潮溝與潮灘地形地貌演變之模擬

Simulations for Geomorphologic Dynamics of Tidal Creeks and Mudflats

鄭庭宇(Ting-Yu Cheng)

指導教授 ： 施上粟

國立臺灣大學/工學院/土木工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201903339

摘要

河口濕地是全世界最高生產力的生態系統之一，也是許多生物賴以維生的重要棲地。但因其為處河川下游，濕地生態系統品質及棲地樣貌同時受到上游河川及潮汐影響。濕地內重要地景如潮溝、潮灘之形貌變化直接影響河口植物競爭、棲地條件、物質能量傳輸效率，故瞭解潮溝潮灘的形貌演化動態及與植物的互動機制，有助於建立河口濕地的經營管理策略。臺灣北部淡水河流域的紅樹林濕地提供豐富的營養鹽類吸引水陸生動物覓食，並成為河口生態系中重要的棲息環境之一。但紅樹林過度的擴張，將影響水理以及泥砂運移之特性，逐漸使濕地陸域化，也造成紅樹林濕地環境單一化。過去曾為了增加濕地的生物多樣性，藉由適當地人為疏伐與營造潮溝與潮灘地之工法維持紅樹林濕地地貌。然而，紅樹林濕地內的潮溝潮灘地貌之變化機制尚未釐清，因此本研究建立垂直二維潮溝潮灘演化模式，且考慮紅樹林與水理及泥砂的交互作用，並以淡水河社子島紅樹林濕地為主要研究地點。本研究發展之河口濕地潮溝潮灘演化模式包含三個模組：水理模組、泥砂模組、植生模組，「植生模組」在本研究中主要針對水筆仔紅樹林（Kandelia obovata），但可擴充或替換為草澤植物或其他紅樹林植物；本模式以Exner equation作為主要的統御方程式，以Fourth Order Runge-Kutta數值方法解算，用以預測不同邊界條件、不同參數條件或不同植生情況下的潮溝及周邊潮灘形貌變化。模式的水位邊界條件是藉由SRH-2D模擬社子島水位變化作為模式的邊界入流條件，模擬平常潮汐以及颱洪時期水動力對於紅樹林濕地地貌演變過程。研究發現潮汐水位對於濕地地貌泥砂沉積速率約為0.0046~0.0108 m/month之間，颱洪時期的泥砂沉積速率約為0.01~0.024 m/day，由此可知河川在颱洪時期夾帶的大量泥砂可造成河口濕地潮溝潮灘短期內形貌大幅度變化。模式的驗證結果顯示此模式現階段已具有預測能力，可反應整體地貌的垂向抬升，但尚無法有效的模擬潮溝最低點橫向位置的變化。本研究也針對模式中泥砂參數與植生參數進行敏感度分析，發現孔隙率的改變將影響整體橫斷面地貌的變化，而侵蝕常數越大則潮溝的垂向侵蝕現象越明顯，有無植生的改變使潮灘的泥沙沉積量增加。本研究也嘗試分析模式的應用價值，假想案例為於社子島引進控制與減少潮差的工法(controlled reduced tidal, CRT)，模擬長時間有無應用CRT工法控制潮汐水位情況下，整體潮溝橫斷面之演變，結果顯示若無應用CRT工法，經過五年後潮溝將逐漸淤積成為潮灘，而應用CRT工法後，經過五年依然能維持潮溝與潮灘地形貌及功能，主要原因除了CRT可藉由設計堤防上下開口位置而控制潮差外，也能在洪汛時期控制進入濕地的泥砂量，因而可讓潮溝形貌維持較久的時間。本研究所發展的潮溝演化模式能依據不同地區的水位條件、泥砂參數條件以及植生參數變化，模擬潮溝地形長時間之變化，亦能展示水理對於地貌之影響並計算泥沙沉積速率，預期未來有機會作為潮溝與潮灘營造工程效益評估的有效工具。

關鍵字

潮溝 ； 紅樹林 ； 地貌 ； 水動力 ； 泥砂 ； 河口濕地

並列摘要

Tidal creeks and mudflats serve as critical habitat areas for shorebirds and fish in subtropical coastal wetlands. Construction efforts for mangrove deforestation, mudflat maintenance, and tidal creek construction, can promote a greater diversity of habitat types and attract shorebirds. The study aims to develop an evaluation model for quantifying the geomorphological changes of tidal creeks and mudflat in coastal wetlands. This study established a vertical 2D mathematical model using the conservation equation and the Exner equation to simulate the evolution of a tidal creek. Variant plant biomass was also incorporated in the model to consider the growth effects of mangroves on the deposition rate of the marsh platform. The boundary conditions of the model were input through a hydrodynamic package, SRH-2D. Simulating geomorphology of wetland under tidal period and typhoon period, it indicated that sedimentation rate in the tidal period ranges from 0.0046 m/month to0.0108 m/month and sedimentation rate in the typhoon period is between 0.01 m/day and 0.024 m/day. A large amount of sediment entrained into the wetland during the typhoon period can cause a substantial change in the tidal creeks of the tidal flats. Sensitivity analysis of the sediment parameters points out that the changes in porosity affect overall cross-sectional geomorphology, and erosion rate affect tidal creek incision. The emergence of vegetation increases deposition in the tidal marsh. Model verification showed that the model is reliable on the prediction of the amount of the deposition but fails on forecasting the horizontal position of the thalweg. The model application on CRT scenarios reveals that the CRT could sustain the geomorphology of the tidal creek and tidal marsh for a more extended period, indicating a useful alternative for constructing and maintaining a riverine wetland. Tidal creek evolution model was proved that it was capable of simulating long-term geomorphologic dynamics based on hydrodynamic parameters, sediment parameters, and vegetation parameters in different regions. The model is thus expected that there will be opportunities in the future to provide as a quantitative tool for evaluating the benefits of tidal creeks and tidal flats.

並列關鍵字

tidal creeks ； mangrove ； geomorphology ； hydrodynamics ； sediment ； coastal wetlands

參考文獻

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