Study On The Formation And Motion Characteristics Of Debris Flow In Small Watershed In Hilly Region Of Loess Area

Debris flow is a common geologic hazard in hilly Loess Region.Debris flow disasters occur frequently in hilly Loess Region of China under the background of the global warming and high-frequencies of extremely climate.Lanzhou city is an important area of economic development for“One Belt and One Road”in China,so it is particularly important to study on debris flow disaster in loess area of Lanzhou.In addition,there were few studies on the formation and motion characteristics of debris flow in the small watershed in hilly Loess Region,which will increase the uncertainly of debris flow and regional security risk.Therefore,the Dasha gully was selected the research object in Lanzhou.The mathematical model,field sampling,indoor test and field simulation test were used to obtain the different classes of geohazard susceptibility area and unstable area.Moreover,the soil physical and water motion characteristics were studied in the unstable area,and the slope failure mechanism of loess slope and the formation process of debris flow from slope failure were revealed.Finally,the motion characteristics of debris flow are explained,and the impact pressure model was modified and the dimensionless impact pressure comprehensive formula was constructed.The mainly results were as following:The proportion of high geologic hazard susceptibility area was always more than44%for the total geologic hazard susceptibility area by using the different methods to calculate the geologic hazard susceptibility area when the time was ranged from 1980to 2015,and the proportion of medium geologic hazard susceptibility area was exceeded about 34%for the total geologic hazard susceptibility area,which indicated that the geologic hazard susceptibility belonged to the high-medium geologic hazard susceptibility class in the Dasha gully,and it also illustrated the slope was in a unstable state.The slope has become unstable state due to the engineering activities based on the field investigate data,and the proportion was 78%for the total slope.Furthermore,the urban area was rapidly increased based on the data of land cover in 35years.Therefore,human activities have strongly affected on the stable of slope.The experimental results of double ring method showed the depth of stable infiltration,the initial infiltration rate and the stable infiltration rate were 27~38 cm,2.29~5.28 min/cm and 0.44~1.75 min/cm,respectively.The depth of stable infiltration was generally increased from upstream to downstream in the Dasha gully.The infiltration coefficient was gradually decreased from upstream to downstream.However,the infiltration coefficient of the left in the Dasha gully was bigger more than the right of infiltration coefficient,and infiltration coefficient of the both side in the main channel were larger than the main channel of infiltration coefficient.The depth of stable infiltration would be influenced on the land cover,forest>grass>arable>deposition material in channel.The infiltration rate showed decreasing trend with the times increasing.Before the slope failure experiment started,the pore water pressure,earth stress,volumetric water content and electrical conductivity of soil were 0~0.1 kPa,0~0.1kPa,0.09~0.14 m~3/m~3 and 0.07~0.39 S/m,respectively.The cracks and fissures were occurred in the slope due to the rainfall erosion and rainwater infiltration at the beginning of experiment,and the pore water pressure,earth stress,volumetric water content and electrical conductivity of soil were increased to 0.22~1.13 kPa,0.25~3.62 kPa,0.09~0.36 m~3/m~3 and 0.07~2.05 S/m,respectively.Meanwhile,the shear strength of slope soil was slowly decreased and the wet weight of soil was raised.These phenomena contributed to the local failure of slope.During the middle period of experiment,the shear strength of soil was further reduced and the ratio of soil pore was becoming small when the rainfall was continued.There was mass slope failure in the experiment when the pore water pressure,earth stress,volumetric water content and electrical conductivity of soil peak up to 3.11 kPa,8.85 kPa,0.57 m~3/m~3 and 4.96 S/m,respectively,and then they would be suddenly gone down.The slope soil was influenced by the rainfall erosion and gravity in the process of slope failure,and the soil would be occurred a series of change process,for example,collision,disintegration and mud-making action,finally,the slope failure was evolved into the debris flow.The shear strength of soil was gradually recovered and the soil compactness was increased at the end of experiment,and the pore water pressure,earth stress,volumetric water content and electrical conductivity of soil were slowly increased again,these indicated the temporary trend of slope was stable.40 groups experiments were carried out to simulate the motion characteristic of debris flow by using the flume physical model under the difference density and difference mixture weight of debris flow,and the results showed that the dynamic viscosity coefficient,yield stress and Froude number were 0.0011~0.0041 Pa s,0.25~30.68 Pa and 2.03~29.23,respectively.The velocity,the flow depth,the pore water pressure and the impact pressure were 1.23~3.62 m/s,2.7~13.4 cm,0.15~4.5 kPa and 1.23~28.41 kPa,respectively.The comprehensive relationship equation between the peak depth,peak velocity,peak pore water pressure,peak impact pressure and density,mixture weight were constructed base on the experimental results.The empirical parameters of hydrodynamic and hydrostatic impact pressure model were modified based on experimental results,and the values were 5.17 and 9.99,respectively,which were significantly different from the empirical parameters of debris flow impact pressure model in earth-rock area.The new hydrodynamic model can perform very well to impact pressure calculation that with high velocities and relatively high Froude number,and the new hydrostatic model was relatively appropriated for impact pressure calculation that with low velocities and low Froude number.Furthermore,the comprehensive relationship between peak impact pressure and Froude number,Reynolds number were built based on the new model.This study could provide the relevant parameters of numerical simulation for the landslides and debris flow in the loess area,and scientific basic for preventing and controlling debris flow disaster.More importantly,it could help scholars to further understand the mechanism of landslide and debris flow in the loess area,and provide reference for regional land use planning and development.