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Hally-Rosendahl, K, Feddersen F, Guza RT.  2014.  Cross-shore tracer exchange between the surfzone and inner-shelf. Journal of Geophysical Research-Oceans. 119:4367-4388.   10.1002/2013jc009722   AbstractWebsite

Cross-shore tracer exchange between the surfzone and inner-shelf is examined using temperature and dye measurements at an approximately alongshore-uniform beach. An alongshore-oriented plume is created by releasing dye continuously for 4.5 h in a surfzone alongshore current. The plume is sampled for 13 h from the release point to 700 m downstream, between the shoreline and 250 m offshore (6 m water depth). Within the surfzone (<= 2 m depth), water is relatively warm, and dye is vertically well mixed. On the inner-shelf (3-6 m depth), alongshore currents are weak, and elevated temperature and dye co-occur in 25-50 m wide alongshore patches. Within the patches, dye is approximately depth-uniform in the warm upper 3 m where thermal stratification is weak, but decreases rapidly below 3 m with a strong thermocline. Dye and temperature vertical gradients are correlated, and dye is not observed below 18 degrees C. The observations and a model indicate that, just seaward of the surfzone, thermal stratification inhibits vertical mixing to magnitudes similar to those in the ocean interior. Similar surfzone and inner-shelf mean alongshore dye dilution rates are consistent with inner-shelf dye properties being determined by local cross-shore advection. The alongshore-patchy and warm inner-shelf dye is ejected from the surfzone by transient rip currents. Estimated Stokes drift driven cross-shore exchange is small. The transient rip current driven depth-normalized heat flux out of the surfzone has magnitude similar to those of larger-scale shelf processes. Dye recycling, from the inner-shelf back to the surfzone, is suggested by relatively long surfzone dye residence times.

Omand, MM, Feddersen F, Clark DB, Franks PJS, Leichter JJ, Guza RT.  2009.  Influence of bubbles and sand on chlorophyll-a fluorescence measurements in the surfzone. Limnology and Oceanography-Methods. 7:354-362.   10.4319/lo.2011.56.3.0787   AbstractWebsite

Continuous chlorophyll-a (Chla) measurements in the surfzone (region of wave-breaking adjacent to the shoreline) would increase understanding of harmful algal blooms, food supply for intertidal invertebrates and fishes, and the fate of terrestrial runoff pollution. Optical measurements of Chla fluorescence in the surfzone are affected by bubbles and suspended sand. Here, errors in surfzone Chla fluorescence measurements (using WET Labs ECO Triplet fluorometers) are estimated by comparing observed (Chla(raw)) with known (Chla(true)) Chla concentrations in laboratory tests with controlled amounts of bubbles and suspended sand (characterized with concurrently measured optical turbidity, tau). For both bubbles and sand, Chla raw and t are linearly correlated, and the regression line slope depends on Chla(true). When Chla(true) is low, Chla(raw) is biased high, and when Chla(true) is high, Chla(raw) is biased low. Fluorometers were also deployed in a natural surfzone, and for the limited range of field Chla observed, the field and laboratory tau-Chla relationships were largely consistent. Mechanisms responsible for these biases are proposed, and correction procedures using the observed tau-Chla relationship are developed and applied to surfzone Chla(raw) observations. For the moderate Chla(true) concentrations (2-4 mu g L(-1)) encountered, errors in hourly mean and instantaneous Chla(raw) are less than 5% and 15%, respectively. Larger errors are expected for Chla(true) outside this range. Although further testing is needed, the results suggest that in situ, optical Chla(raw) from other turbid environments (e.g., estuaries, bays) should also be interpreted cautiously.

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.

Spydell, M, Feddersen F, Guza RT, Schmidt WE.  2007.  Observing surf-zone dispersion with drifters. Journal of Physical Oceanography. 37:2920-2939.   10.1175/2007jpo3580.1   AbstractWebsite

Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10-20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height H-s was equal to 0.5 m and mean alongshore currents vertical bar(v) over bar vertical bar were moderate (<0.1 m s(-1)). On the second day, the obliquely incident waves were larger, with Hs equal to 1.4 m, and at some surf-zone locations vertical bar<(v)over bar>vertical bar was greater than 0.5 m s(-1). The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along-and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like D-2(t) similar to t(3/2) and that the relative diffusivity is scale-dependent as mu similar to l(2/3), with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D-2(t) similar to t(3) and mu similar to l(4/3)]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5-50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths.

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.

