Study on earthquake and typhoon induced regional debris flows in Ta-Chia River, Taiwan

Large-scale debris flow hazards occurred in Ta-Chia River watershed during typhoons that passed through Taiwan from 2001 to 2005 without forewarning. Especially, the Minduli typhoon event in 2004 hit Taiwan which caused severe property damage and inflicted heavy casualties. Though landslide-induced debris flows present a hazard that is being increasingly recognized, such a large-scale debris flow hazard in Ta-Chia River watershed still appears to be particular. In this paper, we present a detailed study on the occurrence of regional debris flow hazard in Ta-Chia River watershed and reveal the trigger mechanism of the landslide and debris flow. Our findings indicate that the regional debris flow hazards were mainly caused by the huge amount of sparsely deposited materials from landslides triggered by Chi-Chi earthquake. Rapidly increasing water pressure caused by typhoon events provided a powerful force that moved the sparsely deposited materials into gullies and then triggered the debris flow movement. A strong coupling between the spatial distribution of rainfalls and the occurrence of regional debris flows is also addressed.