| Water-wind interaction zone in the Loess Plateau is the most severe soil erosion area on the Loess Plateau and the main source of the lower reaches of Yellow River bed, this area have special topography which hilly region covered by Aeolian sand, exist in the sheet sand under loess of binary structure profile, this paper took slope which in hilly region covered by sand of the Loess Plateau as the research object. Through investigating and sampling in the wild to determine this side slope sand binary structure below the difference of soil physical and chemical properties. On this basis, the slope, rainfall intensity and the thickness of covered sand for impact factor, design different experiment combination, on sand binary structure slope artificial simulated rainfall experiment, study the sediment transportation characteristics of the runoff erosion, the response characteristics of different impact factors and slope runoff velocity variation, water and sand coupling relationship, etc. The main results as follows:(1) Variation of sand and loess properties of binary structure profile in hilly region covered by sand of the Loess Plateau: The results showed that sand and loess of binary structure profile in hilly region covered by sand physical properties were significantly different at soil structure,bulk density, particle composition, saturated hydraulic conductivity and moisture characteristic curve. The significant differences of the physical properties of sand and loess layer for special water infiltration, runoff, soil erosion, material migration process laid a solid foundation on this slope lead to this special slope erosion process.(2) Characteristic analysis of the runoff of sand and loess binary structure profile slope:Different thickness of covered sand slope on different effects, after runoff, runoff rate at the0.5cm and 2cm thickness of covered sand slope with rainfall tends to quasi steady state after increasing first, at 5cm and 10 cm thickness of covered sand slope, runoff rate along with the rainfall by the rapid increase first, then slowly reduced, about 15 ~ 25 cm thickness of covered sand slope, runoff rate sharply steep fall with the rainfall. Slope runoff rate under the conditions of different rainfall intensity with the change of rainfall increasing the overall trend is first,diminishing volatility or rapid decline after the peak, with the increase of rainfall intensity, runoff rate increased; Under different slope runoff rate have obvious difference: 10 °,15 ° slope runoff change tendency of the rate of close to, small flow rate, runoff yield increasing slowly, after stabilizing. 20 °, 30 ° slope runoff rate change trend close, increases first, reaches the peak afterbegan to decrease. 25 ° runoff rate is the most special, runoff rate, steep, its peak is larger than other slope. Time within 60 min rainfall, thickness and cumulative sand production flow into extremely significant negative correlation, rainfall intensity and runoff is significantly related to the slope and runoff into extremely significant positive correlation and critical existing in about25 ° slope. The influence of the rainfall intensity changes on the production flow, the largest contribution rate of 64%, followed by the thickness of the coated sand, the effect of slope is minimal. Runoff and slope, rainfall intensity, thickness of sand regression equation is: Q = 0.065I1.342 S0.513(1 + H- 0.227)(R2 = 0.756,N=43).(3) Characteristic analysis of sediment yield of sand and loess binary structure profile slope:Under different conditions covered sand slope have different sediment yield rate, thinner covered sand slope erosion and sediment yield rate change is small, the peak is not obvious. With increase of thickness of covered sand, the erosion process was infiltration-interflow–collapses,the process of sediment yield rate is bigger, steep drop, has an obvious peak value. Overall,covered sand slope erosion rate with the thickness of covered sand, rainfall intensity, slope gradient increase shows increasing trend. Under the strong rain, covered sand slope erosion,under the strong rainfall intensity, covered sand slope erosion rate increased. Covered sand slope critical slope erosion rate, the slope value at about 25 °. Under the same slope and rainfall intensity, covered sand slope within 60 minutes rainfall surface accumulated sediment yield increases with increase of thickness of covered sand, the thinner thickness. Sediment yield rate increased faster with the thickness of sand, increase rate is reduced. Under the same rainfall duration, covered sand slope erosion and the thickness of coated sand, rainfall intensity and slope of the partial correlation coefficient of 0.532, 0.599 and 0.342 respectively, bilateral significance test was significantly variance analysis showed that the influence of rainfall intensity significantly sediment yield changes, contribution by 30%. sand thickness changes significantly affect the sediment yield, the contribution rate is 14.24%. The effects of gradient on sediment yield changes was not significant. Slope sediment yield relationship with slope grade, rainfall intensity and thickness of sand could described with regression equation is: M = 0.005 I1.204S0.801(1 + H0.669)(R2= 0.659,N = 43).(4) Relationship analysis of the runoff and sediment yield:The result showed there was a same increase between runoff rate and sediment yield rate in rainfall process as increasing of sand thickness, slope and rainfall intensity. After a certain time, a similar increasing tendency between runoff and sediment yield under lesser impact factor condition, then stable runoff and decreasing trend of sediment yield rate. When enhancing the impact factor condition, sediment yield rate and runoff rate increased at first, then reduced after peak. Runoff and sediment yieldrate had good consistency, which had a same increased trend under different rainfall and slop in1 hour precipitation. However, because of the controlling of sand thickness on the changing of runoff rate and sediment yield rate, opposite tendency which showed small flow rate and large sediment yield rate between them under different sand thickness. According to changing of runoff rate, slop and its length, there was a regression equation: M=0.249Q0.745S0.315(1+H0.799).(5)Difference of rainfall-runoff-sediment yield between bare slope and covered sand slope:Rainfall-runoff-sediment yield had smooth process on bare loess slope, but significant difference with different slope in slope covered by sand, the process of the runoff and sediment yield has obvious formula, bare loess slope in the process of rainfall rate less than rainfall intensity, sand and loess binary structure slope in the process of rainfall instantaneous rate is greater than, runoff coefficient is greater than. When binary structure slope covered thinner thickness of sand erosion process is similar bare loess slope’s erosion process, however the process was infiltration-interflow-collapses-the change of runoff scouring-shallow gully erosion under the thickness covered thicker sand. |
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