Study On The Mechanism Of Forage Grass Distribution Patterns For Regulating Flow Retardation And Sediment-Trapping Processes On The Slope Land

The hydrodynamic characteristics of overland flow under the effects of vegetation landscape pattern and its response mechanism to sediment-trapping processes via vegetation are not only the weakness but also the hotspot of current soil erosion research.Due to the diversity of vegetation species,the difference of landscape patterns,the influence of rainfall conditions,the physicochemical properties of soil,micro-topography,and other factors,the internal mechanism of water-sediment evolution processes on the vegetation-covered soil slope is still not clear.There are still many doubts about the mechanism of vegetation landscape pattern on the water-sediment control of overland flow.In this study,indoor immovable-bed scouring tests,artificial simulated rainfall tests,and movable-bed sediment interception tests of transverse grass strips were conducted.The effects of different grass distribution patterns（GDPs）were explored as the main line,and the parametric analyses of grass distribution patterns were taken as the starting point.Based on the benefits of grass distribution patterns on flow retardation and sediment interception,the effects of different grass distribution patterns on the water-sediment evolution process were quantified and the best pattern for flow retardation and sediment interception was recommended.The results of this study can enrich the regulation and control scheme of water-sediment processes,which are driven by returning farmland to forests,on the soil slope of loess area.And it can also provide theoretical evidence and technical reference for ecological environment protection on the Loess Plateau of China.The main conclusions of this study are as follows:（1）The influence laws of grass distribution pattern on the hydrodynamic characteristics of overland flow were clarified.Immovable-bed scouring experiments were conducted in a hydraulic flume across eight flow discharges(3–45 L·min^-1),six slope gradients（2–12°）,and five GDPs—a banded pattern perpendicular to slope direction（BP）,a vertical strip pattern parallel to the slope direction（VP）,chequerboard pattern（CP）,small patches distributed in an’X"-shape（XP）,and a random pattern（RP）.An immovable-bed was chosen here to ensure that the water flow did not affect the underlying surfaces,in order to focus on the interaction between vegetation factor and the hydrodynamic characteristics of overland flow.The relative hydrodynamic parameters（relative velocity u_r,relative water depth h_r,relative Reynolds number Re_r,relative Froude Fr_r,and relative resistance coefficient f_r）were introduced to quantify the differences between the hydraulic parameters on the slope covered by five grass distribution patterns with those on the bare slope（BS）.Under the influences of five patterns,relative flow velocity,the relative water depth,relative Froude number,and relative resistance coefficient were positively affected by flow discharge while negatively affected by slope gradient significantly（p<0.01）;whereas the effects on the relative Reynolds number were inverted.Compared with VP,CP,XP,and RP,the effects of BP on five relative hydrodynamic parameters was absolutely different.For instance,the mean of relative Reynolds number under the effect of BP were the smallest as valley values at all slopes,where the minimum values were-0.029,-0.018,-0.009,-0.006,-0.002,and 0.0033 at2°,4°,6°,8°,10°,and 12°,respectively.Through parametric analysis of grass distribution patterns,based on patch density P_dand path connectivity P_ci,nonlinear regression prediction models of relative flow velocity and relative Froude number were established with high accuracy.BP was recommended as the best pattern for flow retardation because it can effectively slow down the flow velocity,raise the water depth,reduce the turbulence of water flow and increase the resistance.If grass cover is going to be laid on the soil slope of loess area,it can be planted in BP,which can effectively retard the hydrological process and weaken the hydraulic erosion.（2）The response laws of runoff and sediment yield on the slope surface to grass distribution pattern were expounded.Artificial simulated rainfall tests were conducted in an experimental hall across five rainfall intensities(60–120 L·min^-1),three slope gradients（5°–15°）,and five GDPs—a banded pattern perpendicular to slope direction（BP）,a vertical strip pattern parallel to the slope direction（VP）,chequerboard pattern（CP）,small patches distributed in an’X"-shape（XP）,and a random pattern（RP）.By considering the underlying surfaces,the effects of five grass distribution patterns on the characteristics of runoff and sediment yield on the slope surface were compared under different erosion conditions on the movable-bed by changing rainfall intensities.According to the variation range of mean flow velocity/runoff yield,and erosion rate/sediment yield,the characteristics of runoff and sediment yield process on the slope surface were quantified.The four parameters were mainly affected by the interaction of rainfall intensity,slope gradient and grass distribution pattern,which have significant differences（p<0.01）.The corresponding patterns of four parameters from large to small were VP,CP,RP,XP and BP,respectively.The relationship between erosion rate and flow power was significant power function,and the erosion rate model based on the flow power was evaluated with satisfying result.Based on boundary density B_dand path connectivity P_cm,a model considering the effect of grass distribution pattern on the sediment interception of slope surface was established.BP was recommended as the best pattern for flow retardation and sediment interception,because it can effectively reduce runoff and sediment yield,and has significant flow reduction and erosion resistance.If grass cover is going to be laid on the soil slope of loess area,it can be planted in BP,so as to effectively control the development of rainfall erosion on the slope and the confluence process of small watershed units.（3）The benefits of flow retardation and sediment interception of transverse grass strip（TGS）under different combinations of numbers and widths was explored.According to the above immovable-bed scouring tests and artificial simulated rainfall tests,BP was recommended as the best pattern.Further,the movable-bed scouring tests under the cover of five combinations of transverse grass strip（2.4 m TGS,1.2 m TGS,0.6 m TGS,0.3 m TGS and 0.15 m TGS）were carried out in an experimental hall across three flow discharges(15–30 L·min^-1)and three slope gradients（5°–15°）.Under the threshold of grass coverage50%,the difference of the effects of five combinations of transverse grass strip on flow retardation and sediment interception were compared.According to the variation curves of interval flow velocity and section water depth on the slope,which significantly responded to flow discharge and slope gradient（p<0.01）,the"stepped"backwater effect of transverse grass strip was clarified.Based on the diachronic variation of erosion rate and the total amount of eroded sediment,the"layer-by-layer"sediment interception effect of transverse grass strip was quantified.In the 95%range of experimental flow discharge,0.15 and 0.3 m TGS had the most significant flow retardation sediment interception effect,and when the slope gradient reached the maximum designed value,0.3 m TGS was verified to have the best flow retardation sediment interception effect.The enrichment rate and aggregation ratio of eroded sediment particles calculated by sediment particle size distribution revealed the property of water flow on selecting specific sediment particles and their transport in the slope erosion process.Based on the analyses of the characteristic parameters of transverse grass strip,a multi-factor regression model of sediment interception was established based on the number of transverse grass strip n and the corresponding width W of transverse grass strip.If transverse grass strip are going to be laid on the soil slope of loess area,the number of grass strips can be increased to form a"stepped"grass-covered slope,and the sediment particles can be intercepted by"layer-by-layer"via flow retardation,thus greatly reducing the sediment carried by the confluence process of overland flow.