The Role of Infragravity Waves in Near-Bed Cross-Shore Sediment Flux in the Breaker Zone

Samantha Kularatne,Charitha Pattiaratchi

doi:10.3390/jmse2030568

Samantha Kularatne, Charitha Pattiaratchi

Open Access

https://doi.org/10.3390/jmse2030568

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Abstract

Results from a series of field experiments, conducted to investigate the influence of infragravity waves (from wave groups), ripple type and location relative to the breaker line on cross-shore suspended sediment flux close to the sea bed in nearshore environments, are presented. The field data were collected from Cable Beach (Broome) and Mullaloo Beach in Western Australia and Chilaw in Sri Lanka. These beaches experience different incident wave, tidal and morphological conditions, with Cable Beach having a 10-m spring tidal range, whilst the other two beaches have tidal ranges <1.0 m. Measurements included simultaneous records of surface elevation, two-dimensional horizontal current velocities and suspended sediment concentrations, together with half-hourly observations of the seabed topography. Although most of the data sets were obtained just outside of the surf zone, a few results from inside of the surf zone were also included. A significant correlation between wave groups and suspended sediment concentration was found at all of the measurement sites, either with or without bed ripples. The direction and magnitude of cross-shore suspended sediment flux varied with location with respect to the breaker line; however, other parameters, such as bed ripples and velocity skewness, could have influenced this result. In Broome, where the measurement location with respect to the breaker line varied with the tidal cycle, the cross-shore sediment flux due to swell waves was shoreward inside and just outside of the surf zone and seaward farther offshore of the breaker line. Further, sediment flux due to swell waves was onshore when the seabed was flat and offshore over post-vortex ripples. Sediment flux due to swell waves was onshore when the normalised velocity skewness towards the shore was high (positive); the flux was offshore when the skewness was lower, but positive, suggesting the influence of other parameters, such as ripples and grain size. The net cross-shore sediment flux was onshore when the Dean number was less than 1.67 and offshore when the Dean number was greater than 1.67. Nevertheless, the Dean number did not account for the influence of ripples or velocity skewness. The cross-shore sediment flux at the infragravity frequency was mainly offshore outside of the surf zone, whereas it varied between onshore and offshore inside of the surf zone.

Highlights

With rising global sea levels and rapidly increasing population densities along coastal stretches, coastal stability has become a major issue for coastal communities and managers
This paper describes results obtained through a series of field measurements undertaken in different nearshore environments under various conditions, such as differing tide, grain size, bed geometry and cross-shore location
A series of field measurements of hydrodynamics and sediment suspension, together with bed topography, was collected at several nearshore locations to investigate the influence of infragravity waves, ripple type and location relative to the breaker line on cross-shore suspended sediment flux close to the bed

Summary

Introduction

With rising global sea levels and rapidly increasing population densities along coastal stretches, coastal stability has become a major issue for coastal communities and managers. It has been noted that longshore transport is predominantly due to the mean motion (mean over several wave periods) [4], whereas a range of mean (tides, undertow and rip currents) and oscillatory components (wind waves, swell, wave groups and infragravity oscillations) drives cross-shore transport [5]. Each of these frequency components uniquely influences the direction and magnitude of cross-shore sediment flux under different conditions [6]. An improved understanding of the processes of sediment re-suspension and flux due to the different oscillatory components is essential to predict cross-shore sediment transport and, coastal stability, accurately

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