Study On Hydrodynamic Characteristics And Erosion Processes Of Freeze-thaw Slopes In High Altitude Regions

Meltwater erosion is an important soil erosion form in high altitude regions of China.Seasonal and diaual thawed soils can be severely eroded by snow-and ice-melting runoff to produce great impacts on ecological systems,industries,agriculture and various manmade infrastructures as well as people's lives.The hydrodynamics of water flow along an eroding rill over frozen and thawed slopes are vital to understanding rill erosion mechanism.This is of great importance for hydrologic and erosion processes.This study focuses on hydrologic,erosion processes,and the methodology and facility for simulating the thawing process of frozen soil.Laboratory experiments were performed to measure velocity of concentrated water flow over frozen and non-frozen slopes of soil and soil-stone mixture using electrolyte tracer method under Pulse Boundary Model.Laboratory experiments were also conducted to investigate the effects of slope gradient,flow rate on soil erosion by concentrated meltwater flow over thawed and non-frozen soil surfaces.The following main progresses and conclusions were achieved in the study.(1)The facility and procedure were advanced and designed to simulate the process of soil thawing from top to bottom.The one-dimensional thawing process was demonstrated through laboratory experiments.The results provides feasible facility,method and operation process for simulating one-dimensional soil thawing to serve research on the effect of thawed soil depth on erosion process.(2)A new methodology was advanced in this study for measuring sediment concentration accurately so as possibly to replace the traditional oven-daring method.This method can be adopted in field.The sediment-laden water and sediment particle mass density are used to calculate the mass of water being replaced by sediments.Then the mass of sediments and further the sediment concentration of the sample were calculated.(3)Hydrodynamics of water flow over frozen and thawed soil slopes.Laboratory experiments were made to determine the effects of soil freezing on water flow velocity.The results indicated that on frozen soil slope,flow velocity increased with flow rate and slope gradient.Flow velocity on the steep slope or at the high flow rate over the thawed slope changed slightly with increases in slope gradient and flow rate.Head cuts formed on thawed slopes influenced the increases in flow velocity.This indicates that water flow over a frozen slope runs much faster than that over a thawed slope.(4)Impacts of stone content on hillslope hydrodynamics over frozen and thawed slopes.Flow velocity on frozen soil slopes increased constantly with increase in slope gradient and flow rate on frozen slopes,but decreased with increase in stone content.Maximum velocity of water flow was reached at a stone content of about 15%,on non frozen slope.The flow velocities over frozen stony slopes were 1.21 to 1.30 times those over non-frozen slopes under different slope gradients and flow rates.(5)Laboratory experiments were conducted to assess the effects of slope gradient and flow rate on soil erosion by concentrated meltwater flow over thawed and non-frozen soil surfaces.Sediment concentrations at the seven slope lengths were measured to form a rill erosion process.Sediemnt concentration along the rill increased exponentially before approaching a limiting value.The sediment concentrations were positively correlated with flow rate and slope gradient.The maximum sediment concentrations in water flow over thawed slopes were slightly higher than those over non-frozen slopes.(6)Meltwater erosion processes of frozen soil in high altitude and cold region as affected by thawed depth were simulated under concentrated flow.The impacts of thawed depth,slope gradient,and flow rate on soil erosion by concentrated meltwater flow were measured.Sediment-laden water samples were collected at the lower end of the flume for determination of sediment concentration.Results showed that it increased exponentially with rill length to approach a maximum value.The sediment concentrations were closely correlated with thawed soil depth,flow rate,and slope gradient.Shallower thawed depths produced more sediments than thick thawed depths.However,the effect of flow rate on sediment concentration was not as great as slope gradient,which declined with increases in slope gradient.This study helps to understand hydrodynamics of water flow over thawed soil slow and the mechanism of soil erosion in cold regions.