Study Of Water Erosion Pricess In The Loess Slope With Aeolian Sand

The Wind-Water Erosion Crisscross Region is the most serious soil erosion region on the Loess Plateau.The wind erosion is dominated in this region during winter and spring,however,water erosion occurred mainly in summer and autumn,thus,the soil erosion is superposed in time and interleaved in space.The severe erosion not only can supply rich sand material for the downstream area included plentiful and coarse sand but also become the main source area of river bed sediment in downstream.The loess soil hill region covered by layers of aeolian sand is the typical geomorphic types in the Wind-Water Erosion Crisscross Region on the Loess Plateau.The different ways and thicknesses of covered sand formed in different geomorphologies due to the changes of wind power,topography and ground cover.Sand layer was separated as different particles by all kinds of exogenic force during transportation and deposition process,and thus formed the covered sand slope of different scale in different region.The study of soil erosion characteristics under different covered sand conditions can provide a theoretical basis for the prediction and control of soil and water loess in the Wind-Water Erosion Crisscross Region.The object of this paper was to study the power characteristics of runoff-producing and flow concentration of sand and soil sand binary structure slope under different ways,thicknesses,particle composition and watershed scale of covered sand,the main results was as followed:(1)The runoff and sediment yield of overlying loess slope with different thickness and particle size of overlying sediment are analyzed.Results show that overlying aeolian sand layers delayed the initiation of runoff and reduced the generation rate and volume of runoff,but increased the yielding rate and volume of sediment on loess slopes.During the rainfall process,runoff and sediment fluctuated sharply in rate and volume and the fluctuation intensified with the aeolian sand layer increasing in thickness.The effect of particle size composition of the sand layer on runoff and sediment yield varied with thickness of the sand layer.On slopes covered with a sand layer 2 cm in thickness,runoff and sediment yield tended to increase with the sand layer increasing in particle size during the initial period of rainfall,but no such a tendency was observed during the late period of rainfall.On slopes covered with a sand layer 5 cm in thickness,runoff increased in rate with the sand layer increasing particle size.The slopes covered with coarse sand was higher in erosion rate than the slopes covered with find sand during the initial period of rainfall,and it went reversely in the late period of rainfall.On slopes covered with a sand layer 10 cm in thickness,runoff and sediment yield did not vary much with particle size composition of the sand layer.The runoff production process on aeolian sand-covered loess slopes goes as vertical infiltration of rainwater-flow at the interface between sand layer and loess layer – seepage at the toe of the sand layer – overland runoff,while the erosion process goes as seepage erosion at the toe of the sand layer – retrogressive collapse caused by gravity and flow – surface runoff transport,which are completely different from the pattern of runoff yield under excessive infiltration and the erosion development process of splash erosion – sheet erosion-rill erosion on bare loess slopes.The velocity fluctuation of overlying sand slope is larger than that on Loess slope.Fine particle size(a,b)of the sand slope velocity is significantly greater than the coarse grain size(c,d and e)of the sand slope velocity,particle size is fine(a,b)the sand erosion rate is less than the coarse grain size(c,d and e)the erosion rate of the sand slope.The velocity and erosion rate of slope increase and decrease at the beginning of rainfall,and change at the end of rainfall,and tend to be stable,and the erosion rate decreases obviously.(2)The initial runoff and sediment process of covered sand loess slope with different covered sand ways are analyzed.Results show that the initial runoff time was shorter and the runoff rate was all lower than that of rainfall intensity in all rainfall intensities and slopes for no covered sand loess slope,as well as the runoff rate and erosion rate increased as the rainfall intensity increased,meanwhile,runoff rate and erosion rate were first increased and then decreased as the slope increased.The significant differences of the initial runoff time exist as the different covered sand ways,that is the initial runoff time of the slope thickening(A)and thinner(B)was shorter than that of the covered flatly slope(C).The runoff rate of B was significant higher than that of A and C at the beginning of the initial runoff,and then lower than that of C at the end of the initial runoff,the peak of runoff rate in all kinds of covered sand ways may be higher than rainfall intensity.The accumulative runoff amount under different covered sand ways was in the following order: B > A> C as a whole.The erosion of B was belong to the erosion in all slope,and the erosion rate increased at first and then decreased with the rainfall duration.The retrogressive erosion occurred when the way of covered sand was C,and was obvious higher than that of B.The erosion rate of A sharply increased in a short time at first and then sharply decreased with the rainfall duration.The accumulative erosion amount in short time under different covered sand ways was in the following order: B > A> C as a whole,and the accumulative erosion amount in long time under different covered sand ways was in the following order: B < A< C,The runoff rate and that of fluctuant of C was highest,however,the lowest runoff rate and fluctuant occurred at B.For B,the erosion rate and runoff rate increased to the peak,and then decreased to the stable state.Along with A and C,erosion rate reached to the peak in a short time at the initial stage,and then declined,but the overland flow velocity increase gradually.In the late stage of runoff production,the erosion rate decreases and tends to be stable state,and of slope velocity the volatility changes and eventually tends to be stable.(3)The initial runoff and sediment process of covered sand loess slope with different scales are analyzed.The initial runoff time of large scale covered sand loess slope was shorter than that of smaller scale covered sand loess slope;the runoff rate of large scale covered sand loess slope significantly higher than that larger scale covered sand loess slope under the slope of 10° and 20°,the trend was similar under the slope of 30° at the first runoff,the scale has no obvious effect on the runoff rate at the last period.The fluctuant of small scale covered san loess slope was slight,however,the contrast result was produced in the larger scale slope.The erosion rate in large scale covered sand loess slope has similar changeable trend with that of small scale slope during the rainfall process.Compared with the small scale covered san loess slope,the erosion rate of large scale covered san loess slope was obviously higher,and the fluctuant in the first stage of rainfall was higher than that of the last stage,as well as higher than that of smaller scale covered sand loess slope.The overland flow velocity of large scale covered sand loess slope is larger than that of small scale slope under the small slope,the difference of the overland flow velocity between large scale slope and small scale slope is larger at the beginning of runoff production and smaller at the later stage of runoff production under large slope.Under different experimental conditions,the average flow velocity of overlying land on different scales is basically similar.The velocity of flow and the rate of erosion are different in the early and late stages of rainfall,and the velocity of flow and the rate of erosion have the same increasing and decreasing consistency in the middle stage of rainfall.