Publications

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2011
Gorrell, L, Raubenheimer B, Elgar S, Guza RT.  2011.  SWAN predictions of waves observed in shallow water onshore of complex bathymetry. Coastal Engineering. 58:510-516.   10.1016/j.coastaleng.2011.01.013   AbstractWebsite

SWAN model predictions, initialized with directional wave buoy observations in 550-m water depth offshore of a steep, submarine canyon, are compared with wave observations in 5.0-, 2.5-, and 1.0-m water depths. Although the model assumptions include small bottom slopes, the alongshore variations of the nearshore wave field caused by refraction over the steep canyon are predicted well over the 50 days of observations. For example, in 2.5-m water depth, the observed and predicted wave heights vary by up to a factor of 4 over about 1000 m alongshore, and wave directions vary by up to about 10, sometimes changing from south to north of shore normal. Root-mean-square errors of the predicted wave heights, mean directions, periods, and radiation stresses (less than 0.13 m, 5 degrees, 1 s, and 0.05 m(3)/s(2) respectively) are similar near and far from the canyon. Squared correlations between the observed and predicted wave heights usually are greater than 0.8 in all water depths. However, the correlations for mean directions and radiation stresses decrease with decreasing water depth as waves refract and become normally incident. Although mean wave properties observed in shallow water are predicted accurately, nonlinear energy transfers from near-resonant triads are not modeled well, and the observed and predicted wave energy spectra can differ significantly at frequencies greater than the spectral peak, especially for narrow-band swell. (C) 2011 Elsevier B.V. All rights reserved.

2007
Thomson, J, Elgar S, Herbers THC, Raubenheimer B, Guza RT.  2007.  Refraction and reflection of infragravity waves near submarine canyons. Journal of Geophysical Research-Oceans. 112   10.1029/2007jc004227   AbstractWebsite

[1] The propagation of infragravity waves ( ocean surface waves with periods from 20 to 200 s) over complex inner shelf ( water depths from about 3 to 50 m) bathymetry is investigated with field observations from the southern California coast. A wave-ray-path-based model is used to describe radiation from adjacent beaches, refraction over slopes ( smooth changes in bathymetry), and partial reflection from submarine canyons ( sharp changes in bathymetry). In both the field observations and the model simulations the importance of the canyons depends on the directional spectrum of the infragravity wave field radiating from the shoreline and on the distance from the canyons. Averaged over the wide range of conditions observed, a refraction-only model has reduced skill near the abrupt bathymetry, whereas a combined refraction and reflection model accurately describes the distribution of infragravity wave energy on the inner shelf, including the localized effects of steep-walled submarine canyons.

2006
Thomson, J, Elgar S, Raubenheimer B, Herbers THC, Guza RT.  2006.  Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone. Geophysical Research Letters. 33   10.1029/2005gl025514   AbstractWebsite

The strong tidal modulation of infragravity (200 to 20 s period) waves observed on the southern California shelf is shown to be the result of nonlinear transfers of energy from these low-frequency long waves to higher-frequency motions. The energy loss occurs in the surfzone, and is stronger as waves propagate over the convex low-tide beach profile than over the concave high-tide profile, resulting in a tidal modulation of seaward-radiated infragravity energy. Although previous studies have attributed infragravity energy losses in the surfzone to bottom drag and turbulence, theoretical estimates using both observations and numerical simulations suggest nonlinear transfers dominate. The observed beach profiles and energy transfers are similar along several km of the southern California coast, providing a mechanism for the tidal modulation of infragravity waves observed in bottom-pressure and seismic records on the continental shelf and in the deep ocean.

