水流經挺水性水生植物之阻力研究

Abstract

水流經水生植物所產生之阻力可用以計算水位、流速變化、水流能量消散和滯留時間、輸砂變化、擴散速率等；為水利、生態與其它水相關工程於設計、操作與維護上之重要資訊。為以直接量測法研究水流經臺灣天然挺水性水生植物之瞬時流力阻力，本研究分成以下四個階段進行：挺水性水生植物之實地觀測與選取、建設小型種植區培育選取之水生植物、設計與建造一嵌入水槽式電子阻力量測儀器、及水槽中挺水性水生植物流阻控制試驗與無因次數據分析。 實驗設計量測並分析水流於五種流速變化下，流經蘆葦、香蒲、大安水蓑衣與燈心草在四種不同植生密度變化下之阻力。本研究所開發之電子阻力量測儀器可直接量測並記錄水流經一陣列不同生長密度之天然(根系與土壤完整)挺水性水生植物所產生的瞬時(秒)阻力數據。實驗結果發現當流速增加，植物流線型因子（流阻係數與植物投影面積之積）隨之下降，顯示這四種挺水性水生植物具有流線型化的能力，於一定流速範圍內，可降低總體植物阻力。試驗研究進一步發現於固定流速下，挺水性水生植物如蘆葦具有其最低植物流線型因子(最高流線型化)所對應之最佳種植密度。 無因次分析水流經蘆葦的實驗數據，發展出二條以流線型係數及阻力係數之自然對數為應變數之高相關性(R2=0.94)無因次方程式[Eq. (3-55)及(3-67)]；並推導出阻力粗糙係數如：達西-威斯巴f值、蔡司C值及曼寧n值。此二式可應用於含有天然挺水性水生植物的明渠流、漫地流、輸砂與海岸淺水波流等控制方程式。

This research is an experimental study on the drag force of four types of emergent macrophytes: Phragmites australis, Typha orientalis, Hygrophila pogonocalyx, and Juncus effusus, in aquatic flows with four varying plant densities under five different flow rates. There are four phases of this experiment: field observation and selection of emergent macrophytes, cultivation of selected emergent macrophytes, design and construction of a direct drag force measurement system to obtain drag force per second of an array of natural emergent macrophytes with roots and soil intact, and execution of controlled flume experiments with dimensional data analysis. Experimental results revealed a drop of the streamline factor (product of drag coefficient and plant projected frontal area) as flow velocity increases, indicating the streamlining ability of these emergent macrophytes which in a range of velocity, reduces overall drag force. In addition, an optimum plant density is observed corresponding to a lowest streamlined factor (highest streamlining ability) for Phragmites australis under each flow velocity. Dimensional analysis on the data sets from flume experiments for Phragmites australis arrives at two dimensionless equations [Eq.(3-55) and (3-67)] achieving high correlativeness (R2=0.94), with the natural logarithm of the streamlined coefficient and the drag coefficient as dependent variables. Three friction factors, Darcy-Weisbach f, Chezy’s C, and Manning’s n are derived from each of these two dimensionless equations, which allow further applications on the governing equations for open-channel flow, overland flow, sediment transport, and coastal shallow water flow through emergent macrophytes.