Transport Process And Characteristics Of Sediment And Nitrogen Of Grassed Waterway In Rare Earth Tailings In Southern Jiangxi

Large area of rare earth tailings are distributed in Ganzhou,Jiangxi Province,where vegetation is seriously damaged and a large number of ore leaching agents remain,resulting in soil erosion,non-point source pollution and ecological environment deterioration,etc.As an ecological measure widely used in ecological restoration and treatment,grassed waterway has a good effect in intercepting sediment and reducing non-point pollution.At present,the research on the characteristics of sediment and nitrogen transport in grassed waterway under the condition of continuous scouring is not deep enough.Therefore,taking the rare earth tailings in southern Jiangxi as the soil sample collection area,this paper selected the dominant grass species suitable for grassed waterway,and consecutive simulated scouring experiments on grassed treatment and bare soil treatment（control）were carried out.This paper discussed the mechanism of runoff and sediment yield,sorting and transportingof particle,hydrodynamics and erosion dynamic characteristics under the conditions of different flow(20 L·min^-1,30 L·min^-1,40L·min^-1)and slope（5°,10°）.The transport characteristics of total nitrogen（TN）,ammonia nitrogen（NH₄-N）and nitrate nitrogen（NO₃-N）in surface runoff,sediment and subsurface flow were explored.The simulation predictions and effect comparison were carried out by using multiple regression and BP neural network model,which provides an important reference basis for the application of grassed waterway technology in the prevention and control of soil erosion and non-point pollution in rare earth tailings,and has important theoretical significance for perfecting the study of soil erosion process in"slope-trench-region".The main results of this paper are as follows:（1）Through the comparison of 9 native herb varieties in Jiangxi Province,Eremochloa ophiuroideswas selected as the dominant variety.The growth status under different flooding time and depthand the reviving after floodingwere studied.The aboveground biomass in the root tipflooding treatment was the highest（6.71 g）,for the highest underground biomass was in the control treatment（2.60 g）,followed by root tip flooding（1.77 g）.The increase of root length under root tip flooding was second only to the control treatment,with 12.86 maverage total root length.For whole root flooding treatment,it was 12.52 m,and the lowest was for whole plant flooding treatment（11.91 m）.The total root length increased slowly at the initial stage of flooding,and grow rapidly after30 days of flooding.The largest root diameter was for whole plant flooding（0.56 mm）,followed by whole root flooding（0.55 mm）,root tip flooding（0.53 mm）and control treatment（0.50 mm）.Under the flooding time of 90 days,except for the whole plantflooding,Eremochloa ophiuroides showed better growth condition aboveground and underground.After the end of the flooding experiment,during the 30-day green period,all the experimental treatments could turn green quickly.（2）The characteristics of runoff and sediment transport in continuous scouring process were compared and analyzed under the conditions of different flow and slope.The discharge time of grassed waterway was longer than that of bare soil waterway.The runoff rate increased with the increase of scouring duration and then remained stable,but different scouring events and slope had no significant effect on the runoff rate of grassed waterway.The drainage of grassed waterwaywas more stable,while that of bare soil waterwaywas fluctuating.The sediment yield rate of the two kinds of waterwayswere significantly different,and the flow and slope had great influence on them,among which the slope had a greater influence.Under the condition of grassed waterway,the sediment yield rate at 5°slope,20 L·min^-1,30 L·min^-1 and 40 L·min^-1 flow were 1.48～9.50,4.10～7.11 and11.76～14.04 g·min^-1,respectively,while those at 10°slope were 5.06～35.01,26.40～39.60,95.79～303.54 g·min^-1 respectively.Under the condition of bare soil waterway,the sediment yield rate at 5°slope,20 L·min^-1,30 L·min^-1 and 40 L·min^-1 flow were34.69～1028.95,184.28～2221.89 and 159.31～3686.12 g·min^-1,respectively,while those at10°slope were 53.87～4241.79,1945.67～7076.39,671.71～11529.91 g·min^-1 respectively,which directly reflected the obvious sediment control effect of grassed waterway.The sediment particles in grassed waterway and bare soil waterwaywere consistent as a whole:The E/U value of clay and fine particlewas less than 1,and the E/U values of coarse silt,sand and gravel were all greater than or close to 1.The particle size of sediment in grassed waterway showed bimodal distribution,the particles<0.068 mm and 0.094～0.171mm dominated the eroded sediment,and almost all the coarse particles>0.955 mm were deposited.The distribution of sediment particles in bare soil waterway is multi-peak,and the sorting characteristics are not obvious.More than 50%of the sediment in grassed waterwaywas transported in the form of suspension/saltation,and the relative contribution rate decreases with the increase of scouring events and flow.The transport rates of suspended/saltation and traction forms of sediment in bare soil waterway were approximately equal.