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Abstract A physics‐informed deep conditional generative model driven with remotely sensed surface currents is shown to estimate surfzone water depths (bathymetry). The model encodes measured flow data as latent Gaussian parameters and decodes these distributions to estimate water depths over the domain, progressively refining its predictions via a loss‐minimization strategy. The model performance is evaluated on in‐distribution and out‐of‐distribution data sets collected in Duck, North Carolina, demonstrating promising site‐specific results, especially given the limited training data set used here (6 bathymetries with a total of 8 flow realizations). However, broader applicability requires transfer learning across a wider range of bathymetric observations. 

Abstract Turbulent anisotropy was estimated using two vertically stacked acoustic Doppler velocimeters (ADVs) deployed in the surfzone (2-m mean water depth) for 18 days with offshore wave heights ranging from 0.8 to 5.7 m. Consistent with prior observations, [c_t], a depth-averaged parameter for cross-shore vertical [two dimensional (2D)] anisotropy, is greater than 1, which is larger than the suggested value (0.22) based on plane wake turbulence. The estimated 2D anisotropy increases weakly from the inner to the outer surfzone (with an increasing ratio of local to offshore wave energy flux) and decreases weakly with vertical shear of the cross-shore current. Only near-surface 2D anisotropy is correlated weakly with wave directional spread, suggesting that the processes affecting anisotropy are depth-dependent. At low frequencies (f< 0.05 Hz), 2D anisotropy is larger than that in the higher-frequency inertial subrange and decreases toward the seafloor and with increasing vertical shear of the cross-shore current, approaching that for wave–current bottom-boundary-layer turbulence when the vertical shear is large. The three-dimensional turbulence Reynolds stress tensor is related to the total vertical current shear, as well as to the directions of both mean currents and waves. Horizontal turbulence length scales are larger than the vertical length scales, consistent with previous studies. 

Abstract Ocean dynamics related to large‐scale circulation, such as the Gulf Stream, and smaller local ocean currents are an important driver of coastal sea‐level variability along the U.S. East Coast. A relevant circulation feature in Southern New England is the Shelfbreak Jet (SBJ). The SBJ flows equatorward from the Labrador Sea toward the Gulf Stream at Cape Hatteras, following the shelf break along the Northeast U.S. Coast. The SBJ and sea level are highly correlated along the Southern New England Coast, especially at timescales of 1–15 days. Since this frequency band coincides with meteorological timescales, we explore the implications for coastal flooding. We find that SBJ transport explains, on average, about 30% of the storm surge variance along Southern New England, in a statistical sense. For a specific Nor'easter storm in March 2018, SBJ dynamics are responsible for more than 90% of the storm‐surge height observed during a flood 4 days after the peak of the storm. Our results suggest local ocean dynamics are an important component of storm surges in Southern New England and can contribute, in some cases, to lingering flooding after a storm has passed. Thus, our results suggest that focusing only on large‐scale circulation, such as the Gulf Stream or meridional overturning, may not be complete for understanding the dynamics essential for coastal impacts. We recommend that the role of local ocean dynamics in floods should be investigated further in other regions. 

Abstract Currents in the swash zone on a sandy Atlantic coast beach estimated with near-field optical remote sensing by tracking breaking-wave-generated foam with particle image velocimetry (PIV) are similar to those measured with in situ acoustic Doppler velocimeters (ADVs). The observations were obtained from a 40-m-tall tower located about 60 m inland of the beach over a 2-month period, yielding 180 h of data during a wide range of incident wave and local foam conditions. A smaller, overlapping set of observations also was obtained from a drone. The remote sensing estimates of mean alongshore flows, and the magnitude and phase of cross-shore flows, are highly correlated with the in situ measurements. However, the remote sensing estimates tend to underestimate the in situ measurements of downrush flows and overestimate the uprush flows, with the differences becoming smaller as the amount of foam increases. Analysis with drone-based images suggests that higher image resolution improves agreement between remotely sensed and in situ measurements, particularly for time-averaged cross-shore flows, but a nadir view angle does not eliminate persistent cross-shore bias. The remote sensing estimates allow for swash zone currents to be estimated both across and along the swash zone with relatively high spatial resolution, and show strong cross-shore gradients, including direction reversals in alongshore currents. In addition, the remote sensing estimates indicate flow patterns associated with the formation of beach cusps during times of submergence when low-tide topographic surveys are not feasible. 

Abstract Wave reflection near the beach affects the energy reaching the shore, surfzone wave conditions, and sediment transport. Many prior studies have shown strong reflection of infragravity (0.01 < frequency < 0.05 Hz) wave energy from steep foreshore slopes. Here, swell‐frequency bores were observed propagating offshore across a shallow sandbar crest at low tide. Reflection coefficients (ratio of outgoing to incoming energy) estimated from pressure and velocity observations were large (∼0.8) at the frequency of the sea‐swell peak (0.07 Hz), especially at low tide when the water depth on the bar crest relative to that in the trough is smallest. The high observed sea‐swell reflection may have been at least partly owing to interactions of the incident waves with the surfzone bathymetry, which included a steep drop from the bar crest into a narrow bar‐trough at the alongshore location where the offshore bore propagation was observed. 

