Catastrophic Landslide Disasters Triggered by Record-Breaking Rainfall in Japan: Their Accurate Detection with Normalized Soil Water Index in the Kii Peninsula for the Year 2011

In September 2011, catastrophic landslide disasters triggered by record-breaking rainfall due to Typhoon 1112 (Talas) caused enormous damage in the Kii Peninsula, Japan. We analyzed cumulative event rainfall, maximum hourly rainfall intensity, Soil Water Index (SWI), and Normalized SWI (NSWI) for 30 landslide events occurring in Nara Prefecture. The first two parameters are classical variables used for landslide hazard assessment. SWI represents the theoretical soil water content calculated by using a hydrological model, while NSWI is normalized SWI divided by the largest value over the past ten years (2001-2010) at the location. The distributions of the cumulative event rainfall, maximum hourly rainfall intensity, and SWI do not well correspond to landslide locations; p-values obtained from Wilcoxon rank-sum tests between landslide and non-landslide cells were 0.98, 0.95, and 0.91 for these distributions, respectively. However, landslides occurred in an area where the maximum NSWI was high (p-value < 0.01). Heavy rainfall occurs frequently in the eastern part of the Kii Peninsula where the mean annual precipitation is higher than 3800 mm. That is, in regions of frequent rainfall, the relative value of NSWI (in comparison with historical records) is more useful than the other rainfall variables for landslide hazard assessment.