In this study, rainfall-induced debris flows in the Houyenshan gravelly gullies,Miaoli, and the corresponding rainfall threshold conditions are explored. Based on the in-situ rain gauge data from the monitoring system and field surveys during the period from 2006 to 2012, we found the debris flow tends to occur when the total rainfall exceeds 57.5 mm and the subsequent rainfall intensity is higher than 3 mm/hr. Field survey and terrestrial LiDAR measurement indicate the talus deposition in the source areas and outbursts of landslide dams on the gully bed are the dominant factors inducing debris flows. Finally, HHT spectrum analysis of geophone signals for stony debris
flows depicts the peak frequencies within the range of 10-50 Hz. The temporal intensity distribution of geophone signals are determined by both the magnitude of debris flows and travel distance with respect to the monitoring station.
Key Words : Houyenshan, gravelly gullies, debris flows, critical rainfall line, dam break.