Elgar Steve

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Elgar

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Steve

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0000-0001-5034-7027

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Observations of swash zone velocities : a note on friction coefficients

(American Geophysical Union, 2004-01-30) Raubenheimer, Britt ; Elgar, Steve ; Guza, R. T.

Vertical flow structure and turbulent dissipation in the swash zone are estimated using cross-shore fluid velocities observed on a low-sloped, fine-grained sandy beach [Raubenheimer, 2002] with two stacks of three current meters located about 2, 5, and 8 cm above the bed. The observations are consistent with an approximately logarithmic vertical decay of wave orbital velocities within 5 cm of the bed. The associated friction coefficients are similar in both the uprush and downrush, as in previous laboratory results. Turbulent dissipation rates estimated from velocity spectra increase with decreasing water depth from O(400 cm2/s3) in the inner surf zone to O(1000 cm2/s3) in the swash zone. Friction coefficients in the swash interior estimated with the logarithmic model and independently estimated by assuming that turbulent dissipation is balanced by production from vertical shear of the local mean flow and from wave breaking are between 0.02 and 0.06. These values are similar to the range of friction coefficients (0.02–0.05) recently estimated on impermeable, rough, nonerodible laboratory beaches and to the range of friction coefficients (0.01–0.03) previously estimated from field observations of the motion of the shoreward edge of the swash (run-up).
Model-data comparisons of shear waves in the nearshore

(American Geophysical Union, 2005-05-27) Noyes, T. James ; Guza, R. T. ; Feddersen, Falk ; Elgar, Steve ; Herbers, T. H. C.

Observations of shear waves, alongshore propagating meanders of the mean alongshore current with periods of a few minutes and alongshore wavelengths of a few hundred meters, are compared with model predictions based on numerical solutions of the nonlinear shallow water equations. The model (after Özkan-Haller and Kirby (1999)) assumes alongshore homogeneity and temporally steady wave forcing and neglects wave-current interactions, eddy mixing, and spatial variation of the (nonlinear) bottom drag coefficient. Although the shapes of observed and modeled shear wave velocity spectra differ, and root-mean-square velocity fluctuations agree only to within a factor of about 3, aspects of the cross-shore structure of the observed (∼0.5–1.0 m above the seafloor) and modeled (vertically integrated) shear waves are qualitatively similar. Within the surf zone, where the mean alongshore current (V) is strong and shear waves are energetic, observed and modeled shear wave alongshore phase speeds agree and are close to both V and C lin (the phase speed of linearly unstable modes) consistent with previous results. Farther offshore, where V is weak and observed and modeled shear wave energy levels decay rapidly, modeled and observed C diverge from C lin and are close to the weak alongshore current V. The simulations suggest that the alongshore advection of eddies shed from the strong, sheared flow closer to shore may contribute to the offshore decrease in shear wave phase speeds. Similar to the observations, the modeled cross- and alongshore shear wave velocity fluctuations have approximately equal magnitude, and the modeled vorticity changes sign across the surf zone.
The role of tides in beach cusp development

(American Geophysical Union, 2004-04-10) Coco, Giovanni ; Burnet, Tom K. ; Werner, B. T. ; Elgar, Steve

Field measurements of morphology and swash flow during three episodes of beach cusp development indicate that tides modulate the height and cross-shore position of beach cusps. During rising tide, beach cusp height decreases as embayments accrete more than horns and the cross-shore extent of beach cusps decreases. During falling tide, beach cusp height increases as embayments erode more than horns and cross-shore extent increases. A numerical model for beach cusp formation based on self-organization, extended to include the effects of morphological smoothing seaward of the swash front and infiltration into the beach, reproduces the observed spacing, position, and tidal modulation. During rising tide, water particles simulating swash infiltrate, preferentially in embayments, causing enhanced deposition. During falling tide, exfiltration of water particles combined with diversion of swash from horns causes enhanced erosion in embayments. Smoothing of beach morphology in the swash zone seaward of the swash front and in the shallow surf zone accounts for most of the observed tidal modulation, even in the absence of infiltration and exfiltration. Despite the qualitative, and in some cases quantitative, agreement of the model and measurements, the model fails to reproduce observed large deviations of horn orientation from shore normal, some aspects of beach cusp shape, and deviations from the basic tidal modulation, possibly because of the simplified parameterization of cross-shore sediment transport and the neglect of the effects of sea surface gradients on flow.
Wave-driven along-channel subtidal flows in a well-mixed ocean inlet

