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1. Field Evidence of Inverse Energy Cascades in the Surfzone

   https://doi.org/10.1175/JPO-D-19-0327.1  

   Elgar, Steve; Raubenheimer, Britt (August 2020, Journal of Physical Oceanography)

   null (Ed.)

   Abstract 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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2. Elevated Momentum Flux in the Surfzone during a Storm

   https://doi.org/10.1175/JAS-D-24-0090.1  

   Potter, Henry; Lyu, Meng; Yang, Xin (July 2025, Journal of the Atmospheric Sciences)

   Abstract Drag coefficient parameterizations, which are largely developed from homogenous deep ocean data, are ineffective nearshore where conditions are nonuniform. This is problematic because operational forecast accuracy depends upon reliable quantification of air–sea momentum transfer. This is especially important for storms which threaten coastal life and property. To help fill this knowledge gap, direct flux measurements were collected from the beach and pierhead in Duck, North Carolina, as part of the During Nearshore Event Experiment (DUNEX). The footprint analysis shows these fluxes were sourced in the surfzone and offshore, representing very different conditions. During a weeklong storm, wind speeds and significant wave heights were 20 m s⁻¹and 4 m, leading to a broad, vigorous surfzone. The drag coefficient in the surfzone was twice the offshore value, explained by increased roughness due to wind stress and bathymetric changes. The Charnock parameter is well predicted by wave age, but it is expected this is site-specific due to unique bathymetry. A horizontal wind speed gradient was observed and attributed to the high surfzone roughness. The wavelengths of the turbulent eddies in the surfzone were smaller than offshore or predicted by universal scaling. This research offers novel insights that can contribute to a crucial collective effort to develop robust coastal flux models, leading to improved forecasting. Significance StatementWhen wind blows over the ocean, the energy associated with its motion is moved from the air into water. This energy transfer helps grow waves and drive currents which, via many pathways, alter the characteristics of the upper ocean and lower atmosphere. In turn, this affects weather and climate, so it is critical this energy exchange is accounted for in forecasts. Energy transfer is reasonably well understood in the deep ocean, but not nearshore where conditions are nonuniform and change quickly, especially in storms where very few measurements are made. To remedy this, data were collected in May 2022 during a storm in Duck, North Carolina, which had wind speeds of 20 m s⁻¹and 4-m wave heights. The extreme conditions created a very wide and energetic surfzone. Wind measurements were made on the beach and approximately 500 m offshore. Due to the rough surface, twice the energy was transferred from the air into the ocean in the surfzone than offshore and the wind speed decreased as it crossed the surfzone. Finally, the wavelengths of the wind that transfer energy into the ocean are much smaller than offshore or predicted by previous research. 

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3. Observations of two-dimensional turbulence in the surfzone

   https://doi.org/10.1063/5.0159170  

   Elgar, Steve; Dooley, Ciara; Gorrell, Levi; Raubenheimer, Britt (August 2023, Physics of Fluids)

   Low-frequency, many-minute-period horizontal surfzone eddies are an important mechanism for the dispersion of material, transporting larvae, pollutants, sediment, and swimmers both across and along the nearshore. Previous numerical, laboratory, and field observations on alongshore uniform bathymetry with no or roughly uniform mean background flows suggest that the low-frequency eddies may be the result of a two-dimensional inverse energy cascade that transfers energy from relatively small spatial-scale vorticity injected by depth limited breaking waves to larger and larger spatial scales. Here, using remotely sensed high-spatial resolution estimates of currents, those results are extended to surfzones with strong complex mean circulation patterns [flows O(1 m/s)] owing to nonuniform bathymetry. Similar to previous results, wavenumber spectra and second-order structure functions calculated from the observations are consistent with a two-dimensional inverse energy cascade. The size of the largest eddies is shown to depend on the surfzone width and the spatial scales of the mean currents. Third-order structure functions also are consistent with an inverse cascade for spatial scales greater than ∼50 m. At smaller scales, the third-order structure functions suggest a mixture of inverse and forward cascades. 

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4. The Roles of Bathymetry and Waves in Rip‐Channel Dynamics

   https://doi.org/10.1029/2023JF007389  

   Christensen, D. F.; Raubenheimer, B.; Elgar, S. (January 2024, Journal of Geophysical Research: Earth Surface)

   Abstract The behavior and predictability of rip currents (strong, wave‐driven offshore‐directed surfzone currents) have been studied for decades. However, few studies have examined the effects of rip channel morphology on the rip generation or have compared morphodynamic models with observations. Here, simulations conducted with the numerical morphodynamic model MIKE21 reproduce observed trends in flows and bathymetric evolution for two channels dredged across a nearshore sandbar and terrace on an ocean beach near Duck, NC, USA. Channel dimensions, wave conditions, and flows differed between the two cases. In one case, a strong rip current was driven by moderate height, near‐normally incident waves over an approximately 1‐m deep channel with relatively little bathymetric evolution. In the other case, no rip was generated by the large, near‐normally incident waves over the shallower (∼0.5 m) channel, and the channel migrated in the direction of the mean flow and eventually filled in. The model simulated the flow directions, the generation (or not) of rip currents, and the morphological evolution of the channels reasonably well. Model simulations were then conducted for different combinations of the two channel geometries and two wave conditions to examine the relative importance of the waves and morphology to the rip current evolution. The different bathymetries were the dominant factor controlling the flow, whereas both the initial morphology and wave conditions were important for channel evolution. In addition, channel dimensions affected the spatial distribution of rip current forcings and the relative importance of terms. 

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5. Estimating Surfzone Currents with Near-Field Optical Remote Sensing

   https://doi.org/10.1175/JTECH-D-24-0015.1  

   Dooley, Ciara; Elgar, Steve; Raubenheimer, Britt; Gorrell, Levi (January 2025, Journal of Atmospheric and Oceanic Technology)

   Abstract Currents transport sediment, larvae, pollutants, and people across and along the surfzone, creating a dynamic interface between the coastal ocean and shore. Previous field studies of nearshore flows primarily have relied on relatively low spatial resolution deployments of in situ sensors, but the development of remote sensing techniques using optical imagery and naturally occurring foam as a flow tracer has allowed for high spatial resolution observations (on the order of a few meters) across the surfzone. Here, algorithms optical current meter (OCM) and particle image velocimetry (PIV) are extended from previous surfzone applications and used to estimate both cross-shore and alongshore 2-, 10-, and 60-min mean surface currents in the nearshore using imagery from both oblique and nadir viewing angles. Results are compared with in situ current meters throughout the surfzone for a wide range of incident wave heights, directions, and directional spreads. Differences between remotely sensed flows and in situ current meters are smallest for nadir viewing angles, where georectification is simplified. Comparisons of 10-min mean flow estimates from a nadir viewing angle with in situ estimates of alongshore and cross-shore currents had correlationsr²= 0.94 and 0.51 with root-mean-square differences (RMSDs) = 0.07 and 0.16 m s⁻¹for PIV andr²= 0.88 and 0.44 with RMSDs = 0.08 and 0.22 m s⁻¹for OCM. Differences between remotely sensed and in situ cross-shore current estimates are at least partially owing to the difference between onshore-directed mass flux on the surface and offshore-directed undertow in the mid–water column. 

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