Runoff Erosion Process On Slope Of Engineering Accumulation Caused By Production And Construction Projects

With increasing or diversifying in numbers and in types of production and construction projects, various construction activities produce a large number of Engineering accumulation during the process of engineering construction, which have changed the original landform and been shaped into special artificial landform units. As a comprehensive mixture of soil and rock fragments with uneven material composition, engineering accumulation is characterised by bare soil, loose structure, smaller internal friction angle and cohesive strength and lower vegetation cover and organic matter, and is the main landform occurred artificial water and soil loss. Such severe soil loss not only cause land degradation and ecological destruction but also pose a great threat to the long-term benefit and safely operation of the Three Gorges Project, and so do the security for downstream area. Because of its different producing sources and soil-rock ratios, the underlying surfaces from disturbed soils accumulation often characterize unique soil erosion process under same runoff scouring condition.Taking the ubiquitous purple soil deposits and yellow sand deposits associated with different soil-rock ratio as examples, this paper analyzed its source characteristics and physical properties variations by field investigation and soil test method; the paper thoroughly studied the difference in hydraulic properties, water infiltration, runoff and sediment yield and rill evolution process on slopes with field scouring experiments combined with Hydraulics, Sediment transport mechanics theories, and then established the water sediment relationship equation and the critical erosion condition; based on analsis on soil physical properties, hydrodynamic parameter, runoff and sediment and rill evolution characteristics, the paper analyzed thoroughly the runoff erosion process on slopes of engineering accumulation with different soil-rock ratio, and further explored the mechanism of slope failure, which could provide some important parameters and technical basis for predicting soil and water loss accurately and arranging reasonable measures for soil and water conservation.The main conclusions were as follows:(1) The soil physical properties of engineering accumulations in slope with different soil-rockratio was obviously different. The higher the soil (less than 10mm diameter) content was, the smaller the soil bulk density, soil noncapillary porosity was and the greater the soil capillary porosity was, which were benifical to the recovery of slope vegetative cover but its rich soil or fine particles were the sediment source of runoff erosion; however, the rock (more than 10mm diameter) showed the contrary and the soil erosion risk was lower. The soil content of both Purple soil and Yellow soil deposits presented such order as pure soil deposit> partial soil deposit> rocky deposit, in which that of Purple soil deposits was 86.47%,79.19% and 57.16%, that of Yellow soil deposits was 76.88% and 60.69% but the rock content showed the contrary, some deposits presented uniform particle distribution and good gradation exept for Purple soil deposits with pure soil; for Purple soil deposits, the soil bulk density of pure soil deposit was the minimum (1.317g/cm3) and that of rocky deposit was the maximum (1.562g/cm3), while the soil bulk density of pure soil deposit was greater than rocky deposit for Yellow soil deposits; the initial water content of both Purple soil and Yellow soil deposits showed the order as pure soil deposit> partial soil deposit> rocky deposit, the accumulations containing greater soil content was prone to erode than the lower soil content. Various landform units caused by engineering construction reduced the soil water retention function of original landform in the project area, in which 3a,2a and 1a engineering accumulations displayed decrease of 12.3%,16.8% and 22.7%, respectively.(2) The variation characteristics of hydrodynamic parameter differed from different engineering accumulations with different soil-rock ratio during erosion process. Runoff regime on all engineering accumulation slopes manifested as turbulent and subcritical flow and some hydrodynamic parameters presented different fluctuations during runoff scouring process, in which flow velocity υ presented a "stonge-weak" fluctuation with time progressed, Darcy-Weisbach/showed a "weak-stronge" fluctuation, flow shear stress had a "weak-stronge-weak" and runoff power was a "stonge-weak" fluctuation; the υ of both Purple soil and Yellow soil deposits increased with the flow discharge by power function, the maximal and minimal exponent were 0.526 and 0.179, respectively; the υ of both pure soil deposit and partial soil deposit under same flow discharge conditions, and the slope gradient and slope length played little effect onυ at greater rock content condition. For a certain condition,f increased with decreasing soil-rock ratio, that was pure soil deposit< partial soil deposit< rocky deposit.τ , affected greatly by soil-rock ratio and topographic conditions (slope gradient or slope length), was increased with increasing flow discharge and Yellow soil deposit presented a rapider increasing rate than Purple soil deposit, in which Purple soil deposit ranged from 24.571 to 83.743 Pa and Yellow soil varied between 22.000 and 57.154 Pa. P also increased with increasing flow discharge and the P of above two deposits showed the order as pure soil deposit or partial soil deposit> rocky deposit.