A study of the landslide potential along the mountain road using environmental indices

Abstract:

Utilization of slope land in recent years is rapid as a result of the dense population and limit of land resources in Taiwan. Therefore, mountain road plays an essential role for the necessity of human life. However, landslide disaster resulting in road failure occurred frequently in Taiwan on the slope land due to earthquake and typhoon. Previous studies found that the extreme rainfall coupled with the property of fragile geology could cause landslide. Nevertheless, the landslide occurrence might be affected by the drainage of the road side ditches.

Taiwan Highway No.21 in Chi-Shan watershed and the forest roads located in Xiao-Lin Village, which failure during the hit of Typhoon Morakot in 2009, were selected for exploring the potential of vulnerable to landslides. Topographic Wetness Index (TWI) and Road Curvature (RC) were extracted along the road to indicate the potential sites which are vulnerable to slope failure. The surface runoff diverted by the road side ditches could spoil the sites with high RC due to the straight movement characteristics of the diverted runoff and cause the downslope collapse.

The sites with higher mean value and lower standard deviation of Normalized Difference Vegetation Index (NDVI) derived from the SPOT imagery taken in dry and/or rainy seasons could be implied as the vegetation stands showing highly buffer effects in environmental stress due to having deeper soil layer, and are hardly interfered by the drought. The stands located in such sites once collapsed are often resulting in huge volumes of debris. Drainage Density (DD) index could be applied as the degrees of geologic fragile in the slope land. A road across the sites with higher mean value and lower standard deviation of NDVI and/or higher DD should be paid more attention because of having highly vulnerable to deep seated landslide. This study is focusing on extracting and analyzing the environmental indices such as TWI, RC, NDVI and DD for exploring the slope stability along the mountain road. The results could be used as the references of related authorities for understanding the potential landslides along a road.