Feddersen, F, Guza RT.  2003.  Observations of nearshore circulation: Alongshore uniformity. Journal of Geophysical Research-Oceans. 108   10.1029/2001jc001293   AbstractWebsite

Nearshore circulation, observed for 4 months on a 200-m-long stretch of natural beach during the SandyDuck field experiment, is shown to be alongshore uniform. An alongshore momentum balance between (wind and wave) forcing and bottom stress, cross-shore integrated between the shoreline and approximately 4 m water depth, holds on each of five instrumented cross-shore transects (skillgreater than or equal to0.87). The corresponding five best fit drag coefficients are similar, consistent with the assumption that terms in the momentum balance associated with alongshore nonuniformity are negligible. In addition, the alongshore nonuniformity of the circulation and bathymetry were examined at five cross-shore locations. Except near the shoreline, the circulation and bathymetry were rarely strongly alongshore nonuniform, and the circulation nonuniformities were usually no larger than expected from current-meter noise alone. Near the shoreline, the bathymetry was more irregular and the circulation was often detectably nonuniform, although no relationship between bathymetric and circulation nonuniformities was found. The closure of the alongshore momentum balances on cross-shore transects, and the observed alongshore uniformity of the circulation on four of five alongshore transects, demonstrates that the simplified dynamics of alongshore uniform circulation are valid during the experiment.

Feddersen, F, Guza RT, Elgar S, Herbers THC.  1998.  Alongshore momentum balances in the nearshore. Journal of Geophysical Research-Oceans. 103:15667-15676.   10.1029/98jc01270   AbstractWebsite

The one-dimensional, time-averaged (over many wave periods) alongshore momentum balance between forcing by wind and breaking waves and the bottom stress is examined with field observations spanning a wide range of conditions on a barred beach. Near-bottom horizontal currents were measured for 2 months at 15 locations along a cross-shore transect extending 750 m from the shoreline to 8-m water depth. The hourly averaged bottom stress was estimated from observed currents using a quadratic drag law. The wave radiation stress was estimated in 8-m depth from an array of pressure sensors, and the wind stress was estimated from an anemometer at the seaward end of a nearby pier. The combined wind and wave forcing integrated ever the entire cross-shore transect is balanced by the integrated bottom stress. The wind stress contributes about one third of the forcing over the transect. Analysis of the momentum balances in different cross-shore regions shows that in the surf zone, wave forcing is much larger than wind forcing and that the bottom drag coefficient is larger in the surf zone than farther seaward, consistent with earlier studies.

George, R, Flick RE, Guza RT.  1994.  Observations of Turbulence in the Surf Zone. Journal of Geophysical Research-Oceans. 99:801-810.   10.1029/93jc02717   AbstractWebsite

Turbulence generated by waves breaking on a natural beach is examined using hotfilm anemometer data. Turbulence intensity is estimated from dissipation rates determined from wavenumber spectra of short (1/8 s) hotfilm time series. The resulting Froude-scaled turbulence intensities are relatively uniform between the seabed and the wave trough level and are similar in vertical structure but lower in magnitude than in existing laboratory studies. The magnitudes of the turbulence intensities observed in both the field and laboratory are consistent with an existing macroscopic model of bore dissipation in the surf zone. Scaling by this bore model relates turbulence intensities generated by monochromatic waves in small-scale laboratory experiments to those generated by random waves in the natural surf zone.

Herbers, THC, Guza RT.  1993.  Velocity Observations above a Rippled Bed Using Laser-Doppler Velocimetry - Comment. Journal of Geophysical Research-Oceans. 98:20331-20333.   10.1029/93jc01757   AbstractWebsite
Herbers, THC, Guza RT.  1992.  Wind-Wave Nonlinearity Observed at the Sea-Floor .2. Wave-Numbers and 3rd-Order Statistics. Journal of Physical Oceanography. 22:489-504.   10.1175/1520-0485(1992)022<0489:wwnoat>;2   AbstractWebsite

This is Part 2 of a study of nonlinear effects on natural wind-generated surface gravity waves in 13-m depth, 30 km offshore of Virginia. At the sea floor in this depth, free surface gravity waves are only weakly attenuated at sea and swell frequencies (0.05-0.30 Hz) but are very strongly attenuated at frequencies higher than about 0.35 Hz. Hence, above 0.35 Hz, relatively long wavelength forced waves, excited by nonlinear interactions between directionally opposing free wind waves, are exposed at the sea floor. An array of pressure transducers at middepth was used to estimate the frequency-directional spectrum of (free) primary sea and swell waves, and the associated (forced) secondary pressure fluctuations were measured with an array on the sea floor. In Part 1, it was shown that forced-wave energy levels at the sea floor increase sharply in response to directionally opposing wind waves, in agreement with weakly nonlinear theory. In Part 2, wavelengths, propagation directions, and non-Gaussian phase coupling between free and forced waves are examined on three occasions with relatively high forced-wave energy levels. A root-mean-square wavenumber magnitude and a vector-averaged mean wave propagation direction (both functions of frequency) can be expressed accurately in terms of the pressure array cross-spectra. The wavenumber estimates at the sea floor show the theoretically expected sharp transition between a 0.05-0.30 Hz frequency range dominated by free sea and swell waves and a 0.35-0.60 Hz range dominated by forced waves with wavelengths that are long relative to free waves of the same frequency. In the "free-wave frequency range," wavenumber estimates are usually well within 10% of the linear dispersion relation and wave direction estimates are in excellent agreement with the directional spectra extracted from the middepth an-ay. In the "forced-wave frequency range," wavenumber and direction estimates agree with nonlinear theory predictions, confirming that the observed forced waves have the sum vector wavenumber of the interacting directionally opposing wind waves. Phase coupling between free and forced waves is examined through third-order statistics of the sea floor pressure data. Consistent with theory, the normalized bispectrum has small imaginary parts scattered approximately randomly about zero and relatively large negative real parts at frequencies that correspond to directionally opposing seas and swell. Estimates of the bispectrum integrated for constant sum frequency confirm that nearly all the energy at double sea frequencies is nonlinearly coupled to directionally opposing wind waves. In qualitative agreement with nonlinear theory predictions, bispectral levels are sometimes significantly reduced by directional spreading of the interacting free waves.

