紅樹林作為海岸防護自然解方的擴散機制及復育策略

Abstract

全球氣候變遷加劇極端氣象事件發生頻率，顯著提升沿海區域災害風險，紅樹林因具備波浪消能、固砂、維持生態多樣性及碳儲存等多重生態系統服務功能，故被廣泛視為取代傳統硬體結構物作為海岸防護的自然解方(Nature-based Solution, NbS)，可提供兼具防災及生態效益的生態系統防減災永續策略（Ecosystem-based DRR）。水筆仔(Kandelia obovata)為臺灣北部的優勢種紅樹林植物，主要分布於淡水河中下游河段兩岸，於局部區域甚至因過度生長導致通洪能力降低。相對的，臺灣北部海岸帶卻鮮見紅樹林的分布，因此，探討紅樹林在海岸帶的擴散機制，並提出具水理及胎生苗擴散基礎的復育策略，有助於檢討臺灣北部沿海永續治理方略。 本研究旨在透過整合多種數值模式及經驗公式制定出適合臺灣北部海岸帶的紅樹林復育策略，分別以水文特性評估水筆仔於臺灣北部海岸帶的潛在棲地生長環境及以水理特性分析水筆仔胎生苗的擴散軌跡。使用SRH-Coast水理模式模擬淡水河系中下游及其鄰近之北臺灣海岸帶的水動力，該模式整合了「風場、潮汐、波浪及河道流場」的交互作用，並根據此水理模擬結果銜接PTM粒子追蹤模式模擬水筆仔胎生苗的擴散軌跡。 結果顯示，由淡水河道內紅樹林釋放的水筆仔胎生苗有機會擴散至河道內的灘地並建立新棲地，但受限於河口地區複雜潮波流的水動力流場，不易直接擴散至海岸帶。因此，本研究再引入SSA墊腳石法概念，在數值模式中於研究區域特定地點釋放水筆仔胎生苗，以觀察其是否能夠透過水動力的方式擴散至海岸帶的潛在棲地，模擬結果發現於大部分區域釋放水筆仔胎生苗，其主要軌跡呈現胎生苗在小範圍內進行循環往復漂移的現象，僅有少數具有明顯離岸流區域釋放水筆仔胎生苗，才能夠實現長距離擴張(Long Distance Dispersal, LDD)並抵達多個海岸帶潛在棲地。在前述選定之八個潛在棲地中，僅有少數能夠實現透過胎生苗的飄流入植而達成種源之間的交換，大部分的潛在棲地，都受限於地形、水流及大型結構物等因素，導致其漂移範圍僅限於釋放源附近。綜上所述，當水筆仔胎生苗由海岸帶特定地點釋放並抵達潛在棲地著生後，僅有很小的機率實現長距離擴張LDD並再次抵達其他潛在棲地而有機會著生。本研究同時以HSI棲地適合度指標評估海岸帶潛在棲地，發現於淺水灣沙灘及寶斗厝沙灘的向海側HSI指標較高，而其餘潛在棲地則只有少數區域有較高的HSI指標，因此建議可以優先針對這些區域進行復育，有助於提升成功機率。 最後，假設紅樹林成功建立於海岸帶潛在棲地，以真實颱洪事件進行水理模擬，評估極端氣候條件下紅樹林對於海岸防護的效果，並比較不同的紅樹林覆蓋率及排列方式對水理條件的影響，期望能針對各個海岸帶潛在棲地率定出最佳分布及覆蓋率。根據評估結果，無論覆蓋率高低，在三種不同的紅樹林分布情境下，發現當紅樹林全面分布於潛在棲地時，各項海岸防護指標表現最差，顯示流速、剪應力上升，且浸淹體積增加。而最佳配置情境則與各潛在棲地的地形及流場等複雜綜合條件有關，需依個別場址進行討論分析。

Global climate change has increased the frequency of extreme weather events, significantly increasing disaster risk in coastal zones. Mangroves provide multiple ecosystem services, including wave energy dissipation, sediment retention, maintenance of ecological diversity, and carbon storage. In recent years, mangroves have been widely regarded as a Nature-based Solution (NbS) to replace traditional engineered structures for coastal protection, representing a sustainable strategy with both disaster mitigation and ecological benefits. Kandelia obovata is the dominant mangrove species in northern Taiwan, mainly distributed on both sides of the middle reach and downstream of the Tamsui River. In contrast, mangroves are rarely distributed in the coastal zone of northern Taiwan. Therefore, exploring the dispersal mechanisms of mangroves and proposing a restoration strategy based on hydraulic conditions can help achieve NbS of both coastal protection and habitat restoration, as well as provide a crucial foundation for the sustainable management of the coastal zone in northern Taiwan. This study aims to develop a mangrove restoration strategy for the coastal zone of northern Taiwan by integrating multiple numerical models and empirical formulas. The hydrological characteristics were used to evaluate the suitability of the potential habitats for Kandelia obovata in the coastal zone of northern Taiwan, and the hydraulic characteristics were utilized to analyze the dispersal trajectories of Kandelia obovata propagules. The hydraulic model SRH-Coast was constructed to simulate the hydrodynamics of the middle and lower reaches of the Tamsui River and the coastal zone of northern Taiwan. The model integrates the comprehensive hydrodynamics induced by the combined effects of wind, tide, waves, and river currents. The particle tracking model PTM was then executed to estimate the dispersal trajectories of Kandelia obovata propagules based on the hydrodynamic simulation results. The results showed that Kandelia obovata propagules released from mangroves in Tamsui River have the potential to drift to mudflats on both sides of the river and colonization. However, complex estuarine hydrodynamics restrict their transport capacity and cannot drift to the coastal zone. Therefore, this study combined the Stepping Stone Approach (SSA) concept to simulate the artificial release of Kandelia obovata propagules at specific locations in the coastal zone and observe whether they can reach potential habitats in the coastal zone through hydrodynamics. According to simulation results, Kandelia obovata propagules released from most areas can only drift in a small range. Long-distance dispersal (LDD) is possible and can reach multiple potential habitats in the coastal zone, only releasing from a few specific locations. Among eight potential habitats, only a few can achieve propagule exchange. Kandelia obovata propagules released from most potential habitats are restricted by factors such as terrain, hydrodynamics, and large structures, which limit the dispersal range only around the release source. In summary, when Kandelia obovata propagules are released from specific locations in the coastal zone and reach potential habitats for colonization, there is only a small probability that they can achieve LDD and reach other potential habitats again for colonization. This study also utilized the habitat suitability index (HSI) to evaluate potential habitats in the coastal zone and found that the HSI is higher on the seaside of Qianshuiwan Beach and Baodoucuo Beach. In comparison, only a few areas in the remaining potential habitats have a higher HSI. Restoration can be initiated in these areas to enhance the opportunity for successful colonization. Finally, assuming that mangroves successfully colonize the potential habitats in the coastal zone, hydraulic simulations are conducted under a real typhoon event to evaluate the mangroves' capability in coastal protection under extreme climate conditions. The optimal distribution and coverage ratio of each potential habitat can be determined. According to the scenario simulation results, regardless of the mangrove coverage rate, the configuration with whole coverage of potential habitats consistently exhibited the poorest performance across coastal protection indicators. This condition was associated with increased flow velocity and shear stress, as well as greater inundation volume. The optimal mangrove distribution configuration varies depending on the specific topographic and hydrodynamic conditions of each potential habitat and thus requires site-specific analysis and discussion.