圖書典藏及影音數位平台

臺灣河川多為多砂河川，河川沖淤變化大，因此常有疏濬需求以維河川通洪能力，但現行的河川及水庫疏濬相關規範均尚未納入生態考量，河道減糙及疏濬能有效維持河川的通洪、容洪功能，但也可能破壞重要棲地環境或減損生態系統功能。以淡水河系為例，近年部分渠段之河道及高灘地有逐年淤積趨勢，需要經常性進行河道疏濬及植生整理，以保持足夠的通洪斷面，及適度維持河道粗糙度，且為因應氣候變遷之嚴峻防洪需求，此項工作尤其重要及迫切。然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，故進行河道整理、疏濬或植生檢討等工作時，勢需針對上述之河川及濕地生態清疏工作，提出更全面且具體可行的對策。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，因此應進行補充調查，及清疏方式、技術的研究，本案本計畫是以生態系統功能及服務的角度，針對計畫範圍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評估，將通疏洪能力納入其中的「調節」功能及服務項中，評估不同河道減糙或疏濬方案的生態系功能權衡關係（trade-off），包括河防安全、景觀生態、棲地生態等，提出有效且可行的河道減糙及疏濬策略，以有效強化淡水河及周邊河道容洪、通洪、防洪能力，並能確保計畫河段之生態功能。 107年已針對臺北防洪計畫大漢溪、二重疏洪道與新店溪部分河段進行清疏兼顧生態之策略研擬，108年主要將針對淡水河主流，並將107年研究範圍尚待進一步探討部份納入計畫內深入研究。

River flooding is one of the major hazards in Taiwan due to the dense population living along the riverside. The Taipei metropolitan area is the largest city with more than one-third of the total population of Taiwan. In this area, numerous severe flooding disasters caused by typhoon events have occurred and resulted in heavy losses. The Tanshui River system passes through the Taipei metropolitan area with a mainstream length of 159 km and a watershed area of 2,762 km2 consisting of three main tributaries: Dahan Creek, Hsindian Creek, and the Keelung River. The Taipei metropolitan area encountered numerous severe floods from the Tanshui River system, and these resulted in heavy losses. The main challenge in the Tanshui River system is that a bottleneck occurred at the smallest river point (which had a width of 450 m) near the Taipei Bridge (T024A). This river point could not allow water from a 200-year return period flood to pass. The Erchung Floodway was thus established to divert some of the floodwaters. It is 450 m to 650 m in width and uses a fixed weir as an inlet control. The weir crest is EL. 4.0 m, the external part of the weir is EL. 3.0 m, and the internal part of the weir is EL. 2.5 m. The specific goal for the construction of the Erchung Floodway was to arrive at 9,200 m3/sec division under the 200-year flood event. The Q200-Tanshui River could thus be decreased from 23,500 to 14,300 m3/sec. Even though projects such as the Taipei Flood Prevention System are effective in mitigating flood disasters, the function of flood prevention might be changed over time by urban development and multiple demands of the usage of floodplains. Therefore, this project aims to examine anthropogenic effects on the major region of the Taipei Flood Prevention System using numerical model simulations by incorporating field surveys. Several scenarios such as river excavation and partial plant removal on floodplain were setup and simulated. Structural and non-structural measures are provided and assessed in the perspectives of ecological services and ecosystem-based disaster risk reduction (Eco-DRR). Four reaches in Tanshui River are investigated for the strategies evaluation provided by this project, including the Tanshui River from its downstream estuary to Jiangzicui, Keelung River from its downstream estuary to Bailing Bridge, Erchong Floodway, and Dahan Creek from cross-section 46 to confluence of Sanxia River. The elevation of the main channel and floodplain of the Tanshui River is getting higher from the sediment deposit, and part of the vegetation on floodplains gradually grows taller and denser. Those are increasing the risk of flood inundation. For water quality improvement on purifying domestic sewage, there are some constructed wetlands of importance on floodplains, where are a sanctuary in urban and paradise of wild animals that we would like to conserve. However, the dense vegetation in wetlands would have an adverse effect on flood management. The river roughness is investigated by reviewing previous reports and recognizing landscape from aerial/satellite photos. The aerial and satellite photos help gather information about the landscape of the study area. The data can be converted to Manning’s roughness coefficient based on base roughness and vegetation by employing GIS technique. Manning’s roughness coefficient of the tidal area in the Tanshui River is examined in unsteadily flow routing by using the HEC-RAS model. Historical flood hydrographs of stage and conveyance from three typhoon events are collected to calibrate the roughness of floodplain. The entire river system of the Tanshui River is taken into consideration for the model verifying. The flood stage and flood conveyance of Tanshui River are investigated via 1-D model, HEC-RAS and 2-D model, SRH2D, for assessing the efficiency of flood control. River excavation and plant thinning are included in the model simulations. The sensitivity analysis indicates that there are high sensitivity measures of the stage at Taipei Bridge in terms of roughness and deposit in Tanshui River from cross-section 9 to 12 and in Tanshui River from cross-section 13 to 20, whose roughness and deposit should be kept as low as possible for the safety of Taipei Metropolitan Area. To alleviate the impact of conducting river excavation and vegetation removal on ecosystem services, strategies are proposed through counting ecological resources, evaluating potential effects on the ecological environment. The integrating opinions from stakeholders revise the plans and consequently provide the impact mitigation on ecological habitat. A compromise and trade-off between ecology and flood management are suggested, including partially removing