Depth Dependence of Nearshore Currents and Eddies

Henderson, Stephen M.; Arnold, Joshua; Özkan-Haller, H. Tuba; Solovitz, Stephen A.

doi:10.1002/2016jc012349

Cited by 7 publications

(14 citation statements)

References 84 publications

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“…The three‐dimensionality of VLF currents, which may impact material exchange and dispersion, is not well documented. Similar to the small number of observations of VLF motion vertical structure (Henderson et al., 2017; Lippmann et al., 2016), VLF motion simulated with SWASH varies in the vertical near the bar crest (Figure 9). Cross‐shore energy density decays with depth, with over an 60% drop in squared coherence over the water column, and with large phase shifts near the bottom (up to 50°) relative to near‐surface velocities (Lippmann et al., 2016).…”

Section: Discussionsupporting

confidence: 80%

“…However, two recent studies on a barred beach measured low‐frequency motions with vertically stacked electromagnetic current sensors (Lippmann et al., 2016) and acoustic Doppler profilers (Henderson et al., 2017). These studies found that low‐frequency cross‐ and alongshore velocities are weakly vertically dependent in the outer surf zone (Henderson et al., 2017; Lippmann et al., 2016). Analytic solutions based on bottom boundary layer theory indicate complex vertical structure of low‐frequency motions in the presence of a horizontally sheared alongshore current (Lippmann & Bowen, 2016).…”

Section: Introductionmentioning

confidence: 99%

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Modeled Three‐Dimensional Currents and Eddies on an Alongshore‐Variable Barred Beach

et al. 2021

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Circulation in the nearshore region, which is critical for material transport along the coast and between the surf zone and the inner shelf, includes strong vortical motions. The horizontal length scales and vertical structure associated with vortical motions are not well documented on alongshore‐variable beaches. Here, a three‐dimensional phase‐resolving numerical model, Simulating WAves till SHore (SWASH), is compared with surfzone waves and flows on a barred beach, and is used to investigate surfzone eddies. Model simulations with measured bathymetry reproduce trends in the mean surfzone circulation patterns, including alongshore currents and rip current circulation cells observed for offshore wave heights from 0.5 to 2.0 m and incident wave directions from 0 to 15° relative to shore normal. The length scales of simulated eddies, quantified using the alongshore wavenumber spectra of vertical vorticity, suggest that increasing wave directional spread intensifies small‐scale eddies (scriptO(10) m). Simulations with bathymetric variability ranging from alongshore uniform to highly alongshore variable indicate that large‐scale eddies (scriptO(100) m) may be enhanced by surfzone bathymetric variability, whereas small‐scale eddies (scriptO(10) m) are less dependent on bathymetric variability. The simulated vertical dependence of the magnitude and mean length scale (centroid) of the alongshore wavenumber spectra of vertical vorticity and very low‐frequency (f ≈ 0.005 Hz) currents is weak in the outer surf zone, and decreases toward the shoreline. The vertical dependence in the simulations may be affected by the vertical structure of turbulence, mean shear, and bottom boundary layer dynamics.

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Section: Discussionsupporting

confidence: 80%

Section: Introductionmentioning

confidence: 99%

Modeled Three‐Dimensional Currents and Eddies on an Alongshore‐Variable Barred Beach

et al. 2021

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“…For the shallow water depths in the surf zone, eddies with horizontal scales greater than a few meters can be considered two-dimensional (2-D), possibly becoming quasi-2-D in the deeper water near the outer edge of the surf zone where there is evidence for weak vertical structure in higher-frequency nearshore eddies (Henderson et al, 2017;Lippmann et al, 2016). In contrast to three-dimensional turbulence, twodimensional flows have an inverse cascade where energy from stirring at small scales is transferred to larger scales (Boffetta & Ecke, 2012;Kraichnan, 1967;Tabeling, 2002).…”

Section: 1029/2018gl081106mentioning

confidence: 99%

Extremely Low Frequency (0.1 to 1.0 mHz) Surf Zone Currents

Elgar

Raubenheimer

Clark³

et al. 2019

Geophysical Research Letters

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Low‐frequency surf zone eddies disperse material between the shoreline and the continental shelf, and velocity fluctuations with frequencies as low as a few mHz have been observed previously on several beaches. Here spectral estimates of surf zone currents are extended to an order of magnitude lower frequency, resolving an extremely low frequency peak of approximately 0.5 mHz that is observed for a range of beaches and wave conditions. The magnitude of the 0.5‐mHz peak increases with increasing wave energy and with spatial inhomogeneity of bathymetry or currents. The 0.5‐mHz peak may indicate the frequency for which nonlinear energy transfers from higher‐frequency, smaller‐scale motions are balanced by dissipative processes and thus may be the low‐frequency limit of the hypothesized 2‐D cascade of energy from breaking waves to lower frequency motions.

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“…For the shallow water depths in the surfzone, eddies with horizontal scales greater than a few meters can be considered two-dimensional (2D), possibly becoming quasi-2D in the deeper water near the outer edge of the surfzone where there is evidence for weak vertical structure in higher-frequency nearshore eddies (Lippmann et al 2016;Henderson et al 2017). In contrast to three-dimensional turbulence, two-dimensional flows have an inverse cascade where energy from stirring at small scales is transferred to larger scales (Kraichnan 1967;Tabeling 2002;Boffetta and Ecke 2012).…”

Section: Hypothesesmentioning

confidence: 99%

Field Evidence of Inverse Energy Cascades in the Surfzone

Elgar

Raubenheimer

2020

Journal of Physical Oceanography

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Low-frequency currents and eddies transport sediment, pathogens, larvae, and heat along the coast and between the shoreline and deeper water. Here, low-frequency currents (between 0.1 and 4.0 mHz) observed in shallow surfzone waters for 120 days during a wide range of wave conditions are compared with theories for generation by instabilities of alongshore currents, by ocean-wave-induced sea surface modulations, and by a nonlinear transfer of energy from breaking waves to low-frequency motions via a two-dimensional inverse energy cascade. For these data, the low-frequency currents are not strongly correlated with shear of the alongshore current, with the strength of the alongshore current, or with wave-group statistics. In contrast, on many occasions, the low-frequency currents are consistent with an inverse energy cascade from breaking waves. The energy of the low-frequency surfzone currents increases with the directional spread of the wave field, consistent with vorticity injection by short-crested breaking waves, and structure functions increase with spatial lags, consistent with a cascade of energy from few-meter-scale vortices to larger-scale motions. These results include the first field evidence for the inverse energy cascade in the surfzone and suggest that breaking waves and nonlinear energy transfers should be considered when estimating nearshore transport processes across and along the coast.

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Depth Dependence of Nearshore Currents and Eddies

Cited by 7 publications

References 84 publications

Modeled Three‐Dimensional Currents and Eddies on an Alongshore‐Variable Barred Beach

Modeled Three‐Dimensional Currents and Eddies on an Alongshore‐Variable Barred Beach

Extremely Low Frequency (0.1 to 1.0 mHz) Surf Zone Currents

Field Evidence of Inverse Energy Cascades in the Surfzone

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