Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/34312
標題: The Mechanism of Large Woody Debris Initial Entrainment in a Flume Experiment
漂&;#63946;木初始運動機制之試驗研究
作者: 王啟榮
Wang, Ci-Rong
關鍵字: large woody debris;wood stability factor;wood incipient motion prediction model
出版社: 水土保持學系所
摘要: 
鑑於吾人對於漂流木在河道中的運動機制瞭解有限;因此,本研究藉由渠槽試驗模擬單根漂流木在定床渠道之初始起動情形,並探討不同漂流木長度、直徑、與水流夾角及底床粒徑對漂流木初始起動水流條件之影響。另外,本研究亦藉由水流作用在單一漂流木的力學平衡,建立漂流木初始起動之預測模式,並以渠槽試驗之結果對預測模式進行驗證。
試驗發現,當漂流木與水流方向平行時,隨著水深逐漸增加,漂流木所受之浮力隨之增加,摩擦阻力則逐漸減小,直至漂流木開始發生運動,此時漂流木會以半浮動半滑動的方式向下游移動;而漂流木與水流方向傾斜及垂直時則皆為滾動之方式向下游移動。綜合試驗與模式預測之結果得知,漂流木能否穩定停留在渠道中,主要跟漂流木與水流之夾角、漂流木密度、直徑、渠道坡度及底床粗糙度等因子有關,而與漂流木長度及拖曳力係數較無關係。雖然在許多漂流木相關研究中皆指出,漂流木長度係影響漂流木穩定性的重要因子,但從試驗結果及預測模式皆顯示,長度因子對於漂流木穩定性並無顯著之影響,是由於本研究之漂流木長度均小於渠寬。
本研究結合漂流木滑動(合力平衡)與滾動(合力矩平衡)兩種不同力學平衡機制,建立漂流木初始起動預測模式:
式中,ρlog、Dlog、Llog及θ分別代表漂流木密度、直徑、長度及與水流夾角, 為漂流木與水流夾角θ相應之摩擦角,ρw及dw分別代表水密度與漂流木初始起動所需之水深,S為渠道坡度(S=tanα),ds為底床粒徑,CD為拖曳力係數。
此預測模式主要適用在漂流木與水流方向之任意夾角;由已知之漂流木性質(水流夾角、密度、直徑及長度)和渠道性質(坡度及底床粒徑),即可預測漂流木初始起動所需之水深。針對預測模式進行驗證之結果顯示,預測模式與試驗結果之誤差範圍佔漂流木直徑之10%,尤其在漂流木與水流方向傾斜及垂直時,其誤差範圍只佔漂流木直徑之5%。上述結果顯示,本研究所提出之漂流木初始起動預測模式之預測能力良好。

There has thus far been relatively little research into large woody debris (LWD) motion. The research was designed as a fixed bed flume experiment to explore the state of LWD critical entrainment, and discussed the initial flow condition was influenced by different length, diameter, orientation and bed roughness. Additionally, the theoretical model was developed to predict wood entrainment and compare predictor with experiment results.
The experiment results show that different orientations of wood will lead to different types of wood movement. For wood parallel to flow, the mechanism of motion features started by semi-floating and semi-sliding, because flow was gradually raised with the buoyant force increased and the friction force decreased until the wood moved to downstream. For wood oblique or perpendicular to flow, the mechanism of motion features started by rolling. Both the models and the experiments indicate that stable wood is significantly associated with wood angle relative to flow direction, the density of wood, wood diameter, channel slope and bed roughness. The wood stability is less sensitive to the choice of the apparent drag coefficient and wood length. Although previously reported as the most important factor in wood stability, wood length did not significantly affect the threshold of movement in the experiments or the model predictions, for wood shorter than channel width.
This research consists of sliding and rolling equilibrium equations to establish a prediction model of wood entrainment:
where dw is the flow depth for wood incipient motion, Llog is the wood length, Dlog is wood diameter, ρlog and ρw are the densities of wood and water, respectively, S is the channel slope (S=tanα), CD is the drag coefficient of the wood in water, ds is the bed grain size, θ is the angle of the wood relative to flow, and is the friction angle between wood and channel bed.
This prediction model is helpfully used for multiple angles of the wood relative to flow, and the flow depth for wood incipient motion could be predicted under the wood and channel characteristics are given. To compare model predictors with experiments, the bias account for about 10% of the wood diameter. Moreover, for wood oblique or perpendicular to flow, the bias only account for about 5% of the wood diameter. According to the results, the prediction model is relative successfully at predicting depths for wood incipient motion.
URI: http://hdl.handle.net/11455/34312
Appears in Collections:水土保持學系

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