| The continuous increase in soil erosion has become a key environmental issue worldwide.Freeze-thaw soil can produce strong soil erosion under the action of melting water,wind and gravity,and it is one of the important forms of soil erosion.The Loess Plateau is an area with strong seasonal freeze-thaw effects.The contribution of freeze-thaw erosion in the sand-rich areas in the middle reaches is huge,while the research on freeze-thaw erosion lags behind water erosion and wind erosion.Based on the watershed hydrological and sediment data during the spring thawing period,this paper analyzes the influence of freezing and thawing on the hydrological process of the basin by monitoring the water changes before and after freezing and thawing in typical areas of the watershed,and further combining indoor control experiments to study different combinations of thawing and flow.Erosion and dynamic process,analyze the influence of freeze-thaw on the hydrological connectivity of water erosion process based on topographic data,and analyze the characteristics of sediment in each period of the erosion process with the laser particle size method,and clarify the sediment particles in the process of freeze-thaw-hydraulic combined erosion.The distribution law and the transportation method of different grain sizes of sediment will help reveal the process and mechanism of freeze-thaw and hydraulic complex erosion,and lay the foundation for the establishment of an erosion model with physical significance.The main conclusions of this research are as follows:(1)The runoff and sand content during the snow-melt period changed more drastically compared with the whole year and the influence of the freeze-thaw process on the migration of soil water content.The depth of snowmelt runoff is in the order of flat and lateral sandy years>more water and more sandy years>less water and less sandy years,and the snowmelt erosion modulus also presents the same law.Based on the change law of runoff and sediment in the snowmelt period,it is found that the change law is strong in the flat lateral sand year,while the change in the year with less water and less sand,and the year with more water and more sand is relatively complex,and the variation of the water and sediment law is large,which also leads to snowmelt Changes in runoff and erosion characteristics are complex and unstable.At the same temperature,the soil moisture content of each layer of the dam is greater than that of the slope farmland.Under the same soil layer,during the freezing process,the soil water content of the dam land decreased faster than that of the slope farmland;similarly,the soil surface water content during the melting process showed an increasing trend,and the growth rate of the soil water content of the dam land was greater than the slope farmland.Compared with dam land,slope farmland is preferentially frozen and melted in the four soil layers.(2)It is clarified that erosion and sediment yield process and the effect of freeze and thaw on the separation of sediment particles at different sizes level under different flow rates of unfrozen slope,shallow thawed slope and frozen slope.Under the runoff of 1 and 2 L/min,the average erosion modulus appears as frozen slope>shallow melting slope>unfrozen slope.Under 4 L/min runoff,the average erosion modulus of shallow melting slope Performance is the biggest.Under the same slope,the growth rate of erosion modulus(k)increases with the increase of runoff.With the extension of the scouring duration,the content of the three kinds of sediment particles under the unfrozen,shallow melting and frozen slopes all showed powder particles>sand particles>clay particles.Compared with the undisturbed soil,the clay particles and silt particles were stripped and lost under different conditions,while the sand particles were deposited.The runoff rates of unfrozen slopes and shallow melting slopes have a significant impact on the accumulation of clay,silt and sand particles.(3)The spatial distribution characteristics of the connected areas of different sediment structures on the slope surface after the water release test were revealed,and the influence of freezing and thawing on the process of sediment function connectivity was explored from a three-dimensional perspective.Under the same discharge flow,the runoff coefficient of different soil freeze-thaw states increased rapidly with the extension of the erosion duration,and the increase of the frozen slope was the most obvious;under the same soil freeze-thaw state,the runoff coefficient gradually increased with the increase of the discharge flow.According to the spatial distribution of the high connected areas on the slope,the connectivity of the sediment structure on the slope is obviously enhanced with the increase of the discharge flow.Under different discharge flows and soil freezing and thawing conditions,the laws of the vertical and vertical connectivity rates of the slope are relatively similar,both increase first and then stabilize,while the lateral connectivity rate first decreases and then stabilizes.From the perspective of the lateral connectivity rate,the erosion form at the beginning of the test was mainly surface erosion;as the test progressed,the erosion form gradually changed from surface erosion to rill erosion.The correlation between longitudinal and vertical sediment connectivity and sediment transport rate is relatively good,while the correlation between lateral sediment connectivity is relatively weak.(4)The runoff energy consumption mechanism is clarified under different slopes,and a freeze-thaw erosion model is established based on runoff energy consumption.The sediment particle transport mechanism was analyzed,and the contribution rate of the sediment transport form was quantitatively identified.The energy consumption of runoff increases with the increase of runoff.Under the same runoff,the average value of runoff energy consumption on the three slopes is as follows:unfrozen slope>shallow melting slope>frozen slope.Erosion modulus has a good linear relationship with runoff energy consumption.For shallow melting slopes with dual structure,the sand production capacity based on runoff energy consumption is the same at runoff rates of 1 and 2 L/min.Under the flow rate,the sediment production capacity based on runoff energy consumption increases significantly.In this test,more than 60%of the sediment particles are transported in the form of suspension jumping,and particles smaller than 0.027 mm are preferentially transported,accounting for about 40%of the particle content.For shallow melting slopes,with the increase of runoff rate,the particle size value of the minimum transport rate gradually increases,indicating that the upper limit of the particle size of particles transported by suspension jumping method increases with the increase of runoff rate.For unfrozen slopes and frozen slopes,with the increase of runoff rate,the relative contribution rate of suspended load jump load shows a decreasing trend,while the shallow melting slope shows the opposite trend.Under the same runoff energy consumption,freezing and thawing have a significant effect on the relative contribution rate of suspended load jumping load and rolling transport in the slope erosion process. |
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