The Formation Mechanism Of Debris Flow In Bailong River Basin

Geological disasters, such as landslides and debris flows are multiple and frequent in Bailong river catchment, lead this region to be one of the top four area with high incidence of geological disasters in China. Intensive and wide distributed landslides and debris flows have become main obstacles of the local economic and social development. Especially after the earthquake happened in 2008 in Wenchuan, the situation of disaster like landslides and debris flows have become more serious, such as the large debris flow disasters on 2010 8.8, Zhouqu. Because of its large scale, vast damage ability and wide spread, the study of debris flow disaster has become a priority part of work of the prevention and control of geological disasters in this region.In this paper, based on existing research results, remote sensing monitoring and field survey methods were used to investigate the temporal and spatial distribution of debris flow disasters in Bailong river catchment. Then, the developmental characteristics combined with the background of natural environment are discussed. Based on this, appropriate influential factors were selected. Then, the factors were analyzed in two evaluation unit scale with information value model, analytic hierarchy process, logistic regression, artificial neural network model and support vector machine model to estimate the susceptibility of debris flow disaster in Longjiang catchment. Then, information value model was picked up as it possesses strong abilities on learn and prediction to accomplish susceptibility zonation in Bailong river catchment and furthermore, and to quantitatively analyse the forming conditions of the debris flows by referring to the contribution factor of each factors. Guided by the qualitative, quantitative studies on the causes of the debris flows and the results of susceptibility assessment, Goulinping which located in the high susceptibility regions in the left bank of the middle reach of Bailong river catchment were selected to do this study. Slope runoff and sediment erosion test were done under artificial simulated rainfall in two test fields to simulate the processes of surface runoff, sediment transportation, formation of slope debris flows downhill and the starting processes of gully debris flows in realistic natural conditions. In the end, integrate all the results above, a in-depth study was made to reveal the formation mechanism of debris flows in Bailong river catchment.The main work and fruits of the studies are:(1) Multi-spectral and high precision remote sensing image and the platform of GIS were used to accomplish the preliminary results of automatic interpretation and artificial recognition on the spatial distribution of debris flows in Bailong river catchment. Then, referencing the existing disaster history and regional survey data the results were further confirmed and complemented to the temporal and spatial distribution characteristics of debris flows within the whole catchment. Integrate the data on topography, lithology, geological structures, vegetation cover, rainfall characteristics and regional basic conditions, qualitative analysis of the causes of debris flow disaster in Bailong river catchment were made.(2) Based on the spatial distribution of debris flows and background environmental factors in Bailong river catchment,5 kinds of regional disaster evaluation model were adopted to evaluate the catchment units in two scales. Evaluation and grading of the susceptibility of debris flows were accomplished based on the results above. Through the analysis of ROC curve and the comparison of the probability of happened debris flows in the regions that was regarded as high susceptible, information value model has proved to be good in estimating of debris flows in two kinds of scales. Thus, information value model could guide the work on classification and regionalization of debris flows in Bailong river catchment.Based on the parameters of the factors that has been used in information value model, quantitative estimations could be done when comparatively analyse the causes, conditions, internal classification, and weight in causing the debris flows in Bailong river catchment. Results showed that the indicators:5-8mm rainfall per 10 minute,10-50km2, the catchment area of 5-10km2, the relative elevation >2000m and between 1600 to 2000m, slope degree<15 degrees, low NDVI value, weak rock, average slope of watershed 25-30 degrees, showed increasing importance in forming debris flows one after another.(3) Through artificial rainfall simulation experiment with the runoff field test, the study analyzes the relationships between runoff producing the rainfall characters on two main types of slopes:loess slopes and weathering phyllite slopes. Then, searching for the mechanism erosion and failure. The particles composition of slope materials determines its permeation characteristics. This leads to the phenomenon that on the low permeability loess slope, erosion is the essential break form which starts from the slope surface, then develop to gully erosion, and eventually, lead to debris flows on slope. While, on the weathering phyllite slopes with high permeability, seepage aggregates fine particles in deep and form a water resisting layer, which leads to an increase of pore water pressure but a decrease of the effective stress of slopes and the shear strength. In the end, sudden creep deformation happened and may developed to slope debris flows.(4) In the gully debris scouring simulation test, comparison and analysis were made on pore water pressure, pressure, transportation of groundwater and surface erosion accumulation records, to rebuild the complete process of gully runoff formation and the mechanism of triggering debris flows. Analysis shows that the change of pore water pressure caused the vibration liquefaction of inside bed material and the attenuation of effective stress. Which, leads to loose accumulation materials on the valley bed of damage and become important factors for the formation of debris flowsPressure on river bed could response the transportation and accumulation of surface bed materials well. Pore water pressure correlates to gully runoff depth, both of them could be used to monitor the flood and debris flows in gully. They can also be used in debris flows’ warning and forecasting, as the index.