Flow and Bed Form Variations on Arranged Vegetation Experiments

Abstract

本研究以具倒伏透水性之模擬植生為基材，共進行動床清水沖刷及定床加砂落淤實驗；於動床清水沖刷實驗中，固定渠槽坡度(S=0.001)及流量(Q=0.011cms)條件，本研究將植生擺設為橋台、丁壩、江心洲與固床工四種型態，並以四種植生排列密度(D=0.056、D=0.127、D=0.226、D=1(硬式結構物))觀察植生影響水流紊動與床砂面發展型態之交互關係；根據實驗結果之水理分析發現，四種擺設型態所造成阻水斷面比、邊壁阻擋與植生倒伏程度三者，為影響水流紊動強度與擴散方式差異主因；植生透水特性降低水流紊動強度，亦因植生倒伏擺盪而將紊流動能往近液面與下游傳遞，此現象具有減緩沖刷能力集中效果；而流速減緩探討上，橋台擺設在上游迎水面產生最大流速減緩率，江心洲擺設則在下游流速減緩具有明顯效果，尤其在植生密度D=0.127時，平均流速降低166%，紊流動能值高達80，顯示落淤最為集中；根據床面型態分析發現，橋台、丁壩、江心洲與固床工型態所對應沖刷坑外型分別為：狹長型、L型、橢圓與寬窄型，植生密度僅造成沖刷程度差異，於沖刷坑型態上無明顯變化；而在沖刷體積探討上，結構物鄰近帶為主要沖刷發展區，於各型態下其沖刷量均可達總沖刷50%以上，且橋台與丁壩型態易造成沖刷上游擴張，江心洲具導向沖刷能力，固床工則僅於結構物鄰近帶產生沖刷，影響範圍為四種型態最小。 在定床落淤實驗，主要則以固床工之擺設觀察底床植生對植生緩流落淤現象，實驗結果發現植生密度為影響植株叢上游湧水與下游迴流現象產生程度差異之主因，進而改變囚砂效率；本研究於植生密度D=0.226時，囚砂率高達80%。

The study used the simulated vegetation to proceed the mobile bed and non-mobile bed experiments with fixed channel slope (0.001). In mobile bed experiments, the simulated vegetation were arranged as abutment, spur dike, sand bar and sill with four density (D=0.056, 0.127, 0.226 and 1) and fixed discharge (0.011 cms). According to the mobile bed experiments data, the arranged types, wall effect and vegetation proneness are the main reasons to affect the turbulence intensity and diffusion. Porous and flexible properties of vegetation results in a decrease of turbulence kinetic energy and for a proportion of the surface region flow above. The upstream and downstream mean velocity have the most reduction in abutment and sand bar arranged type experiments, respectively. Especially, the sand bar arranged type experiment with D=0.127, the reduction of the downstream mean velocity is 166%. The shapes of scour hole in abutment, spur dike, sand bar and sill are slender, L-shaped, ellipse and narrow, respectively. Different vegetation density only affects the scour volume and has no obvious influence in shape of scour hole. More than 50% of the total scour volume centralizes within 15cm to any vegetation structure in the experiments. The scour zone develops toward upstream in the abutment and spur dike arranged type experiments and downstream in the sand bar arranged type experiments. In the non-mobile bed experiments, the study observes the sediment deposition caused by vegetation. According to the non-mobile bed experiment data, the vegetation density is the main reason to affect the surge in the upstream, the clockwise vortices in the downstream and the sediment deposition volume. The experiment in D=0.226 has the largest sediment deposition volume, 80% of the total sediment, in all non-mobile bed experiments.