Wave Boundary Layer Hydrodynamics and Sheet Flow Properties Under Large‐Scale Plunging‐Type Breaking Waves

Abstract

Wave boundary layer (WBL) dynamics are measured with an Acoustic Concentration andVelocity Profiler (ACVP) across the sheet flow-dominated wave-breaking region of regular large-scale waves breaking as a plunger over a developing breaker bar. Acoustic sheet flow measurements are first evaluated quantitatively in comparison to Conductivity Concentration Meter (CCM+) data used as a reference. The near-bed orbital velocity field exhibits expected behaviors in terms of wave shape, intrawave WBL thickness, and velocity phase leads. The observed fully turbulent flow regime all across the studied wave-breaking region supports the model-predicted transformation of free-stream velocity asymmetry into near-bed velocity skewness inside the WBL. Intrawave concentration dynamics reveal the existence of a lower pickup layer and an upper sheet flow layer similar to skewed oscillatory sheet flows, and with similar characteristics in terms of erosion depth and sheet flow layer thickness. Compared to the shoaling region, differences in terms of sheet flow and hydrodynamic properties of the flow are observed at the plunge point, attributed to the locally enhanced wave breaker turbulence. The ACVP-measured total sheet flow transport rate is decomposed into its current-, wave-, and turbulence-driven components. In the shoaling region, the sand transport is found to be fully dominated by the onshore skewed wave-driven component with negligible phase lag effects. In the outer surf zone, the total net flux exhibits a three-layer vertical structure typical of skewed oscillatory sheet flows. However, in the present experiments this structure originates from offshore-directed undertow-driven flux, rather than from phase lag effects. Plain Language SummaryWe focus here on novel wave boundary layer hydrodynamics and sheet flow properties obtained with the Acoustic Concentration and Velocity Profiler measurement technology. It is the first time this advanced acoustic instrumentation is used for high-resolution measurements of wave-driven sheet flows under large-scale breaking waves. The wave boundary layer hydrodynamics and, in particular, the detailed properties of the sheet flow dynamics in terms of pickup layer, bedload, and suspended sand transport are investigated. Finally, sand fluxes (as transport rates) decomposed into the undertow-, the wave-and the turbulence-driven contributions allow a new insight into the underlying sand transport mechanisms in the scientifically challenging, coastal wave-breaking region. These results are compared to sheet flow properties obtained in (nonbreaking) oscillatory flows (from experiments in U-tube facilities) in order to show how the wave-breaking process impacts the internal and external sheet flow dynamics.FROMANT ET AL. 75

Stabilized RANS simulation of surf zone kinematics and boundary layer processes beneath large-scale plunging waves over a breaker bar

“…Finally, Figure 12c shows that the asymmetry decays towards the bed. This is again consistent with results from previous research (Berni et al, 2013;Henriquez et al, 2014;Fromant et al, 2019). The asymmetry of the model maintains a qualitatively similar vertical structure as the experiments, though it is slightly underestimated.…”

Sand transport processes and bed level changes induced by two alternating laboratory swash events

“…Sheet flow layer (SFL) dynamics have been extensively studied in oscil-latory flow tunnels (Ribberink and Al-Salem, 1995;Hassan and Ribberink, 2005) and in wave flumes involving non-breaking (Dohmen- Hanes, 2002, 2005;Schretlen, 2012) and breaking (Mieras et al, 2017;van der Zanden et al, 2017;Fromant et al, 2019) waves. In such conditions, sediment is eroded from the bed and brought upward during maximum velocity magnitudes, and settles when the velocity forcing reduces.…”

The Influence of Slipface Angle on Fluvial Dune Growth

“…These two near-bed echoes can be separated when the acoustic intensity profile is derived from the demodulated Doppler signals since the acoustic scatters constituting the nonmoving polystyrene bed produce a constant voltage with negligible signal variance. This results in the time evolution of the nonmoving polystyrene bed, the overlaying high sediment concentration layer (the bed load layer), and the suspended load layer including both the two-component flow velocity and the sediment concentration (Fromant et al, 2019;Naqshband et al, 2014bNaqshband et al, , 2017. The ACVP was mounted on a measurement carriage and positioned at a fixed location along the flume (x = 6 m, see Figure 1d) with dunes migrating underneath.…”

“…The ACVP technology was previously used to investigate the contribution of both bedload and suspended load to migrating sand dunes in equilibrium (Naqshband et al, 2014c), to quantify sediment transport distribution during dune transition to upper stage plane bed (Naqshband et al, 2017), and to study boundary layer flow and sediment transport dynamics under gravity current-driven and wave-driven sheet flows (Fromant et al, 2018(Fromant et al, , 2019Revil-baudard et al, 2015Revil-baudard et al, , 2016. In the present study, we deploy the ACVP to investigate dynamics of flow separation in the dune lee side and associated sediment transport gradients during dune growth from an initial flatbed.…”