Abstract A field‐calibrated morphodynamic model (MIKE21) is used to investigate the importance of the dimensions of a rip channel across an outer sand bar to the hydrodynamics and morphological evolution of an inner sand bar and rip channel for a range of initial bathymetries and wave conditions. The model was driven with offshore wave conditions and idealized bathymetry representative of field conditions near Duck, NC, USA during which strong rips and associated channel erosion were observed to occur over an inner bar. Consistent with prior results, the strength of the hydro‐morphological coupling between the two bars depends on the dimensions of the outer‐bar perturbation, as well as the wave forcing. The results suggest that in double‐barred systems, a single moderate‐scale perturbation (O(0.1 m deep, 10 m wide)) in the outer‐bar elevation can lead to the generation of a rip current and associated erosion of a rip channel across the inner bar. The simulations suggest that the magnitude of the inner‐bar rip flow, the depth to which the inner‐bar channel is eroded, and the alongshore position of the inner‐bar rip relative to the outer‐bar perturbation depend on the non‐dimensional outer‐bar channel depth, the transverse rip‐channel slope, and the wave height, period and directional spreading. For deep and narrow outer‐bar channels, the outer‐inner bar coupling is strong. In contrast, for shallow and wide outer‐bar channels, the system may alternate between being coupled and uncoupled with unstable locations of the inner‐bar rip. 

Abstract Interactions between surface flows and groundwater in beaches can influence erosion and accretion, wave overtopping, groundwater levels and salinization, and transport of nutrients and pollutants. Laboratory experiments using transparent crushed quartz and optically matched mineral oil as proxies for sand and water allow the degree of saturation to be computed at pore‐scale (0.7 mm resolution) enabling detailed investigations of the wave runup driven infiltration into a beach in a wave flume for a range of slopes and flow boundary conditions. The evolution of the wetting front resulting from wave runup on an initially unsaturated beach is described in detail, including the formation of an infiltration wedge in the subsurface of the swash zone and the wave‐driven rise in fluid elevation inside the beach. The elevation of the runup for each event is found to be related closely to the saturation of the beach face, reaching an equilibrium state once the subsurface in the swash zone reaches capacity. The back wall boundary condition in the flume has a significant role in how subsurface flows increase saturation within the beach, especially with boundary head elevations greater than the initial phreatic surface. The results of these novel experimental observations are used to develop dimensionless relationships between the surface wave runup and the subsurface saturation rates. To improve monitoring and interpretation of future coastal groundwater studies, three distinct cross‐shore regimes are defined for assessing change in subsurface fluid elevation in the beach. 

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. 

Abstract Sea‐level change threatens the U.S. East Coast. Thus, it is important to understand the underlying causes, including ocean dynamics. Most past studies emphasized links between coastal sea level and local atmospheric variability or large‐scale circulation and climate, but possible relationships with local ocean currents over the shelf and slope remain largely unexplored. Here we use 7 years of in situ velocity and sea‐level data to quantify the relationship between northeastern U.S. coastal sea level and variable Shelfbreak Jet transport south of Nantucket Island. At timescales of 1–15 days, southern New England coastal sea level and transport vary in anti‐phase, with magnitude‐squared coherences of ∼0.5 and admittance amplitudes of ∼0.3 m Sv⁻¹. These results are consistent with a dominant geostrophic balance between along‐shelf transport and coastal sea level, corroborating a hypothesis made decades ago that was not tested due to the lack of transport data. 

Abstract The cross‐shore transformation of breaking‐wave roller momentum and energy on observed barred surfzone bathymetry is investigated with a two‐phase Reynolds Averaged Navier Stokes model driven with measured incident waves. Modeled wave spectra, wave heights, and wave‐driven increases in the mean water level (setup) agree well with field observations along transects extending from 5‐m water depth to the shoreline. Consistent with prior results the roller forcing contributes 50%–60% to the setup, whereas the advective terms contribute ∼20%, with the contribution of bottom stress largest (up to 20%) for shallow sandbar crest depths. The model simulations suggest that an energy‐flux balance between wave dissipation, roller energy, and roller dissipation is accurate. However, as little as 70% of the modeled wave energy ultimately dissipated by breaking was first transferred from the wave to the roller. Furthermore, of the energy transferred to the roller, 15%–25% is dissipated by turbulence in the water column below the roller, with the majority of energy dissipated in the aerated region or near the roller‐surface interface. The contributions of turbulence to the momentum balance are sensitive to the parameterized turbulent anisotropy, which observations suggest increases with increasing turbulence intensity. Here, modeled turbulent kinetic energy dissipation decreases with increasing depth of the sandbar crest, possibly reflecting a change from plunging (on the steeper offshore slope of the bar) to spilling breakers (over the flatter bar crest and trough). Thus, using a variable roller front slope in the roller‐wave energy flux balance may account for these variations in breaker type.