(John Wiley & Sons, 2014-05-20) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve

Observations of waves, flows, and water levels collected for a month in and near a long, narrow, shallow (∼ 3000 m long, 1000 m wide, and 5 m deep), well-mixed ocean inlet are used to evaluate the subtidal (periods > 30 h) along-inlet momentum balance. Maximum tidal flows in the inlet were about 1.5 m/s and offshore significant wave heights ranged from about 0.5 to 2.5 m. The dominant terms in the local (across the km-wide ebb shoal) along-inlet momentum balance are the along-inlet pressure gradient, the bottom stress, and the wave radiation-stress gradient. Estimated nonlinear advective acceleration terms roughly balance in the channel. Onshore radiation-stress gradients owing to breaking waves enhance the flood flows into the inlet, especially during storms.
Modeled three-dimensional currents and eddies on an alongshore-variable barred beach

(American Geophysical Union, 2021-06-26) Baker, Christine M. ; Moulton, Melissa ; Raubenheimer, Britt ; Elgar, Steve ; Kumar, Nirnimesh

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 ( (10) m). Simulations with bathymetric variability ranging from alongshore uniform to highly alongshore variable indicate that large-scale eddies ( (100) m) may be enhanced by surfzone bathymetric variability, whereas small-scale eddies ( (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.
Flow separation effects on shoreline sediment transport

( 2017-01) Hopkins, Julia ; Elgar, Steve ; Raubenheimer, Britt

Field-tested numerical model simulations are used to estimate the effects of an inlet, ebb shoal, wave height, wave direction, and shoreline geometry on the variability of bathymetric change on a curved coast with a migrating inlet and strong nearshore currents. The model uses bathymetry measured along the southern shoreline of Martha’s Vineyard, MA, and was validated with waves and currents observed from the shoreline to ~10-m water depth. Between 2007 and 2014, the inlet was open and the shoreline along the southeast corner of the island eroded ~200 m and became sharper. Between 2014 and 2015, the corner accreted and became smoother as the inlet closed. Numerical simulations indicate that variability of sediment transport near the corner shoreline depends more strongly on its radius of curvature (a proxy for the separation of tidal flows from the coast) than on the presence of the inlet, the ebb shoal, or wave height and direction. As the radius of curvature decreases (as the corner sharpens), tidal asymmetry of nearshore currents is enhanced, leading to more sediment transport near the shoreline over several tidal cycles. The results suggest that feedbacks between shoreline geometry and inner-shelf flows can be important to coastal erosion and accretion in the vicinity of an inlet.
Fortnightly tides and subtidal motions in a choked inlet

(Elsevier, 2014-04-12) MacMahan, Jamie ; van de Kreeke, Jacobus ; Reniers, Ad ; Elgar, Steve ; Raubenheimer, Britt ; Thornton, Ed B. ; Weltmer, Micah ; Rynne, Patrick ; Brown, Jenna

Amplitudes of semi-diurnal tidal fluctuations measured at an ocean inlet system decay nearly linearly by 87% between the ocean edge of the offshore ebb-tidal delta and the backbay. A monochromatic, dynamical model for a tidally choked inlet separately reproduces the evolution of the amplitudes and phases of the semi-diurnal and diurnal tidal constituents observed between the ocean and inland locations. However, the monochromatic model over-predicts the amplitude and under-predicts the lag of the lower-frequency subtidal and fortnightly motions observed in the backbay. A dimensional model that considers all tidal constituents simultaneously, balances the along-channel pressure gradient with quadratic bottom friction, and that includes a time-varying channel water depth, is used to show that that these model-data differences are associated with nonlinear interactions between the tidal constituents that are not included in non-dimensional, monochromatic models. In particular, numerical simulations suggest that the nonlinear interactions induced by quadratic bottom friction modify the amplitude and phase of the subtidal and fortnightly backbay response. This nonlinear effect on the low-frequency (subtidal and fortnightly) motions increases with increasing high-frequency (semi-diurnal) amplitude. The subtidal and fortnightly motions influence water exchange processes, and thus backbay temperature and salinity.
Field observations of the evolution of plunging-wave shapes

(American Geophysical Union, 2021-07-21) O'Dea, Annika ; Brodie, Katherine L. ; Elgar, Steve

