| The aeolian landform and water-erosion landforms in the Wind-water Erosion Crisscross Region of the Loess Plateau are staggered,the vegetation is sparse,and the soil erosion process is complex and intense,which is a typical ecologically fragile zone.There is a unique sand-covered loess slope this area.The production methods of runoff and sediment are unique,and the erosion process is complex.The erosion mechanism remains to be further studied.In this paper,the simulated rainfall event combined with soil moisture and pore water pressure monitoring technology was carried out to study the sand layer instability and soil erosion process on the sand-covered loess slope.The conditions were analyzed under different conditions(rainfall intensity: 1 mm/min,1.5 mm/min and 2 mm/min;slopes gradient: 10%,15%,20% and 25%;sand layer thicknesses: 2 cm,5 cm and 10 cm),runoff rate and soil loss rate on the sand-covered loess slope,total runoff and sediment yield of 90 mm rainfall,the total runoff and sediment yield caused of 60 min rainfall,the soil water content and the pore water pressure at different monitoring points in the slope;and quantitatively calculate the effects of rainfall intensity,sand layer thickness and slope gradient on the change of initial runoff time,the total runoff and sediment yield of 90 mm rainfall,the total runoff and sediment yield of 60 min rainfall,the soil water content and pore water pressure corresponding to the monitoring points of the sand-covered loess slope when the runoff begins to occur,and the maximum soil water content and pore water pressure at each monitoring point during the rainfall process,and pore water pressure change at each monitoring point during rainfall were studied.The following conclusions were made:(1)The greater the rainfall intensity,the shorter the initial runoff time of the sand-covered loess slope,the larger the maximum runoff rate and the maximum erosion rate.When the rainfall intensity is constant,the thickness of the sand layer on the sand-covered loess slope is positively correlated with the initial runoff time,the maximum runoff rate and the maximum soil loss rate.The steeper the gradient of the sand-covered loess slope,the shorter the average initial runoff time.The contribution rate of each influencing factor to the change of initial runoff time was rainfall intensity(56.46%)> sand layer thickness(19.20%)> slope gradient(9.01%),and the error term contribute rate was 15.33%.(2)The thickness of the sand layer changes the erosion mode of the sand-covered loess slope.On the loess slope covering sand with a thickness of 5 cm,the effect of rainfall intensity on the total runoff and sediment yield is greater than that of rainfall.The runoff pattern of the slope is mainly filled with runoff,the runoff is deposited from the surface of the sand layer,and a small amount of sediment is carried.When the thickness of the sand layer is more than 5 cm,and the rainfall determines the total runoff and sediment yield on the slope,and the soil-sand interface flows during the rainfall process,causing the collapse of the front edge of the sand layer.The more severe the erosion of the sand-covered loess slope,the greater the change in the rate of runoff and erosion.(3)With the increase of rainfall intensity,the average total runoff and sediment yield of the 90 mm rainfall on the sand-covered loess slope increased,and the distribution was more concentrated.With the increase of the thickness of the sand layer,the total runoff and sediment yield of the 90 mm rainfall increased first and then decreased.On the slope gradient of 10% ~ 20%,with the increase of the slope gradient,the average total runoff and sediment yield of 90 mm rainfall increased,and the distribution was more concentrated.The contribution rate of each influencing factor to the total runoff variation of 90 mm rainfall is slope gradient(31.72%)>rainfall intensity(18.09%)> sand layer thickness(17.27%).Uncontrollable factors have a great influence on the total runoff 32.92%,and the contribution rate is 32.92%.The contribution rate of each influencing factor to the sediment yield of 90 mm rainfall is slope gradient(60.4%)> rainfall intensity(11.11%)> sand layer thickness(10.95%),and the contribute rate of uncontrollable factors is 17.55%.(4)The total runoff and sediment yield generated by 60 min rainfall increased with increasing rainfall intensity.The total runoff of 60 min rainfall decreased first and then increases with increase of sand layer thicknesses,and the larger the average total runoff of 60 min rainfall,the more concentrated the distribution.The thicker the sand layer,the grater the sediment yield of 60 min rainfall,the more severe the erosion.With the increasing of slope gradient from 10% to 20%,the average total runoff and sediment yield of 60 min rainfall increased,and the distribution is more concentrated.However,the average total runoff and sediment yield of 60 min rainfall on the slope with gradient of 25% is between 20% and 15%.The contribution rate of each influencing factor to the total runoff variation of 60 min rainfall is rainfall intensity()83.27%> slope gradient(6.24%)> sand layer thickness(0.89%).The rainfall intensity absolutely controls the total runoff of 60 min rainfall.The contribution rate of each influencing factor to the sediment yield of 60 min rainfall is rainfall intensity(52.20%)> error > slope gradient(17.4%)> sand layer thickness(6.92%),the contribute rate of uncontrollable factors is 23.48%,indicating the production process of sediment is complex.(5)When the sand layer is thinner,the soil water content in the sand-covered loess slope responds more quickly to the rainfall,and the soil water content corresponding to the runoff is higher.The greater the rainfall intensity,the faster the response of the soil water content of each slope monitoring point to rainfall intensity,and the greater the soil water content of each monitoring point on the slope.The saturated water content of the old loess layer is about 0.32,the maximum water content at the interface of between loess and sand is about 0.36,and the saturated water content of the sand layer is 0.34,that is,the maximum water content at the sand-soil interface is greater than that of the sand layer and the old loess layer.(6)On the loess slope with the same thickness of sand layer covering,the pore water pressure on the slope under different rainfall intensity is not much different.The thinner(5 cm)slope of sand covering,the greater the rainfall intensity,the more severe the pore water pressure changes in the slope.The severity of the change of the pore water pressure corresponds to the soil water content,and the more the soil water content,the more serve the pore water pressure changes.On the sand-covered loess slope,the slope gradient has no significant effect on the pore water pressure corresponding to the runoff,the maximum pore water pressure,and the pore water pressure change.The uncontrollable factor has the largest contribution rate to the pore water pressure when the runoff begins occur,maximum pore water pressure and the variation of pore water pressure at each monitoring point during rainfall,indicating that the pore water pressure is not sensitive to the variation of rainfall intensity,slope gradient and sand layer thickness. |
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