Slope movements in a dynamic environment—A case study of Tachia River, Central Taiwan

It is well established that landslides are extensively triggered as a result of large seismic events in upland areas. Some authors have postulated that sediment moves through the drainage system as a ‘slug’, whilst others have noticed only limited impact as material remains stored in the drainage basin. What has almost never been addressed is the continued evolution of the landslides themselves after the seismic event, and their continued role in releasing slope materials. This paper examines the Tachia River basin in western Taiwan, which was extensively affected by landslides during the 1999 Chi-Chi earthquake. Using vertical cartographic aerial photographs and satellite images, this study demonstrates that the area affected by landslides increased substantially after the main earthquake as a result of high intensity rainfall events. Analysis of suspended sediment data from downstream of the area affected by landslides, show that sediment concentration steadily increases with time, in the period after the initialization of slope movements. A model in which sediment production increases immediately after the earthquake and then steadily reduces through time appears to be over simplistic, and should be replaced by a model that incorporates complex responses to the initial and subsequent events.