment in overland flow inhibited the roll wave formation, meanwhile, the criticalconditions for roll wave disappearance were also given.(3) The influence of rainfall runoff hydraulics on slope erosion forms was revealed,and the concept of erosion gully hydraulic geometry relation on slope was introduced. Inthe natural watershed, specific erosion forms occurred under specific slope and specificconfluence conditions. In order to research the relationship between the slope erosion formsand slope characteristics as well as rainfall runoff, the concept of erosion gully hydraulicgeometry relation was introduced, namely, the specific erosion forms would occur on thespecific slope under the specific upstream runoff and rainfall conditions, and thecharacterization parameters of erosion form such as gully width, water depth and gradientcorrelated with these slope characteristics and rainfall runoff.On this basis, the index and criteria were given to distinguish the different erosion forms.The optimal index was width to depth ratio parameters (ζ), and the suitable range of ζ for rill,ephemeral gully and gully were5~15,2~5, and0.2~1.5, respectively. The second optimalindex was hydraulic geometry index, and the exponents1,2, and3were0.3,0.4and0.3for rill,0.4,0.4, and0.2for ephemeral gully and0.5,0.4, and0.1for gully, respectively. Thethird was the hydraulics parameters. With the development of slope erosion forms from sheeterosion to rill, ephemeral gully and gully erosion, the corresponding Re and Fr increased andresistance coefficient decreased, and the flow regime for all erosion forms was rapid flow.(4) An incipient velocity formula for non-cohesive sediment on slope under rainfallconditions was proposed and the effect of rainfall on sediment incipient motion wasanalyzed. A mathematical expression of raindrop impact stress on individual sedimentparticle was deduced based on the principle of conservation of momentum, and then the formula with clear physical meaning and reasonable structure was established based on forcesanalysis. The formula indicated that rainfall could reduce the critical overland flow velocityand made the sediment particles easier to be entrained into motion. The rainfall impact largelydepends on the rainfall intensity, sediment particle diameter and the ratio of flow depth toraindrop diameter (h/dy). The larger the rainfall intensity, the easier the sediment started tomove, while the larger the sediment particle diameter or h/dy, the less influence the rainfallhad. The formula was applied in the prediction of sediment transport capacity of overlandflow under rainfall impact and the simulation results agreed well with the measured ones. Theeffect of soil cohesion on sediment incipient velocity should be taken into account whenapplying this formula on the Loess Plateau.(5) The unified resistance of erosive runoff on slope was preliminarily explored, anda coupled model that could reflect spatio-temporal variation of runoff generation andsediment transport processes on different land types was developed. The resistance oferosive runoff was preliminarily unified and divided into four resistance zones based on thesimulation experiments on different underlying surfaces and collection data. Then the modelwas developed. The model with fewer parameters had simple calculation and could satisfy theaccuracy requirement. The characteristics of runoff generation and sediment transport on theLoess Plateau were predicted using the model and the results showed that soil erosion wouldbe greatly controlled on5-year-old forestland or grassland with coverage more than60%under the normal r ainfall. Only the10-year-old forestland could provide a good erosioncontrol effect while that effects of grassland and bare land were limited under the once inten-year rainfall.