Henderson, SM, Guza RT, Elgar S, Herbers THC, Bowen AJ.  2006.  Nonlinear generation and loss of infragravity wave energy. Journal of Geophysical Research-Oceans. 111   10.1029/2006jc003539   AbstractWebsite

[1] Nonlinear energy transfers with sea and swell (frequencies 0.05-0.40 Hz) were responsible for much of the generation and loss of infragravity wave energy (frequencies 0.005-0.050 Hz) observed under moderate- and low-energy conditions on a natural beach. Cases with energetic shear waves were excluded, and mean currents, a likely shear wave energy source, were neglected. Within 150 m of the shore, estimated nonlinear energy transfers to ( or from) the infragravity band roughly balanced the divergence (or convergence) of the infragravity energy flux, consistent with a conservative energy equation. Addition of significant dissipation (requiring a bottom drag coefficient exceeding about 10(-2)) degraded the energy balance.

2005
Elgar, S, Raubenheimer B, Guza RT.  2005.  Quality control of acoustic Doppler velocimeter data in the surfzone. Measurement Science & Technology. 16:1889-1893.   10.1088/0957-0233/16/10/002   AbstractWebsite

Acoustic Doppler velocimeter measurements in the surfzone can be corrupted by bubbles and suspended sediment, lack of submergence during the passage of wave troughs, biofouling, blockage (e.g., from kelp on instrument mounting frames) of the flow field near the current meter or of the path between the sampled fluid volume and the acoustic transducers, and by insufficient distance between an accreting seafloor and the sample volume. Individual bad acoustic Doppler velocity values can be detected (and subsequently replaced) from low along-beam signal-to-noise ratios and from low coherence between successive acoustic returns used to estimate velocity. In addition, corrupted data runs can be identified from ratios of pressure to velocity variance that deviate from linear theory, and from low coherence between time series of collocated pressure and wave-orbital velocities. Unmeasured vertical tilts of a current meter can be estimated from horizontal and vertical velocities, and corrected for numerically.

Sheremet, A, Guza RT, Herbers THC.  2005.  A new estimator for directional properties of nearshore waves. Journal of Geophysical Research-Oceans. 110   10.1029/2003jc002236   AbstractWebsite

The infragravity wave (periods between roughly 20 and 200 s) energy balance in shallow, nearshore waters is believed to be effected by generation by groups of sea and swell, dissipation, shoreline reflection, and refractive trapping. Observations obtained with alongshore oriented arrays of current meters or pressure gauges have been previously used to identify concentrations of energy at the frequency-alongshore wavenumbers of refractively trapped edge waves, but seaward and shoreward propagating waves were not differentiated. Surfzone dissipation theoretically limits edge wave growth, and a different analysis (using the approximation of shore-normal propagation) shows that the energy flux of shoreward propagating infragravity waves decreases owing to surfzone dissipation. Here an estimator is developed that yields the alongshore wavenumber-frequency spectra of seaward and shoreward propagating waves, using the WKB approximation and observations from an alongshore-oriented array of pressure and velocity sensors. Example spectra, estimated using data from the spatially sparse and relatively short SandyDuck arrays, suggests that strong dissipation of shoreward propagating infragravity waves occurs over a wide range of alongshore wavenumbers, effectively suppressing the excitation of edge wave modes.

2003
Herbers, THC, Orzech M, Elgar S, Guza RT.  2003.  Shoaling transformation of wave frequency-directional spectra. Journal of Geophysical Research-Oceans. 108   10.1029/2001jc001304   AbstractWebsite

[1] 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.

2002
Sheremet, A, Guza RT, Elgar S, Herbers THC.  2002.  Observations of nearshore infragravity waves: Seaward and shoreward propagating components. Journal of Geophysical Research-Oceans. 107   10.1029/2001jc000970   AbstractWebsite

[1] The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross-shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross-shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross-shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 and 1.5.

