Investigation on the Generation of Infragravity Waves Using Group Waves as Sources

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Project title

Code/計畫編號

NSC97-2221-E019-041-MY3

Translated Name/計畫中文名

群波為亞重力波來源之研究

Project Coordinator/計畫主持人

Cheng-Han Tsai

Funding Organization/主管機關

National Science and Technology Council

Department/Unit

Department of Marine Environmental Informatics

Website

https://www.grb.gov.tw/search/planDetail?id=1988433

Year

2010

Start date/計畫起

01-08-2010

Expected Completion/計畫迄

31-07-2011

Bugetid/研究經費

557千元

ResearchField/研究領域

土木水利工程

每年夏季都會受到颱風侵襲，當颱風侵襲時花蓮港因港外亞重力波由港口進入港區內，會造成港池發生共振的現象。國內不少研究探討此問題。大部份研究認為共振週期主要在100-200秒間。其振幅放大係數亦常被討論。至於港外亞重力波來源則還未有定論。最近邊緣波曾被認為可能是亞重力波來源，但此說法很難用實測資料證實。本計畫擬探討另一可能來源。外海傳來之颱風波浪常含有群波，而群波已被認為會造成海灘的surf beat，而surf beat已知會造成臨近港口之振盪。有人認為當群波到達海灘時其所帶之bound long wave會因碎波而被釋放，再受海岸之反射回外海成長週期leaky wave。另有研究認為群波傳至近岸其set-down wave即可造成港池振盪。本計畫將以三年時間探討颱風波浪之群波是否為花蓮港港池振盪之波源。第一年將分析港灣研究所在蓮港港外之颱風波浪，找出會造成港內共振之群波特性。並探討群波之振幅與週期和港內共振波之關係。第二年將用Boussinesq type模式探討港外群波與共振之問題。當對造成港池振盪之群波特性有了解後，進而可用上述模式探討港南若設置離岸堤是否可降低群波造成之共振。Resonance in the Hualien Harbor has been studied for quite some time. Most studies seem to find the resonant periods are between 100-200 s, and amplitude amplification in this range has been investigated by various means. However, the source of infragravity wave that excites the resonance in the harbor has yet to be thoroughly studied. Recently, edge wave has been suggested to be the source of the resonance in the harbor, but it is difficult to verify in the field measurements. This study will look at another likely source of the harbor resonance. Storm generated waves often come in groups. Shoring of group waves has been known to generate surf beats. It has been suggested that surf beats are one of the reasons for harbor surges. Other studies showed that breaking of group waves release their bound long wave, which is reflected toward offshore as leaky wave. Even shoring of group waves on the beach without breaking has been shown to generate infragravity wave, which excites resonance at the harbor nearby. Hence this three-year study investigates whether group wave is a source of the resonance at the Hualien Harbor during typhoon season. In the first year, field data measured outside and inside the Hualien Harbor during past typhoons are examined for their group wave properties, e.g. period and amplitude. These properties are also investigated for their relation with the resonant period and amplitude for available data measured inside the harbor. Boussinesq type model are used in the second year for further investigation on the group waves on the resonance of the harbor. The model used is MIKE 21 BW, which has been shown to simulate harbor resonance as well as surf beat at the near shore successfully. In the final year, simulations with different configurations of offshore jetties are tested for their effect in reducing the effect of incident group wave, from various directions, on the resonance of Hualien Harbor.

Keyword(s)

群波
颱風波浪
群波參數
波浪包絡線
BoussinesqBoussinesq模式
離岸堤
Group waves
typhoon waves
groupiness parameters
wave envelope
Boussinesq wave model
offshore jetties

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Investigation on the Generation of Infragravity Waves Using Group Waves as Sources

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