In this paper, we present a detailed study on the occurrence of regional debris flow hazards in storms after strong seismic disturbance. To explore the coupling between the Chi-Chi earthquake and sequential regional debris flow hazards in the study area, analyses of rainfall characteristics, rainfall-induced landslides, and debris flow formation from rainfall-induced landslides were conducted. Our findings indicate that the regional debris flow hazards were mainly caused by the huge amount of sparsely deposited materials from landslides triggered by the Chi-Chi earthquake. Rapidly increasing water pressure caused by typhoon events with specific rainfall intensity provided a powerful force that moved the sparsely deposited materials into gullies and then triggered the debris flow movement.
It is widely recognized that most shallow landslides in Taiwan occur as a result of heavy rainfall and consequent pore pressure increases in the near subsurface. Historically typhoon events with high-intensity, longduration rainfall often triggered shallow, rapidly moving landslides (Anderson and Sitar, 1995, Chen et al., 2005), i.e. debris flows, resulting in casualties and property damage in the Da-Chia river watershed over the past few decades. A number of studies (Iverson et a;., 1997, Chen et al., 2006) have demonstrated that rainfall-induced landslides can be transformed into debris flows as they move downslope (Iverson et al., 2000). After the 1999 Chi-Chi earthquake, large-scale geohazards occurred in the Da-Chia river watershed during typhoons that passed through Taiwan from 2001 to 2008 without forewarning (Central Weather Bureau, 2008). Especially, the Minduli typhoon event which hit Taiwan in 2004 causing severe property damage and inflicting heavy casualties, as shown in Fig. 1 and Fig. 2. (refer to the full paper)Though landslide-induced debris flows present a hazard that is being increasingly recognized (Delmonaco et al., 2003), such a large-scale debris flow hazard induced by a earthquake-typhoon sequence in the Da-Chia river watershed still