2001
Elgar, S, Gallagher EL, Guza RT.  2001.  Nearshore sandbar migration. Journal of Geophysical Research-Oceans. 106:11623-11627.   10.1029/2000jc000389   AbstractWebsite

Field observations suggest that onshore sandbar migration, observed when breaking-wave-driven mean flows are weak, may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface waves. Large accelerations (both increases and decreases in velocity magnitudes), previously suggested to increase sediment suspension, occur under the steep wave faces that immediately precede the maximum onshore-directed orbital velocities. Weaker accelerations occur under the gently sloping rear wave faces that precede the maximum offshore-directed velocities. The timing of strong accelerations relative to onshore flow is hypothesized to produce net onshore sediment transport. The observed acceleration skewness, a measure of the difference in the magnitudes of accelerations under the front and rear wave faces, is maximum near the sandbar crest. The corresponding cross-shore gradients of an acceleration-related onshore sediment transport would cause erosion offshore and accretion onshore of the bar crest, consistent with the observed onshore migration of the bar crest. Furthermore, the observations and numerical simulations of nonlinear shallow water waves show that the region of strongly skewed accelerations moves shoreward with the bar, suggesting that feedback between waves and evolving morphology can result in continuing onshore bar migration.

1999
Herbers, THC, Elgar S, Guza RT.  1999.  Directional spreading of waves in the nearshore. Journal of Geophysical Research-Oceans. 104:7683-7693.   10.1029/1998jc900092   AbstractWebsite

Observations of surface gravity waves shoaling between 8-m water depth and the shoreline on a barred beach indicate that breaking results in an increase in the directional spread of wave energy, in contrast to the directional narrowing with decreasing depth predicted by refraction theory (Snell's law). During low-energy wave conditions, when breaking-induced wave energy losses over the instrumented transect are small, the observed mean propagation direction and spread about the mean both decrease with decreasing depth, consistent with the expected effects of refraction. Nonlinearity causes high-frequency components of the spectrum to become directionally aligned with the dominant incident waves. During high-energy wave conditions with significant wave breaking on the sand bar, the observed mean directions still decrease with decreasing depth. However, the observed directional spreads increase sharply (nominally a factor of 2 for values integrated over the swell-sea frequency range) between the outer edge of the surf zone and the crest of the sand bar, followed by a decrease toward the shoreline. Observations on a nonbarred beach also show directional broadening, with spreads increasing monotonically from the outer edge of the surf zone to a maximum value near the shoreline. Although the mechanism is not understood, these spatial patterns of directional broadening suggest that wave breaking causes significant scattering of incident wave energy into obliquely propagating components.

1997
Elgar, S, Guza RT, Raubenheimer B, Herbers THC, Gallagher EL.  1997.  Spectral evolution of shoaling and breaking waves on a barred beach. Journal of Geophysical Research-Oceans. 102:15797-15805.   10.1029/97jc01010   AbstractWebsite

Field observations and numerical model predictions are used to investigate the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach. Nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic (about 0.8-m significant wave height), narrowband swell propagated without breaking across an 80-m-wide, nearly flat (2-m depth) section of beach between a small offshore sand bar and a steep (slope = 0.1) beach face, where the waves finally broke. These nonlinear energy transfers are accurately predicted by a model based on the nondissipative, unidirectional (i.e., reflection is. neglected) Boussinesq equations. For a lower-energy (wave height about 0.4 m) bimodal wave field, high-frequency seas dissipated in the surf zone; but lower-frequency swell partially reflected from the steep beach face, resulting in significant cross-shore modulation of swell energy. The combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations. Boussinesq model predictions on the flat section (where dissipation is weak) are improved by decomposing the wave field into seaward and shoreward propagating components. In more energetic (wave heights greater than 1 m) conditions, reflection is negligible, and the region of significant dissipation can extend well seaward of the sand bar. Differences between observed decreases in spectral levels and Boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum pf breaking wave induced dissipation between adjacent measurement locations. The inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates.

