Analysis on Debris Flow Non-rectilinear Motion—From Case Study to Hazard Zone Delimitation Discussion

This study aims to discuss the possibility of debris flow non-rectilinear motion and the current debris flow torrent delimitation method. The recent debris flow disaster in Taiwan triggered by torrential rainfall caused turning curve at alluvial fan and damaged community which was thought out of disaster potential. In order to clarify the factors causing this phenomenon and recognize torrents with similar susceptibility, numerical simulations were carried out by Flo-2D. In the previous study, simulations were given to the disaster case during Typhoon Soula and verified with investigation data. The result shows possible factors may trigger debris flow non-rectilinear motion, including slope gradient difference, geologic materials, and sediment material at the flooding part. In order to clarify the similar potential of non-rectilinear motion of the other torrents, this study selected five torrents in Kao-Ping basin for further analysis. Three accumulation types will occur by comparing the last and the first of accumulation areas. The type of entirely different accumulation areas after three times of debris flow occurrence was the one with non-rectilinear motion susceptibility, and the other types include extended accumulation area by more debris flow occurs and the same accumulation area but increasing depth. These types caused completely different risks to the communities inside or nearby the alluvial fan of the torrent. The qualitative analysis of flowing and accumulation parts of each torrent was given. Torrents with debris slide potential distributing along the flowing part, gradient difference between flowing and accumulation parts, and obvious gully terrain, have higher susceptibility of non-rectilinear motion. The geologic material that easily generates large border is also a significant factor, but it relies on detailed onsite investigation. This study successfully built an analysis process to clarify the possibility of non-rectilinear motion of debris flow torrents and some conclusions were therefore summarized.