單根漂流木啟動之動床渠槽試驗

The Mobile Bed Flume Experiment of Single Woody Debris Initial Entrainment

梁閔智(Min-Chih Liang) ； 陳樹群(Su-Chin Chen)

《中華水土保持學報》 49卷4期 (2018/12) Pp. 254-261

https://doi.org/10.29417/JCSWC.201809_49(4).0005

摘要

本研究運用渠槽試驗模擬單根漂流木在動床渠道之啟動情形，以不同長度、直徑之漂流木並配合與水流方向三種夾角變化，探討漂流木與底床的互動關係、運動方式、表面流場與流速變化，以及比較定床與動床之漂流木啟動穩定因子。研究內容主要分為三部分：第一部分是運用雷射掃描數化地形資料，以判釋結果說明漂流木啟動前渠床微對地形的影響，以及漂流木之運動方式；第二部分是，利用PTV追蹤表面顆粒繪製出表面流場與流速；第三部分是運用漂流木臨界啟動之水力條件來計算漂流木穩定因子。試驗結果顯示，漂流木周圍之流場與流速會因不同擺放方向而改變，漂流木在動床中會受地形影響而改變導致漂流木之運動方式、初始啟動門檻值。漂流木置於平行水流方向時，其運動方式可分為軸轉動和滑動兩種模式。而漂流木與水流傾斜與垂直時，亦有兩種運動模式，分別為軸轉動與滑動同時發生與只發生滾動。漂流木在定床渠道中，與水流方向平行漂流木的啟動主要是藉浮力啟動，動床結果顯示受浮力與拖曳力同時影響，漂流木傾斜與垂直水流方向，會比定床條件更難啟動，因此需要較大的水流條件。

關鍵字

漂流木 ； 漂流木啟動 ； 動床 ； 渠槽試驗

並列摘要

This study conducted flume experiments that simulated the initial entrainment of single woody debris in a moving channel bed. Woody debris of different lengths, diameters and orientations to flow direction were used to explore their interactions with the moving channel bed. In addition, their mode of transport and variation of the flow field were investigated. Results of test were compared to the initial entrainment of woody debris on a fixed channel bed. For the sake of comparison, this study was divided into three parts. In the first part, laser scanning was used to compare channel topography before and after woody debris movement. The second part used particle tracking velocimetry (PTV) to establish the fields of surface flow and velocity. The final part assessed the stability of the woody debris stability factor. The results of these experiments indicate that woody debris orientation influences the flow field, flow velocity and the final bed topography. Topography changes influenced the mode of wood transport and their entrainment threshold. Woody debris oriented parallel to flow had two modes of transport; rotation and sliding to downstream. Under oblique and transverse placements, woody debris movement can also be divided into two; rotation and sliding two modes and rotation is majority. A notable difference between the fixed and the moving bed channel was that entrainment of woody debris was primarily initialized by buoyancy in the fixed bed, whereas both buoyancy and drag-force initiated movement in the moving bed channels. Finally, our results demonstrated that woody debris in a moving bed channel needed larger discharge to initiate movement than that in fixed bed channels.

並列關鍵字

woody debris ； wood entrainment ； mobile bed ； flume experiment