Investigation about rainfall-induced shallow landslides in CYL and TWR watersheds, Taiwan

Landslides identified from aerial photos and satellite images between 1988 and 2009 in Chenyulan (CYL) and Tsengwen Reservoir (TWR) watersheds in Taiwan were investigated. These watersheds have similar area, but different percentage of landslide area due to their own watershed characteristics and extreme triggers (earthquakes and typhoons). The Chi–Chi earthquake in 1999 increased about 3 % of landslide area in CYL, had obvious influence on rainfall-induced landslides in subsequent 5 years, while the Typhoon Morakot in 2009 increased 2.53 % of landslide area in TWR. The incremental landslide area by rainfalls, especially brought by typhoons indicates that the rainfall-induced shallow landslide depends not only on the rainfall amount but also on intensity. Two quadratic equations of the percentage of incremental landslide areas in terms of the rainfall erosivity factor that is composed of rainfall amount and intensity were developed. The number and size of landslides caused Typhoon Morakot in CYL and TWR were different, but they followed a similar power-law frequency–area distribution. Extreme triggers play the most important roles in the evolution of landslides in these watersheds.