1994
Herbers, THC, Guza RT.  1994.  Nonlinear-Wave Interactions and High-Frequency Sea-Floor Pressure. Journal of Geophysical Research-Oceans. 99:10035-10048.   10.1029/94jc00054   AbstractWebsite

Linear wave theory predicts that pressure fluctuations induced by wind-generated surface gravity waves are maximum at the ocean surface and strongly attenuated at depths exceeding a horizontal wavelength. Although pressure fluctuations observed at the seafloor in deep water are indeed relatively weak at wind-wave frequencies, the energy at double wind-wave frequencies is frequently much higher than predicted by applying linear wave theory to near-surface measurements. These double-frequency waves can in theory be excited by nonlinear interactions between two surface wave components of about equal frequency, traveling in nearly opposing directions. Observations from a large aperture, 24-element array of pressure sensors deployed in 13-m depth are presented that quantitatively support this generation mechanism. As in previous studies, dramatic increases in the spectral levels of seafloor pressure at double wind-wave frequencies (0.3-0.7 Hz) frequently occurred after a sudden veering in wind direction resulted in waves propagating obliquely to preexisting seas. The observed spectral levels and vector wavenumbers of these double-frequency pressure fluctuations agree well with predictions obtained by applying second-order nonlinear, finite depth wave theory (Hasselmann, 1962) to the observed directionally bimodal seas. High-frequency seafloor pressure spectral levels also increased in response to directionally narrower but more energetic seas generated by strong, steady or slowly rotating winds. Bispectral analysis suggests that these pressure fluctuations are generated by nonlinear mechanisms similar to the veering wind cases.

1993
Elgar, S, Guza RT, Freilich MH.  1993.  Dispersion, Nonlinearity, and Viscosity in Shallow-Water Waves. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 119:351-366.   10.1061/(asce)0733-950x(1993)119:4(351)   AbstractWebsite

The roles of frequency dispersion, nonlinearity, and laminar viscosity in the evolution of long waves over distances of many wavelengths in constant water depth are investigated with numerical solutions of the Boussinesq equations. Pronounced frequency doubling and trebling is predicted, and the initial evolution to a wave shape with a pitched-forward front face and peaky crests is followed by development of a steep rear face and a nearly symmetric crest/trough profile. While reducing overall energy levels, laminar viscosity acts to prolong cycling of third moments and to inhibit the onset of disordered evolution characteristic of nonlinear, inviscid systems. Preliminary laboratory results show some qualitative similarities to the numerical simulations. However, these laboratory experiments were not suitable for detailed model-data comparisons because dissipation in the flume could not be accounted for with either laminar or quadratic damping models. More carefully controlled experiments are required to assess the importance of viscosity (and the accuracy of the Boussinesq model) in the evolution of nonlinear waves over distances of many wavelengths.

1992
Elgar, S, Guza RT, Freilich MH, Briggs MJ.  1992.  Laboratory Simulations of Directionally Spread Shoaling Waves. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 118:87-103.   10.1061/(asce)0733-950x(1992)118:1(87)   AbstractWebsite

Field observations of a shoaling, nonbreaking, directionally spread wave field are simulated in a laboratory basin to determine whether laboratory artifacts cause significant distortions of the shoaling process. The laboratory wave field is measured with scaled arrays of surface-elevation sensors similar to the arrays used for the field observations. However, differences in the laboratory and field beach slopes (0.033 and 0.025, respectively) do not allow precise replication of the field conditions in the laboratory. Therefore, a nonlinear wave propagation model with no adjustable parameters (previously successfully compared to a wide range of field data) is used to show that differences between the laboratory and field data sets are caused primarily by the different beach slopes. The observations demonstrate, in agreement with the model, that it is possible to compensate partially for differences in beach slope by altering the initial conditions. With such compensation, the evolution of surface-elevation power spectra, bispectra, and skewness and asymmetry are remarkably similar in the laboratory and field. Frequency-directional spectra measured just outside the surf zone also show similar nonlinear effects in both field and laboratory data. Based on this case study, the laboratory directional wave basin appears to be useful for investigating the linear and nonlinear evolution of random, two-dimensional waves on beaches.