Effects Of Grass Coverage And Spatial Pattern On Slope Runoff And Sediment Regulation Under Simulated Rainfall Conditions

The hydrodynamic characteristics of overland flow and the process of runoff and sediment movement under the grass covered slope are the dynamic basis of hydraulic erosion and the focus of the current research.However,due to the influence of vegetation,rainfall,topography,soil and other factors,the regulation mechanism of runoff and sediment is complex,and the observation means of overland flow under the condition of rainfall are not perfect,so there are still many doubts about the regulation mechanism of vegetation on slope runoff and sediment.Through the indoor simulated rainfall experiment,the diachronic process and mean change law of slope hydrodynamic parameters under the influence of grass coverage and patterns were studied,and the flow resistance model of grass covered slope is constructed.The effects of coverage and patterns on the characteristics of runoff and sediment yield on slope were analyzed,and the coupling relationship between erosion rate and flow intensity index was established.And the regulation effect of grass on runoff and sediment on slope was discussed.To clarify the particle size distribution（PSD）of erosion sediment on slope under different grass patterns.The research results can provide theoretical basis and scientific support for slope erosion control and high-quality development of ecological environment on the Loess Plateau.The main research results are as follows:（1）The response law of overland flow hydrodynamic parameters to grass coverage and patterns were explored.The ranges of mean flow velocity and Darcy-Weisbach resistance coefficient are respectively 0.01-0.27 m s^-1 and 0.21-5.42.The flow regime belongs to laminar flow.With the increase of slope,the flow pattern changes from slow to jet.The time required for mean flow velocity to converge to the steady-state phase is overall positively correlated with coverage and L_f.The time at which Reynolds and Froude numbers converge to the steady-state phase is overall negatively correlated with L_f.The Reynolds numbers all increased sharply with rainfall ephemeris and then stabilized.The greater the grass coverage,the smaller the fluctuation in the calendar time.The Froude number decreases and then stabilizes with rainfall duration for high grass coverage,diamond shaped,step strip and banded grass strip patterns,and showed the opposite trend in other cases.Under the same slope and rainfall intensity,the mean flow velocity,Reynolds number and Froude number of overland flow decrease exponentially and power function with grass coverage and L_f,respectively.The flow regime index m ranges from 0.315-0.899,which is directly proportional to grass coverage and L_f and inversely proportional to rainfall intensity.On the whole,the resistance coefficient decreases first and then stabilizes with the rainfall duration,and the fluctuation range of resistance coefficient decreases with the increase of coverage and L_f.The resistance coefficient were exponentially related to coverage and L_f.The slope resistance coefficient decreases with the increase of Reynolds number.Finally,this study constructs a slope resistance model including particle resistance,morphological resistance and rainfall resistance.（2）The process characteristics of runoff and sediment production on grass covered slope were studied and the coupling relationship between runoff and sediment yield and flow intensity index was established.The variation trend of runoff rate with rainfall duration can be divided into two stages,which first increases and then tends to dynamic stability.The infiltration rate decreases at first and then stabilizes with rainfall duration,the erosion rate duration trend varies with the slope,and the sediment concentration of slope under different coverage and transverse grass belt patterns decreases at first and then stabilizes with rainfall duration,and the trend is opposite under other conditions.The time at which Runoff rate,erosion rate and sediment concentration converge to the steady-state phase is overall positively correlated with coverage,L_f.The ranges of runoff rate and erosion rate are respectively 0.22-2.40 L m^-2 min^-1 and 7.48-48.63 g m^-2 min^-1,the sediment concentration decreases by a range of 29.5%-45.2%.Runoff rate,erosion rate and sediment concentration were positively correlated with coverage and L_f,respectively.The greater the intensity of rain,the easier it is to form a crust on the soil surface.There is a very significant positive correlation between erosion rate and stream power and unit energy of cross section.The erosion rate has a power function and exponential relationship with flow shear force and stream power,respectively.The prediction effect of the erosion rate index model based on the unit energy of the cross section is the best.（3）The effect of grass patterns on the particle size distribution and transport law of slope erosion sediments were clarified.The clay-sized particle content of the eroded sediments was controlled by the original soil and the grass distribution patterns,and its effective particle size content was only 3.55%–13.06%.In the bare slope,random,checkerboard,and vertical strip pattern,the fine silt-and sand-sized particle contents decreased and increased with rainfall duration,respectively,but the opposite trend was found in the step strip and transverse grass belt pattern.In addition,the fine silt-and sand-sized particle contents showed a negative linear correlation and a positive linear correlation with stream power,respectively,which indicates that stream power has an associated threshold level.The degree of eroded sediment particle enrichment was significantly affected by the grass distribution patterns and rainfall intensity.Under a rainfall intensity of 60 mm h^-1,the finer particles（clay and fine silt）in bare slope were preferentially enriched as aggregates,while the sand-sized particles of vertical strip pattern,step strip pattern,and transverse grass belt pattern were preferentially enriched as aggregates,and the PSDs of the eroded sediments in random patch and checkerboard patch were close to that of the original soil.In addition,coarse silt-sized particles were consistently transported as primary particles.When the rainfall intensity increased to 120 mm h^-1,sand particles were enriched and clay particles were enriched only under random patches and step strip pattern.（4）The regulation effects of grass coverage and patterns on slope runoff and sediment were investigated.The deceleration effect,runoff reduction effect,sediment reduction effect and sediment concentration reduction effect of grass coverage are-19.81%-86.68%,-10.12%-75.83%,-23.53%-70.62%,-10.38%-62.01%,respectively.The deceleration effect was positively correlated with coverage and L_f,respectively.The effect of runoff reduction increased exponentially and logarithmically with the increase of coverage and L_f,respectively.The effects of sediment reduction and sediment concentration reduction effect on slope increased logarithmically with the increase of coverage and L_f.60%grass coverage can be used as the critical value of slope runoff reduction effect.The order of the effects of runoff and sediment reduction is Transverse grass belt pattern>Step strip pattern>Diamond shaped pattern>Random patch>Checkerboard patch>Vertical strip pattern.The vertical strip pattern aggravates the degree of slope soil erosion.The increase of rainfall intensity and slope leads to the decrease of the deceleration effect,runoff reduction effect,sediment reduction effect,sediment concentration reduction effect by 22.3%-48.6%,13.5%-46.1%,25.9%-52.1%,38.5%-68.3%and 16.7%-56.6%,24.7%-44.5%,62.7%-63.0%,39.1%-51.2%,respectively.