There are few high-resolution field observations of the water surface during breaking owing to the difficulty of collecting spatially dense measurements in the surf zone, and thus the factors influencing breaking-wave shape in field conditions remain poorly understood. Here, the shape and evolution of plunging breakers is analyzed quantitatively using three-dimensional scans of the water surface collected at high spatial and temporal resolution with a multi-beam terrestrial lidar scanner. The observed internal void shapes in plunging breakers agree well with previously developed theoretical shapes at the onset of breaking, and become more elongated and less steep as breaking progresses. The normalized void area increases as the local bottom slope steepens and as the breaking depth decreases. The void shape becomes more circular as the local bottom slope and the ratio of breaking water depth to wavelength increase, as well as in conditions with opposing winds.
Estimating nearshore bedform amplitudes with altimeters

( 2005-01-14) Gallagher, E. L. ; Elgar, Steve ; Guza, R. T. ; Thornton, Ed B.

Estimates of the heights of large (0.1-0.4 m heights and 1-10 m horizontal lengths) migrating bedforms on a sandy beach made with fixed, single-point altimeters are similar to heights estimated from profiles across the bedforms made with altimeters mounted on an amphibious vehicle that traversed the surf zone. Unlike many profiling systems, the robust, fixed altimeters can measure bedforms in bubbly, sediment-laden surfzone waters nearly continuously, including during storms, thus allowing investigation of the relationships between bedform heights and near-bottom velocities to be extended to a wide range of wave conditions. The fixed-altimeter observations of migrating bedforms suggest a sandy surfzone seafloor is not always smooth during energetic conditions with strong mean currents and large wave-orbital velocities.
Wave dissipation by muddy seafloors

(American Geophysical Union, 2008-04-12) Elgar, Steve ; Raubenheimer, Britt

Muddy seafloors cause tremendous dissipation of ocean waves. Here, observations and numerical simulations of waves propagating between 5- and 2-m water depths across the muddy Louisiana continental shelf are used to estimate a frequency- and depth-dependent dissipation rate function. Short-period sea (4 s) and swell (7 s) waves are shown to transfer energy to long-period (14 s) infragravity waves, where, in contrast with theories for fluid mud, the observed dissipation rates are highest. The nonlinear energy transfers are most rapid in shallow water, consistent with the unexpected strong increase of the dissipation rate with decreasing depth. These new results may explain why the southwest coast of India offers protection for fishing (and for the 15th century Portuguese fleet) only after large waves and strong currents at the start of the monsoon move nearshore mud banks from about 5- to 2-m water depth. When used with a numerical nonlinear wave model, the new dissipation rate function accurately simulates the large reduction in wave energy observed in the Gulf of Mexico.
Evaluation of video-based linear depth inversion performance and applications using altimeters and hydrographic surveys in a wide range of environmental conditions

(Elsevier, 2018-03-12) Brodie, Katherine L. ; Palmsten, Margaret L. ; Hesser, Tyler J. ; Dickhudt, Patrick J. ; Raubenheimer, Britt ; Ladner, Hannah ; Elgar, Steve

The performance of a linear depth inversion algorithm, cBathy, applied to coastal video imagery was assessed using observations of water depth from vessel-based hydrographic surveys and in-situ altimeters for a wide range of wave conditions (0.3 < significant wave height < 4.3 m) on a sandy Atlantic Ocean beach near Duck, North Carolina. Comparisons of video-based cBathy bathymetry with surveyed bathymetry were similar to previous studies (root mean square error (RMSE) = 0.75 m, bias = −0.26 m). However, the cross-shore locations of the surfzone sandbar in video-derived bathymetry were biased onshore 18–40 m relative to the survey when offshore wave heights exceeded 1.2 m or were greater than half of the bar crest depth, and broke over the sandbar. The onshore bias was 3–4 m when wave heights were less than 0.8 m and were not breaking over the sandbar. Comparisons of video-derived seafloor elevations with in-situ altimeter data at three locations onshore of, near, and offshore of the surfzone sandbar over ∼1 year provide the first assessment of the cBathy technique over a wide range of wave conditions. In the outer surf zone, video-derived results were consistent with long-term patterns of bathymetric change (r2 = 0.64, RMSE = 0.26 m, bias = −0.01 m), particularly when wave heights were less than 1.2 m (r2 = 0.83). However, during storms when wave heights exceeded 3 m, video-based cBathy over-estimated the depth by up to 2 m. Near the sandbar, the sign of depth errors depended on the location relative to wave breaking, with video-based depths overestimated (underestimated) offshore (onshore) of wave breaking in the surfzone. Wave speeds estimated by video-based cBathy at the initiation of wave breaking often were twice the speeds predicted by linear theory, and up to three times faster than linear theory during storms. Estimated wave speeds were half as fast as linear theory predictions at the termination of wave breaking shoreward of the sandbar. These results suggest that video-based cBathy should not be used to track the migration of the surfzone sandbar using data when waves are breaking over the bar nor to quantify morphological evolution during storms. However, these results show that during low energy conditions, cBathy estimates could be used to quantify seasonal patterns of seafloor evolution.
Resonances in an evolving hole in the swash zone

