Change of topography and prediction of sediment disaster after the Chi-chi earthquake in Taiwan

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The Chi-Chi Taiwan Earthquake on September 21, 1999 killed many people and involved great property loss. Also, it triggered many slope failures and debris flows in the mountainous regions. The resulting sediment movement caused sediment disasters at every heavy rain. This study aimed to examine the prediction of sediment disaster and the change of topography following the earthquake based on the field investigations. The results are summarized as follows :1. Changes of topography of slopes and river channels following the earthquake ware clarified quantitatively in the study area (10.76km^2)in central Taiwan. More than 131 slope-failure scars have been identified through aerial photographic interpretation. The slope-failure area is 1.35km^2, representing 12.5% of the study area. The sediment from slope failures flows out to the lower reaches of the river at every heavy rain. The outflow of the sediment changes the river topography at the same time as it causes sediment disasters.2.19 courses of traverse were installed in altitude 800m-1100m, and change of the topography was surveyed every year in the main river channel of the study area. The long-term topography changes following the earthquake were quantitatively clarified based on the survey results.3. Although debris flows had occurred about once per year in the study area before the earthquake, they occurred 20 times or more after the earthquake. Older and more recent debris-flow terraces are observed along the river in the study area. In Taiwan, severe earthquakes have occurred repeatedly in the past, and sediment movement resulting from earthquakes is considered to be the main cause of change in mountain and river topography.4. In order to research the characteristics of debris flows after the earthquake, debris flow was observed using the video camera. The big debris flow was observed also in 2007.