Erosion Mechanism And Its Modeling Of Cultivated Black Soil Slopes Affected By Thawed Depth

Meltwater erosion is an important soil erosion type in high latitude and altitude regions.Accelerated soil erosion in north-eastern China causes reduction of the thickness of cultivated black soil,to affect grain productivity and to threaten ecologic safety.The meltwater erosion is the hot spot of soil erosion in the cold regions.Seasonal and diurnal fluctuations of air temperature cause the top soil layer to freeze and thaw repeatedly,which also cause fluctuations of the runoff intensities generated from snow and glacier melting to lead to serious erosion of shallowly-melted soil layer.It is of great significance to investigate soil erosion mechanism for model prediction and to prevent soil erosion,in cold regions.Based on the seasonal and diurnal fluctuations of melted runoff and thawed depth of frozen soil,simulated experiments were used to investigate the erosion mechanism of partially-thawed soil by concentrated melt water flow,in terms of sediment delivery processes,rill detachment rate,sediment transport capacity,soil erodibility and critical shear stress,as well as hydrodynamic parameter as water flow velocity.Soil erosion process model can be established under partially thawed soil to simulate melt soil erosion on hillslope in the cold regions.Research results of the research are presented as the following.1.Measurement of water flow velocity over frozen and thawed soil slopes with analytical model of electrolyte tracer transportation in water flow.Flow velocities were measured under different slope gradients(5°,10°,15°and 20°),and flow rates(1,2 and 4 L/min)over frozen and non-frozen soil slope.Results showed that velocities over frozen soil slopes increased with flow rate and slope gradient.Flow velocities over non-frozen soil slopes increased with flow rate and slope gradients from 5° to 15° and stabilized at slope gradient higher than 15°.Flow velocities over frozen soil slopes were 30%,54%,71%,and 91%higher than those over non-frozen ones at the four slope gradients.Flow velocities over frozen soil slopes under different flow rates were 52%,59%,and 79%higher than those over non-frozen soil,respectively.Water flow velocity was also measured with leading edge method over frozen and non-frozen soil slopes,which can be used to determine the correction coefficient of leading edge method,by comparing the measured velocities with those measured by electrolyte tracer method.Based on the measured data,the correction coefficient was achieved to be 0.8.2.The erosion process of non-frozen black soil.Laboratory experiments were applied to research rill erosion processes of non-frozen soil slopes by concentrated water flow,under four slope gradients(5°,10°,15° and 20°)and three flow rates(1,2 and 4 L/min).The series data of sediment concentration and slope length were achieved under different hydraulic conditions.The sediment concentration increased exponentially with slope length until it reached to the sediment transport capacity.3.Methodology for rill erosion process of partially thawed soil slope by concentrated water flow.The experiment involved four slope gradients and three flow rates(the same as above)as well as three thawed depths(1,2 and 5 cm).The sediment concentration as functions of slope length were experimentally measured.Results show that thawed depth had major influences on rill erosion process.Under shallow thawed depths of 1 and 2 cm,sediment concentration of rill erosion initially increased linearly and then exponentially with slope length after a critical slope length,and finally reached to a sediment concentration higher than that over non-frozen soil slopes.The sediment concentration along an eroding rill under a thawed depth of 5 cm tended to be similar to that along a rill on non-frozen soil slope,which increased exponentially with slope length.4.Identification of detachment rate as functions of rill length and sediment concentration on partially thawed soil slopes.Soil detachment rate was calculated with the experimental data under different thawed depths.The result shows that when the thawed soil depth was shallow(1 and 2 cm),the relationships between detachment rate and slope length and sediment concentration were both piecewise functions.The detachment rate,as function of slope length,initially kept constant and then decreased exponentially with slope length.The detachment rate was also a piecewise function with sediment concentration.With the increase of sediment concentration,detachment rate initially kept constant and then decreased linearly with sediment concentration.5.New methodology for determination of sediment transport capacity based on soil detachment rate.Conceptually,when sediment concentration in flow water reaches to the maximum,the rill detachment rate decreased to zero.Thus,the sediment transport capacity can be determined by the point where detachment rate was zero.The calculated sediment transport capacities were compared with the measured ones,to produce proportionality factor of 0.98 which indicates the high agreement of the two data sets.This demonstrates the rationality of the new method.6.Computational methodology for soil erodibility and critical shear stress of partially thawed soil.According to rill erosion equation in WEPP,soil detachment rates were used to fit with flow rates under a given slope gradient to produce soil erodibility and critical shear stress.It is found that soil erodibility of partially thawed soil slope was higher that of non-frozen soil,and the critical shear stress of partially thawed soil was lower than that of non-frozen soil.This may interpret the reason why partially thawed soil slope was more prone to be eroded.The results and data from this research are significant for understanding soil erosion mechanism over thawed black soil slope and calculating the parameters of soil erosion predict model.