Export 6 results:
Sort by: Author Title Type [ Year  (Desc)]
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.

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.  2006.  Refraction of surface gravity waves by shear waves. Journal of Physical Oceanography. 36:629-635.   10.1175/jpo2890.1   AbstractWebsite

Previous field observations indicate that the directional spread of swell-frequency (nominally 0.1 Hz) surface gravity waves increases during shoreward propagation across the surf zone. This directional broadening contrasts with the narrowing observed seaward of the surf zone and predicted by Snell's law for bathymetric refraction. Field-observed broadening was predicted by a new model for refraction of swell by lower-frequency (nominally 0.01 Hz) current and elevation fluctuations. The observations and the model suggest that refraction by the cross-shore currents of energetic shear waves contributed substantially to the observed broadening.

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.

Noyes, TJ, Guza RT, Elgar S, Herbers THC.  2004.  Field observations of shear waves in the surf zone. Journal of Geophysical Research-Oceans. 109   10.1029/2002jc001761   AbstractWebsite

Alongshore propagating meanders of the mean alongshore current in the surf zone called shear waves have periods of a few minutes and wavelengths of a few hundred meters. Here shear wave properties are estimated with arrays of current meters deployed for 4 months within 300 m of the shoreline of a sandy beach. Shear wave velocity fluctuations are approximately horizontally isotropic, with root mean square values between 10 and 40% of the mean (3-hour-averaged) alongshore current V. Cross-shore variations of the time-averaged shear wave momentum flux are consistent with shear wave energy generation close to shore where the breaking wave-driven mean alongshore current V and current shear V-x are strong and with shear wave energy dissipation and transfer back to the mean flow farther offshore where V and V-x are weak. In case studies where V is a narrow jet near the shoreline the observed strong decay of shear wave energy levels seaward of the jet, and the cross-shore and alongshore structure of shear waves within the jet, are similar to predictions based on the linearly unstable modes of the observed V. Shear wave energy levels also are high in a marginally unstable case with a strong, but weakly sheared, V.

Noyes, TJA, Guza RT, Elgar S, Herbers THC.  2002.  Comparison of methods for estimating nearshore shear wave variance. Journal of Atmospheric and Oceanic Technology. 19:136-143.   10.1175/1520-0426(2002)019<0136:comfen>;2   AbstractWebsite

Shear waves (instabilities of the breaking wave-driven mean alongshore current) and gravity waves both contribute substantial velocity fluctuations to nearshore infragravity motions (periods of a few minutes). Three existing methods of estimating the shear wave contribution to the infragravity velocity variance are compared using extensive field observations. The iterative maximum likelihood estimator (IMLE) and the direct estimator (DE) methods use an alongshore array of current meters, and ascribe all the velocity variance at non-gravity wavenumbers to shear waves. The ratio (R) method uses a collocated pressure gauge and current meter, and assumes that shear wave pressure fluctuations are small, and that the kinetic and potential energies of gravity waves are equal. The shear wave velocity variance [q(sw)(2)] estimated from the relative magnitudes of the total (shear plus gravity wave) pressure and velocity variances. Estimates of root-mean-square shear wave velocity fluctuations root [q(sw)(2)] from all three methods are generally in good agreement (correlations >0.96), supporting the validity of their underlying assumptions. When root [q(sw)(2)] is greater than a few centimeters per second, IMLE and DE estimates of root [q(sw)(2)] differ by less than 10%. The R estimates of root [q(sw)(2)] are usually higher than the IMLE and DE estimates, and on average the R method attributes 15% more of the total horizontal velocity variance to shear waves than is attributed by the IMLE method. When mean currents and shear waves are weak, all three estimators are noisy and biased high.