On the Use of Directional Wave Spectra to Identify Distant Swells Approaching a Pacific Atoll

Laura Cagigal1, Ana Rueda2, Alba Ricondo2, Giovanni Coco1 and Fernando Javier Méndez Incera2, (1)University of Auckland, School of Environment, Auckland, New Zealand, (2)University of Cantabria, Ciencias y Tecnicas del Agua y del Medio Ambiente, Santander, Spain
Understanding the generation areas and the local sea states generated from the wave directional spectra is a key issue in the low-lying nations of the Pacific area. Distant swells are the main cause of coastal flooding as, after travelling thousands of kilometres (Snodgrass et al., 1966), its dissipation generates infragravity waves that increase the impact of the local sea waves when they reach the coast. ESTELA (Evaluating the Source and Travel-time of the wave Energy reaching a Local Area, Perez et al., 2014) is an available methodology used to define the wave generation area and travel time to any location. Nevertheless, this methodology is not able of identifying all the different swells that are simultaneously present in a directional spectrum, that can be up 10 at some instants and locations.

The aggregation of the full spectrum into single parameters such as significant wave height (Hs), peak wave period (Tp), and mean wave direction (Dir) often results in the loss of a significant amount of detail related to energy propagation. For this reason, what we propose is the analysis off the spectral wave energy arriving towards Majuro atoll with the aggregation of the information from 4 different points surrounding the atoll into a central “super-point”. With the aggregated information we perform a swell partitioning using the methodology proposed in Hanson and Philips (2001) in order to track each individual swell over the ocean basin (Portilla et al., 2015), and link them with its generating pressure field. The aim is to be able of reproducing swell behaviour over time by using the relationship found between the weather and the enhancement and decay parameters that characterize the shape of each individual swell in terms of Hs, Tp, Dir and directional spreading.