Infragravity waves at the Hambantota port located in the North Indian Ocean

Abstract

Infragravity (IG) waves are the primary cause of harbor oscillations. The Hambantota port located in the North Indian Ocean is taken as an example to study the IG waves in the vicinity and interior of a complex-shaped real harbor and to reveal the energy transfer of IG waves toward the harbor. The research employs a combination of in-situ wave measurement and numerical simulations conducted using the spectral wave model WAVEWATCH III. The latter could provide a long-term estimation of the level of IG wave energy over a broad geographical region, though it cannot reflect the complex mechanism of IG waves near/at the shore. Results indicate that, at the sensor near the harbor entrance, the free IG wave heights HFIG generally exceeds the bound IG wave height HBIG. However, during energetic sea states (significant wave height HS > 2.5 m in this study), HBIG increases significantly and occasionally exceeds HFIG. The spatial distributions of HS, HBIG, and HFIG indicate that the narrow continental shelf surrounding Sri Lanka has a marginal effect on attenuating swell energy. Consequently, the Hambantota port experiences a substantial impact from both swell and accompanying IG waves. If solely considering the maximum acceptable HS, the downtime for large ships would be greatly underestimated. The process of wave propagation into the harbor enables the energy transfer from bound IG waves to free IG waves that finally excite harbor oscillations. Albeit the tidal range is low, the tidal level has a noteworthy impact on harbor oscillations. The IG spectral density during low tides is generally greater than that during high tides due to the enhanced nonlinear interactions of short waves under low-tide conditions.