Near‐Bed Turbulent Kinetic Energy Budget Under a Large‐Scale Plunging Breaking Wave Over a Fixed Bar

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

doi:10.1002/2017jc013411

Abstract

AbstractHydrodynamics under regular plunging breaking waves over a fixed breaker bar were studied in a large‐scale wave flume. A previous paper reported on the outer flow hydrodynamics; the present paper focuses on the turbulence dynamics near the bed (up to 0.10 m from the bed). Velocities were measured with high spatial and temporal resolution using a two component laser Doppler anemometer. The results show that even at close distance from the bed (1 mm), the turbulent kinetic energy (TKE) increases by a factor five between the shoaling, and breaking regions because of invasion of wave breaking turbulence. The sign and phase behavior of the time‐dependent Reynolds shear stresses at elevations up to approximately 0.02 m from the bed (roughly twice the elevation of the boundary layer overshoot) are mainly controlled by local bed‐shear‐generated turbulence, but at higher elevations Reynolds stresses are controlled by wave breaking turbulence. The measurements are subsequently analyzed to investigate the TKE budget at wave‐averaged and intrawave time scales. Horizontal and vertical turbulence advection, production, and dissipation are the major terms. A two‐dimensional wave‐averaged circulation drives advection of wave breaking turbulence through the near‐bed layer, resulting in a net downward influx in the bar trough region, followed by seaward advection along the bar's shoreward slope, and an upward outflux above the bar crest. The strongly nonuniform flow across the bar combined with the presence of anisotropic turbulence enhances turbulent production rates near the bed.

Highlights

The wave bottom boundary layer (WBL) is defined as the lowest part of the water column where orbital velocities are significantly affected by the presence of the bed (Nielsen, 1992)
Because the present study focuses on the concurrent effects of breaking-generated and bed-shear-generated turbulence on near-bed turbulent kinetic energy (TKE), it is important that the WBL flow is in a turbulent flow regime that is representative for prototype waves at natural beaches
Measurements for the present study were collected during the same experimental campaign as that reported by van der A et al (2017), who focused on the outer-flow hydrodynamics

Summary

Introduction

The wave bottom boundary layer (WBL) is defined as the lowest part of the water column where orbital velocities are significantly affected by the presence of the bed (Nielsen, 1992). Turbulence is initially generated at the bed during the accelerating flow phase in the form of longitudinal low-speed streaks, which break up into smaller-scale vortices that merge and produce a burst of turbulence during the decelerating stage of the half-cycle (Carstensen et al, 2010; Costamagna et al, 2003; Hayashi & Ohashi, 1982; Scandura et al, 2016; Vittori & Verzicco, 1998). Turbulent intensities and Reynolds stresses show a progressive time lag with elevation, relative to their phase behavior at the bed (e.g., Hayashi & Ohashi, 1982; Hino et al, 1983; Jensen et al, 1989; van der A et al, 2011). The precise spatial and temporal turbulent behavior of oscillatory flows depends on Reynolds number and bed roughness (e.g., Jensen et al, 1989)

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