Study on the Cross-Shore Sedimentation for Coarse Grain Size Beach

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Project title

Code/計畫編號

NSC96-2221-E006-331-MY3

Translated Name/計畫中文名

粗粒徑底床質向離岸輸砂特性及輸砂量推估之研究

Project Coordinator/計畫主持人

Tai-Wen Hsu

Funding Organization/主管機關

National Science and Technology Center for Disaster Reduction

Department/Unit

Department of Hydraulic and Ocean Engineering,NCKU

Website

https://www.grb.gov.tw/search/planDetail?id=1751346

Year

2009

Start date/計畫起

01-08-2009

Expected Completion/計畫迄

31-07-2010

Bugetid/研究經費

778千元

ResearchField/研究領域

土木水利工程

粗粒徑底床質向離岸輸砂特性及輸砂量推估之研究本計畫擬透過水工模型試驗探討不同粒徑、比重之底床質在波浪作用下之向、離岸方向輸砂特性及其輸砂量推估，以補足過往多著重在粒徑0.1~1.0mm 砂質海灘而對粗粒徑(1.0mm 以上粗砂、卵礫石海岸)底床輸砂研究之缺乏。由於礫石底床因孔隙率高，其滲透性較砂質海岸大，波浪於灘面溯上過程中，水體自孔隙間滲漏而使得灘面回流下刷力較小，再者粗顆粒底床質受波浪作用之起動、安定條件及運動行為亦與細砂明顯不同，因此粗粒徑之礫石海灘不論是型態或坡度皆與砂質海灘明顯不同。據此，本計畫擬進行水工模型試驗探討粗粒徑底床質向離岸方向漂砂海灘地形變化特性，供斷面地形變動數值模式之用。計畫共分三年進行。第一年將利用於波動水槽中配合質點影像追跡法 (PIV) 量測水槽中規則波浪通過不同管徑組成之透水斜坡底床模型之流場，同時在試驗中並利用容量式波高計量測孔隙底床之波浪變化特性，藉由觀測波浪通過透水孔隙底床之波高變化及水粒子運動變化機制，以瞭解輸砂動力於透水性較高之粗粒徑底床之特性，並提供數值模式校核。第二年將將利用各種不同粒徑之均勻底床質於大型斷面波動水槽中探討粗粒徑底床質之地形變化、運動型態、影響厚度並建立與粒徑相關之向離岸方向輸砂公式，以建立粗粒徑底床之輸砂模式。第三年將承繼前兩年度有關粗粒徑透水底床波流及輸砂模式研究之成果，進行卵礫石海岸地形特徵之參數模式研究，並探討粗粒徑底質之海灘地形變遷特徵。綜合三年之成果，探討粗粒徑底床質海岸在各種波浪條件作用下之運動行為及輸砂量關係，藉以掌握粗粒徑海岸地形變動機制，並從成果釐清與現有細粒徑砂質漂砂量特性及地形變動之異同，其研究成果可提供工程單位對卵礫石海岸地形變遷機制有進一步認知及裨益海岸防護工程規劃參考。本計畫為台灣與日本名古屋大學之共同研究計畫，配合日本方面對透水性孔隙底床波流場及地形變動模式之開發，可加速對礫石海灘地形變遷機制及模式預測之研究。 Study on the cross-shore sedimentation for coarse grain size beach In recent years coastal erosion is one of the largest serious problem from the perspective of land preservation. Coastal erosion often results in inherent damages in living the coast. As typhoon comes accompanied with storm surge attack the shore, the cross-shore sediment transport is dominant that causes the shoreline migrates landward, destroying homes built and properties located too close to the sea. Taiwan is an island having a coastal length around 1139km and more than half of the sandy beach are suffering from coastal erosion. It is desirable to conduct some proper protective countermeasures of the coast. The changes of beach profile are the main characteristics coast brought about by the cross-shore sediment transport. This is particularly apparent in the variation of a bar profile with the increase of wave energy. The estimation of cross-shore sediment transport and its corresponding net transport is if great importance to determine the beach profile changes. The cross-shore sediment transport rate relates to the local wave conditions, superimposed currents and sediment grain sizes. The well-known formulas for calculating the cross-shore sediment transport rate under oscillating flow are based on experiments or fine grain sizes. However, some beaches have field data for the coarsest grain sizes and these models have not been accurately accounting for the cross-shore sediment transport. This is due to the fact that the wave energy attenuation in shallow water caused by high porosity is difficult to move coarse sediment particles. It is not easy to derive reliable formulas of cross-shore sediment transport for coarse sediments. Here we will investigate the cross-shore sediment rate for different grain sizes from the flow field and beach profile evolution from laboratory experiments. A series of small and large wave-flume experiments will be conducted to detect the water-particle velocities and beach profile changes for a given grain size. The PIV (Particle Image Velocimetery) is employed to measure the flow velocity in the vicinity of grain bed. The equation of continuity provides an useful tool for converting beach profile data into the estimation of cross-shore sediment transport. This task is a 3 year project to achieve our goal.

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Study on the Cross-Shore Sedimentation for Coarse Grain Size Beach

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