Integrating Rainfall Distribution Patterns and Slope Stability Analysis in Determining Rainfall Thresholds for Landslide Occurrences: A Case Study

After a series of rainfall-related slope incidents that threatened immediately protected entities, the Taipei City government initiated a slope deformation monitoring and investigation program for potential landslides in its administrative districts and a review of its current rainfall thresholds for landslide occurrences, which is the aim of this study, in 2021 for better preparedness in facing the extreme weather- and climate-related natural hazards. Due to the limited availability of historical data, this study employed a physically based method to derive rainfall thresholds for landslide occurrences by integrating different rainfall distribution patterns into infiltration and slope stability analyses. The study examined four rainfall patterns—uniform, intermediate, advanced, and delayed—to assess their impact on slope failure mechanisms. Results indicate that advanced rainfall patterns (where peak rainfall occurs early) trigger the fastest failures, while delayed rainfall patterns lead to gradual groundwater accumulation, causing slope destabilization over longer durations. The study also found that short-duration rainfall (24 h) mainly triggers shallow landslides, whereas prolonged rainfall (72 h) leads to deep landslides. The study’s findings are crucial for early landslide warning systems, which provide different mitigation strategies based on the expected rainfall duration and provide a scientific basis for authorities to revise and integrate new rainfall thresholds into their real-time landslide warning systems.