Toward nearshore, bathymetry induced wave amplification in False Bay, South Africa

A seamount called Rocky Bank (RB) has been suspected to spatially focus wave energy toward the eastern periphery of False Bay, South Africa, where it is not uncommon for fishermen to get washed off by waves. Situated at the bay's mouth, the submerged seamount acts like a refracting lens for incoming long-crested swell. The present study aims at understanding the wave transformation effects of RB and the nearshore bathymetry using the phase-averaging spectral wave model simulating waves in the nearshore. Open ocean conditions around the Cape Peninsula, derived from National Center for Environmental Prediction reanalysis products, were used as model boundary conditions. To establish RB's influence, a bathymetry without RB was created. The model output indicated that coastal significant wave heights were 9% greater when RB was removed. This difference was largest for smaller incident significant wave height and increasing incident peak period. RB also causes wave convergence via refraction and presents evidence for spatial focusing, a coincidental interference, extreme wave generating mechanism. This result, together with recorded incidents and anecdotal evidence, suggests that the wave transformation effects of RB may be leading to extreme waves along the eastern periphery. Evidence for spatial focusing toward the eastern periphery occurred for peak wave directions between 210 degrees and 245 degrees true north and for peak periods greater than 13 s. The likelihood of a wave leading to an incident is greater in smaller, long-crested seas based on the recent literature for the region. To extend this study to predict rogue wave events within False Bay, a phase-resolving model would be required. (c) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

The study found that Rocky Bank (RB) influences extreme wave occurrences along the eastern periphery of False Bay in South Africa, where wave energy may spatially converge around the seamount and cause wave convergence through refraction. In addition, evidence from recorded incidents and anecdotal evidence suggests that the wave transformation effects of RB may lead to extreme waves in the region.