Spatial Relationships between Landslides and Topographical Factors at the Liukuei Experimental Forest, Southwestern Taiwan after Typhoon Morakot

Abstract

Typhoon Morakot produced copious amounts of rainfall and triggered enormous landslides in southern Taiwan when it hit Taiwan on 8~9 August 2009. The Liukuei Experimental Forest (LEF) managed by the Taiwan Forestry Research Institute was located in the peak rainfall region and inescapably incurred severe landslides and road destruction. This paper presents a statistical approach to study spatial relationships between landslides and their geographic factors in the LEF after typhoon Morakot. The normalized difference vegetation index (NDVI) of satellite images SPOT2 and SPOT4 photographed before and after typhoon Morakot were compared to extract landslide patches. Those extracted patches, consisting of 10 x 10-m digital elevation model (DEM) data and the related coverage of vegetation of LEF, were examined using the WinGrid computer software for the topographic analysis. Results indicated that the extremely heavy rainfall was the main factor that triggered landslides during the typhoon. There were 204 locations which experienced identifiable landslides, and their total area was about 804.49 ha. The elevation and types of vegetation cover were not factors related to the failure of the slopes. Steepness, aspect, and distances to roads and streams were factors that contributed to instability of the slopes. About 70.2% of the total area of landslides occurred on slopes with steepness in the range of 60~80%, and this showed that steep slopes are prone to collapse. Landslides with distance of the lower edge to a stream of ＜ 10 m occurred in 49 locations, and their areas accounted for 33.6% of the total landslide areas. In addition, 34 landslides (24.7% of all landslides areas) occurred within 50 m of a forest road, and the average area of those landslides was 5.79 ha. The deeply concentrated surface flows resulting from the extreme rainfall in streams and on road surfaces striking against the foot of hillslopes that contained saturated soil water were the main factors causing slides of large area. This also indicated that landslides in the LEF were closely related to stream systems, and forest road construction. The aspect of slope was not the main factor related to landslides in most cases. However, slopes with southerly and southeasterly aspects directly faced the strong winds and heavy rainfall brought by typhoon Morakot, and more landslides occurred on those slopes than on slopes with other aspects.