碎波帶漂沙與波動特性研究; Study of sediment transport and infragravity waves in the surf zone

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题名:	碎波帶漂沙與波動特性研究;Study of sediment transport and infragravity waves in the surf zone
作者:	林柏青;Poching Lin
贡献者:	土木工程研究所
关键词:	碎波帶;群波;長波振盪;Hilbert轉換;薄層流;漂沙;Hilbert transform;infragravity waves;wave groups;surf zone;sheet flow;sand drift
日期:	2003-09-26
上传时间:	2009-09-18 17:05:33 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本研究分別在台灣西海岸中部與南部地區，以波浪儀、流速計及濁度計等多種儀器觀測碎波帶附近之波浪、流及底床上之懸浮質濃度分佈，並安裝水下攝影機同步觀察底床附近之懸浮質與薄層流輸沙，以深入分析並探討碎波帶的漂沙活動。碎波帶附近波浪因淺化碎波，能量劇烈交換引致漂沙現象。而外海入射波浪常具有群波結構，此等隱含長週期波能振盪的群波，也會直接影響底床輸沙；另外因地形或氣候因素在近岸水域常發生長週期之水面波動，此等長週期之水位振盪現象因含有相當份量之能量對於碎波帶之漂沙活動自有其重要性。本研究分析方法除了以平滑化瞬時波能法(SIWEH)解析群波現象外，另以Hilbert 轉換及內含模組函數(Intrinsic Mode Function)解析能譜分佈在時間歷程之變化，可以更清楚瞭解碎波帶附近波能變化與漂沙活動之關係。資料分析結果顯示近岸底床附近之懸浮質濃度具有一固定背景值，其變化則有週期性之驟增現象，並與入射波浪之群波特性有密切關係。底床附近之懸浮質濃度驟增量可達背景濃度值的2~3倍，影響近岸懸浮質輸沙量估算結果甚鉅。此外，波浪由碎波帶外向岸接近時其群波特性會因波浪碎波而逐漸減弱。在碎波帶外波浪群波特性較強時，由群波主控底床之懸浮質濃度變化，但在碎波帶內此種現象則較不明顯。為能進一步探討波浪在碎波帶之輸沙行為，本研究結合現場波浪、三維流速、濃度剖面觀測與同步影像分析，俾能在定性與定量上探討底床漂沙活動時作清楚明確的闡述。本研究並由同步現場攝影發現碎波帶內之主要漂沙機制來自沙漣結構上的薄層流及懸浮質輸沙；在粒徑0.27mm水深約1.7m波高0.34m沙漣高5.5cm底流超過40cm/s時即可引發薄層流。由於懸浮載、薄層流以及底移載在碎波帶內常是三者並存，其實際分佈與量化分析仍有待進一步觀測與探討。 Field experiments were conducted to investigate the wave, current and sediment suspension across the surf zone by deploying wave gauges, 3D-ACM current meters and OBS turbidity meters at Taichung and Ampin, in the west coast of Taiwan. A synchronized video camera was also installed underwater to observe sheet flows and sediment movement on ripple beds. Generation of sand drift in surf zone is mainly due to energy released from shoaling waves. Wave groups in approaching waves and surf beats usually imply energy undulation of low frequencies, which may affect the movement of bed sediments. A new approach using Hilbert transform and Intrinsic Mode Functions analysis to explore the effect of energy grouping and undulating on sediment suspension is presented. An interesting bursting phenomenon of near-bed sediment concentration was found dominating in surf zone, which was closely related to incident wave groups. Peak concentrations of suspended sediment associated with wave groups were 2~3 times larger than that without wave groups. It is found the main mechanisms of sand drift in the surf zone are sheet flows and sediment suspension. The criterion for inception of sheet flow at depth of 1.7m, ripple height of 5.5cm and sand of 0.27mm in diameter, is about 40cm/s in current speed. The suspension load, bed load and sheet flows are found all responsible for the sand drifts in surf zone. This study is far from being complete but should provide readers a good insight on sediment transports in surf zone. It needs further observations to clarify the amount of sand actually moved by sheet flow alone.
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