Maximal Heights of Nearshore Storm Waves and Resultant Onshore Flow Velocities

Nans Bujan,Rónadh Cox

doi:10.3389/fmars.2020.00309

Abstract

Storm waves, after breaking or overtopping, generate strong onshore flows that do significant mechanical work, including eroding and transporting large boulders. The waves can be amplified on approach, and the flows themselves may be further intensified by local topographic effects. These processes are currently poorly parameterised, but are of great importance for understanding the interactions between waves and coasts. We present a highly generalised equation for estimating maximal coastal wave heights and consequent onshore flow velocities. Although very approximate, this method contains no embedded assumptions, and thus provides a more realistic first-order check of storm wave capabilities than previous approaches. Initial analysis suggests that amplified waves may generate maximal onshore flow velocities up to six times the phase speed of the offshore waves. Although the probability of occurrence in any given storm is very low, the possibility of such extreme values cannot be ignored, especially when interpreting ancient deposits of large boulders. The equations presented here can be used as a first-order test for coastal boulder deposits currently interpreted as tsunami deposits, to evaluate whether a storm-wave origin should be reconsidered. This approach could also be employed at coasts in general, to evaluate long-term probabilities of damaging flows, as a component of coastal risk analysis.

Highlights

The behaviour of large waves as they approach steep coasts is one of the great unknowns in marine science
The most widely applied approach in coastal boulder studies is that of Nott (1997, 2003b), which has driven thinking about likely heights and transport capacities of coastal wave flows, so we address it in detail here
Subsequent studies incorporated Nott’s (1997; 2003b) approach in a range of equations attempting to relate flow speeds to wave heights. Such equations are often employed to hindcast wave heights for both modern and ancient coastal boulder deposits. They have frequently been used to argue that coastal boulders could not have been emplaced by storm waves, because the equations call for waves too large relative to measured or inferred Hs (e.g., Kennedy et al, 2007; Mastronuzzi et al, 2007; Scicchitano et al, 2007; Barbano et al, 2010; Boulton and Whitworth, 2017; Roig-Munar et al, 2019b)

Summary

Introduction

The behaviour of large waves as they approach steep coasts is one of the great unknowns in marine science. Collapse on the coastal platform or cliff top yields a bore with velocity Ubore, which may be focussed and accelerated by topography, generating high-energy flows (Umax) with the capability of moving large boulders.

Results

Conclusion