Changes of sediment disaster and topography following the ChiChi Taiwan Earthquake on September 21,1999

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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 changes of sediment disaster and 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.Although debris flows had occurred about once per year in the study area before the earthquake, they occurred 20 times or more in 2000 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.3.The mechanisms of slope failures, expansion of slope-failure scars by rainfall, and the countermeasure were examined for some slope failures (Jiu-Feng-Er-Shan, Tzao-Ling, Huo-Yan-Shan).4.The sediment disasters by typhoons of the period in 2002 to 2004 were investigated in the areas influenced by the earthquake. The relationships between slope failure and rainfall conditions were analyzed before and behind the earthquake.5.The countermeasures and warning evacuation systems for the sediment disaster were compared between Japan and Taiwan.