LARGE SCALE EXPERIMENTS ON BEACH EVOLUTION INDUCED BY BICHROMATIC WAVE GROUPS WITH VARYING GROUP PERIOD

Jose M Alsina,Jan S Ribberink,Joep Van Der Zanden,Ivan Caceres,Tom E Baldock

doi:10.9753/icce.v34.sediment.3

Jose M Alsina, Jan S Ribberink + Show 3 more

Open Access

https://doi.org/10.9753/icce.v34.sediment.3

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Abstract

New large scale experimental data have been presented showing the wave group influence on beach morphodynamics at the surf and swash zones. Bichromatic wave conditions were generated varying the modulation bandwidth but keeping the wave energetic content constant within the experimentation limits. The wave group influence in the surf zone is observed in the form of the cross-shore location of the breaker bar respect to the initial still water level (SWL) location, which has been shown to increase as increases the wave group period. This influence is explained in terms of differences on the surf zone width induced by the varying wave group periods. In the swash zone, time dependent bed level elevation measurements were done using a newly developed conductivity technique, the CCM+ system. Bed level time variations at the swash zone have shown to be composed of a long scale trend and bed level oscillations of shorter frequencies related to the wave group forcing. A good spectral correlation has been found between the water surface elevation and bed level variation at the wave group period for every bichromatic component indicating an important influence of wave groups on the swash zone morphodynamics.

Highlights

Wave grouping and associated bound long waves have been studied to be important for coastal zone sediment transport for many years (Butt and Russell, 2000; O'Hare and Huntley, 1994)
As explained in the Introduction, the main aims of the present experiments were to investigate the role of wave groups and associated bound long waves in the hydrodynamics, sediment transport and beach evolution
New large-scale experimental data have been presented on the influence of wave groups on the beach profile evolution and bed level changes in the swash zone

Summary

Introduction

Wave grouping and associated bound long waves have been studied to be important for coastal zone sediment transport for many years (Butt and Russell, 2000; O'Hare and Huntley, 1994). It is currently assumed that most of the beach evolution and sediment transport is induced by short-scale wind and swell waves, the morphological changes are based on sediment transport gradients that can vary over long-wave time scales (Baldock et al, 2011). The convergences in wave energy density are inherent to the wave groups influencing the shoaling and wave breaking process in the surf zone. Wave groups introduce further unsteadiness and intermittency into the short-wave processes that force sediment transport, i.e. wave-induced currents and turbulence. The hydrodynamics in the inner surf and swash zones is largely affected by infragravity frequencies, when dissipation influences the sea swell wave energy range more than the infragravity energy range. Dissipation of steeper waves at sea swell frequencies occurs mostly due to breaking in shallow water with energy transfer to gently sloping infragravity waves, for which breaking is less likely to occur

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