Near‐bed hydrodynamics and turbulence below a large‐scale plunging breaking wave over a mobile barred bed profile

Abstract

Detailed measurements are presented of velocities and turbulence under a large‐scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross‐shore locations around a mobile medium‐sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near‐bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking‐wave plunge point. This relates possibly to breaking‐induced TKE or to the diverging flow at the shoreward slope of the bar. Outer flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking‐induced turbulence invades the WBL during both crest and trough half cycle. This results in an increase in the time‐averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near‐bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half cycle. The results imply that breaking‐induced turbulence, for large‐scale conditions, may significantly affect near‐bed sediment transport processes.

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

“…In a recent laboratory experiment involving a barred sand bed profile, van der Zanden et al (2016) studied for the first time the WBL flow and near-bed TKE under a large-scale plunging wave. Their measurements showed that wave breaking turbulence invades the WBL in the breaking region, leading to a significant increase in near-bed TKE compared to the shoaling zone, hence reaffirming the aforementioned observations from small-scale wave flumes.…”

“…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.…”

On Bedload and Suspended Load Measurement Performances in Sheet Flows Using Acoustic and Conductivity Profilers

“…Estimates of the quasi‐instantaneous Doppler frequencies using the pulse‐pair algorithm at the two receivers are then converted into the desired ( u , w ) velocity profile considering the transformation matrix associated with sensor geometry and positioning relative to flow orientation. The unique high‐resolution sediment flux measurement ability of the ACVP has recently provided new insights into a variety of wave‐ (Chassagneux & Hurther, ; Hurther & Thorne, ; van der Zanden et al, ) and current‐driven (Naqshband, Ribberink, Hurther, Barraud, et al, ; Naqshband, Ribberink, Hurther & Hulscher, et al, ; Revil‐Baudard et al, , ) boundary layer sediment transport processes.…”

Suspended and Bedload Transport in the Surf Zone: Implications for Sand Transport Models

“…The outer-flow and wave bottom boundary layer (WBL) hydrodynamics are described in detail by Van der Zanden et al (2016); the present section summarizes the main findings. Fig.…”

“…Finally, echo sounders were used to measure the bed profile before each experiment and after every 2 nd run. The experiments and data processing steps are described in more detail in Van der Zanden et al (2016;Submitted). Waves were breaking over the bar crest and were of the plunging type.…”