人工溼地設立與淹水減緩效益之分析雲林沿海地區個案研究

Abstract

雲林沿海地區因地層下陷嚴重，被列為國土復育計畫的重點區域之一。該區因持續之地層下陷導致排水不易，常於颱風或是暴雨期間發生淹水。而河岸堤防與抽水站並無法有效減緩淹水，鑑此，本研究建議將地層下陷嚴重地區轉為溼地，希冀能夠達到減緩淹水的效果。本研究目的為提出一溼地選取模式與效益分析方法，以利相關之淹水防治工作。研究方法上，本研究首先參考相關理論與案例，針對雲林地區之淹水問題，評估出適合做為溼地之環境條件，並且參考Automated Assessment Method for Northeastern Wetlands(AMNEW)的溼地評估方法，據以選出合適設置人工溼地之地區。另外，利用經濟部水利署之軟體SOBEK進行設置溼地之淹水模擬測試，本研究以10年回歸期設計暴雨作為暴雨模擬條件，以四種不同解析度之DEM( 80m , 40m , 20m , 10m )試驗各解析度所需之模擬執行時間與淹水模擬結果的代表性。最後，比較設立溼地前後之淹水情形，進行淹水減緩效益之評估。研究結果方面，本研究於沿海鄉鎮選出三塊適合設置人工溼地及一塊滯洪池的地區，依據SOBEK淹水模擬結果，80m-DEM需時1.5小時，而10m-DEM則需時七天半。本研究考慮預警發佈與地區公共之災害反應，選取20m-DEM需時27小時作為模式模擬之最適解析度。在比較設置人工溼地之前後的模擬淹水情形後，認為本研究所選取之人工溼地對鄰近區域內1000公尺洪水的減緩具顯著成效，對於聚落減緩淹水的效果，以滯洪池的成效最好，有五處聚落受到淹水減緩的影響，減緩的值約在0.1公尺(10CM)。

The coastal area of Yunlin County is listed as one of the focused areas in the National Land Recovery Program because of its severe land subsidence problem. As the drainage of this area continues to worsen due to land subsidence, flooding occurs regularly whenever a typhoon or a storm passes. Dikes and pump stations along rivers have not worked well in reducing the floods. This research investigates the idea of converting seriously subsided areas into wetlands as a solution to the flooding problem, and this research refers to Automated Assessment Method for Northeastern Wetlands(AMNEW). First, we evaluated potential areas for conversion to wetland. Second, we ran SOBEK, a software developed by the WL｜Delft Inc., the Netherlands, with a designed 10years-return-period storm and digital elevation models (DEMs) of four different resolutions (80m, 40m, 20m, and 10m) for the purpose of selecting a resolution that could provide adequate flooding information within an acceptable timeframe for warning assessment. For our study area, a SOBEK run ranged from 1.5 hours with the 80m DEM to 7.5 days with the 10m DEM.The optimal modeling input appeared to be the 20m DEM with 27-hours running time is selected to be the optimal modeling input. Finally, we evaluated three potential sites for wetland conversion and used the overland flow model in SOBEK to simulate the flooding conditions. This last experiment allowed us to compare the before and after conditions and measure the effectiveness of wetland on reducing the flooding problem. After comparison, we found that the flooding problem within 1000 m of the wetland reduced the most. The detention pond had the best effect in reducing the villages’ flooding problems. Five villages had their floods reduced by about 0.1m(10cm).