Herbers, THC, Lowe RL, Guza RT.  1991.  Field Verification of Acoustic Doppler Surface Gravity-Wave Measurements. Journal of Geophysical Research-Oceans. 96:17023-17035.   10.1029/91jc01326   AbstractWebsite

A compact acoustic Doppler current meter, designed for nearshore surface gravity wave measurements, was field tested by comparison to a colocated array of pressure transducers. Both measurement systems were bottom mounted in a water depth of 7 m. Each of four acoustic beams, inclined 45-degrees from vertical, measures the along-beam velocity at a single range (1 m) about 1.5 m above the seafloor. These four velocity beams are used to estimate low-order moments of the frequency-directional wave spectrum and are compared to pressure measurements on four occasions. Predictions of the (nondirectional) bottom pressure spectrum at sea and swell frequencies (0.04-0.30 Hz), based on the velocity measurements and linear theory, are in excellent agreement with directly measured pressure. The general level of agreement (gain errors less than 5%) is somewhat better than results reported from similar (but spanning a much wider range of conditions) intercomparison studies using conventional in situ current meters. Observed cross spectra between colocated pressure and horizontal velocity components, frequently used to separate turbulence and wave orbital velocities (assuming that the coherence of wave velocity and pressure is equal to 1), are compared to predictions based on the pressure array data and linear wave theory. The observed and predicted pressure-velocity cross spectra are in excellent agreement and show that large coherence reductions can occur in natural wind waves owing to wave directional spreading effects, despite relatively low turbulence energy levels. Wave radiation stresses, estimated from the velocity measurements, also agree well with estimates extracted from the pressure array data. Overall, the intercomparisons show that the present acoustic Doppler system has directional resolution comparable to a pitch-and-roll buoy, and they suggest that higher-order directional information as well as weak nonlinear properties of natural wind waves may be examined with a slightly modified compact system.

Herbers, THC, Guza RT.  1991.  Wind-Wave Nonlinearity Observed at the Sea-Floor .1. Forced-Wave Energy. Journal of Physical Oceanography. 21:1740-1761.   10.1175/1520-0485(1991)021<1740:wwnoat>;2   AbstractWebsite

This is Part 1 of a study of nonlinear effects on natural wind waves. Array measurements of pressure at the sea floor and middepth, collected 30 km offshore of Virginia in 13-m depth, are compared to an existing theory for weakly nonlinear surface gravity waves. In this depth, free surface waves (obeying the linear dispersion relation) are weakly attenuated at the sea bed at sea and swell frequencies (0.05-0.3 Hz) but very strongly attenuated at frequencies higher than about 0.35 Hz. Only nonlinearly driven motions can reach the sea floor at these high frequencies. Nonlinear interactions between free (primary) waves of about the same frequency, traveling in nearly opposing directions, theoretically excite long-wavelength, double-frequency forced (secondary) waves that are only weakly attenuated at the sea floor and form a mechanisms for the generation of microseisms at great depth. In 13-m depth, wind-generated free waves and corresponding long-wavelength, high-frequency forced waves can be simultaneously observed on the sea floor, and the coupling between the two examined in some detail. Bottom-pressure spectra observed over a 4-day period show large [O(10(2))] fluctuations in high-frequency (0.35-0.6 Hz) forced-wave energy levels at the sea floor occurring in only a few hours. Correspondingly rapid changes in estimates of the free-wave frequency-directional spectrum show that forced-wave energy levels are weak in unidirectional seas and increase dramatically in response to nearly opposing seas, consistent with the theoretical generation mechanism. On one occasion, directionally opposing seas, and a corresponding double-frequency forced-wave peak, followed a rapidly veering wind. However, comparable increases in forced-wave energy levels were observed in response to the arrival of nonlocally generated seas with directions much different than local winds and seas. Although the accuracy of theoretical forced-wave predictions is limited by the directional resolution of the small aperture (20 m x 20 m) middepth array, predicted and observed forced-wave energy levels agree within about a factor of 2. The observed weak decay between middepth and sea-floor wave pressure at double sea frequencies is also consistent with theoretically expected long wavelengths. Wavelengths, propagation directions, and phase coupling between free and forced waves are examined using the bottom-pressure array data in Part 2.