Overland Flow Hydrodynamic Characteristics And Sediment Transport Mechanism

The research on the overland flow resistance characteristics,the driving mechanism of sediment,and the sediment transport capacity prediction model are the basis for developing the physical process model of soil erosion.The mechanism of runoff and sediment transport on slopes covered by different grass patches is one of the hot and difficult topics in soil erosion research under the new situation of returning farmland to forests and grasslands in the Loess Plateau.Based on the results of the fixed bed resistance test,the test of the overland flow sediment transport capacity and the erosion test of the grass hillslope under the simulated rainstorm conditions,a systematic study was carried out on the variation law of the overland flow surface velocity correction coefficient under the influence of different underlying surface characteristics and rainfall,the overland flow resistance,the characteristics of erosion and sediment transport,and the regulation mechanism of the grass on runoff and sediment.The following new results have been obtained:（1）The evolution law of the correction factor of surface overland flow velocity（α）under different underlying surface characteristics and rainfall conditions was clarified.The effects of surface roughness,rainfall intensity,and grass cover onαwere quantified through indoor experiments with six types of surface roughness,five types of rainfall intensities,ten types of grass cover density,and five types of simulated grass and shrub communities.The research showed that there is a significant negative correlation betweenαand slope and roughness（p<0.001）.The grass density had a significant effect onα.When the grass density（G_d）was less than 3.45%,αincreased with the increase of G_d;when G_dwas greater than 3.45%,αshowed an opposite trend with the increase of G_d.Flow depth（h）was an important parameter for predictingαunder grass-covered conditions.Finally,theαprediction model suitable for rainfall and roughness and theαprediction model under grass cover and grass-shrub community cover were constructed respectively.（2）The influencing factors of the overland flow Darcy-weisbach friction factor（f）were revealed,and the Darcy-weisbach friction factor calculation model was constructed based on dimensional analysis.The power function relationship（f=K/Re^b）between f and Reynolds number（Re）showed that in laminar flow,the power exponent b was 1,and the coefficient K increased with the increase of roughness,which was greater than the fixed value of 24 in open channel laminar flow.The overland flow velocity profile parameter（Γ）had a good power function relationship with slope gradient,Froude number and Re.For water flow,the 97.8%error in f calculated using the final formula was≤±10%.For mixed fluids flow,when Re≥7,the 92.2%error of f calculated according to the final formula was≤±10%.When Re<7,the f calculated by the final formula had 64.4%error≤±20%,and 31.1%error≤±10%.（3）A prediction model of sediment transport capacity（T_c）based on general flow intensity parameter was constructed.Based on the internal relationship of the mean flow velocity,shear stress,stream power and unit stream power calculation formula,a general flow intensity parameter was constructed,and the sediment transport capacity prediction model suitable for different soil textures was established.According to the evaluation index P.O._0.5–_2.0（the percentage of predictions in which the discrepancy ratio falls between 0.5and 2.0,and the discrepancy ratio is the ratio of the predicted and measured T_c）,the prediction performance of different sediment transport capacity equations was analyzed.It is shown that the sediment transport capacity equation derived using large particle size soils generally underestimates the sediment transport capacity of soils with small median particle size.The prediction model based on the general flow intensity parameter had high accuracy,its P.O._0.5–_2.0 is 83%and the calculated P.O._0.5–_2.0>73%for each soil sample.（4）The erosion mechanism of Shenmu loess was revealed.Meanwhile,the unbalanced sediment transport prediction model of hillslope erosion in sandy and coarse sand area was constructed.The erosion mechanism of Shenmu loess was divided into two categories.The first type was the soil detachment-limited in the initial rainfall stage and the sediment transport-limited in the subsequent rainfall stage.Another type was the sediment transport-limited during the entire rainfall process.The stream power was a good predictor of the erosion rate of a single soil sample.However,the prediction accuracy of the erosion rate of the two soil samples using the stream power was low.Based on the unbalanced sediment transport theory and the sediment transport capacity equation,the unbalanced sediment transport model of hillslope erosion was constructed.For the validation dataset,the unbalanced sediment transport model had higher prediction accuracy（R²=0.92,ME=0.88,RRMSE=6.7%）.（5）The regulation mechanism of runoff and sediment on hillslope erosion by grass patch types was clarified,and the prediction equation of grassland soil loss under different types of grass patches was constructed.The soil and runoff retention capacity of the grass patch types from large to small were distributed in the order of horizontal strips,random strips,staggered strips,checkerboard and vertical stripes.The relationship between relative runoff（R_r）and relative soil loss（SL_r）and grass cover（V_C）was in accordance with an exponential function(R_r=e^-η₁V_C,SL_r=e^-η₂V_C.For all grass patch types,the decay indexη₁was between 0.003 and 0.032;the decay indexη₂ was between 0.011 and 0.029.Based on the energy dissipation mechanism during soil erosion,the components of rainfall kinetic energy and surface runoff potential energy were revealed.For all test conditions,the ratio of the effective kinetic energy of raindrops to the total effective energy did not exceed10%.The erosion caused by raindrops and overland flow was the result of the combined effect of the effective kinetic energy of raindrops and the effective potential energy of runoff.Finally,with the effective energy（the sum of the effective kinetic energy of raindrops and the effective potential energy of runoff）as the predictor,the hillslope erosion model under different grass patches was constructed.