Experimental Study On Dynamic Process Of Soil Erosion Of Engineering Accumulation In Steep Slope During The Process Of Engineering Construction

Soil erosion in production and construction areas is a comprehensive reflection ofenvironmental impacts as an integration of various controlling environmental factors,human factors and surface conditions, which alters the original compositions, structures ofsurface materials and morphologies, even bringing about important impacts on the basicrules of occurrence and development of runoff and sediment. As a kind of specialgeomorphic units stacked with spoils produced by artificial activities of production andconstruction, engineering accumulation is an important source of soil erosion newly arisingin production and construction areas. It is of important practical significance for deeplycomprehension and understanding of soil erosion characteristics of different geomorphicunits in production and construction areas to further explore the soil erosion process,hydraulic characteristics of runoff and dynamic process of erosion and sediment yieldunder different disturbance. In the paper, a set of field pouring water scouringexperiments were conducted on steep slope with72.7%gradient and12-meter length oftypical spoil ground along the line of Shenfu freeway to investigate the soil erosion processof deposit slope and its dynamic response to dynamic characteristics of slope runoff undersimulated runoff conditions. Preliminary conclusions drew from the experiments are asfollows:（1） The soil erosion process of engineering accumulationin in steep slope undersimulated runoff conditions is analysed. Under experimental conditions, gravitationalerosion has great influence on the variation of sediment concentration, which has a criticaldischarge of inflow in between20L·min^-1and25L·min^-1; the dominant factors influencingsediment concentration are not the same under different discharges, including run-off V, sediment transport rate Trand scour time t and so on. The average sediment concentrationcan be used as an indicating parameter to characterize the process of soil erosion ofengineering accumulation in steep slope. The process of slope sediment yield falls intothree stages: abrupt, fluctuation and stable. Spatial distribution of sediment yield alongdifferent slope profiles under different runoff conditions can be reduced to two types:steady decrease and violently fluctuating reduction. Soil detachment rate Drthatfluctuates with increasing discharges is linearly related with unit discharge q: D_r=0.693q+3.97（R²=0.781，n=80）, which can also be expressed as the power function ofsediment yield and flow width over different time-intervals: Dr=1.689M^0.671b^-0.669（R²=0.799， n=80）. Sediment yield can be described with powerfunction of run-off over the same time-interval（exponent＞1, linear trend）: M=0.5548q^1.036（R²=0.822，n=80），accumulative sediment yield M_sis linearly related tocumulative run-off Q during each runoff event: M_s=0.687Q+6.123（R²=0.975，n=80）.（2） The hydro-dynamic characteristics of steep slope runoff of engineeringaccumulation under simulated runoff conditions are studied. Average flow velocity V canbe well simulated with a power function of flow discharge qT: V=aq_T^b（R²ï¼ž0.86，n=5）,and the exponent of flow discharge differs under different flow states. For sheet flow, rillflow and the whole runoff process, the value of a is6.69,2.4,3.2respectively; the value ofb is0.28,0.22,0.24respectively. Subjecting to the effects of surface roughness, theexponent of flow discharge for sheet flow is less than0.5. Average flow velocity V ofdifferent slope segments shows S-Curve trend with the increase in slope lengthL:V=ae^b/L（b＜0，R²ï¼ž0.60，n=20）; Compared to flow discharge, slope length has greaterimpact on flow velocity. Flow depth h increases exponentially with flow discharge q, theexponent of flow discharge for sheet flow is somewhat larger than indoor findings; on thecontrary, the exponent of flow discharge for rill flow is somewhat less than indoor findings.It is inappropriate to describe the depth features of slope runoff generated on engineeringaccumulation with the indicator of average flow depth. Flow depth is the leading factorinfluencing hydrodynamic characteristics of slope runoff, the relationships betweenaverage flow depth and slope length vary under different discharges, from which amaximum or minimum value of flow depth along the distance was observed. The flowregime of slope runoff mostly belongs to