紅樹林復育及皆伐對潮灘及潮溝形貌演變之影響

Abstract

紅樹林是河口重要藍碳生態系統，同時具有攔截集水區來砂及陸源汙染與有機質，提供人類社會許多直接及間接的生態系統服務價值。潮溝及潮灘是紅樹林及近岸沼澤、濕地生態系統重要地景及棲地，植生分佈、水理特性、泥砂運移等複雜機制影響潮溝及潮灘的形貌差異，瞭解潮溝、潮灘和植生交互作用下的形貌變化，有助於使溼地管理決策更加完善。 本研究改良垂直二維潮溝及潮灘演化模式，模式中包含三個模組「水動力」、「河川輸砂」、「植生(紅樹林)」，透過水動力模式中建立植生地形，藉以反應紅樹林在灘地上阻礙流動的概念，並透過模式中連續方程式和動量方程式，比對流量值，以此迭代計算得到能量坡度；再進入輸砂模式中，修正河流漲退潮所帶來的泥砂濃度差異，結合水動力所計算出的參數值，算出泥砂沉積、侵蝕濃度。在紅樹林模式中，透過界定灘地、溝地有無植生下的流速、粗糙度等差異性，計算得到生物量，並計算紅樹林捕獲泥砂的沉積速率。最後，將所求得參數代入Exner方程式，得到地形、地貌及濕地高程變化。 演化模式模擬結果顯示，在長時間尺度模擬下，模式能夠有效反應沼澤、紅樹林潮溝發展的前、中、後期階段，呈現潮溝自近乎於平床的灘地形貌，在潮汐及河川水流共同作用下出現下刷、拓寬，再回淤、束縮的過程。另外，本研究亦收集植生水槽實驗資料驗證水動力及植生模組，透過設定斷面植生阻礙物，並提供植生阻力的等效參數，可重現實驗水槽的水面剖線，顯示水動力及植生模式具備可靠性；也收集淡水河社子島紅樹林4處潮溝斷面地形進行輸砂模組驗證，透過模式中灘地、溝地的界定，對於灘地過度淤積的模擬得到良好的改善。本研究也進行模式參數敏感度分析，藉以更深入瞭解潮溝在地貌演化下之機制。

Mangrove forests are crucial blue carbon ecosystems in estuaries, providing numerous direct and indirect ecosystem services of great value to human societies. They intercept sediment land-based pollutants, and organic matter, from the watershed, while serving as essential habitats and landscape features within the mangrove and nearshore marsh wetland ecosystems. The morphology of tidal channels and tidal flats, influenced by complex mechanisms such as vegetation distribution, hydraulic characteristics, and sediment transport, plays a significant role. Understanding the morphological changes resulting from the interactions between tidal channels, tidal flats, and vegetation can greatly enhance wetland management decisions. This study has improved a vertical two-dimensional model for the evolution of tidal channels and tidal flats. The model consists of three modules: hydrodynamics, sediment transport, and vegetation (mangrove). By incorporating vegetation topography into the hydrodynamics module, the model is able to capture the concept of vegetation hindering flow on tidal flats. This study utilizes the continuity and momentum equations in the model to iteratively calculate energy slopes by comparing investigated flow values. The sediment transport module adjusts the sediment concentration differences caused by tidal current and river flows, incorporating parameters calculated from the hydrodynamics module to determine the critical sediment deposition and erosion. In the mangrove module, we calculate biomass by considering flow velocity and roughness differences between vegetated and unvegetated areas, and estimate the sediment trapping rate by mangroves. Finally, the obtained parameters are used in the Exner equation to simulate changes in topography, geomorphology, and wetland elevation.

The simulation results of the evolution model demonstrate its effectiveness in capturing the early, middle, and final stages of mangrove tidal channel development over long-term scales. It shows the gradual development of tidal channels from nearly flat beds, undergoing scouring, widening, subsequent infilling, and constriction processes under the combined effects of tides and river flow. Additionally, this study collected data from vegetation flume experiments from Freeman et al. (2000) to validate the hydrodynamics and vegetation modules. By setting up vegetation obstacles in the flume and providing equivalent parameters for vegetation resistance, the model successfully reproduces the water surface profiles observed in the experiments, demonstrating the reliability of the hydrodynamics and vegetation modules. We also collected data from four tidal channel cross-sections in the Shezi Island mangrove in the Tanshuei River to validate wetland morphological dynamics. By defining tidal flats and channels in the model, we significantly improved in simulating excessive sediment accumulation on the intertidal flats. This study also conducted a sensitivity analysis of model parameters to gain deeper insights into tidal channel geomorphic evolution mechanisms. This study offers a quantitative tool for effectively maintaining tidal channels and tidal flats that may enhance mangrove swamps’ integrated and adaptive management.