( 2011-08-06) Elgar, Steve ; Raubenheimer, Britt ; Thomson, James M. ; Moulton, Melissa

Water oscillations observed in a 10-m diameter, 2-m deep hole excavated on the foreshore just above the low-tide line on an ocean beach are consistent with theory. When swashes first filled the initially circular hole on the rising tide, the dominant mode observed in the cross-shore velocity was consistent with a zero-order Bessel function solution (sloshing back and forth). As the tide rose and swash transported sediment, the hole diameter decreased, the water depth inside the hole remained approximately constant, and the frequency of the sloshing mode increased according to theory. About an hour after the swashes first reached the hole, it had evolved from a closed circle to a semi-circle, open to the ocean. When the hole was nearly semi-circular, the observed cross-shore velocity had two spectral peaks, one associated with the sloshing of a closed circle, the other associated with a quarter-wavelength mode in an open semi-circle, both consistent with theory. As the hole evolved further toward a fully semi-circular shape, the circular sloshing mode decreased, while the quarter-wavelength mode became dominant.
Comparison of rip current hazard likelihood forecasts with observed rip current speeds

(American Meteorological Society, 2017-08-28) Moulton, Melissa ; Dusek, Gregory ; Elgar, Steve ; Raubenheimer, Britt

Although rip currents are a major hazard for beachgoers, the relationship between the danger to swimmers and the physical properties of rip current circulation is not well understood. Here, the relationship between statistical model estimates of hazardous rip current likelihood and in situ velocity observations is assessed. The statistical model is part of a forecasting system that is being made operational by the National Weather Service to predict rip current hazard likelihood as a function of wave conditions and water level. The temporal variability of rip current speeds (offshore-directed currents) observed on an energetic sandy beach is correlated with the hindcasted hazard likelihood for a wide range of conditions. High likelihoods and rip current speeds occurred for low water levels, nearly shore-normal wave angles, and moderate or larger wave heights. The relationship between modeled hazard likelihood and the frequency with which rip current speeds exceeded a threshold was assessed for a range of threshold speeds. The frequency of occurrence of high (threshold exceeding) rip current speeds is consistent with the modeled probability of hazard, with a maximum Brier skill score of 0.65 for a threshold speed of 0.23 m s−1, and skill scores greater than 0.60 for threshold speeds between 0.15 and 0.30 m s−1. The results suggest that rip current speed may be an effective proxy for hazard level and that speeds greater than ~0.2 m s−1 may be hazardous to swimmers.
Nearshore vertical pore pressure gradients and onshore sediment transport under tropical storm forcing

(American Society of Civil Engineers, 2022-09-14) Florence Matthew ; Stark Nina ; Raubenheimer Britt ; Elgar Steve

Colocated sediment pore pressures at depths of approximately 0.02 and 0.22 m below the sand surface and near-bed water velocities were measured for approximately 2 weeks in approximately 1 m mean water depth on an ocean beach near Duck, North Carolina. These measurements suggest that storm wave-driven liquefaction processes may enhance local shoreward sediment transport. During the passage of tropical storm Melissa, wave heights in 26-m water depth (NDBC 44100) were 1–4 m, and storm surge (approximately 1 m) and wave-induced setup increased the water depth on the beach. Upward vertical gradients in pressure heads between the sensors increased with the storm approach, with the largest values observed before the maxima in local wave heights, wave periods, and water depths. The large gradients in pore pressure exceeded several liquefaction criteria and usually occurred when near-bed velocities were upward- and shoreward-directed.
Hydrodynamic and sediment transport modeling of New River Inlet (NC) under the interaction of tides and waves

(John Wiley & Sons, 2015-06-07) Chen, Jia-Lin ; Hsu, Tian-Jian ; Shi, Fengyan ; Raubenheimer, Britt ; Elgar, Steve

The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3-D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semidiurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves.
Physical linkages between an offshore canyon and surf zone morphologic change

(John Wiley & Sons, 2017-04-29) Hansen, Jeff E. ; Raubenheimer, Britt ; Elgar, Steve ; List, Jeffrey H. ; Lippmann, Thomas C.

The causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model (Delft3D/SWAN). Numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross-shore integrated (between 0 and 6.5 m water depths) accretion and erosion observed in a ∼300 m alongshore region, a few hundred meters from the canyon head. The observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients. The location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon, suggesting that bathymetric features on the inner shelf can have first-order effects on short-term nearshore morphologic change.
Field evidence of inverse energy cascades in the surfzone

(American Meteorological Society, 2020-08-01) Elgar, Steve ; Raubenheimer, Britt

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.
A surfzone morphological diffusivity estimated from the evolution of excavated holes

(John Wiley & Sons, 2014-07-14) Moulton, Melissa ; Elgar, Steve ; Raubenheimer, Britt

Downslope gravity-driven sediment transport smooths steep nearshore bathymetric features, such as channels, bars, troughs, cusps, mounds, pits, scarps, and bedforms. Downslope transport appears approximately as a diffusive term in the sediment continuity equation predicting changes in bed level, with a morphological diffusivity controlling the rate of seafloor smoothing. Despite the importance of surfzone sediment transport and morphological evolution, the size of the downslope transport term in nearshore models varies widely, and theories have not been tested with field measurements. Here observations of the infill of large excavated holes in an energetic inner surf zone provide the first opportunity to infer the morphological diffusivity in the field. The estimated diffusion coefficient is consistent with a theoretical bedload morphological diffusivity that scales with the three-halves power of the representative bed shear stress.
Currents in a small tidal-flat channel

( 2010-06) Elgar, Steve ; Raubenheimer, Britt

Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a shallow (0.25 to 0.40 m deep) channel. For water depths between 0.5 and 2.5 m (when both current meters are submerged), current speeds 0.13 m above the bed on the flat are about 30% greater than those observed 0.13 m above the bed in the channel, and are approximately equal to those observed 0.58 m above the channel bed (0.26 m above the flat elevation). Flow directions on the flat are similar to those in the channel. For flows parallel to the channel axis, the ratio of speeds 0.13 m above the bed on the flat to those 0.13 m above the bed in the channel decreases from about 1.4 to about 1.1 with increasing water depth, consistent with conservation of mass. For flows directed across the channel axis, the ratio of speeds increases from about 1.3 to about 2.2 with increasing water depth. The corresponding ratio of the vertical velocity variances (a proxy for turbulence) decreases from about 1.5 to about 0.2, suggesting that the turbulence near the bed of the channel is greater than that near the bed of the flat for water depths greater than about 1.0 m. In contrast, for along-channel flows, the channel and flat turbulence levels are similar. Drag coefficients estimated with the vertical velocity variance or with a cross-shore momentum balance are approximately 70% larger in the channel than over the visually smoother flat.
Curvature‐ and wind‐driven cross‐channel flows at an unstratified tidal bend

(John Wiley & Sons, 2018-04-19) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve

Observations of currents, water levels, winds, and bathymetry collected for a month at an unstratified, narrow (150 m), shallow (8 m), 90° tidal inlet bend are used to evaluate an analytical model for curvature‐driven flow and the effects of local wind on the cross‐channel circulation. Along‐channel flows ranged from −1.0 to 1.4 m/s (positive is inland), and the magnitudes of cross‐channel flows were roughly 0.1–0.2 m/s near the outer bank of the bend. Cross‐channel observations suggest the lateral sea‐surface gradients and along‐channel flows are tidally asymmetric and spatially variable. The depth‐averaged along‐channel dynamics are consistent with a balance between the surface tilt and centrifugal acceleration. The vertical structure and magnitude of cross‐channel flows during weak winds are consistent with a one‐dimensional depth‐varying balance between centrifugal acceleration, bottom stress, and diffusion. Low‐passed (to remove tides) surface and bottom cross‐channel flows are correlated (r2 = 0.5–0.7) with cross‐channel wind velocity, suggesting that winds can enhance or degrade the local‐curvature‐induced, two‐layer flow and can drive three‐layer flow. The observed flow response to the wind is larger than that expected from a one‐dimensional balance, suggesting that two‐dimensional and three‐dimensional processes may be important.

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