the transition flow condition, shifting from laminar sheet flow to transitional rill flow as scour time continues; turbulence conditionoccurs with the increase of water discharges, critical discharge controlling thetransformation of flow regime from transition to turbulence is between20L·min^-1and25L·min^-1, different flow regimes exist along the distance（laminar for sheet flow all along,changes from transition to turbulence for rill flow）. The value of Fr which hasexponential descending relation to time t (Fr=12.576t^-0.719, R²=0.894, n=80) is mostlygreater than1, indicating slope runoff belongs to the supercritical flow condition, shifting tosubcritical flow condition with the water supply process gradually. Average resistancecoefficient along the distance obeys exponential function of slope length L to increase:f=me^K/L（k＞0，R²ï¼ž0.70，n=20）; Relationship between resistance coefficient f and Froudemnuber Fr accords with negative power function: f=4.70Fr-2, relationship betweenresistance coefficient f and Reynolds number Re accords with positive power function: f=0.0024Re^0.9592（R²=0.7956, n=80）.（3） The relationships between sediment concentration, sediment production andhydrodynamic parameters of slope runoff are investigated. Relationships betweensediment concentration and main hydraulic parameters such as hydrodynamic radius, unitstream power, Reynolds number, resistance coefficient in accordance with the equations ofpower function, exponential function, logarithmic function and so on, of which sedimentconcentration is positively correlated with unit stream power and is negatively correlatedwith other3parameters（sig＜0.001）. Relationships between sediment yield overdifferent time-intervals and Reynolds number, flow shear stress, stream power appeardifferent trends including power-function ascending exponential ascending and exponentialascending respectively（sig＜0.001）, sediment yield process of engineering accumulation insteep slope can be better described with runoff energy parameters.（4） The dynamic process of soil erosion in steep slope of engineering accumulation issimulated by using different theories. In the flow shear stress model, the soil erodibilityparameter is5×10^-3s·m^-1, and critical shear stress of carrying rickle on surface is0.02paunder conditions of sheet flow erosion; the soil erodibility parameter is3×10^-3s·m^-1, andcritical shear stress of rill erosion is12.8pa under conditions of sheet flow erosion. Inthe stream power model, average stream power of soil erosion is1.14N·m^-1·s^-1, andaverage soil erodibility parameter is7.6×10^-3s2·m-2under test conditions; critical unit stream power of rill erosion is0.09m·s^-1and Soil erodibility parameter is1.15kg·m^-3;preliminary results show that sediment transport process of sheet flow erosion can not beeffectively depicted with unit stream power. In terms of the whole experimental process,soil erosion rate Drcan be expressed as a logarithmic function of runoff kinetic energy F:D_r=ln （F）+b （R²=0.5035, n=80）. Unit energy of water-carrying section can be used todescribe dynamic process of sediment transport of rill erosion, which can be expressedwith the following equation: Dr=a （E–b）（R²=0.7624, n=65）; critical unit energy ofwater-carrying section of rill erosion is0.53cm under test conditions.（6） Soil erosion process in production and construction areas has different forms andshows different development characteristics, it is advisable to carry out relevant researchseparately on the basis of clarifications of disturbance and intensity, not copying thegeneral experience; theories of slope runoff regulation can still provide important guidancefor comprehensive control of soil erosion in engineering and construction areas; dynamicprocess of rill erosion in steep slope of engineering accumulation can be well simulatedwith current models and control equations, in terms of fitting effects, all theories testedshould be ranged in the order of stream power theory＞unit energy of water-carryingsection theory＞unit stream power theory＞flow shear stress theory＞runoff kinetic energytheory; mechanics indexes and energy indexes have different advantages on descriptions ofdynamic process of soil erosion in steep slope of engineering accumulation, whichshould be determined on practical considerations instead of lumped together. It seemsthat the dynamic process of sheet flow erosion and rill erosion ought to be investigatedfrom the perspectives of mechanics indexes and energy indexes respectively.