Numerical modeling of harbor oscillations induced by typhoon-generated swell waves: a pre-post study of harbor modification

Typhoons pose significant threats to harbors and coastal infrastructures through intense waves that can cause direct damage. Even typhoons located at a distance can generate swells that propagate into harbors, leading to harbor oscillations. Such oscillations cause moored ship motions with consequences for the operational downtime of dock operation. This study focuses on an offshore liquefied natural gas port in the southwestern Taiwan, Yongan Harbor, completed in 1990. Since its completion, the harbor has faced typhoon-generated swells, induced long-period wave oscillations and overtopping, thereby compromising dock operation safety. To mitigate this problem, additional breakwaters were constructed in 2005 to narrow the harbor entrance for better protection. However, instead of improving harbor tranquility, this modification unexpectedly prolonged wave oscillations and amplified wave amplitudes, as reported by longshoremen, though no field data available to confirm this phenomenon. To investigate the wave characteristics, a wave-resolving model is applied to simulate the spatial distribution of swells and infragravity waves before and after the harbor modifications. The model results indicated that waves concentrated energy along the breakwater walls and corners, intensifying harbor oscillations and affecting the safety and efficiency of port operations. Further simulations of typhoon-induced swells revealed that the primary oscillation periods within the harbor remained unchanged before and after the new constructions. However, the post-construction harbor configuration amplified the energy of these wave periods within the harbor. This amplification shows that while the harbor modifications aimed to enhance protection, they inadvertently intensified specific natural resonance periods, leading to increased oscillation durations. The findings highlight the importance of comprehensive wave hydrodynamics in harbor design and modification. It is crucial to consider not only the immediate protective benefits to narrow the harbor entrance but also their potential to alter wave patterns within the harbor. Future harbor designs should integrate advanced simulation models to predict and mitigate adverse oscillation effects, ensuring dock safety and operational efficiency.