Distributions of landslides, vegetation, and related sediment yields during typhoon events in northwestern Taiwan

Abstract

This study examined landslides caused by typhoon events in the upstream area of the Tao-Cheng River in northwestern Taiwan during a 13-year period, 1996 to 2008. Data from six typhoon events were compiled to analyze the relevant characteristics of landslides, vegetation, and changes in sediment discharge with particulate carbon in rivers in this area. The landslide ratio, the ratio of the landslide area to the total study area, was determined to be between 0.60% and 1.29%, and a high new generation ratio and low reactivated ratio of landslides were noted. Analysis of different bands of satellite images taken over a 10-year period disclosed that the Normalized Difference Vegetation Index (NDVI) in the upstream area of the Tao-Cheng River ranged from 0.47 to 0.63 before typhoon events and from 0.38 to 0.46 after typhoon events. The low landslide ratio and high new generation ratio in this area indicated that sporadic landslides tend to occur in new areas and do not have a great impact on vegetation conditions. Thus, the decrease in NDVI after typhoon events was caused by the seasonal effect with withering of vegetation. On the other hand, landslides in this area tend to occur on nonforest land because of the interventions of external forces, such as human developments and reclamation.Geological materials produced by landslides and vegetation flushed by heavy rainfall during typhoon periods have a great effect on the amount of sediment discharge and particulate carbon discharge in the rivers. Furthermore, during typhoon periods, the amount of accumulated rainfall has a greater impact than the peak rainfall intensity on the increase of sediment and particulate carbon in rivers and on the decrease in the NDVI. Analysis of the particulate carbon from the rivers in this area revealed that in the rainy season, high strength and accumulated rainfall will lead to more sediment discharge and particulate carbon discharge in rivers. However, the particulate carbon is limited with further increases in rainfall because of the limited supply of particulate carbon in the upstream of the Tao-Cheng River.