Waves running up and down the beach (‘swash’) at the landward edge of the ocean can cause changes to the beach topology, can erode dunes, and can result in inland flooding. Despite the importance of ...swash, field observations are difficult to obtain in the thin, bubbly, and potentially sediment laden fluid layers. Here, swash excursions along an Atlantic Ocean beach are estimated with a new framework, V-BeachNet, that uses a fully convolutional network to distinguish between sand and the moving edge of the wave in rapid sequences of images. V-BeachNet is trained with 16 randomly selected and manually segmented images of the swash zone, and is used to estimate swash excursions along 200 m of the shoreline by automatically segmenting four 1-h sequences of images that span a range of incident wave conditions. Data from a scanning lidar system are used to validate the swash estimates along a cross-shore transect within the camera field of view. V-BeachNet estimates of swash spectra, significant wave heights, and wave-driven setup (increases in the mean water level) agree with those estimated from the lidar data.
•A new framework, V-BeachNet, provides estimates of alongshore variable swash motions using fully convolutional networks.•The study identifies sources of error in the calculation of swash using video images and computer vision algorithms.•V-BeachNet enables efficient extraction of swash data from archived or real-time video footage of the shoreline.
The role of tides in beach cusp development Coco, Giovanni; Burnet, Tom K.; Werner, B. T. ...
Journal of Geophysical Research - Oceans,
April 2004, Letnik:
109, Številka:
C4
Journal Article
Recenzirano
Odprti dostop
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.
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 ~200m 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.
•Erosion and deposition around a curved shoreline (“a corner”) is investigated with a field-verified Delft3D numerical model.•Model simulations reproduce the observed separation of strong ebb flows from the curved coastline.•Simulations test the effects of waves, currents, and a nearby inlet on shoreline sediment transport.•Simulations suggest shoreline sediment transport depends more on corner shape (a proxy for flow separation) than on the effects of a nearby inlet.
Tidal water table fluctuations observed for 27 days in a gently sloped ocean beach are predicted well by numerical models based on the Boussinesq equation driven with the observed 10 min‐averaged ...shoreline (ocean‐beach intersection) motion. Diurnal and semidiurnal water table fluctuations are almost completely damped 100 m landward of the mean shoreline location on this fine‐grained sand beach, but fluctuations at spring‐neap periods (≈14 days) are attenuated less. Comparison of the observations with the predictions suggests that the asymmetries in the water table level time series measured in this study result from nonlinearity owing to the large (relative to the wavelength) horizontal shoreline excursions, rather than from nonlinearity owing to finite‐amplitude water table fluctuations. Cross‐shore variations of the aquifer depth are predicted to have a small effect on the landward decay rate of the water table fluctuations. The seepage face width is predicted accurately and depends on the nonplanar beach profile. In general, the development of a seepage face is predicted to have little effect on the water table level landward of the intertidal region.
Fluid flows consistent with low‐mode edge waves were evident in video observations of swash motions during a field experiment in which beach cusps developed on an initially smooth beach. As beach ...cusps grew, energy lying along low‐mode dispersion curves increased. The most energetic edge‐wave propagation direction changed from upcoast to downcoast as the orientation of the cusp horns rotated. These observations suggest a coupling between morphodynamics and hydrodynamics, and are evidence that beach cusp evolution might control low‐mode edge wave dynamics.
The relationship between microwave imaging radar measurements of fluid velocities in the surf zone and shoaling, breaking, and broken waves is studied with field observations. Normalized radar cross ...section (NRCS) and Doppler velocity are estimated from microwave measurements at near‐grazing angles, and in situ fluid velocities are measured with acoustic Doppler velocimeters (ADVs). Joint histograms of radar cross section and Doppler velocity cluster into identifiable distributions. The NRCS values from pixels with large NRCS and high Doppler velocities (>2 m/s) decrease with decreasing bore height to the shoreline, similar to scattering from a cylinder with decreasing radius. The Doppler velocities associated with these regions in the histograms agree well with theoretical wave phase velocities. Radar and ADV measurements of fluid velocities between bore crests have similarly shaped energy density spectra for frequencies above about 0.1 Hz, but energy levels from the radar are an order of magnitude higher than those of the ADV data. Instantaneous interbore Doppler velocities are correlated with ADV measured fluid velocities but are offset by 0.8 m/s. This offset may be due to Bragg wave phase velocities, wind drift, range and azimuth sidelobes, the finite spatial resolution of the radar, and differences between mean flows measured at the surface with radar and flows measured below the surface with ADVs. Shoaling and breaking waves measured through radar grating lobes significantly affect both the Doppler velocities near the edges of the images and the scattering from the rear faces of waves, causing large Doppler velocities to be observed in these regions.
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.
Key Points
Downslope sediment transport is important for smoothing of steep bathymetry
Excavated surfzone holes filled at rates correlated with bed shear stress
Morphological diffusivity inferred from observations is consistent with theory
A Boussinesq model for the nonlinear transformation of the frequency‐directional spectrum and bispectrum of surface gravity waves propagating over a gently sloping, alongshore uniform beach is ...compared with field and laboratory observations. Outside the surf zone the model predicts the observed spectral evolution, including energy transfers to harmonic components traveling in the direction of the dominant waves, and the cross‐interactions of waves traveling in different directions that transfer energy to components with the vector sum wavenumber. The sea surface elevation skewness and asymmetry, third‐order moments believed to be important for sediment transport, also are predicted well. Effects of surf zone wave breaking are incorporated with a heuristic frequency‐dependent dissipation term in the spectral energy balance equation and an empirical relaxation of the bispectrum to Gaussian statistics. The associated coefficients are calibrated with observations that span a wide range of surf zone conditions. With calibrated coefficients, the model predicts observed surf zone frequency spectra well and surf zone skewness and asymmetry fairly well. The observed directional spectra inside the surf zone are broader than the predicted spectra, suggesting that neglected scattering effects associated with the random onset of wave breaking or with higher‐order nonlinearity may be important.
Abstract 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.
Ocean surface gravity waves with periods between 20 and 200 s were observed to reflect from a steep‐walled submarine canyon. Observations of pressure and velocity on each side of the canyon were ...decomposed into incident waves arriving from distant sources, waves reflected by the canyon, and waves transmitted across the canyon. The observed reflection is consistent with long‐wave theory, and distinguishes between cases of normal and oblique angles of incidence. As much as 60% of the energy of waves approaching the canyon normal to its axis was reflected, except for waves twice as long as the canyon width, which were transmitted across with no reflection. Although waves approaching the canyon at oblique angles cannot propagate over the canyon, total reflection was observed only at frequencies higher than 20 mHz, with lower frequency energy partially transmitted across, analogous to the quantum tunneling of a free particle through a classically impenetrable barrier.
PDF