mangrove trees to create mudflat. The previous mangrove-removal experiment in Shizi island demonstrated that the creation of a small patchwork of mudflats dramatically increased the species richness of wintering shorebirds. In cost-effective practice in Mai Po Ramsar site, WWF Hong Kong suggested to carry out mangrove management works to remove mangrove seedlings that are growing on tidal mudflat for sustaining mudflat area where benthic fauna is abundant. We conclude that the reaches of the high sensitivity of roughness and deposit should be restricted from high roughness. The Tanshui River mouth is the only downstream reach of the Tanshui River that lacks levees or dikes because the flood risk therein is low due to its broad channel cross-section. However, the growth of the population and economy in this area have increased the need to prevent flood hazards by improving the associated flood mitigation and management measures. Nineteen villages with a total area of 38 km2 are situated along the Tanshui River mouth. Considering increases in the population and precipitation, this project evaluated the capability of these strategies to protect against flood inundation in the Tanshui River estuary. Non-structural flood protection measures are provided by establishing warning water levels based on variant scenario simulations. The project provides a nonstructural flood prevention measure by integrating inundation maps, evacuation route maps, and warning water levels to effectively mitigate the inundation impacts at this region. The results of simulations showed that the decrease in the drainage system capacity induced by elevated river water is significant when considering river flooding effects. The capacities of the current flood mitigation strategies to prevent riverbank overflow and drainage system overflow do not reach the designed flood protection standard. As a consequence of the establishment and expansion of numerous settlements, the current flood mitigation strategies must rely on non-structural flood management. The results highlighted the need to improve the official guidelines for producing potential inundation maps and establishing warning water levels. Unfortunately, there are no gauging stations to acquire water level data with which to establish the warning water level in the warning system in most of the settlements along the river. The simple linear regression model helped establish the warning water level at each settlement. Two suggestions were proposed: (1) the impacts of downstream river water should be considered to identify all potential inundation areas, and (2) a warning water level should be established at each settlement to enable appropriate and prompt reactions in those settlements. The study also suggests utilizing the Eco-DRR approach by incorporating environmental and socioeconomic factors, and the flood prevention and mitigation strategies provided by this project. Mangrove habitats are important carbon reserve sites. In the Tanshuei Estuary, the mangroves are mainly distributed on the convex bank in meandering bends. The four major mangrove-vegetated wetlands are the Wazwei, Zhuwei, Guandu, and Shezi wetlands. With the exception of the Shezi wetland, all the wetlands have been designated as reserves under the Taiwan Culture Heritage Reservation Act. They are all composed of the species Kandelia obovata. The earliest record of the mangrove species K. obovata in the Guandu wetland can be dated to 1978. These habitats possess the largest population of K. obovata in the Northern Hemisphere and represent the species’ northernmost geographical distribution. However, the overgrowth or over-expansion of mangroves may increase the risk of flooding, thus affecting human wellbeing. Such uncontrolled expansion of mangroves results in a series of alterations to both the hydrological regime and biotic community, such as increasing flood risks, replacing shorebirds with tree birds, diminishing benthic diatom production, changing the dominance of benthic communities from polychaetes and amphipods to crabs, which led to the local extinction of the sedge Cyperus malaccensis, and confining the distribution of the Taiwan endemic fiddler crab Uca formosensis. To decrease flood risks and restore biodiversity, partial mangrove removal has been proposed as a managerial action. Our results show that the ratios of the effective reduction in flood level (benefit) and the loss of mangrove C stocks (cost) in Guandu mangrove wetland were only higher under the condition of the removal of aboveground structures of mangrove trees than the ratios under the condition of the removal of both above- and belowground structures. The highest ratio of the effective reduction in flood level and the loss of mangrove C stocks occurred under the condition of removal of 20% of aboveground structures of mangrove trees, indicating the optimal removal area for mangrove management in the Tanshuei River. However, the partial mangrove removal is not allowed in Guandu so far due to the Act prohibition. In the current stage, the project follows the suggestion of the project entitled “Investigation of Ecological Dredging Strategy for Flood Management Plan of Taipei Metropolitan Area”, which aims to investigate the competition between flood management and ecological conservation, and proposes general strategies of dredging sludge in a river channel or thinning vegetation in the floodplain.