After Typhoon Morakot, a considerable amount of unstable sediment was distributed on mountain slopes and river channels comprising the watershed in Taiwan. During heavy rainfall, such sediment can turn into debris flows, posing a risk to downstream residents and critical public infrastructure. Taiwan has a disaster prevention and
warning mechanism for debris flow. Although this system has been in place for many years, investigations of unstable
sediment in watersheds remain limited. Unstable sediment is primarily distributed in the watersheds of mountainous
areas, making studies time-consuming and labor-intensive. In this study, we compiled data from the event-based landslide
inventory regarding the area and frequency of newly formed landslides in 2006 to 2022 and established a physiographic
fragility index for Taiwan. Furthermore, we identified unstable sediment watersheds by using the tributaries
of rivers managed by governments. This study considered both the sediment production potential and the degree of
hazard within these watersheds to calculate the propensity for unstable sedimentation (P), and 312 watersheds were
found to be highly susceptible to unstable sedimentation. This study also identified 38 bridges located in or adjacent to these high-priority watersheds as a reference for disaster prevention authorities.
Keywords: Event-based landslide inventory, Unstable sediment, Physiographic fragility, QPE