往昔學者針對不同的沙丘形式進行漂沙、侵蝕過程與越波的研究，提出相關的侵蝕機制、漂沙方程式與數值模擬結果，並提出波浪侵蝕時有最好防護力的沙丘型式為有平台的沙丘(Berm dune，簡稱BD)之結論。然而對於此型式之沙丘，並未對其做較完整的內部流場與渦度探討，因此本文應用質點影像測速法 (Particle Image Velocimetry，簡稱PIV)量測技術探討不同週期之波浪於沙丘前側坡與背側坡的流場特性，並呈現其流場與渦度的時空分布。
本實驗在成功大學水工試驗所流體力學實驗室中的中型玻璃水槽進行，為了理解越波時沙丘之水動力機制的變化情形，本實驗採用壓克力板製作沙丘模型並進行定床模型試驗。受到沙丘模型頂部為薄層流之影響，因此只拍攝了模型前側與模型背側部分，並以能清楚拍到質點之最大畫面來進行拍攝，而每次拍攝均取一個完整週期的成熟波進行分析。本實驗在相機與波高計完成同步後進行影像資料的擷取，並將影像資料使用canny method進行自由液面之偵測，取出感興趣之內部流場。最後，在每個周期與拍攝區域，均進行穩定收斂次數為20次之重複試驗並配合整體平均法分離出平均速度，試驗時之水位誤差約0.2到0.3%。
本研究呈現了沙丘模型前後側在五種不同週期且深海波高均為5cm時之流場與渦度的時空分布情形。模型前側部分，各週期波浪均於溯上時夾帶大量氣泡，且流場十分紊亂，並於流速分離時，在底床不同位置產生一流速停滯區。模型後側部分，短周期波浪在水體液面處產生激烈的主流速與渦度變化，而長週期波浪，除了水體液面處之流速與渦度變化劇烈外，底床部分也產生明顯且深度較深之掏刷情形，並產生渦流轉向之情形。最後本研究整理出在不同週期下之主渦流的流場與渦度變化，並將各週期之渦度變化值與越波量做計算與統整。

Over the past years, researchers have done lots of research on sand drift, sand erosion and overtopping in different kinds of sand dunes. They submitted erosion mechanisms, sand drift equations and the results of simulation, and finally conclude that the dune with berm has the best protection against wave erosion. But they didn’t conduct any investigation about the internal flow field and vortex of this kind of sand dune. So this paper is to present the characteristic of flow field about the forward and backward of this type of sand dune, and show the temporal and spatial distribution of flow filed and vortices.
The experiment is conducted in a medium-sized glass tank at Tainan Hydraulics Laboratory. The sand dune model is made of acrylic material to understand the change of hydrodynamic mechanism when the water wave overtops the sand dune. Because the top layer the sand dune is too thin, it can only take pictures from the forward and backward of the sand dune. Yet, this experiment tries to get the clearest particles within the maximum range of our target. And this experiment choses a mature wave with a complete wave period to get substantial analysis. This experiment uses an optical gate to control the camera and wave gauges synchronously to get the image data. After getting the image data, we analyze the data by using the canny method to detect water surface line to get the internal flow field we are interested in. Besides, this experiment repeated 20 tests in every wave period test and shooting area to get average velocity with ensemble average method. The variation of surface elevation is less than 0.3% in every test.
This research shows the temporal and spatial distribution of flow filed and vortices in front slope and back slope of BD model in five kinds of period with 5 cm deep sea wave high. All kinds of waves of front slope of the BD model runs up with lots of bubbles, and the flow field is chaotic so as to set apart while moving, which results in stagnation in the model bed. The short wave of the rear part of the BD model produces violent current and vortex in the surface of the liquid. As to long wave, besides the dramatic change in the speed and vortex of the surface liquid, there is obvious and deeper overwash in the bottom part, which leads to diverse direction of vortex. To sum up, this report presents the changes and calculation of flow field and vortex of wave in various periods.

1. Jens Figlus, Nobuhisa Kobayashi, Christine Gralher, and Vicente Iranzo, “Wave Overtopping and Overwash of Dunes”, Journal of Waterway, port, coastal, and ocean engineering, ASCE., 2011.
2. Steven A. Hughes, and Justin M. Shaw, “Continuity of Instantaneous Wave Overtopping Discharge with Application to Stream Power Concepts”, Journal of Waterway, port, coastal, and ocean engineering, ASCE., 2011.
3. Kuang-An Chang and Philip L.-F. Liu, “Experimental investigation of turbulence generated by breaking waves inwater of intermediate depth”, Journal of Waterway, port, coastal, and ocean engineering, ASCE., 2011.
4. Hans Hanson, Magnus Larson, and Nicholas C. Kraus, “Calculation of beach change under interacting cross-shore and longshore processes”, Coastal Engineering 57, 610–619, 2010.
5. Ana Matias , Ana Vila-Concejo , O´scar Ferreira, Brad Morris, and Joao Alveirinho Dias, “Sediment Dynamics of Barriers with Frequent Overwash”, Journal of Coastal Research, 25(3), 768–780. West Palm Beach (Florida), ISSN 0749-0208.
6. Ana Matias , Ana Vila-Concejo , O´scar Ferreira, Brad Morris, and Joao Alveirinho Dias, “Short-term morphodynamics of non-storm overwash”, Marine Geology 274 , 69–84, 2010.
7. Nobuhisa Kobayashi, Ali Farhadzadeh, Jeffrey Melby, Bradley Johnson, and Mark Gravens Source, “Wave Overtopping of Levees and Overwash of Dunes”, Journal of Coastal Research, Number 265:888-900. 2010.
8. Jens Figlus, Nobuhisa Kobayashi, Christine Gralher, and Vicente Iranzo, “WAVE-INDUCED OVERWASH AND DESTRUCTION OF SAND DUNES”, Journal of Waterway, port, coastal, and ocean engineering, ASCE., 2011.
9. J.M. Chyan and H. H. Hwung, (1993), On the interaction of a turbulent jet with waves, Journal of Hydraulic Research, IAHR. Vol. 31, No. 6, pp. 791-810.
10. 許文陽，「溢波碎波帶之內部流場研究」，國立成功大學水利及海洋工程研究所碩士論文，2006。
11. 林呈，「應用流場可視化法及LDV探討斜坡上波動內部流場及底部邊界層之特性」，國立成功大學水利及海洋工程研究所博士論文，1989。
12. 張振禎，「DIP、PIV及BIV應用於複合式平台之碎波流場特性研究」，國立成功大學水利及海洋工程研究所碩士論文，2010。
13. 張振禎、謝志敏、歐善惠、許泰文、林建鋒、鄭詠翰，「波浪通過人工潛礁變形實驗量測與數值模擬」，第三十一屆海洋工程研討會，基隆，pp. 219-224，2010。
14. 黃志誠，「碎波帶動力及紊流特性之試驗研究」，博士論文，國立成功大學水利與海洋工程研究所，2010。