圖書典藏及影音數位平台

近年氣候異常，超大豪雨或局部區域豪雨事件頻傳，然石門水庫於颱風豪雨期間如若啟動緊急洩洪工作，亦影響大臺北地區防洪安全，故如何維持正常功能運作、減輕下游洪水負擔，為各界關切焦點。是以本案針對石門水庫進行分洪入海計畫進行初步勘查規劃，進行分洪計畫之路線規劃、大臺北地區防洪機能效益及跨域加值效益等整體性評估，爰提本計畫書。

1、Background Keelung, Xindian and Danyang Rivers are part of the Danshui River Basin. These rivers each have systems in place to prevent the impact of the flood on the over-developed Taipei Basin where these rivers also intersect. Shimen Dam is one such system located in the upper reaches of Dahan River. During typhoon season, mandated discharge from the dam causes a rapid rise in water levels at the River resulting in undue concerns to the locals. Similar concerns were also expressed as extreme weather events become more frequent. Studies showed that existing levees in Taipei will not be unable to accommodate for the ever-increasing 200-year flood flow. For these reasons, this project investigated the effect of flood diversion at Dahan River to alleviate potential flood loading at Danshui River. Findings from these studies will be used to formulate climate change strategies for future projects. 2、Conclusions (A) Hydrograph Analysis 1.The “Design Flow” scenario studied existing flood-mitigating systems (Danshui River and Erchung Flood Diversion Channels) and levee designs - whereby a 26cm increase in the height of the levee was proposed to satisfy the requisite 1.5m freeboard - at the section between Danshui River and Taipei Bridge. Our study showed that with a flood diversion capacity of 1500 m3/s, the levee design will not only increase the base cost of the project, it will also make it uneconomical. 2. The “Climate Change” scenario considered 2,800 m3/s of flood diversion capacity was able to accommodate for the overflow at Danshuei Chiao section of the levee. However, to account for systematic errors or deviations associated with various studies: hydrological analysis, hydraulic modelling, and studies on the effect of flood detention, 3,000 m3/s was chosen instead. 3. During “Extreme Weather Events”, flowrate at Danshuei River will exceed the 200-year flood, and flooding is inevitable. However, due to its relatively low probability of occurrence, this scenario was not considered. (B) Cost-Benefit Analysis 1. Primary Benefits: Flooding of levee due to climate change incurred a cost of NT$ 7.33 billion. The equivalent annual cost was around NT$367 million/year; and all of the flood diversion methods had a benefit-cost ratio (B/C ratio) was around 0.01. 2.Added Benefits: Shimen Reservoir was originally intended for flood and sediment prevention. However, its actual benefit were not great significant enough to be deemed valuable, and thus should not be considered. 3. Marginal Benefits: Construction waste material were found to be suitable for use in reclaiming lands for artificial lakes and wind farms. The estimated B/C ratio was around 0.07. III. Recommendations (A) Despite imposing a 200-year flood (accounting for climate change) for most part of the rivers, the nature of the surrounding terrains and the highly-developed nature of Taipei Basin meant that levee will not be sufficient to prevent additional flood water from entering the levee. During extensive flood events, ponding will occur to the detriment of the levee and aggravate the existing ponding within the Basin. Further studies were recommended to evaluate suitable mitigation measures. (B) Due to the heterogeneous nature of climate change, the Intergovernmental Panel on Climate Change (IPCC) re-evaluates the hydrology of Danshui River every six years. We believe it is necessary to be update the latest hydrological results. (C) Shimen Reservoir was found to be effective at mitigating flood loading at Danshui River. However, the preliminary estimation showed that the B/C ratio of the 24km diversion tunnel, including 8km of open channel, will be as low as 0.01. Re-purposing construction waste material to form reclaimed lands and artificial lakes was not advised.