Risk, Vulnerability Assessment For Post-earthquake Coincident Debris Flows In Longxihe River Basin, Dujiangyan City

Induced by heavy rains, debris flows broke out several times at mountains and hills in the earthquake disaster area. Debris flows damaged the residential areas of settlements and caused a huge economic loss. A statistic shows the great earthquake induces collapses and landslides far more than 50000. Beyond 10000 sites could be classified to geological hazard sites, and about 1000 gullies have high probability of developing debris flows. Collapses and landslides can provide a plenty of source for gullies to develop debris flows. If an extreme rainfall occurred at Wenchuan Earthquake area, a group of large-scale debris flows could occur. Therefore, risk, vulnerability assessment are important for preventing disasters induced by debris flows in Wenchuan Earthquake area. They have a great significance to protecting life and properties of people in that area.Debris flow disasters in this area have typical characteristics of earthquake area’s geo-hazard. The geological property of debris flows in this area is elaborately investigated before writing the paper. Performance in the research of debris flow’s risk, vulnerability assessment, and early warning technique are obtained by this thesis.Criterion for risk assessment of debris flow is corrected and modified. “Single debris flow risk assessment architecture” and “Multiple debris flow risk assessment architecture” are constructed. Risk degree of single debris flow and multiple debris flows in Longxihe River Basin are evaluated. The gully and source conditions of debris flows are analyzed. Main gullies develop at the deep cutting part of a mountain. The debris material comes from deposit of landslides and collapses in the source area of the gullies. The crucial factors, such as impact pressure, depth, output span, are obtained by using numerical simulation. The risk assessment index map of debris flows can be made according to the parameters generated by numerical simulation. Furthermore, the risk assessment result of Longxihe River Basin could be acquired.“Watershed and rainfall analysis debris flow risk assessment architecture” is constructed. Fuzzy-logic-based method is used to describe debris flows’ initiating factors. Adaptive Inverse Distance Weighting Method and Multiple Linear Regression Method are introduced in the procedure of debris flow risk assessment. The topological condition and rainfall condition are analyzed in the process of calculating the risk index. By using Geologic Information System, a risk assessment index map can be created. This research has a practical value for the geological hazard administrative department.Evaluation method for physical vulnerability has been modified and expanded. Debris flow’s vulnerability refers to the maximum potential loss of life, property and facility in an area threatened by debris flow hazard, in a given period. Major physical parameters, such as output span, depth, velocity, and impact pressure can obtain by using numerical simulation to reconstruct entire outbreak events of a debris flow. After the outbreak procedure of the debris flow is simulated, the damage degree of buildings in the town can be estimated. The physical damage can be quantified for each affected structure by the functions of vulnerability, which could be calculated as a ratio between a monetary loss and a reconstruction value. Three different vulnerability functions are generated, which are functions of debris flow impact pressure, depth, and kinematic viscosity, respectively.An architecture based on SOM for evaluating vulnerability in Wenchuan Earthquake area is constructed. In the research of vulnerability, an analysis for consequences of a disaster is far more important than an analysis of the hazard procedure. This thesis takes Longchi Town as a research sample to develop calculation methods for debris flow’s vulnerability. SOM is used to sieve vectors of vulnerability that are constructed from research objects, such as human, building, road in the research area. The vulnerability index map can be made based on a SOM sieving result. The research purpose is to realize a quantitative management for vulnerability and disasters.