紅樹林作為國土保育自然解方之潛力評估

Abstract

因溫室氣體濃度上升導致的氣候變遷在全球各地造成不同程度與類型的災害事件，紅樹林生態系除了具備吸碳、固碳、底床土壤累積碳含量等優異碳匯能力外，紅樹林濕地的地形抬昇特性有機會對抗海平面上升帶來的衝擊，包括海岸線退縮。本研究區域位於桃園市新屋溪河口，屬於紅樹林與藻礁生態複合共生系統，本研究假設紅樹林生態系統具備囚砂能力，是作為國土保育的重要生態工程策略，並可藉此降低排入下游藻礁生態系統的砂量及懸浮泥砂濃度，呼應近年國際間大力倡議之自然解方精神(nature-based solution, NbS)。 本研究進行地形、水位、流速等現地調查，包括UAV、壓力式水位計、都普勒流速剖面儀等，並收集水文、泥砂、空照圖資等歷史資料以建立數值模式並進行模式驗證；結果顯示，本研究建立的模式在常時水位驗證下具有非常良好的品質(NSE=0.9821)具備可信度，同時在高流量測試下所設置之結構物功能可在不影響模式水流分布下達成預設的阻力，而模式選用之適當輸砂公式透過模式建議之採用公式、歷年正射影像資料以及輸砂公式原理選定。另外，本研究進行9種情境之模擬分析：3種「25年重現期入流泥砂濃度」搭配3種「下游河口水位」條件；情境模擬結果顯示，入流泥砂濃度是影響濕地沖淤深度及範圍最大的因子，入流濃度提升淤積越多越廣，至於下游水位上升的影響程度相對而言屬較次要因子，研究也發現入流泥砂濃度提升至兩倍後對灘地整體高程變化為0.27 mm/yr，因未考量到紅樹林本身的生物堆積作用，故尚不足以達到平均海平面上升速率(2.35mm/yr)，但紅樹林確實提供近岸藻礁生態系自然囚砂機制，而此固砂效益也展現其作為延緩國土流失自然解方的潛力。

Climate change caused by increased greenhouse gas concentrations has resulted in varying degrees and types of disasters worldwide. The mangrove ecosystem, in addition to its excellent carbon sequestration capacity, such as carbon absorption, fixation, and accumulation in the sediment, has been able to counteract the impacts of rising sea levels, including coastal erosion. This study hypothesizes that the mangrove ecosystem has sediment-trapping capability, making it an essential ecological engineering strategy for land conservation. Reducing the sediment load and suspended sediment concentration entering the downstream algal reef ecosystem aligns with the spirit of nature-based solutions (NbS), which has been strongly advocated internationally in recent years. The study area is located at the mouth of Xinwu River in Taoyuan City, with a complex symbiotic system of mangroves and algal reefs. We conducted field surveys on topography, water levels, and flow velocities using UAVs, pressure transducers, and ADCP. Historical data on hydrology, sediment, and aerial photographs were also collected and incorporated with the filed investigation to establish a numerical model and the relevant model validation. The results showed that the model developed in this study exhibited excellent quality (NSE=0.982) and reliability when validated against steady-state water levels. The structural elements installed for high-flow testing achieved the intended resistance without affecting the water flow distribution. The appropriate sediment transport formulas were selected based on the literature recommendations, historical orthophotos, and principles of sediment transport formulas. Furthermore, the study conducted simulations for nine scenarios involving the 25y-return-period inflow sediment concentration combined with downstream water levels. The simulation results indicated that inflow sediment concentration was the most influential factor affecting wetland siltation depth and extent. As the inflow concentration increased, siltation became more extensive. The impact of downstream water level rise was relatively minor. The study also found that doubling the inflow sediment concentration resulted in an overall elevation change of 0.27 mm/yr in the intertidal zone. Although it is insufficient to meet the average rate of sea-level rise (2.35 mm/yr), the mangrove forest provides a natural sediment-trapping mechanism for nearshore algal reef ecosystems. The sediment retention benefits demonstrated its potential as a nature-based solution to mitigate the land loss caused by the increased inundation under rising sea levels.