Ephemeral Gully Erosion Processes Based On Different Erosive Force Factors And Numerical Simulation Of Ephememral Gully Flow

Based on the long-term field investigation,field survey,three-dimensional laser scanning and photogrammetry technologies,ephemeral gully development processes in Chinese loess hilly and gully region were analyzed;Based on the field survey and photogrammetry technology,soil pipe collapse impacts on ephemeral gully development were studied;Based on the controlled indoor rainfall and runoff simulation experiments,rainfall intensity,upslope and lateral inflow impact on ephemeral gully erosion were clarified;Base on the indoor runoff simulation experiment,seepage and soil pipe flow contribution to ephemeral gully headcut migration processes were quantified;Based on the numerical simulation of CCHE2D model,spatial and temporary distribution of surface flow hydrodynamic characteristics were analyzed.The results of this dissertation provided important scientific bases for establishing the process based ephemeral gully erosion prediction model and hillslope soil erosion prevention.The main results were as follows:?1?Ephemeral gully development in Chinese loess hilly and gully region were analyzed.From 2003 to 2015,ephemeral gully length with different upslope drainage areas increased12.1%²9.3%,annual growth rate were 51.5⁸2.3 cm yr^-1;cross section areas increased22.5%⁶5.1%,annual growth rate were 500.0⁴076.7 cm² yr^-1.When upslope drainage area increased 34%and 160%,ephemeral gully length increased 40%and 138%,cross section areas increased 20%and 75%,ephemeral gully depth increased 32%and 71%.The relationship between soil loss from ephemeral gully catchment and rainfall characteristics was established.The equation between drop sills distance and local slope was also established.?2?Three different soil pipe collapse features impact on ephemeral gully development were investigated in Goodwin Creek Watershed of the typical Loess region in USA.Compared with the pipe collapse features in 2013,the soil loss volumes caused by the sinkhole,flute hole and gully window increased 3.4%¹3.6%,10.2%³1.4%and 36.3%³7.9%in 2017,respectively.Total volume of soil pipe collapse features reached to 36.3⁷1.26 m³,which was 36.3.%⁴4%larger than that in 2013,reflecting the soil pipe collapse contributions to ephemeral gully development.With the contribution of seepage and soil pipe flow,the annual growth rate of average depth,surface area and volume of soil pipe collapse feature were 11.6 cm yr^-1?1.28 m² yr^-11 and 0.608 m³ yr^-1,respectively.?3?Rainfall intensity impacts on ephemeral gully processes and morphology were investigated.Under simulation experimental condition,when rainfall intensity increased from50 mm h^-1 to 100 mm h^-1,soil loss rate increased 1.1 to 3.3 times,average width and depth increased 1.4 and 0.6 times respectively,gully density,surface dissected degree and tortuosity complexity of ephemeral gully increased 10.1%³0.1%?25.0%³8.5%and 7.4%²6.2%,respectively.Directional derivatives were generated from original DEMs after rains according to the relationship between the neighbor grids,and it can be concluded that counter map of directional derivatives grids can reflect the length,surface area and gully bottom position of ephemeral gully channel and rills on the lateral slope.?4?Upslope inflow impacts on ephemeral gully erosion processes were studied.In 50,75and 100 mm h^-1 treatments,when upslope inflow rate increased from 1.3 L min^-1 to 20.0 L min^-1,the flow velocity with upslope inflow increased 22.7%to 79.4%.,and upslope inflow induced soil loss occupied 52.2%⁷4.1%,48.3%⁷1.4%and 29.5%⁶6.7%to the total soil loss.A soil loss prediction equation decided by upslope inflow topography and rainfall intensity was established and calibrated,the calibration results showed that determination coefficient and model efficiency coefficient were 0.80 and 0.87,respectively,which meant that the model was applicable.?5?Upslope and lateral inflow contributions to ephemeral gully erosion were separated.When lateral inflow was applied,lateral inflow contributed 45.4%⁶6.2%to the total soil loss.When upslope inflow was applied,upslope inflow contributed 72.3%⁹0.0%to the total soil loss.When upslope and lateral inflow were applied,lateral inflow contributed8.8%¹1.1%and upslope inflow contribute 64.7%⁸1.0%to the total soil loss.The results showed the great impacts of upslope inflow,but lateral inflow generated rill networks were important paths for surface flow concentrating into ephemeral gully channel,which increased the soil loss and could not be neglected.?6?Seepage and soil pipe flow contributions to the headcut migration were identified.Under simulation experimental condition,compared with free drainage condition,when water table was raised to headcut position,migration rates increased 42%and sediment rate increased 46.5%.when water head was set over the headcut,the migration rates increased 3.8to 8.2 times compared with free draniage condition and increased 1.7 to 4.8 times compared with the condition when water table at headcut position;sediment rate increased 2.1 times compared with free draniage condition and increased 43.8%compared with the condition when water table at initial headcut position.When soil pipe flow was conducted at seepage condition,the time of surface flow transferred to subsurface flow advanced 5.4 min,the headcut migration rate increase 5.3 times,sediment concentration increased 13.7 times.?7?Slope surface flow in ephemeral gully catchment was numerically simulated in CCHE2D model.The calibration results showed good accuracy in both ephemeral gully channel areas and interrill areas,which meant the model can be used to simulate flow in complex topography and inflow conditions.Velocity vectors distributions near the headcut were displayed and the result showed that,when upslope concentrated flow was concentrated in the headcut grid,the flow velocity was increased,while when flow entered into the plunge pool structure,the flow velocity was decreased,then flow velocity gradually increased with slope,this phenomenon showed the flow energy transfer and dissipation processes in the ephemeral gully channel.In rill and ephemeral gully channel areas,the measured velocities had no relationship to the Manning's equation,concentrated flow with a complex topography can only be predicted by solving general shallow water equations.While the kinematic wave method works well only in interill areas where the sheet flow prevails.