Typhoon Soudelor brought high-intensity, short-duration rainfall and was the largest typhoon event to strike Taiwan in 2015. However, the effects of the typhoon on natural forested areas have been little reported. This study investigated the spatial distribution of new landslides in the Tsukeng catchment in Fushan Experimental Forest and compared their effects through satellite images and field investigations. Based on satellite image analysis, 62 new landslides were induced by the typhoon. The surface area of the landslides increased with slope gradient, except for
areas of stream-bed fluctuation. The characteristics and types of landslides were different along three field investigation lines. The Shuiguan trail showed smaller landslides but included four types, namely cracks, collapses, gully expansions,and compounding. For 56% of the landslides occurring at downslope locations along the Tsukeng stream, cracks or collapses were also found at corresponding locations in the upslope direction along the Shuiguan trail. It suggests the consecutiveness of mass movements between the upslope and downslope, as well as impending movements at these locations in the near future. Since these effects are barely detectable from satellite images alone, it is necessary to use them in combination with field investigations to collect the greatest amount of information possible.
Keywords: Field investigation, landslide type, natural forested catchment, satellite image analysis.