Risk assessment of debris flows in Songhe Stream, Taiwan

Abstract Typhoon Mindulle was the most severe typhoon across the Tachia River watershed following the 921 Chi-Chi earthquake, and resulted in extremely high precipitation of 1431 mm that induced many landslides, debris flows, and debris floods in affected areas. This paper analyzes the debris flows in the Songhe area induced by Typhoon Mindulle by employing a numerical model for debris flow simulation. The FLO-2D numerical program was adopted to simulate the flow conditions of debris flows in the Songhe area, including flow depths, flow velocities, and sediment depositions. Comparing the field data with simulation results, this paper defines an Index of Accuracy to examine simulation accuracy. Ignoring the influence of houses in the alluvial fans, the simulation accuracy was 79.3% and 61.4%, respectively for the First and Second Branches of the Songhe Stream. Considering the house effect in the Songhe area raised the simulation accuracy to 82.3% and 75.5%, respectively for the First and Second Branches. In addition, the simulation considering the hydraulic structures established after Mindulle was executed. This study also adopted 24 h accumulated precipitation at various recurrence intervals (10, 100, and 500 years) to perform scenario simulation of debris flows. Based on the method of Swiss Disaster Degree Classification, consisting of influential intensity and occurrence probability parameters, this study classified the risk degree of hazardous debris-flow areas into three categories, including high, medium, and low. The proposed approach generates the risk distribution map that may be used for setting up a disaster mitigation strategy for the Songhe area.