Research About The Mechanism Of Debris Flow(Loess Mudflow)in Yan’an Area

Typical debris flow valleys in Yan’an City are research object. Based on field survey, field simulated rainfall experiment and theoretical analysis such as Hydromechanics, we research in-depth the mechanism of starting and formation of debris flow in the area. The study has made some valuable results.Firstly, debris flow has been classified depending on the classifica-tion standard and methods. We have classified debris flow by landforms, the largest number is ravine and the second largest number is hillside and the least is valley. Based on the material composition of debris flow, and the most widely distributed type is loess mudflow and the distribution of debris flow is less than loess mudflow, but this region has not the water-stone flow. According to inducing conditions of debris flow, the mainly type is storm.Secondly, forming conditions of debris flow were analyzed in detail based on field surveys, what including basic conditions (topography, source and water) and priming condition. Terrain conditions including some factors, such as drainage area, hillside slope, gully slope and vegetation. The main water source is rainfall. Recharge mode solids are landslides, collapses, diarrhea and other slip and spoil wasteland. We have combined with the previous researching data and historical disasters of debris flow. The mainly type is storm in Yan’an area. Debris flow concen-trated in June to September every year.Finally, we research the mechanism about formation and starting of debris flow through rainfall experiment. About conditions of debris flow’s forming,we have summed up the12single factors. Comprehensive and environmental conditions of valleys have been evaluated valley by quan-titative and semi-quantitative mathematical methods. In the last, the integrated environmental values were calculated. We design a simulated rainfall experiment and measure soil pressure, pore pressure, suction and many other parameters. The testing process disclosure accurately the characteristics of infiltration and changes of soil feature in the process of experiment. Research starting condit-ions of debris flow, conditions should be obtained to meet the standard which including terrain should be6-15°slope, and a large number of loose solids accumulate in the ditch, and a certain depth of runoff and a certain thickness of saturated soil are needed,this condition equivalent to satisfy the equation1.15<(?)<1.6.Using the theory of hydromel-chanics, we research the starting rule and analysis starting mode.Fine particulate matter starts displacing when the pore water pressure increment reached4.6KPa. According to the conclusions of the above studies, we can get the formational mechanism of debris flow in Yan’an area.