The assessment and prediction of the landslides and debris flows in Ta-Chia river after Taiwan Chi-Chi earthquake

ABSTRACT: After September 21, 1999, Chi-Chi Taiwan earthquake, the subsequent typhoons and,follow-up rainfalls caused ,the heavy ,landslides and ,debris flows ,in the ,Ta-Chia river watershed, the sediment yields from the landslides were transported into main river. There were lots of damages in the watershed including dams, power plants, bridges, villages, amusement parks, and other infrastructure. In order to assess the impact of sediment yields from landslides and debris flows and to investigate the strategies of mitigating the geohazards, quantitative assessment,was conducted ,by using ,aerial photos ,and satellite ,images ,obtained ,at 6 stages of major earthquake ,and typhoon ,events. In order not only ,to estimate ,the volume ,of the ,sediment yields from landslides and debris flows, but also to establish the relationships between the volumes of sediment yields, the rainfalls intensity, and the discharge. The HEC-6 program was applied to simulate,the flushing and deposit,of Ta-Chia river in the near future. The results show the highest level of riverbed ,around ,the Chin-Shan area ,would ,raise more ,than 20m in addition. Among the branch rivers of Ta-Chia main river, Ji-Ler river and Pi-Ya-Sun river brought the most sediment yields from landslides in the sub-watershed. In conclusion, there are at least 40% of the total sediment yields from landslides still remain in the study area. Therefore, the sediment will transported out in the near future, and monitoring should be conducted continually to mitigate the hazards.