Study On Starting Mechanism And Early Warning Model Of Debris Flows

Debris flow in Beijing area, whose harmfulness always be ignored, is the typical north low frequency debris flow. Once erupted, it will cause huge loss, such as disaster caused by the “7.21” torrential rain. The research on debris flow in Beijing has a practical significance. This paper is based on the program, the key technology research on forecast and precaution of debris flow and flush flood in Beijing, financial aided by Science Organization of Beijing. Taking Damo gully in Mentou area for research object, do geological survey and evelopment features of debris flow, analysis the composition of loess materials, structural character and mechanical properties. And on this basis, the physical simulation test is carried out, and the erosion characteristics of the source accumulation body are studied. To analyze the main influencing factors of the starting of debris flow of Damo gully, and obtain the critical excitation rainfall of different rainfall types. The early warning model of debris flow in Damo gully is established based on the critical excitation rainfall and the occurrence data of historical debris flow. Many main results and achievements of this staudy are obtained as follows:(1)Based on the geological survey and Mapgis treatment, the characteristics of source areas, flow paths and tributary of Damo gully were obtained. There are 69 of unstable sources with 9763.6×104m3and 40 of less instable sources with 47399.8×104m3. The deposits soil, more than 20 mm and grain size soil mass percentage is 49.93% and defined as granule soil by Code for Investigation of Geotechnical Engineering, or defined as mass rock and soil by Department of Geology and mineral resources DT-92 rules. The coal gangue, more than 20 mm and grain size soil mass percentage is 53.13% and defined as gravel by Code for Investigation of Geotechnical Engineering, or defined as mass rock and soil by Department of Geology and mineral resources DT-92 rules. According to deposits soil and coal gangue, the environment energy of debris flow formation is high, the skewness is extremely positive, kurtosis is very sharp and narrow, turbidite facies, rolling and suspension transport.(2)The development characteristics of debris flow of Damo gully were evaluated scientifically. It is a gully debris flow with the trigger of rainstorm, waste provenance and slide matter, with high activity and disaster-causing ability. The main gully is in its decline phase, with an easy happening degree and middle risk. The village near Damo gully, where branch gully a main gully mixed up, is a high risk area. The easy-damage degree Vd is 0.71 and the village among Damo gully is a middle or higher risk area, the right band of main gully is a middle risk area. Damo gully is middle risk area with a risk degree R is 0.24.(3)Through the design of similarity relation, source processing method, rear catchment, rainfall intensity, and combination type, artificial rainfall system to build the outdoor physical simulation test platform with 1:60 and the indoor physical simulation test platform with 1:30. Model dimensions are 1.03m×0.52 m, 3.72m×1.86 m. Through three different rainfall condition of outdoor physical simulation test and five different rainfall condition of indoor physical simulation test to get the source bulk erosion failure characteristics, volumetric water content, pore water pressure, erosion amount out of the variation.(4)The erosion failure process of Damo gully is as follows infiltration, surface erosion, rill erosion, slope undercutting, gully erosion. Provenance from begin to runoff supplement at the rill erosion, slope undercutting, gully erosion, which formed the eruption range. The time of occurrence of gully erosion is the starting point of debris flow.(5)Rainfall test under different type shows that the heavier the rainfall, the easier for debris flow to startup. The higher the initial water content, the faster the debris flow to start-up, with heavier runouts and faster rate. Comprehensive analysis the starting mechanism of Damo gully debris flow, it is found that the soil physical function(antecedent rainfall), rear catchment erosion, rainfall rainfall intensity, rainfall duration are the major factors in the starting of debris flow.(6)The critical excitation rainfall of saturated soil and once in a century years rainfall condition is 70.92 mm, and direct once in century years rainfall condition is 86.70 mm, and once in ten years of rainfall turn once in a century years rainfall condition is 117.78 mm by outdoor experiments. The critical excitation rainfall of saturated soil and once in a century years rainfall condition is 55.70 mm, and direct a once in century years rainfall condition is 89.12 mm, and once in ten years of rainfall turn once in a century years rainfall condition is 97.76 mm, and direct once in fifty years rainfall condition is 107.36 mm, the critical excitation rainfall of antecedent rainfall and once in a super century years rainfall condition is 81.06 mm by indoor experiments.(7)Using the critical excitation rainfall as the main data source, combined with the historical debris flow occurrence data to create a rainstorm type classification of early warning model and a antecedent rainfall type classification of early warning model by the "one rainfall process" to determine the type of rain.The research in this paper, analysis the developing feature of debris flow in Damo gully comprehensively. Investigated the starting mechanism of Damo gully debris flow through physical simulation experiment, get the critical rainfall, build the early warning models of geological in Damo gully watershed at last, which enriched the research work on debris flow in Beijing area and will do a demonstration for the low frequency debris flow in north.