Erosion Mechanism Of Colluvial Deposits In Graniet Benggang

In the tropical and subtropical South China, Benggang is widely distributed. Benggangmainly consists of colluvial deposits with high erosivity. The erosion of colluvial deposits caninfluence the redistribution of the materials from clasping walls and slope surfaces, and thusfurther affect the degree of erosion damage in Benggang. Through the field survey on thegranite colluvial deposits in Yang Keng Village of Longmen town in Anxi County, FujianProvince, the types and morphological characteristics of colluvial deposits are categorized;Though laboratory experiments, the soil characteristics of colluvial deposits are analyzed. Therelationships between the overland flow hydraulic, dynamic parameters and soil detachmentof scattered colluvial deposits are analyzed on the conditions of the flow discharge (1.1,2.2,3.3,4.4,5.5L·min^-1) and slope gradient （20,25,30,35,40°）. The erosion mechanisms ofscattered colluvial deposits are stutied via the artificial rainfall tests under various conditionswhich includ different rainfall intensities (1.00,1.33,1.67,2.00,2.33mm·min^-1) and slopes（20,25,30,35,40°）, different crusts degree （uncrust,5min curst,10min crust）, and repetitiverainfalls (1.00,1.67,2.33mm·min^-1three times respectively). The main conclusions are asfollows:（1） The types and morphological characteristics. Colluvial deposits are divided intoscattered and slide types, and dominated by the scattered type. The slide type has steep slope,up to40-70°, while the slope of the scattered type is between20°and40°, and mainly in thevicinity of30°.（2） The soil characteristics. There is very low organic matter content in colluvial deposits.The soil particles are mainly gravel, sand and sediment, and the clay content is low; colluvialdeposits have loose soil which results in low soil shear strength and soil hardness, weakerosion corrosion resistance, but strong infiltration. However, loose soil can easily cause soildisintegration.（3） The relationship between the characteristics of overland flow hydraulic parameters andsoil detachment. Flow velocity, water depth and the resistance coefficient are mainly affectedby the flow control; Resistance coefficient shows a power function decrease with the increaseof Reynolds and Frode number. It is feasible to simulate the soil detachment rate throughtraffic and slope, depth and slope. The soil detachment rate of colluvial deposits and the flow velocity/shear stress exit an exponential function relationship, but the soil detachment rate ofcatchment slope （nature soil） performs a power function relationship with them. Therefore,the colluvial deposits is more easily separated by water currents than natural soil. Theexponential equation of detachment rate and shear stress is the preferred equation to analyzethe flow detachment on colluvial deposits.（4） The runoff and sediment characteristics of colluvial deposits under different conditionsby artificial rainfall experiments. The erosion characteristics under different rainfall intensityand slop conditions: The infiltration processes are fitted by the model: Ir=aT-n+If, and therunoff processes are fitted by the model: Rr=-bT-m+Rf. There are two types of changes in theyield and sediment content: single peak and multiple peak models. The infiltration, runoff andsediment yield increase with increasing rainfall intensity with a liner, exponential, powerfunction respectively; The infiltration decreases with increasing slope gradient with a linerfunction; the runoff and sediment erosion have a critical slope gradient of erosion:³0°and>25°respectively; The sediment concentration increases with increasing rainfall intensity,while the sediment content of different rainfall intensities varies according to the slopegradient. The average rate of runoff exhibit an exponential function relationship with theaverage yield and a power function with the sediment concentration, and the average sedimentconcentration has a linear function relationship with the average sediment yield. Theinfiltration, runoff, sediment yield and sediment content of colluvial deposits can be wellforecasted by using rainfall intensity and slope. The runoff and sediment shows thesecharacteristics under different crust conditions: The presence of crusts can slow erosion, butwith the increase of the crust, the ability of surface runoff erosion grows, which results inincreased erosion. The runoff and sediment appears the characteristics under repetitiverainfalls: the diversification of slope terrain and erosion patterns lead to different sedimenterosion of various rainfall intensities. Under three times of rainfalls whose intensities are1.00and1.67mm·min^-1, there is no significant difference, but under2.33mm·min^-1rainfallintensity, the sediment erosion increasingly intensifies with the rainfall time increase.（5） The characteristics of rill erosion under different conditions. Under the conditions ofdifferent slops and different rainfalls, the time that the erosion happens is shorten as the slopand rainfall intensity increases; when the slop and rainfall intensity increase, the rill density and width increase; when the slop increases, the width-depth ratio decreases; and when theslop and rainfall intensity increases, the main erosion pattern changes from sheet erosion torill erosion. Under the conditions of different crusts, crust delays the happening of rill erosion;the rill density as well as the rill width of crust slop is less than those of the uncrust slop; andthe crust rill is deeper than uncrust. The amount of the erosion generated by crust slop is lessthan uncrust slop. The more crusts are on the surface of the slop, the more serious the rillerosion is. This indicates that the crust to some extent retards the rill erosion. The rill showsmany characteristics under the conditions of different rainfalls: when the rainfall intensityreaches1.00and1.67mm·min^-1because of the different ways of erosion, the width-depthratio has decreasing trends with the increasing rainfall times, but it turns out opposite resultwhen the rainfall intensity reaches2.33mm·min^-1. Moreover, the rill erosion will be muchserious when the rainfall time increases.（6） The sorting characteristics and transport mechanism of sediment particle in colluvialdeposits. The sediment particle under different rainfall density and gradient shows manycharacteristics: When the rainfall density and slope increase, both the content and enrichmentratios of coarse particle in sediment increase, and the losing of sediment tends to be coarseparticleing. The sediment particle gradually changes to coarse in the course of the interrillerosion-rill erosion and finally keeps in accordance with the tested soil’s particlecomposition. The saltation plasmid level of sediment particle is high:>0.537mm. During theinterrill erosion, the contents of suspention load and saltation load are between71.26%and97.64%, after the rill erosion occurs, the proportions of bed load rise up to10.93-41.74%. Thesediment particle under different surface crusting shows some characteristics: The crustingslope has the feature of transporting small particles. Runoff transportation of sediment particlealso chooses the coarse particle in the late period of erosion. Moreover, the selectiontransportation of the10min high-degree crusting slope is earlier than that of the5min one.Under the light rainfall density, the transportation of runoff tends to choose coarse particlegradually as the number of precipitation increases, but in strong rainfall density allprecipitation shows the “overall-transportation” feature of the test soil.（7） The dynamic mechanism of colluvial deposits under the rainfall conditions. The flowvelocity, the flow depth, the Reynolds number, the Froude number and other dynamic parameters are smaller between rills than those of colluvial deposits, but the resistancecoefficient and the roughness coefficient have the opposite results. The relationships amongthe rate of sediment and flow velocity, rough coefficient and resistance coefficient represent apower function between rills. Only the interrill rate of sediment and the flow velocity have alinear relationship. The mean rate of sediment with flow velocity, resistance coefficient androughness coefficient have a power relationship, but the mean rate of sediment with flowdepth, Reynolds number and Froude number are shown by a linear relationship. The sedimentrate has linear relationships with five dynamic parameters. The unit stream power，energyconsumption per unit width and specific energy in sections can be the primary choices to bethe dynamic parameters of colluvial deposits’ sediment transportation under rainfallconditions.