Study On The Effects Of Typical Pollutants On The Flow Variation Characteristics Of Erosive Water Flow

With the development of the"Returning Farmland to Forest(Grassland)"project and the promotion of high-standard farmland construction on the Loess Plateau,the three-dimensional development mode from slope to valley has gradually been applied in small watersheds.However,under this mode,the underlying surface has undergone drastic changes due to engineering measures such as building slope-separated terrace,terrace greenhouses,gully land consolidation,and warp land dam.Due to the large use of agricultural fertilizers,combined with frequent extreme rainfall,the raindrop splash can cause the sediment content in the low-lying areas to reach over 500 kg/m~3.The high-sediment water flow formed by the runoff of rainfall containing soil nutrients such as nitrogen,phosphorus,and potassium will produce greater erosion force,often resulting in severe erosion with an erosion modulus of more than 50,000 t/(km~2·a),posing a great threat to the ecological environment and socio-economic development.What are the fluid properties of the sediment-laden water flow polluted by soil nutrients such as nitrogen,phosphorus,and potassium through rainfall runoff,and why does it have such a great erosion effect?These are important scientific questions that need to be addressed in the study of erosion fluid.This study utilized experimental instruments such as a rotational viscometer,field emission scanning electron microscope,and a dual-vertical-tube rheometer developed by the affiliated institution.The research focused on investigating the flow variation characteristics of highly polluted and sediment-laden water under various combinations of urea concentrations ranging from0 to 511 kg/m~3,calcium superphosphate concentrations ranging from 0 to 417 kg/m~3,potassium sulfate concentrations ranging from 0 to 458 kg/m~3,and sediment content ranging from 0 to 800 kg/m~3.The main achievements obtained are as follows:1.Constructed a calculation model for the viscosity coefficient of high-suspended sediment water flow contaminated with nutrients.There is a lack of research on the viscosity of contaminated high-suspended sediment water flow in the Loess Plateau,and there is an urgent need to clarify the influencing factors in production.In this study,the physical and chemical properties of nitrogen,phosphorus,potassium,and sediment water flow at different concentrations were measured,and combination rheological experiments were conducted to explore the physical characteristics and influencing factors of erosion-polluted water flow.The experimental results showed that:(1)The physical and chemical properties such as solubility,density,acidity,and conductivity of nitrogen,phosphorus,and potassium water flow were influenced by factors such as temperature and concentration.At room temperature,water flows of different concentrations of sediment,urea,calcium dihydrogen phosphate,and potassium sulfate exhibited alkalinity(p H=7～8.77),alkalinity(p H=7～8.60),acidity(p H=7～2.74),and acidity(p H=7～4.03),respectively,and their conductivity values were 0.12～0.14,0.01～64.50,0.01～3.00,and 0.01～232.00m S/cm,respectively,with the ionization degree being potassium sulfate>calcium dihydrogen phosphate>urea>sediment;(2)Under constant temperature conditions(25℃),the viscosity coefficient of erosion-polluted water flow was mainly affected by the concentration and physical and chemical properties of nitrogen,phosphorus,potassium,and sediment.With the same sediment content,the viscosity coefficient increased by 62.64%～187.92%,53.69%～164.43%,and11.19%～129.75%for every 0.1g/cm~3 increase in nitrogen,phosphorus,and potassium concentration,respectively.The viscosity coefficient of erosion water flow increased exponentially with the increase in nitrogen,phosphorus,and potassium concentrations;(3)A calculation model for the viscosity coefficient with sediment concentration,particle size distribution,and pollutant concentration was established and verified.2.A calculation model for the yield stress of nitrogen,phosphorus,potassium,and sediment-laden water flow and the critical threshold for fluid variation was established.There is a need for further research on the rheological properties of high sediment-laden water flows contaminated by nitrogen,phosphorus,and potassium on the Loess Plateau.Based on the above research,this study investigated the fluid type and influencing factors of eroded polluted water flows.The results showed that:(1)High-concentration eroded sediment-laden water flows were influenced by pollutants such as nitrogen,phosphorus,and potassium,exhibiting characteristics of yielding dilatant fluid or Bingham fluids.