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

Abstract

Hydrodynamics 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.

Field Observations of Turbulence, Sand Suspension, and Cross‐Shore Transport Under Spilling and Plunging Breakers

“…In two cases, TKE increased steadily from the velocity zero-downcrossing and peaked at velocity reversal, followed by a steady decrease (Figure 13d). This is also consistent with observations reported by van der Zanden et al (2018). For PL36, TKE did exhibit a local maximum at velocity reversal, but a larger maximum occurred under the wave crest.…”

“…Reynolds stress (τ; Figure 13b) exhibited a distinctive pattern with large upward directed stresses on the lower front face of the wave (t/T ≈ 0.42) immediately prior to velocity reversals. A similar phenomenon was observed under regular plunging breakers by van der Zanden et al (2018) and ascribed to local turbulence production and a bed-parallel TKE-influx from shoreward locations immediately prior to the arrival of the plunging jet. This is followed by downward directed stresses at velocity reversal (t/T ≈ 0.55) caused by the plunging jet (van der Zanden et al, 2018), and there is some evidence for turbulent roll-up at t/T ≈ 0.62-0.72.…”

“…A similar phenomenon was observed under regular plunging breakers by van der Zanden et al (2018) and ascribed to local turbulence production and a bed-parallel TKE-influx from shoreward locations immediately prior to the arrival of the plunging jet. This is followed by downward directed stresses at velocity reversal (t/T ≈ 0.55) caused by the plunging jet (van der Zanden et al, 2018), and there is some evidence for turbulent roll-up at t/T ≈ 0.62-0.72. These latter fluctuations bear the signature of large coherent turbulent vortices associated with plunging breakers.…”

“…Recently, several large-scale laboratory experiments using regular van der Zanden et al, 2016van der Zanden et al, , 2018 and irregular (Brinkkemper et al, 2016) waves have revealed details on the turbulent velocity field, in particular under plunging breakers. Plunging breakers may generate large-scale coherent vortices, which exhibit a rotational velocity field with brief but intense downward motion from the plunging jet on the front face of the wave, followed by upward motion from the ascending vortex limb (Aagaard & Hughes, 2010;Ting, 2013).…”

“…Plunging breakers may generate large-scale coherent vortices, which exhibit a rotational velocity field with brief but intense downward motion from the plunging jet on the front face of the wave, followed by upward motion from the ascending vortex limb (Aagaard & Hughes, 2010;Ting, 2013). At the wave breakpoint, turbulent kinetic energy (TKE) increases steadily from the wave trough and reaches a peak at the zero-velocity upcrossing on the wave front (van der Zanden et al, 2018). Laboratory experiments by LeClaire and Ting (2017) have shown that the vortex timescale under plunging waves is large, typically in the order of T/2 where T is the wave period, while the vortex diameter is in the order of 0.5 h, where h is the water depth.…”