Experimental Study On Runoff And Infiltration In The Slope As Affected By Gravels

Gravel is different from soil particles in density, size, and water permeability, surface andchemical structure, this will affect and change the structure and the physical properties for soiland the exsitace of rock fragments in soil lead to differences of the movement of watercompared to that in homogeneous soil, In this paper, study on runoff and infiltration in theslope as affected by gravels, the main conclusions were drawn as follows:(1) In this study, simulated rainfall experiments were carried out in order to investigate soilhydraulic parameters and infiltration for soil with five gravel contents (massive fractions as0,10%,20%,30%and40%) under three slope gradients (5°,10°and15°) and rainfall intensity of120mm/h. The results showed that Fronde number and overland flow velocity for theexperimental soils increased and runoff depths and Darcy-weisbach friction coefficientsdecreased with increasing slope gradient. The Reynolds number and the Darcy-weisbachfriction coefficients for the soils containing gravels were larger than that of the soil withoutgravels. The runoff type of the soils belonged to laminar flow and then the runoffs changed fromslow-flow into torrent flow with the increase of gravel content on the slope of5°. The overlandflows of the soils on the slopes of10°and15°were torrent flows. The infiltration amount priorto runoff generation decreased with the increase of gravel content. Reduction of infiltration wassignificantly linearly related to gravel content. the infiltration after runoff containing gravel of10%is larger than that of the soil containing gravels of0,20%,30%and40%. Both of Kostiakovinfiltration model and Philip equation could describe the rainfall infiltration process well．(2) In this study, the flow velocity and the processes of runoff and sediment generationsalong the slopes for five soils (gravel mass contents were0,10%,20%,30%, and40%,respectively) were investigated under rainfall simulation experiments in order to improve theunderstanding of soil and water loss process in the soil containing gravels. The resultsindicated that the flow velocity for soil containing gravels was larger than the soil withoutgravels. During the initial0-10min of the rainfall, sediment yields in the soils with gravelsincreased sharply with time and the peak period of sediment generations appeared around10min. Then, sediment yields decreased gradually. At the later stage of the rainfall (30-60min), the sediment yields for the soils with gravels increased again especially for the soils with30%and40%gravel contents their sediment yields enhanced significantly and fluctuated with time.In the experiments, the total sediments yield was positively linked to the gravel content in thesoil and the sediment concentration was positively linearly related to the sediment yield.(3) Investigation on the process of runoff and sediment in stony soil can help tounderstand soil and water process in this kind soil and also provide significant soil parametersfor soil erosion process-modeling. In this study, the processes of runoff and sediment in soilcontaining gravels under three slope gradients were analyzed by simulated rainfallexperiments. The results indicated that the runoffs in stony soil increased obviously during0-20min and then became steady. The effect of gradient on runoff was weakened with theincreasing of gravels in the soils. The peak period of sediment generation appeared between0-20min. The sediment yields generated during this stage accounted for a considerableproportion to the total. When the gradient reached15°, the sediment yields of soils with thegravel contents of20%,30%and40%increased again after30min. And total soil sedimentyields under this gradient increased greatly compared with others. In the experiments, thegradient of the slope dominated the sediment yield.(4) The sediment yield and infiltration are affected by rainfall intensity based onsimulated rainfalls. The results showed that the sediment delivery rate changed smoothlyunder rainfall intensity of60mm/h and the sediment delivery rate fluctuated due to rill duringrainfall, the appearance of rill need more time of5～20minutes under rainfall intensity of90mm/h than that under rainfall intensity of120mm/h, the total sediment yield was increasedobviously with the addition of rainfall intensity, the increased quantity of total sediment yieldfrom60mm/h of the rainfall intensity to90mm/h was0.83～2.82times more than that from90mm/h of the rainfall intensity to120mm/h; with the increasing of the rainfall intensity, theinfiltration rate was decreasing, The rainfall infiltration process in soil containing gravelsunder three rainfall intensities could be well simulated by the experiential infiltration modelof Kostiakov and Horton. Horton model showed better fitting effect of water Infiltrationprocess under rainfall intensity of60mm/h, Kostiakov model showed the fine fitting effectunder the rainfall intensity of90mm/h and120mm/h.(5) The runoff generation time increases with the increase of gravel coverage degreesunder rainfall intensity of65mm/h and the five different slopes, The runoff generation timewas linearly related to gravel coverage degrees; the runoff and sediment yield are affected bygravel coverage degree, the influence of sediment yield is significant, compared to the soilwith the gravel coverage degree of0, the sediment yield significantly increase when thegravel coverage degree increase from20%to50%, the runoff increase with the increase of gravel coverage degree under the slopes of5°,10°and15°, the runoff is less after trend ofincrease under the slopes of20°and25°, the slope has the important influence for the runoff.