（3）The changing characteristics of hydraulics and erosion dynamics under different treatment conditions of continuous scouringwere discussed systematically.With the increase of scouring times,the flow velocity decreased slowly,and the velocity in the grassed waterway treatment(0.12～0.22 m·s^-1)was significantly lower than that（P<0.05）of the bare soil waterway(0.27～0.47 m·s^-1).The Reynolds of water flow in grassedand bare soil waterway treatmentwas 1368～5756 and 3869～10063,respectively,which showed an overall increasing trend with the times of scouring events,and both were turbulent flow patterns.Froude values decreasedalong with the scouring events,and all of them were slow flow pattern,while the variation of flow resistance coefficient of bare soil waterway was not obvious.The variation ranges of flow shear force,flow power,unit flow power and runoff erosion power in grassed waterway were 12.36～58.48 Pa,1.54～13.03 N·m^-1·s^-1,0.010～0.039 m·s^-1 and 0.80～4.60 m⁴·s^-1·km^-2,respectively.The shear stress increased with the increase of scouring times,slope and flow.Grass coverage had a significant effect on flow power and unit flow power（P<0.05）.（4）The characteristics of nitrogen transport of surface runoff,erosion sediment and subsurface flow in the process of continuous scouring were clarified.The slope had a great influence on the nitrogen loss of sediment andsubsurface flow ingrassed waterway.The change of flow had a great influence on therate and amount of nitrogen loss in surface runoff,while the variance between different slopewith the same flowwas small.The differences of NH₄-N loss in sediment were more obvious under large flow.Under 40L·min^-1flow,the amount of NH₄-N loss in 5°and 10°slope was 15.43 mg and 237.01 mg,respectively.Grassed waterway could effectively reduce the NO₃-N loss of sediment.Except for 10°slope and 40 L·min^-1 flow,the loss amount of NO₃-N in sediment was higher（96.36mg）,and the change of loss amount of NO₃-N in sediment was small under other conditions（5.37 mg～10.97 mg）.The overall nitrogen loss in subsurface flow showed the law that the effect of flow on nitrogen loss increased with the increase of slope.Different scouring times,slope,flow and grass coverage had extremely significant effects on nitrogen transport.Subsurface flow was the main way of TN and NO₃-N transport in grassed waterway treatment,and NH₄-N was mainly transported through surface runoff.The loss of nitrogen in subsurface flow mainly occurred in the last three events of five continuousscourings,in which the loss of TN,NH₄-N and NO₃-N accounted for46.13%～84.20%,25.99%～78.58%and 33.50%～78.58%of the total,respectively.（5）Multiple regression and BP neural network model were used to simulate and predict the water,sediment and nitrogen loss under the condition of continuous scouring.The runoff yield accuracy simulated by multiple stepwise regression method（>0.8）was higher than that of sediment yield simulation（<0.8）.The runoff yield of different slope was mainly affected by runoff erosion power.Unit flow power and runoff width were the main factors affecting sediment yield.The accuracy of nitrogen loss predicted by multiple regression fitting was relatively low,in which the determination coefficient of TN loss model in sediment particle was the largest（0.529）.Runoff erosion power,unit flow power and runoff characteristics were the main influencing factors of nitrogenlossin sediment particle.The overall performance of the multiple regression model was that the higher the slope,the greater the fitting R² value.The relative errors of BP neural network analysis of runoff yield simulation results were all less than 6.5%,and in addition to runoff erosion power,hydraulic radius also played an important role in runoff and sediment production.The relative error of the simulation results of nitrogen loss with BP modelwas less than25%.Hydraulic radius and flow power had a great influence on the nitrogen loss in sediment particles.And generally speaking,BP neural network model was better than multiple regression model.