(2)Under the same sediment content,for every 0.1 g/cm~3increase in nitrogen,phosphorus,and potassium concentrations,the yield stress increased by 0～0.57 N/m~2,0～0.6 N/m~2,and 0～1.67 N/m~2,respectively.The calculation models for the yield stress of different types of eroded polluted water flows were derived through theoretical deduction and verified.(3)Based on the above research,the critical threshold for fluid variation in eroded polluted water flows was determined.For nitrogen-containing eroded water flows,when the sediment content was greater than the critical value of 235 kg/m~3,fluid variation occurred as a yield-thickening fluid.As the concentration of calcium phosphate increased from 0 to 0.45 g/cm~3,the critical sediment content for the transition from a Newtonian to a non-Newtonian fluid decreased from 249 kg/m~3 to 127 kg/m~3,a decrease of 49%.When the concentration of potassium sulfate increased from 0 to 0.45 g/cm~3,the critical sediment content decreased from 275 kg/m~3 to 89 kg/m~3,a decrease of 68%.3.Through rheological experiments on nitrogen,phosphorus,potassium,sediment,and their mixed water flows,variations in resistance characteristics of different erosion-polluted water flows were discovered.Further analysis of the effect of pollutant type and concentration on flow and resistance variation characteristics of water flow was conducted through double vertical tube rheological experiments.The results show that:(1)The resistance loss along the course is influenced by various factors such as temperature,sediment concentration,nitrogen,phosphorus,and potassium concentration.With controlled factors such as temperature,pipe length,and diameter,under a design sediment concentration of 200kg/m~3,the unit flow velocity resistance loss of sediment water flow,nitrogen-sediment water flow,phosphorus-sediment water flow,and potassium-sediment water flow with different pollutant concentrations were 1.03,0.70～1.00,1.33～2.05,and 1.11～1.37,respectively.Except for nitrogen,phosphorus and potassium exhibit an increasing resistance effect on sediment-laden water flow.(2)The resistance coefficient of erosion-polluted water flow follows a classical power function relationship with Reynolds number.As the concentration of nitrogen,phosphorus,potassium,and other pollutants increases,parameters a and b show an increasing and decreasing trend,respectively.Under the same Reynolds number,the order of resistance coefficient along the course of different concentrations and types of erosion-polluted water flow is as follows:sediment-potassium sulfate water flow>sediment water flow>sediment-calcium phosphate water flow>sediment-urea water flow>water.Urea and calcium phosphate have a reducing effect on sediment-laden water flow,while potassium sulfate has an increasing resistance effect.4.The fluid variability mechanism of nitrogen,phosphorus,potassium,and other typical pollutants interacting with sediment was analyzed.The increase in nitrogen,phosphorus,and potassium concentrations leads to an increase in fluid viscosity and yield stress,indicating that the interface coupling effect between the pollutant and water-sediment mixed fluid is enhanced.Changes in the type and concentration of pollutants alter the distribution of electric charges on the surface of water and sediment particles,which disturbs the balance of van der Waals forces and double-layer repulsion and enhances the adhesion between solid particles,thereby affecting the fluid variability behavior of the entire erosion water flow system.The strengthening of the interface attraction between pollutants and sediment in the erosion-contaminated water flow leads to an increase in fluid viscosity and yield stress,causing fluid and flow variability.In summary,the entry of nutrients such as nitrogen,phosphorus,and potassium into water bodies increases the viscosity and yield stress of the fluid,accelerating the transition of the fluid from a Newtonian fluid to a yielding dilatant fluid or a Bingham fluid.This leads to variations in the resistance of erosion runoff and enhances the erosive effect of the runoff.The research results provide a new scientific basis for further constructing the mechanism of erosion runoff transport.