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

Nicholas Salonen,Christo Rautenbach

doi:10.1063/5.0055734

Abstract

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° and 245° 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.

Highlights

False Bay is situated at the south-east coast of Cape Town, found at the south-western coast of South Africa
The eastern arm of False Bay is haunted by their crosses.”3 Some of these incidents have been attributed to extreme wave events, which are known to wash users off of the rocky eastern periphery of False Bay, earning it the nickname “Death Coast.”4 Similar accidents have been recorded in Australia, with rock fishing being classified as one of Australia’s most deadliest sports between 2004 and 2017.5 in an Australian case study, there is a debate scitation.org/journal/adv whether rogue waves are responsible for a majority of such events or if inexperienced rock fishermen are lured into a false sense of security during small, longer period swells
The wave transformation is characterized by wave shoaling over Rocky Bank, a reduction in wave heights in the lee of Rocky Bank (Hm0_coast) and converging wave energy due to wave refraction caused by Rocky Bank

Summary

Introduction

False Bay is situated at the south-east coast of Cape Town, found at the south-western coast of South Africa. The coastal area of False Bay (from Cape Point to Cape Hangklip) attracts both domestic and overseas tourists who contribute an estimated R 900 × 106 (US$ 63 × 106) per annum in direct expenditure with ∼80% and 14% being generated by the Cape Peninsula and the Kogelberg coast, respectively.. The eastern arm of False Bay is haunted by their crosses.” Some of these incidents have been attributed to extreme wave events, which are known to wash users (especially rock fishermen) off of the rocky eastern periphery of False Bay, earning it the nickname “Death Coast.” Similar accidents have been recorded in Australia, with rock fishing being classified as one of Australia’s most deadliest sports between 2004 and 2017.5 in an Australian case study, there is a debate scitation.org/journal/adv whether rogue waves are responsible for a majority of such events or if inexperienced rock fishermen are lured into a false sense of security during small, longer period swells.

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Conclusion