Effect of variations in rainfall intensity and different return period rainfall events on unsaturated slope stability

Rainfall patterns considered a factor in slope stability, are affected by climate change. This study conducts a trend analysis and a rainfall frequency analysis to examine rainfall intensity variations and rainfall amount for various return periods in order to understand the rainfall pattern changes in the Zengwen reservoir catchment area in the future. Then, a coupled hydromechanical framework is used to analyze the effect of rainfall pattern changes on slope stability. The results of the trend analysis reveal an increasing trend from 1990 to 2016. The trend (slope) of the one-day rainfall events is the largest, with an average slope of 0.08 mm/h per year. The slope stability analysis results reveal that if the rainfall intensity continues to rise in the future, precipitation will more easily infiltrate the soil, decreasing stability. For rainfall in 2016, 2050, and 2100, the LFS (Local Factor of Safety) of soil at a slope depth of 2.5 m decreased by 4.8%, 8.1%, and 12.2%, respectively. Moreover, the results of the frequency analysis show that rainfall intensity, and thus the infiltration rate, increased with increasing return period. For rainfall events for a 2-year return period, the LFS of soil at a slope depth of 3.0 m did not show an obvious drop. However, for 10- and 100-year return periods, the stability of soil started to decrease at 48 h and 32 h, respectively. According to a Taiwan Climate Change Projection and Information Platform) report (2016), there is an increase in the probability of extreme rainfall events in the future. The slope data (e.g., slope topography, slope structure, and soil mechanical and hydraulic parameters) obtained from the field in this study can be used to prevent or reduce the hazards of slope disasters.