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Typhoon Morakot first struck Taiwan on 6 August 2009, which led to a much more serious disaster than the notorious ‘August 7th Flood’ which occurred 50 years previously; it broke records for almost all meteorological observing data with an estimated rainfall in five days surpassing Taiwan's annual average rainfall of 2500 mm. Such a huge amount of rainfall caused more serious damage than the 921 earthquake. The typhoon left 620 people dead, 80 people missing, and over NT$90 billion in total direct property losses in 2009. Therefore, within one week after the disaster, the National Science Council requested the National Science and Technology Center for Disaster Reduction to investigate the disasters caused by Typhoon Morakot and to carry out a comprehensive investigation to better understand the type, numbers, and scale of the disasters. Meanwhile, this investigation also aims to better understand the causes and to spot potentially dangerous areas. Our key focuses include: flooding, water facilities, land...

Typhoon Morakot hit Taiwan during August 7-9, 2009. Its record-breaking rainfall caused catastrophic damage, making it the deadliest typhoon to visit Taiwan in the last 50 years. Conducting a three-months and 160-member-strong field investigation of the scale and causes of this disaster, this paper proposes strategies effective to improve flood prevention work in Taiwan. The severe flood disaster triggered by Typhoon Morakot’s excessive rainfall is attributable to four factors: (1) hydraulic system failures, (2) river flow retardation, (3) reservoir release, and (4) land subsidence. Based on these findings, this paper proposes comprehensive improvement strategies in hydraulic facility inspection, emergency response, river basin management, and climate change assessment to improve flood prevention work in Taiwan. This study combines governmental, academic, and public efforts in investigating effective post-disaster flood prevention strategies that we hope will prove to be a useful reference for other countries while facing such issues.

Midstream of the Keelung River Basin in Northern Taiwan has become highly urbanized and densely populated area. Flood inundation along riversides frequently occurred during typhoons or rainstorms. Three protection measures, including constructions of high-level protection levees, a diversion channel, and a detention reservoir, were proposed for flood mitigation. The main purpose of this study is to evaluate the flood mitigation performance of the three proposed structural measures by using combined hydrologic analyses and hydraulic routings. A semi-distributed parallel-type linear reservoirs rainfall-runoff model was used for estimating the surface runoff. Furthermore, a 1-D dynamic channel routing model was coupled with a two-dimensional inundation model to simulate the hydraulic characteristics of river flooding and overland flow. Simulation results of flood stages, runoff peak discharges, and inundation extent under design rainfall scenarios were chosen as the criteria for evaluation. The results showed a diversion channel is superior to the other two measures for flood mitigation of the study area. After the process of environmental impact assessment, a revised diversion channel approach has been approved for construction as the major structural measure.

The purpose of this study is to evaluate the flood mitigation performance of various long-term regulated plans through simulations of both 1-D unsteady flow and 2-D overland flow. The domain of interest is a highly developed valley zone located at the middle-upstream of Kee-Lung River Basin in northern Taiwan. Owing to the development of floodplain control and urban land-use, current riversides have become highly urbanized and densely populated areas. As a result, flooding along riversides often caused losses of lives and damages of properties when typhoons or rainstorms attack the area. To provide better protection for highly developed and densely populated areas against flooding disasters, engineering protection measures were often constructed. Based on field survey and geomorphologic data of river network, various long-term regulated measures, including protecting banks, pumping stations, diversion works, and retention reservoirs, were proposed for performance assessment before engineering constructions. One-dimension unsteady flow model and two-dimension inundation model were employed to evaluate the flood mitigation effect of each measure based on three criteria, including flood stages, runoff peaks, and inundation depths and ranges. From the simulated results of hydrological and hydraulic studies, it is regretful that none of proposed regulated plans has the absolute priority to thoroughly resolve the flood and inundation problem.