Field evidence of inverse energy cascades in the surfzone

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(8),(2020): 2315-2321, doi:10.1175/JPO-D-19-0327.1.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.Funding was provided by a Vannevar Bush Faculty Fellowship [from OUSD(R&E)] and NSF

Observations and model simulations of wave-current interaction on the inner shelf

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016); 198–208, doi:10.1002/2015JC010788.Wave directions and mean currents observed for two 1 month long periods in 7 and 2 m water depths along 11 km of the southern shoreline of Martha's Vineyard, MA, have strong tidal modulations. Wave directions are modulated by as much as 70° over a tidal cycle. The magnitude of the tidal modulations in the wavefield decreases alongshore to the west, consistent with the observed decrease in tidal currents from 2.1 to 0.2 m/s along the shoreline. A numerical model (SWAN and Deflt3D-FLOW) simulating waves and currents reproduces the observations accurately. Model simulations with and without wave-current interaction and tidal depth changes demonstrate that the observed tidal modulations of the wavefield primarily are caused by wave-current interaction and not by tidal changes to water depths over the nearby complex shoals.ONR, NSF, Sea Grant, NDSEG, an MIT Presidential Graduate Fellowship, and ASD(R&E)2016-07-1

A surfzone morphological diffusivity estimated from the evolution of excavated holes

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 4628–4636, doi:10.1002/2014GL060519.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.Funding was provided by the Assistant Secretary of Defense for Research and Engineering, a National Defense Science and Engineering Graduate Fellowship, a National Science Foundation Graduate Research Fellowship, and the Office of Naval Research.2015-01-1

Storm impact on morphological evolution of a sandy inlet

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 5751-5762, doi:10.1029/2017JC013708.Observations of waves, currents, and bathymetric change in shallow water (8 m). A numerical model (Delft3D, 2DH mode) simulating waves, currents, and morphological change reproduces the observations with the inclusion of hurricane force winds and sediment transport parameters adjusted based on model‐data comparisons. For simulations of short hurricanes and longer nor'easters with identical offshore total time‐integrated wave energy, but different peak wave energies and storm durations, morphological change is correlated (R2 = 0.60) with storm intensity (total energy of the storm divided by the duration of the storm). Similarly, the erosion observed at the Sand Engine in the Netherlands is correlated with storm intensity. The observations and simulations suggest that the temporal distribution of energy in a storm event, as well as the total energy, impacts subsequent nearshore morphological change. Increased storm intensity enhances sediment transport in bathymetrically complex, mixed wave‐and‐tidal‐current energy environments, as well as at other wave‐dominated sandy beaches.National‐Security‐Science‐and‐Engineering and Vannevar‐Bush Faculty Fellowships; National Oceanic and Atmospheric Administration Sea Grant; National Science Foundatio

Flow separation effects on shoreline sediment transport

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Coastal Engineering 125 (2017): 23–27, doi:10.1016/j.coastaleng.2017.04.007.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.Funding was provided by NSF, Sea Grant (NOAA), NDSEG, ASD(R&E), and ONR

Wave-driven along-channel subtidal flows in a well-mixed ocean inlet

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 2987–3001, doi:10.1002/2014JC009839.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.Funding was provided by the Office of Naval Research, the Office of the Assistant Secretary of Defense for Research and Engineering, and a National Defense Science and Engineering Graduate Fellowship.2014-11-2

Curvature‐ and wind‐driven cross‐channel flows at an unstratified tidal bend

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 3832-3843, doi:10.1029/2017JC013722.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.Funding was provided by the Office of Naval Research, a National Defense Science and Engineering Graduate award, and a Vannevar Bush Faculty Fellowship from the Office of the Assistant Secretary of Defense for Research and Engineering.2018-10-1

Vorticity generation by short-crested wave breaking

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L24604, doi:10.1029/2012GL054034.Eddies and vortices associated with breaking waves rapidly disperse pollution, nutrients, and terrestrial material along the coast. Although theory and numerical models suggest that vorticity is generated near the ends of a breaking wave crest, this hypothesis has not been tested in the field. Here we report the first observations of wave-generated vertical vorticity (e.g., horizontal eddies), and find that individual short-crested breaking waves generate significant vorticity [O(0.01 s−1)] in the surfzone. Left- and right-handed wave ends generate vorticity of opposite sign, consistent with theory. In contrast to theory, the observed vorticity also increases inside the breaking crest, possibly owing to onshore advection of vorticity generated at previous stages of breaking or from the shape of the breaking region. Short-crested breaking transferred energy from incident waves to lower frequency rotational motions that are a primary mechanism for dispersion near the shoreline.Funding was provided by a National Security Science and Engineering Faculty Fellowship, the Office of Naval Research, and a Woods Hole Oceanographic Institution Postdoctoral Fellowship.2013-06-2

Observations of swash zone velocities : a note on friction coefficients

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C01027, doi:10.1029/2003JC001877.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).This research was supported by ONR and NSF

Comparison of rip current hazard likelihood forecasts with observed rip current speeds

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Weather and Forecasting 32 (2017): 1659-1666, doi:10.1175/WAF-D-17-0076.1.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.Funding was provided by the National Science Foundation (1232910, 1332705, and 1536365), and by National Security Science and Engineering and Vannevar Bush Faculty Fellowships funded by the assistant secretary of Defense for Research and Engineering.2018-02-2