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Analysis of flow pattern and resistance in step-pool streams
|Authors: ||Yi-Liang Chen|
|Contributors: ||NTOU:Department of Harbor and River Engineering|
step-pool;spill resistance;3D fluid dynamic numerical model;nappe flow;skimming flow
|Issue Date: ||2011-06-30T07:54:34Z
|Abstract: ||臺灣地區地文條件特殊，使得山區河川擁有坡陡特性，當河川上游來砂量不足、泥砂級配不均勻、河床處於沖刷狀態及具有難以沖動的巨石等，則於河道中形成階梯-深潭(step-pool)之底床型態。水流於階梯-深潭流動過程，產生跌落、翻滾及水躍等水力特性，此三種連續流動過程構成溢濺阻力(spill resistance)，而消散更多水流能量，因此如何適切決定山區河道糙度係數以供水利設計乃為重要課題。本研究利用三維流體動力數值模式進行變量流河道水流演算，藉由比較有階梯-深潭河道與無階梯-深潭河道之出流量歷線，以暸解階梯-深潭河道對水流阻力影響，並對糙度係數檢定結果進行分析與探討。 研究中以台北縣瑞芳鎮坑子內溪之150m河段，進行山區階梯-深潭河道水流演算。研究結果顯示，水流型態為舌流(nappe flow)時，水流流經階梯-深潭產生跌落及水躍特性，使得河道曼寧糙度係數達0.4；而當水流型態為滑移流(skimming flow)，水流淹沒階梯，且水流於深潭區產生連續翻滾而形成渦流，其曼寧糙度係數為0.1。研究中所檢定後之曼寧糙度係數值，可適切反應階梯-深潭河道於不同水流型態對能量消散與水流阻力之變異。|
Mountain streams in Taiwan usually have steep slope due to the special geomorphologic characteristics in the island. If sediment supply from upstream is insufficient, sediment is highly non-uniform and with immovable boulders, the riverbed will be scoured and result in a step-pool morphology. For water flowing over step-pool, plunge from step to pool, tumbling and hydraulic jump will be produced. These three kinds of flows result in spill resistance, which generate significant energy dissipation. Consequently, it is considered to be an important issue to determine the channel roughness coefficient of mountain streams for hydraulic design work. In this study, unsteady-flow routing by using a 3D fluid dynamic numerical model was performed in a stream with/without step-pool morphology by comparing the outflow hydrographs to determine the flow resistance and to calibrate Manning’s roughness coefficient. A 150m mountain channel reach located at Ken-Tzu-Nei Creek was chosen to perform the step-pool channel routing. The result shows that for stream in a nappe-flow condition, plunge from step to pool and hydraulic jump are produced, so the Manning’s roughness could reach to 0.4. For stream in a skimming-flow condition, the steps are submerged and flow tumbling in continuous pools with vortex, and then Manning’s roughness was calibrated as 0.1. The Manning’s roughness calibrated in this study for the example mountain stream is considered to be adequately reflected energy dissipation and flow resistance in different flow patterns at step-pool streams.
|Appears in Collections:||[河海工程學系] 博碩士論文|
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