(3) The runoff and sediment characteristis was obviously different among slopes of engineering accumulation with different soil-rock ratio. Infiltration process existed decreasing rapidly (before 3min), decreasing slowly (3-20min) and stabilising (20min) 3 stages; the average infiltration rate of both Purple soil deposit and Yellow soil deposit showed the order as pure soil deposit or partial soil deposit> rocky deposit during runoff eroson process. Runoff yield process increased at the beginning and then tended to be constant as scouring time progressed, moreover, that existed different fluctuations and sudden changes which occurred within 7 min after runoff yielded; rill erosion had influence on runoff yield; soil-rock ratio also had a great effect on runoff yield, and the effect was enhaced with increasing flow discharge, the increasing amount of average runoff yield of pure soil deposit was the maximum (19.337 L/min) when flow discharge ranged from 5 to 15 L/min while that of rocky deposit was the minimum. Sediment production process showed a continuously waving variations and changed acutely as flow discharge increase, the collapse on gully wall caused by its gravity on slopes was the important reason causing fluctuations. The influencing degree of hydrodynamic parameters on sediment yield ranged in such order as γQ>γh> γb>γf>γv>γp>γτ, and its correlation coefficient varied from 0.5183 to 0.9284, in which flow discharge played a great influence on sediment yield; the relationship between sediment yield and flow discharge could be described by the equation M=0.0072Q 30287, the average sediment yield of rocky deposit for Purple soil and Yellow soil was the greatest at small flow discharge (5L/min) while it showed the order as pure soil deposit>partial soil deposit>rocky deposit at greater flow discharge.(4) Rill erosion on slope of engineering occumulation mainly occurred 3 min later after runoff yielded, which could be divided into two stages:rill developing stage (3-45min) and rill stable stage (after 45min), the runoff sediment concentration showed a decrease trend with fluctuations and sediment yield was closely related to rill morphology. Yellow soil deposit produced a single main rill after scoured while and Purple soil deposit formed many rills; for rocky Purple soil deposit, the number of rill, average rill width, average rill depth increased with increasing flow discharge overall but the width-depth ratio showed the contrary, and the increasing slope gradient would aggravate cutting effects of runoff; when flow discharge ranged between 10 and 30 L/min, the average rill depth changed from 1.58 to 7.67 cm, the average rill density varied from 1 to 2.27 m/m2. The sediment yield on slopes of engineering accumulation had significantly positive correlation (R2=0.592) with the average rill depth, which could be expressed by the equation M=0.2187 h 29508, however, the correlation between the sediment yield and the number of rill, average rill width, average rill density and width-depth ratio were not obvious.(5) The gravity erosion in rill evolution process appeared at rill banks and rill head, which was not only the important factor influencing sediment yield but also the important reason causing fluctuations of sediment production. The contribution of gravity erosion to the total soil erosion increased with increasing slope gradient, the maximal contribution reached 99.6% and the minimal was 17.4%. For purple soil deposit, the critical erosion conditions was different for slopes with different soil-rock ratio, in which the critical runoff power producing sediment for partial soil deposit (6.699N/(m-s)) was lower than rocky deposit (7.265N/(m-s)); The critical runoff power of Yellow sand soil deposit was lower than Purple soil deposit under same soil-rock ratio condition, which indicated that Yellow sand soil deposit was prone to erosion than Purple soil deposit; The critical erosion slope on slope of engineering accumulation increased with increasing flow discharge, when flow discharge increased from 10 to 30 L/min the critical erosion slope for rock Purple soil deposit were 35°,35°,30°,32.5° and 30°, respectively.(6) Infiltration-Runoff process on slope of engineering accumulation played a great impact on slope stability, in which the runoff scouring process would directly cause surface roughening and rocky and the water infiltration process would cause slope failure because of the increasing of soil water content and bulk density and the decreasing of soil shear strength. Based on SLOPE/W model, the slope safety factor of Purple soil deposit were 2.863 and 1.600, respectively, under non-rainfall condition and moderate rainfall condition (rainfall duration of 18h and precipitation of 23.3mm), in which the most dangerous sliding surface in slope under non-rainfall condition occurred at the place between the rock surface and the soil bottom while that under moderate rainfall condition occurred at the place between the deposit bottom and the soil surface, all of those indicated that the surface soil developed easily into weak plane and slip soil under the effects of water. The slope stability of engineering accumulation was influenced by the combined action of various factors, including such internal factor as engineering properties of soil, side slope shape, geology geomorphology and such external factors as rainfall, vegetation, external force, weathering and human activities.