Annual landslide risk and effectiveness of risk reduction measures in Shihmen watershed, Taiwan

Landslide risk analysis procedures in this study could evaluate annual landslide risk, and assess the effectiveness of measures. Risk analysis encompassing landslide hazard, vulnerability, and resilience capacity was used to evaluate annual landslide risk. First, landslide spatial, temporal, and area probabilities were joined to estimate annual probability of landslides with an area exceeding a certain threshold in each slope unit. Second, different elements were assigned corresponding values and vulnerabilities to calculate the expected property and life losses. Third, the resilience capacities of communities were calculated based on the scores obtained through community checklists and the weights of items, including "the participation experience of disaster prevention drill," "real-time monitoring mechanism of community," "autonomous monitoring of residents," and "disaster prevention volunteer." Finally, the annual landslide probabilities, expected losses, and resilience capacities were combined to evaluate annual landslide risk in Shihmen watershed. In addition, annual risks before and after the implementation of measures were compared to determine the benefits of measures, and subsequently benefit-cost analysis was performed. Communities with high benefit-cost ratios included Hualing, Yisheng, Siouluan, and Gaoyi. The watershed as a whole had a benefit-cost ratio far greater than 1, indicating the effectiveness of measures was greater than the investment cost. The results of factor sensitivity analysis revealed changes in vulnerabilities and mortality rates would increase the uncertainty of risk, and that raise in annual interest rates or reduction in life cycle of measures would decrease the benefit-cost ratio. However, these changes did not reverse the cost-effective inference.