Herbers T. H. C.

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T. H. C.

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Now showing 1 - 9 of 9
  • Article
    Model-data comparisons of shear waves in the nearshore
    (American Geophysical Union, 2005-05-27) Noyes, T. James ; Guza, R. T. ; Feddersen, Falk ; Elgar, Steve ; Herbers, T. H. C.
    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 Özkan-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 (∼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.
  • Article
    Observing directional properties of ocean swell with an Acoustic Doppler current profiler (ADCP)
    (American Meteorological Society, 2010-01) Herbers, T. H. C. ; Lentz, Steven J.
    Acoustic Doppler current profilers (ADCPs) are widely used for routine measurements of ocean currents and waves in coastal environments. These instruments have the basic capability to measure surface wave frequency–directional spectra, but the quality of the estimates is not well understood because of the relatively high noise levels in the velocity measurements. In this study, wave data are evaluated from two 600-kHz ADCP instruments deployed at 20- and 45-m depths on the Southern California continental shelf. A simple parametric estimation technique is presented that provides robust estimates of the gross directional wave properties, even when the data quality is marginal, as was often the case in this benign wave environment. Good agreement of mean direction and (to a lesser degree) directional spreading estimates with measurements from a nearby surface-following buoy confirms that reliable wave information can generally be extracted from ADCP measurements on the continental shelf, supporting the instrument’s suitability for routine wave-monitoring applications.
  • Article
    Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone
    (American Geophysical Union, 2006-03-01) Thomson, James M. ; Elgar, Steve ; Raubenheimer, Britt ; Herbers, T. H. C. ; Guza, R. T.
    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.
  • Article
    Reflection and tunneling of ocean waves observed at a submarine canyon
    (American Geophysical Union, 2005-04-07) Thomson, James M. ; Elgar, Steve ; Herbers, T. H. C.
    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 longwave 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.
  • Article
    Nonlinear generation and loss of infragravity wave energy
    (American Geophysical Union, 2006-12-08) Henderson, Stephen M. ; Guza, R. T. ; Elgar, Steve ; Herbers, T. H. C. ; Bowen, A. J.
    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.
  • Article
    Field observations of shear waves in the surf zone
    (American Geophysical Union, 2004-01-31) Noyes, T. James ; Guza, R. T. ; Elgar, Steve ; Herbers, T. H. C.
    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 Vx are strong and with shear wave energy dissipation and transfer back to the mean flow farther offshore where V and Vx 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.
  • Article
    Refraction of surface gravity waves by shear waves
    (American Meteorological Society, 2006-04) Henderson, Stephen M. ; Guza, R. T. ; Elgar, Steve ; Herbers, T. H. C.
    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.
  • Preprint
    Wave evolution across the Louisiana shelf
    ( 2012-10) Engelstad, Anita ; Janssen, T. T. ; Herbers, T. H. C. ; van Vledder, Gerbrant ; Elgar, Steve ; Raubenheimer, Britt ; Trainor, Lincoln ; Garcia-Garcia, Ana
    Observations and third-generation wave model hindcasts of ocean surface gravity waves propagating across the Louisiana shelf show that the effects of the mud environment on wave evolution are complex and episodic. Whereas low-frequency waves (0.04-0.20 Hz) show a consistent decay similar to earlier studies, the presence of mud also appears to suppress the development of short waves (0.20-0.25 Hz) under fetch-limited growth conditions. Significant suppression of wave development under wind-forced conditions is found to occur almost exclusively during easterly winds when satellite images show the Atchafalaya mud plume extends into the study area. These results suggest that episodic sediment suspension events with high mud concentrations in the upper water column can affect the evolution of wind waves.
  • Article
    Refraction and reflection of infragravity waves near submarine canyons
    (American Geophysical Union, 2007-10-10) Thomson, James M. ; Elgar, Steve ; Herbers, T. H. C. ; Raubenheimer, Britt ; Guza, R. T.
    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.