THE RESPONSE OF A COASTAL REGION WITH IRREGULAR REEF BATHYMETRY TO STRONG GRAVITY WAVE FORCING - A NUMERICAL MODELING STUDY

Computations with a high-resolution, nonlinear numerical Boussinesq model (BOSZ¹) are used to explore the nearshore ocean’s response to highly energetic sea/swell wave forcing along the North Shore coast of O’ahu Island. Model output and observations of sea level and currents are found to be in good agreement, as the model is able to reproduce the correct spectral levels and capture much of the observed spectral details. This modeling effort helps to (i) better understand the infragravity (IG) wave response of a small harbor to strong sea/swell wave forcing; and, (ii) improve wave runup forecasts for reef-protected and un-protected shorelines. The simulations demonstrate the generation of strong currents oscillating at IG periods from 0.5 to 17 minutes. The highest IG current amplitudes close to shore occur at the shortest periods, while near the outer edges of several deep channels the strongest currents occur at the longest IG periods. The simulations reveal standing IG wave patterns modified by coastal morphology, with alongshore symmetry varying with frequency. Such standing wave patterns result in strong currents near the nodal lines in sea level amplitude. The investigated wave and current processes are typical for tropical and subtropical islands in the Pacific and elsewhere and help to improve efforts on coastal hazard mitigation and mariners’ safety.

 ¹BOSZ - Boussinesq Ocean and Surf Zone (Roeber and Cheung, 2012)