Investigations on the relationship between group waves and infragravity waves in the Hualian Harbor based on Boussinesq wave model

• Main Authors: Tzu-Li Yang, 楊自立
• Other Authors: Cheng-Han Tsai
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/36167432898014110088

碩士 === 國立臺灣海洋大學 === 海洋環境資訊學系 === 100 === It is well known that there are infragravity waves in the Hualien Harbor during the passage of typhoons. The infragravity waves may cause cable breakage for the moored ships in the harbor. Past physical experiments and numerical simulations have been conducted by many researchers, trying to understand the cause of the Infragravity waves inside the harbor. Group wave is one of the possible sources for the infragravity waves, and the waves from offshore typhoons often consist of group waves. In this study, we used the Boussinesq model developed by Danish Hydraulic Institute (MIKE21-BW) to investigate whether there is a connection between group waves outside the harbor and the infragravity waves inside the harbor. The infragravity waves can be triggered by the set-down (or set-up) bound to the wave groups. After analyzing the wave records measured outside the harbor during the passage of the Typhoon Haitang in 2005, we found two peak wave periods in the spectrum that can be combined into group waves. We first used the combined group waves and then measured waves as the incident waves to the wave model. Results showed that both simulated waves inside the harbor are similar, and they also agreed with the measured waves inside the harbor. Then we filtered the waves measured outside the harbor into one contains waves with period less than 30s and one with period longer than 30s. These two wave trains were used as incident waves to the BW model separately. The results showed that the typhoon waves with period less than 30 second cause higher infragravity wave energy inside the harbor than using typhoon waves with only waves longer than 30s. To mitigate the long waves inside the harbor, we found that, using measured typhoon waves as incident waves, the infragravity wave energy inside the harbor can be reduced, if the porosity layers are added to the harbor structures such as wharves and breakwaters. Also if the offshore breakwaters were added to the beach outside the harbor entrance, the simulation showed that the long wave energy inside the harbor can also be reduced. Key words : Boussinesq model, infragravity waves, offshore breakwater, porosity layers