Risk assessment of simultaneous debris flows in mountain townships

Many mountain towns in China are located on the joint alluvial fans of multiple and adjacent past debris flows, making them vulnerable to large, multiple, and simultaneous debris flows during heavy rainfall. Without emergency management planning, such flows, often appearing with interconnecting and chain-reaction processes, can lead to extensive loss of life and property. In the Wenchuan earthquake-affected area, such disasters are common. We analyzed the compound effects of simultaneous debris flow events, and proposed three quantitative methods of debris risk assessment based on kinetic energy, flow depth, and inundation depth. Validated using a field study of actual debris flow disasters, these analyses are useful in determining the type, quantity, distribution, economic worth, and susceptibility of hazard-affected objects in a region. Subsequently, we established a method to determine the vulnerability of different hazard-affected objects, particularly concerning the susceptibility indexes of buildings or structures. By analyzing the elements underlying hazard formation conditions, damage potential, and the socio-economic conditions of mountain townships, we proposed a systematic and quantitative method for risk analysis of mountain townships. Finally, the proposed method was applied to a case study of Qingping Township, which was affected by 21 simultaneous debris flows triggered by a 50-year return period precipitation event. The proposed method analyzed the superposition and chain-reaction effects of disasters and divided the affected area of the township into three risk zones. The analysis indicated that the calculated risk zones coincide with the actual distribution and severity of damage in the debris flow event, which suggests that the risk assessment is consistent with results from the actual disaster.