基於自然解決方案之都市河川洪氾平原管理—以二重疏洪道為例

Abstract

河川洪氾溢淹或外水壅高引致內水排放困難是造成都市積淹水主因之一，由於都市高度發展而吸引眾多人口居住在河岸區域，一旦脆弱度及暴露度增加將加乘此洪氾災害風險，因而傳統上多以硬性工程手段搭配疏散避難方式降低風險。但工程手段常被詬病大幅破壞生態環境，也有越來越多研究案例顯示自然環境所提供，作為支持人類社會穩定成長的生態服務功能被長期忽視，許多重要生態系統因而縮減、弱化或消失，因而近年國際間開始倡議生態系統防減災的重要性（Ecosystem-based Disaster Risk Reduction；Eco-DRR），其中自然解決方案（Nature-Based Solution；NBS）的災害管理策略是較被廣為推廣的概念，主要目的在建立韌性而宜居的共生環境。河川主深槽、洪水平原具有不同的水理及生態特性，感潮段的灘地同時具備陸域及水域的生境，與主深槽交界的水際域常是生物多樣性最高、植群結構最複雜的區域，而高灘地的水理主要受到植生結構及特徵的影響，像是植生的種類、覆蓋率、密度、高度、樹冠厚度等，因此植生是NBS洪氾管理策略中重要的生態因子及水理參數。本研究選擇二重疏洪道作為植生管理的研究區域，並使用淡水河系作為水理模式的模擬範圍，淡水河包含了整個臺北都會區，是臺灣非常重要的河流，其面臨的最大挑戰是在臺北大橋段，因其為淡水河最窄的河道段，容易發生水理瓶頸現象。一般水理演算會將植生特徵轉換為曼寧n值作為水理演算的主要參數，n值對水理現象極為敏感，但實務上常有決定n值的困擾。本研究嘗試透過無人飛行載具（Unmanned Aerial Vehicle；UAV）進行調查，使用其得到的數值模型來檢視研究區域的植生特徵，透過最大概似法結合地面物高度對正射影像進行地景的分類，藉此得到地表的基礎曼寧n值與植生位置及高度等特徵數據。並利用HEC-RAS一維模式進行淡水河水位與輸水容量的水理模擬，建立「通洪阻礙物」來產生喬木、灌木等不倒伏植生的阻水效果，並以Freeman試驗配置進行模擬驗證，研究結果表明綜合n值可由底床基礎n值與「通洪阻礙物」取代。另以驗證過的二重疏洪道植生阻礙物模型對淡水河進行模擬，分析被高度人為利用的疏洪道，其植生特徵、位置等條件對洪氾風險的影響程度；結果發現在200年重現期距的洪水下，疏洪道的分洪能力已從當初設計的9200 m3 s-1降為6546 m3 s-1，推測因人為開發及高大植生的生長與增加，影響了疏洪道的分洪能力。為了找出疏洪道糙度敏感區域，針對疏洪道上、中、下游的減糙效果進行敏感度分析，結果發現疏洪道的減糙效果為：上游>中游>>下游，在二重疏洪道植生完全清除的情況下，分洪量可增加571 m3 s-1，臺北大橋水位下降7 cm。本研究在不影響現況分流量下，提出有效的自然解決方案‐在二重疏洪道植生管理下進行濕地復育與設立，只清除上游段植生並於區域1、2、3進行濕地的復育與建立為二重疏洪道最適合的自然解決方案，除了不影響現況分洪能力，還多了生態復育與環境教育的功能。

River flooding is one of the significant hazards in urban areas due to the dense population living along the riverside. Once the vulnerability and exposure increase, disaster risk might be multiplied. Traditional and hard engineering-methods are often criticized for greatly damaging the ecological environment, even they provide effective prevention of flood hazards. Recent studies indicate that the ecological services and functions provided by the natural environment as support for stabilizing human society have been neglected for a long time, causing the ecosystem to shrink, weaken, or even disappear. Therefore, the importance of Ecosystem-based Disaster Risk Reduction (Eco-DRR) has been advocated in recent years. Among them, the Nature-Based Solution (NBS) is a more popular concept on disaster management strategy, which helps to build a resilient and livable symbiotic environment. The floodplain hydraulics are major concerns specifically on variant vegetation characteristics such as their types, coverage ratio, density, height, and canopy. Therefore, vegetation is an essential ecological factor and hydraulic parameter in NBS flood management strategy. This research aims at examining biological effects on impacting river flood protection using numerical model simulations by incorporating UAV surveys. The UAV images were identified as different landscapes and then converted to Manning’s n value, decreased flood conveyance area, and wetted perimeter. The water-blocking effects representing the reduced flood conveyance area by trees and shrubs were investigated through building “Blocked Obstructions” (BOs) in the HEC-RAS model. The modeling results of Freeman’s flume experiments proved that the overall n value is the sum of bed Manning’s coefficient and BOs. We then choose the Erchung Floodway in the Tanshui River system as an NBS research area. The main challenge in the Tanshui River is that a bottleneck occurred at the smallest river point near the Taipei Bridge. The flood stage of the Tanshui River and flood conveyance of Erchung Floodway were investigated. The verified vegetation obstruction model in Erchung Floodway was conducted for analyzing the degree of influence of vegetation characteristics and location on the flood risk. The results indicated that the capacity of the Erchung Floodway diversion capacity has noticeably decreased from 9,200 m3 s-1 to 6,546 m3 s-1 under the 200-year recurrence flood. The sensitivity analysis indicates that the vegetation effects on flood diversion within the Erching Floodway were: upper section > middle section >> lower section. In the case of completely clearing vegetation, the diversion capacity increased by 571 m3 s-1, and the water level of the Taipei Bridge drops by 7 cm. The upstream vegetation removal proposes a useful NBS scenario that has the functions of ecological rehabilitation and environmental education without affecting the current diversion capacity. The wetland restoration for duck and waterbirds habitat was carried out in areas 1, 2, and 3. Our findings also showed that the application of flood mitigation strategies should take into account the local ecology, environment, landscape, and socioeconomic factors, which meet the demand of the ecosystem-based disaster risk reduction.