Hurricane-generated ocean wave propagation and impacts on a fringing reef in Hawai’i

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Zimmerman, Zoe
surface waves , coastal engineering , hurricanes , numerical modeling
The Pacific Ocean surrounding the Hawaiian Islands is a region subject to strong storm events. Hurricane Hector and Hurricane Lane, successive offshore storms, had strong winds that generated large waves and impacted the islands in the summer of 2018. This study aims to investigate the ocean surface wave response to hurricane wind forcing across the ocean at a large scale and to understand the finer-scale processes of energy dissipation and the corresponding wave-driven circulation as waves break over a steep fringing reef. Hydrodynamic and wave modelling frameworks are used to develop two numerical models using SWAN and Delft3D-SWAN at different scales to investigate the ocean and coastal responses to the two hurricanes. In the first part of this study, a large-scale model that encompasses the entire Hawaiian Archipelago is developed and the surface waves generated using four different large-scale spatially varying wind fields are compared. These include a detailed atmospheric model input (CFSv2), a simple parametric hurricane wind model, the parametric model blended with CFSv2 winds, and this blended product with a limited maximum wind speed. The results indicate that the blended and limited wind field produces wave model predictions in the best agreement with bulk wave statistic observations at several ocean wave buoys. The model provides valuable results for the large-scale propagation of hurricane winds and waves, but the relatively low-resolution and large scale limits the use in providing detailed predictions at coastal observation sites. In the second part of this study, a fine-scale coupled hydrodynamic-wave model is developed for the south coast of Moloka’i. This area is comprised of a fringing coral reef and numerical model grids were developed to resolve the detailed reef morphology with high resolution. Results from the two energetic ocean wave events generated by the distant passing hurricanes were validated with wave and current measurements collected at multiple observation stations deployed by the US Geological Survey across a 1200 m wide cross-reef transect. The model, in combination with analysis of the field observations, was used to evaluate wave breaking over a steep reef face and the corresponding wave-driven energy dissipation and circulation to provide predictions of nearshore processes resulting from wave events generated by offshore hurricanes.
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