Classification of Slope Collapse and Deposition after Typhoons in Chenyulan Watershed, Taiwan

Taiwan is annually affected by typhoon due to this island located on the main path of the western Pacific typhoons. Numerous landslides and debris flows often occurred in mountainous watershed during typhoon event, resulting in casualties and properties/infrastructures damage. Among the historical disasters, there were a several typhoon events harmed Chenyulan watershed in Nantou County, central Taiwan, particularly Typhoon Herb in 1996 and Typhoon Morakot in 2009 which brought catastrophic destruction. The typhoons caused extreme rainfall and triggered more than 30 large debris flows that buried many houses and deaths.

This study selected the SPOT satellite images after those typhoon events to explore the hazard maps. The satellite images interpretation could extract the emergent landslides of each typhoon event. Normalized Difference Vegetation Index (NDVI) and Spatial Dispersion Index (SDI) were used to categorize collapse and deposition area of the landslides. Then, the total amount of collapse volume could estimate from slope-depth area calculation.

The result found that the landslide sites with higher NDVI and lower SPI could be implied the headstream collapse or deep-seated landslide, whereas the landslide sites with lower NDVI and lower SPI could be implied the deposition or debris. In summarize, this study performed effective results for rapid collapse/deposition extraction from the landslide disaster. To ensure the safety from the post-disaster, it could be suggested for the related authorities because the collapse/deposition area should be instantly enclosed and classified as the conservation lands which require difference intensive conservation practices.

Keywords: Hazard map, Debris flow, Slope, Normalized difference vegetation index, Spatial dispersion index