圖書典藏及影音數位平台

淡水河系近年部分河道及高灘地有逐年淤積趨勢，未來恐需要經常性進行河道疏浚及植生整理，以保持足夠的通洪斷面，及適度維持河道粗糙度，以因應氣候變遷之防洪需求；另文資法與溼地法等相關法令可能影響對防洪治理之進行，應有專案適切研究。而根據淡水河系河川情勢調查結果及相關生態調查報告指出，淡水河系蘊藏豐富的生態資源，故在進行河道整理、疏浚或植生檢討等工作，應該針對河川清疏工作，研擬更全面且具體可行的對策，相關之清疏技術也需因地制宜研究評估。 淡水河系早年位於台北防洪計畫範圍，除社子島及五股疏左地區因屬洪水平原管制或限制發展區，其餘地區已完成台北防洪計畫保護標準，但是近年來高灘地高度利用，及河道回淤速度受天候自然輸砂及人為排砂等因素，回淤速度有逐年加速之情勢，因此應針對特定河段或區域進行補充調查，及清疏評估方法、技術的研發，本案主要針對現況大漢溪（三峽河匯流口至出口間）河段進行包括河道及高灘地河相變遷、高灘地植生分布及生長情形等，分析工作則是同時由地景生態學、棲地生態學、河相學、水理學等角度，建構本計畫所需之相關理論及模式，並評估各種可行的清疏方法；二重疏洪道分析現況與台北防洪計畫通洪斷面差異，並考量維護生態環境，提出清疏方案初步策略；新店溪（斷面Ｈ017至秀朗橋）評估疏濬工程對生態之影響並提出生態維護之建議。藉此提出適用計畫範圍的河川生態清疏對策，以確保淡水河系容洪、通洪、防洪功能，並維護河川及濕地生態功能。

Although Taiwan has used potential inundation maps as references to set up non-structural strategies for mitigating flood hazards, structural measures still play an important role in decreasing the risk of flooding. Three protection measures, including the construction of high-level protection levees, diversion channels, and detention reservoirs, have been proposed for flood mitigation. To mitigate flood disasters a mega-project, Taipei Flood Prevention System, was implemented in 1963 and completed in 1999. The specific goal was to protect the Taipei metropolitan area against the 200-year return flood period. Approximately, a total of 32 km of dykes and levees have been constructed. However, its function may have changed over time due to urban development, in the form of hydraulic facility construction, and natural alterations resulting from riparian vegetation succession and sediment deposition. Even though the Taipei Flood Prevention System are effective in mitigating flood disasters, it requires considerable time and money to implement. In addition, long-term disturbances by human activities on Taiwan’s river environment have induced ecological degradation and increased the difficulty of river management. The river management of Taiwan government authority used to aim at flood control, flood protection, and water resources allocation, indicating the inconvenient truth that severe competition exists between water conservancy and ecology. Integrated management for river and wetland environment is scarce. In recent years, environmental protection consciousness has been more and more important, there is an urgent need to conduct a comprehensive investigation to make up the shortage of existing material which should be integrated for public reference. This project aims to investigate the tradeoffs between flood control and ecological conservation, and to propose general strategies of river dredging and vegetation thinning as well as standard operating procedure (SOP) of making decision while facing dilemma of balancing ecology and flood management. The analysis of three reaches in Tanshui River were conducted this year, including Dahan Creek from its downstream estuary to the confluence of Sanxia River, Erchung Floodway, and Hsindien Creek from cross section 17 to Xiulang Bridge. The roughness of flow channel in Tanshui River were investigated by reviewing previous reports, recognizing landscape from UAV photos, and calibrating Manning’s roughness coefficient through numerical models. UAV helps gather information of landscape and the height of different vegetation very quickly. With help of recognition via GIS technique, the UAV data can be converted to Manning’s roughness coefficient. An one dimensional model, HEC-RAS, was employed for simulating flood levels of the whole Tanshui River system in different scenarios; while the SRH-2D, a horizontal two-dimensional model, was conducted for determining more complicated and specific flow problems such as the flow direction and the splitting flow discharge diverted into the Erchung Floodway. To alleviate of the impacts of conducting dredging/thinning on ecosystem services, tradeoff strategies were proposed through counting ecological resources, evaluating potential effects on ecological environment, and integrating opinions from stakeholder. The relevant results were exhibited in several NGO workshops and panel discussions. The opinions and feedback were collected for revising the dredging/thinning plans in this project and might thus to provide useful suggestions for future dredging/thinning plans. The modeling results point out that the capacity of the Erchung Floodway diversion has noticeably declined from 9200 m3/sec to about 6300-6400 m3/sec under a 200-year recurrence flood, resulting in rising the flood level of Tanshui River. The current flood levels of Taipei Bridge section in Tanshui River and some sections in Dahan Creek are higher than the planned levels, indicating the vulnerable truth of the Taipei Flood Prevention System. Variant dredging and/or thinning plans were established and incorporated into the revised modeling scenarios, such as: (1) the removal of remains of Sanchong Dump raise the flood water diversion of the Erchung Floodway due to change of flow direction; (2) the removal of the Luzhou Dump reduce the flood stage at Taipei Bridge obviously in both 1-D and 2-D modeling; (3) considering existing levees and piers in channel, optimal dredging plans in Taipei Bridge reach, Erchung Floodway, and Dahan Creek were studied. The dredged reaches all reveal the effects of lowering flood levels and indicates that the plans are feasible in terms of conducting dredging/thinning and avoiding damaging ecosystem services. We conclude the necessity to follow the procedure and strategies in this project and might therefore strike a balance between ecology and flood management in planning dredging/thinning engineering.