Publications

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2008
Apotsos, A, Raubenheimer B, Elgar S, Guza RT.  2008.  Wave-driven setup and alongshore flows observed onshore of a submarine canyon. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004514   AbstractWebsite

The effect of alongshore variations in the incident wavefield on wave-driven setup and on alongshore flows in the surfzone is investigated using observations collected onshore of a submarine canyon. Wave heights and radiation stresses at the outer edge of the surfzone (water depth approximate to 2.5 m) varied by up to a factor of 4 and 16, respectively, over a 450 m alongshore distance, resulting in setup variations as large as 0.1 m along the shoreline (water depth approximate to 0.3 m). Even with this strong alongshore variability, wave-driven setup was dominated by the cross-shore gradient of the wave radiation stress, and setup observed in the surfzone is predicted well by a one-dimensional cross-shore momentum balance. Both cross-shore radiation stress gradients and alongshore setup gradients contributed to the alongshore flows observed in the inner surfzone when alongshore gradients in offshore wave heights were large, and a simplified alongshore momentum balance suggests that the large [O(1 kg/(s(2) m)] observed setup-induced pressure gradients can drive strong [O(1 m/s)] alongshore currents.

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.

Apotsos, A, Raubenheimer B, Elgar S, Guza RT, Smith JA.  2007.  Effects of wave rollers and bottom stress on wave setup. Journal of Geophysical Research-Oceans. 112   10.1029/2006jc003549   AbstractWebsite

[1] Setup, the increase in the mean water level associated with breaking waves, observed between the shoreline and about 6-m water depth on an ocean beach is predicted well by a model that includes the effects of wave rollers and the bottom stress owing to the mean flow. Over the 90-day observational period, the measured and modeled setups are correlated ( squared correlation above 0.59) and agree within about 30%. Although rollers may affect setup significantly on beaches with large-amplitude ( several meters high) sandbars and may be important in predicting the details of the cross-shore profile of setup, for the data discussed here, rollers have only a small effect on the amount of setup. Conversely, bottom stress ( calculated using eddy viscosity and undertow formulations based on the surface dissipation, and assuming that the eddy viscosity is uniform throughout the water column) significantly affects setup predictions. Neglecting bottom stress results in underprediction of the observed setup in all water depths, with maximum underprediction near the shoreline where the observed setup is largest.

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.

2005
Noyes, TJ, Guza RT, Feddersen F, Elgar S, Herbers THC.  2005.  Model-data comparisons of shear waves in the nearshore. Journal of Geophysical Research-Oceans. 110   10.1029/2004jc002541   AbstractWebsite

[1] 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 Ozkan-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 ( similar to 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.

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.

1995
Herbers, THC, Elgar S, Guza RT.  1995.  Generation and propagation of infragravity waves. Journal of Geophysical Research-Oceans. 100:24863-24872.   10.1029/95jc02680   AbstractWebsite

The generation and propagation of infragravity waves (frequencies nominally 0.004-0.04 Hz) are investigated with data from a 24-element, coherent array of pressure sensors deployed for 9 months in 13-m depth, 2 km from shore. The high correlation between observed ratios of upcoast to downcoast energy fluxes in the infragravity (F-up(IG)/F-down(IG)) and swell (F-up(swell)/F-down(swell)) frequency bands indicates that the directional properties of up infragravity waves are strongly dependent on incident swell propagation directions. However F-up(IG)/F-down(IG) is usually much closer to 1 (i,e., comparable upcoast and downcoast fluxes) than is F-up(swell)/F-down(swell), suggesting that upcoast propagating swell drives both upcoast and downcoast propagating infragravity waves. These observations agree well with predictions of a spectral WKB model based on the long-standing hypothesis that infragravity waves, forced by nonlinear interactions of nonbreaking, shoreward propagating swell, are released as free waves in the surf zone and subsequently reflect from the beach. The radiated free infragravity waves are predicted to be directionally broad and predominantly refractively trapped on a gently sloping shelf. The observed ratios F-sea(IG)/F-shore(IG) of the seaward and shoreward infragravity energy fluxes are indeed scattered about the theoretical value 1 for trapped waves when the swell energy is moderate, but the ratios deviate significantly from 1 with both low- and high-energy swell. Directionally narrow, shoreward propagating infragravity waves, observed with low-energy swell, likely have a remote (possibly trans-oceanic) energy source. High values (up to 5) of F-sea(IG)/F-shore(IG), observed with high-energy swell, suggest that high-mode edge waves generated near the shore can be suppressed by nonlinear dissipation processes (e.g., bottom friction) on the shelf.