Large‐scale laboratory study of breaking wave hydrodynamics over a fixed bar

Dominic A Van Der A,Joep Van Der Zanden,David Hurther,Jan S Ribberink,Stuart J Mclelland,Iván Cáceres,Tom O'Donoghue

doi:10.1002/2016jc012072

Abstract

AbstractA large‐scale wave flume experiment has been carried out involving a T = 4 s regular wave with H = 0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at 12 locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time‐averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area, TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar‐trough geometry, which enables larger length scales and time scales of breaking‐generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow‐related flux, whereas the wave‐related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates.

Highlights

Van Rijn et al [2013] identified the surf zone as an important area where our understanding and predictive capability of sediment transport remains poor
Suspended sediment dynamics around the breaker bar in the outer surf zone, near-bed sediment dynamics in the wave boundary layer under breaking waves, vertical structure of sediment dynamics in the inner surf zone, and coherent flow structures and intermittent sediment stirring under breaking waves were highlighted as important topics for future research
The rather long horizontal section was chosen in order to ensure that bed slope effects in the inner surf zone or swash zone processes did not affect the hydrodynamics around the bar

Summary

Introduction

Van Rijn et al [2013] identified the surf zone as an important area where our understanding and predictive capability of sediment transport remains poor. Suspended sediment dynamics around the breaker bar in the outer surf zone, near-bed sediment dynamics in the wave boundary layer under breaking waves, vertical structure of sediment dynamics in the inner surf zone, and coherent flow structures and intermittent sediment stirring under breaking waves were highlighted as important topics for future research. Improving this understanding of the sediment dynamics throughout the surf zone requires a detailed knowledge of the intrawave hydrodynamics and turbulence structure under breaking waves. Turbulence dissipation rates decrease with distance from the free surface [George et al, 1994; Feddersen et al, 2007], which indicates that wave breaking is the dominant source of turbulence dissipation [Grasso et al, 2012]

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Discussion

Conclusion