Suspended sediment transport around a large-scale laboratory breaker bar

Abstract

Pre-print of manuscript published in Coastal Engineering. Full reference:Highlights:  Outer-flow and wave bottom boundary layer suspended sand concentrations and fluxes are measured around a large-scale laboratory breaker bar.  The suspended load is highest above the shoreward-facing slope between the bar crest and trough.  Near-bed reference concentrations correlate significantly with near-bed turbulent kinetic energy.  Depth-integrated suspended transport is dominated by offshore-directed current-related fluxes at outer flow elevations; the onshore wave-related transport is generally confined to the wave bottom boundary layer.  The contributions of horizontal sediment advection and of vertical exchange with the bedload layer (pick-up/deposition) are quantified to explain the complex intra-wave spatiotemporal behavior of near-bed suspended sediment concentrations.

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

“…The results of the present experiment can be compared with observations of sand transport processes during an accompanying mobile-bed experiment involving similar wave conditions and bed profile (van der Zanden et al, , 2017a(van der Zanden et al, , 2017b. The overall distribution of near-bed TKE seen in the present study is qualitatively and quantitatively similar to observations from the mobile-bed experiment .…”

“…This relates to the dominant contribution of wave breaking turbulence to total TKE, which overpowers the differences in bed-shear-generated TKE due to different roughness and mobile versus fixed bed effects between both experiments. The time-averaged 2-D fluid circulation through the near-bed layer may largely explain the cross-shore distribution of suspended sand pick-up, advection and deposition reported in van der Zanden et al (2017a). At the bar trough and lower section of the bar's shoreward slope, TKE is advected toward the bed and enhances local turbulent sand pick-up.…”

“…External turbulence enhances the momentum exchange between the freestream and WBL, hence altering the flow inside the WBL (Fredsøe et al, 2003). Breaking-generated turbulence enhances magnitudes of instantaneous bed shear stresses (Cox & Kobayashi, 2000;Deigaard et al, 1991) and suspended sediment pick-up and transport rates (Nadaoka et al, 1988;van der Zanden et al, 2017a;Zhou et al, 2017). Consequently, numerical simulations of velocities, suspended sediment concentrations, and ultimately net sand transport rates and morphology in the surf zone, require accurate predictions of TKE, especially near the bed, where the majority of sediment transport occurs (van der Zanden et al, 2017a).…”

“…Breaking-generated turbulence enhances magnitudes of instantaneous bed shear stresses (Cox & Kobayashi, 2000;Deigaard et al, 1991) and suspended sediment pick-up and transport rates (Nadaoka et al, 1988;van der Zanden et al, 2017a;Zhou et al, 2017). Consequently, numerical simulations of velocities, suspended sediment concentrations, and ultimately net sand transport rates and morphology in the surf zone, require accurate predictions of TKE, especially near the bed, where the majority of sediment transport occurs (van der Zanden et al, 2017a). However, existing turbulence closure models tend to systematically overestimate TKE levels under breaking waves, both above (Brown et al, 2016;Christensen, 2006;Lin & Liu, 1998) and inside (Fernandez-Mora et al, 2016) the WBL.…”

“…Each plot includes the zero crossings of the water surface for phase reference (dotted lines). A similar presentation of data was used by van der Zanden et al (2016van der Zanden et al ( , 2017a to study the spatiotemporal variation in breaking-generated turbulence and suspended sediment over a mobile sand bed; their analyses are here extended by quantifying the horizontal and vertical influx of TKE. Figure 10a shows the depth-averaged bed-parallel velocity h b u R i.…”