曾文溪口束洪對地形變遷影響之研究

臺灣河川流量豐枯變化顯著，河川輸沙主要發生於豐水期；水庫興建後，更是集中於排洪期。為避免洪水氾濫成災，防洪工程常以興築堤防約束洪水在河槽內，所以河口的洪水束流將淤沙沖往外海深水處。有鑑於此，本研究利用數值模式進行曾文溪口束洪對地形變遷影響之研究。本文蒐集曾文溪口民國98年莫拉克颱風前後的地形水深與颱洪資料，並利用二維水動力與地形變化數值模組模擬莫拉克颱風洪水條件下的河口地形變遷趨勢。模擬成果與實際現象相符，即曾文溪流量較小時，有少量淤沙積聚於溪口；但流量大時，河口洪水流速加快，形成束流將淤沙沖往外海深水處，而使近岸流無法將其帶回鄰近海岸淤積，此應為導致近幾年洪水期曾文溪的大量輸沙反而無法有效補助海岸沙源之主因。最後，將模式應用於河口拓寬，對於減緩束洪、增加河口沉沙之成效模擬，以評估改善曾文溪鄰近海岸沙源供給的可行性。

關鍵字

曾文溪口；束洪；地形變遷；數值模擬

並列摘要

Seasonal river discharge in Taiwan has been changing significantly, causing sediment transport in rivers mainly occur in the wet period. At downstream part of a river the dike has been built to restrict flood in a fixed narrow channel. However, such hydrodynamic facilities would cause remarkable sediment transport and resulting morphological evolution nearby an estuary. The main goal of this paper is to present a numerical evaluation on morphological evolution subject to a flood event nearby Tseng-Wen estuary in Taiwan. Numerical experiments suggest that the flooding flow is able to drive sediments to deposit beyond E. L. -5 m with a faster speed of 2 (m/s), by which the flow is confined within a fixed narrow river channel. Deposition of offshore sand may not be transport by nearshore flow, causing the lack of sediment supply to a coastline. Numerical results also indicate that an adequate sediment transport in a river would be observed within the wave effective area, an expansion of river channel for reducing the effluent speed of flood discharge subject to a typhoon event is recommend.

並列關鍵字

Tseng-Wen River Estuary ； flood under restriction ； morphological evolution ； morphological modeling

國際替代計量

曾文溪口束洪對地形變遷影響之研究

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