摘要
When wave energy builds up, beach berm and even dunes are quickly eroded with this material being removed offshore to form a bar parallel to the beach. It proceeds seawards during the course of the storm to reach some ultimate location prior to being dismantled and moved back to the beach by subsequent swell. Wave heights and wave periods vary continuously during the storm. There is difficulty in defining the steepness associated with storm, although it has commonly been accepted that steeper waves often produce a bar or a barred beach bar position and its crest height for a given storm condition, such as wave period, wave height, duration, etc. Therefore, criteria for offshore and onshore migration direction, i. e. for the formation of bar and berm profiles respectively, need to be clarified. In this paper, the available experimental data of large wave tank (LWT) from U.S. Army Crops of Engineering and Japanese Central Research Institute of Electric Power Industry are used to construct a new relationship. Finally, a criterion for bar and berm formation is found, and new criterion has a quite simple form and widely applicable range.
並列摘要
海岸砂丘在形成及退化過程中，常受波浪能量大小而改變。當波能大時，沙丘受暴風浪（storm）之能量而冲蝕並帶到深海處形成平行於海灘的砂洲排列。而俟暴風雨過後，再由定常的浪濤（swell），將部份砂洲再帶上岸形成砂丘。雖然一般常以波浪之峰度（steepness of waves）來判定砂洲或砂丘，但似乎這不是唯一可靠之因素。身爲海岸工程師者，不但需要明白海灘之侵蝕極限、砂洋最遠位置、已知暴風下之砂洲波高，更須要明白，在已知的海灘條件及海浪現象下，砂洲與砂丘及海灘變遷之判定與區分，俾便作爲海岸保護工程之參考依據。本文中運用美國工兵團（CE）之大波浪槽實驗（LWT）及日本工業電力研究中心（CRIEPI）之實驗資料，建立一組簡單而且實用之判定準則，來區分砂洲與砂丘。更進一步，可利用本準則來檢定︰（1）現有海灘狀況，可承受多大一次頻率之暴風浪而不受冲蝕；及（2）在已知的波浪狀況下，現有海灘是否會受冲刷。