Britt Raubenheimer

The vertical structures of cross‐shore wave, roller, and other momentum forcing terms are examined using a two‐phase Reynolds‐averaged Navier‐Stokes model driven with field and laboratory measured waves and bathymetry. Modeled wave height, setup, and undertow agree well with field observations along a cross‐shore transect extending from the shoreline to about 5‐m water depth, and with measurements in a wave flume. Momentum flux carried by wave, roller, vertical turbulence transfer, pressure slope, and vertical inertia primarily balance near the surface, while wave, vertical turbulence transfer, vertical inertia, and setup primarily balance in the mid‐to lower‐water column. Turbulence transports momentum downward, while vertical inertia transfers momentum upwards. Combined wave and roller forcing agrees with a linear wave theory parametrization offshore of the sandbar and on a near‐planar beach, and combined wave, roller, and vertical inertia agrees with linear theory onshore of the sandbar. Roller thickness is related to local wave height, similar to previous studies. Sub‐surface offshore‐directed mean currents (undertow) can be estimated from the mid‐to lower‐water column momentum balance. Turbulence production dominates the near‐surface turbulence‐energy‐flux balance, and its penetration depth in the sandbar trough is correlated with local wave height.

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.

The dissipation of wave energy is important to nearshore circulation and beach profile evolution. Here, radar measurements of wave dissipation at the water surface across the surfzone are used to estimate water velocities and sediment transport in the lower water column to drive an energetics model for morphological change. The radar‐driven model accurately simulates both the 25‐m onshore and the 50‐m offshore migration of a sand bar observed on an Atlantic Ocean beach with a single set of calibration coefficients. Similar to previous studies, wave asymmetry dominated during mild wave conditions when the bar migrated shoreward, and undertow dominated during energetic conditions when the bar migrated seaward. Model results were improved by accounting for both wave bottom boundary layer effects near the sand bar (especially during onshore migration) and the vertical extent of sediment suspension in the undertow transport (especially during offshore migration).

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.

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 towards the Gulf Stream at Cape Hatteras, following the shelf break along the Northeast US 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 four 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.

Nearshore circulation patterns such as rip currents and eddies transport bacteria, sediment, and people across and along the surf zone, affecting beach safety, coastal morphology, and marine wildlife (Grant et al. 2005). Recent phase-resolving modeling studies suggest nearshore circulation patterns and the associated vorticity are linked to bathymetric and incident wave conditions. Persistent eddies correspond to the length scales of the underlying bathymetric variations (order 100 m), whereas small-scale (order 10 m) vorticity increases with wave directional spread (O’Dea et al. 2021, Baker et al. 2021). However, these numerically generated hypotheses have not been tested with field observations, largely due to the challenge of instrumenting the surf zone with sufficient spatial resolution (Holman and Haller 2013). Here, remote sensing techniques provide high spatial resolution estimates of surface flows in the surfzone for a range of conditions, allowing hypotheses about the impacts of bathymetry and incident waves on surfzone vorticity to be examined with field data.

Swash and beach groundwater influence the morphological evolution of beaches (Bakhtyar, 2011; Masselink, 2012) and impact maximum runup elevations (Delisle, 2023). Numerical modeling and laboratory experiments provide insight into the relationship between surface and subsurface swash zone processes. However, there are few simultaneous observations of these processes in the field. Here, surface (runup, rainfall, and beach profile) and subsurface (pore pressure and moisture content) measurements are used to investigate swash zone flow exchange dynamics for a range of conditions.

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.

In March of 2018, a winter storm hit Southern New England (US East Coast), with most of the coastal cities experiencing minor to moderate coastal floods. However, days after the storm passed and the winds and waves calmed down, the tide-gauge data continued to reach minor flooding levels. What was the cause of this prolonged recurrent flooding? Here we argue that the lingering effects of local ocean current dynamics contributed to this flooding.Along the US East Coast, an important driver of coastal sea-level variability is ocean dynamics, related to both large-scale circulation, such as the Gulf Stream, but also smaller local ocean currents. A relevant circulation feature in Southern New England is the Shelfbreak jet (SBJ). The SBJ flows equatorward from the Labrador Sea towards the Gulf Stream at Cape Hatteras following the shelf break along the Northeast US coast. In a recent study, we showed that the SBJ and sea level along the southern New England coast are highly correlated, especially at timescales of 1-15 days (Camargo et al., 2024). ​​Since this frequency band coincides with the timescales of storm surges, we explore the implication of our findings for coastal flooding. We find that the SBJ explains, on average, about 25% of the storm surge variance for flood days along Southern New England. Specifically, for the March 2018 winter storm, SBJ dynamics are responsible for more than 90% of the storm surge observed 4 days after the peak of the storm. That is, there would have been no flooding so many days after the storm passed if not for SBJ-related-dynamics.Our results suggest local ocean dynamics are an important component of storm surges in Southern New England, and contribute to lingering flooding after a storm has passed. Thus, we advocate that ocean dynamics should be considered in flood studies elsewhere. Furthermore, our results suggest that focusing only on large-scale circulation, such as the Gulf Stream or ocean overturning, may not be satisfactory for understanding the most basic dynamics essential for making meaningful projections of the future.Reference: Camargo, C. M. L., Piecuch, C. G., & Raubenheimer, B. (2024). From Shelfbreak to Shoreline: Coastal sea level and local ocean dynamics in the northwest Atlantic. Geophysical Research Letters, 51, e2024GL109583. https://doi.org/10.1029/2024GL109583