Export 10 results:
Sort by: Author Title Type [ Year  (Desc)]
Fiedler, JW, Smit PB, Brodie KL, McNinch J, Guza RT.  2019.  The offshore boundary condition in surf zone modeling. Coastal Engineering. 143:12-20.   10.1016/j.coastaleng.2018.10.014   AbstractWebsite

Numerical models predicting surfzone waves and shoreline runup in field situations are often initialized with shoreward propagating (sea-swell, and infragravity) waves at an offshore boundary in 10-30 m water depth. We develop an offshore boundary condition, based on Fourier analysis of observations with co-located current and pressure sensors, that accounts for reflection and includes nonlinear phase-coupling. The performance of additional boundary conditions derived with limited or no infragravity observations are explored with the wave resolving, nonlinear model SWASH 1D. In some cases errors in the reduced boundary conditions (applied in 11 m depth) propagate shoreward, whereas in other cases errors are localized near the offshore boundary. Boundary conditions that can be implemented without infragravity observations (e.g. bound waves) do not accurately simulate infragravity waves across the surfzone, and could corrupt predictions of morphologic change. However, the bulk properties of infragravity waves in the inner surfzone and runup are predicted to be largely independent of ig offshore boundary conditions, and dominated by ig generation and dissipation.

Hally-Rosendahl, K, Feddersen F, Clark DB, Guza RT.  2015.  Surfzone to inner-shelf exchange estimated from dye tracer balances. Journal of Geophysical Research-Oceans. 120:6289-6308.   10.1002/2015jc010844   AbstractWebsite

Surfzone and inner-shelf tracer dispersion are observed at an approximately alongshore-uniform beach. Fluorescent Rhodamine WT dye, released near the shoreline continuously for 6.5 h, is advected alongshore by breaking-wave- and wind-driven currents, and ejected offshore from the surfzone to the inner-shelf by transient rip currents. Novel aerial-based multispectral dye concentration images and in situ measurements of dye, waves, and currents provide tracer transport and dilution observations spanning about 350 m cross-shore and 3 km alongshore. Downstream dilution of near-shoreline dye follows power law decay with exponent -0.33, implying that a tenfold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner-shelf dye mass balances close, and in 5 h, roughly half of the surfzone-released dye is transported offshore to the inner-shelf. Observed cross-shore transports are parameterized well ( r2=0.85, best fit slope 0.7) using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference. The best fit cross-shore exchange velocity u*=1.2x10-2ms-1 is similar to a temperature-derived exchange velocity on another day with similar wave conditions. The u* magnitude and observed inner-shelf dye length scales, time scales, and vertical structure indicate the dominance of transient rip currents in surfzone to inner-shelf cross-shore exchange during moderate waves at this alongshore-uniform beach.

Spydell, MS, Feddersen F, Olabarrieta M, Chen JL, Guza RT, Raubenheimer B, Elgar S.  2015.  Observed and modeled drifters at a tidal inlet. Journal of Geophysical Research-Oceans. 120:4825-4844.   10.1002/2014jc010541   AbstractWebsite

Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (> 1 ms(-1)) in two approximately 30 m wide channels that bisect the 1-3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2-D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates mu(in) were small (mu(in) approximate to 0.5 m(2) s (1)). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to <= 0.2 ms(-1), about 1 km from shore), and cluster spreading rates are larger with mu(out) approximate to 3 m(2) s(-1). Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters. The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in mu(out) observed in the outer inlet. However, model spreading rates increase only to mu(out) < 1 m(2) s(-1). Smaller modeled than observed mu(out) may result from using unstratified models. Noncoincident (in space) observations show evidence of a buoyant plume (Delta rho = 1 kg m(-3)) in the outer inlet, likely affecting drifter lateral spreading. Generally, drifter-based model performance is good within the inlet channels where tidal currents are strongest, whereas model-data differences are significant farther offshore.

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.

Spydell, MS, Feddersen F, Guza RT, MacMahan J.  2014.  Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone. Journal of Geophysical Research-Oceans. 119:1022-1037.   10.1002/2013jc009415   AbstractWebsite

Concurrent Lagrangian and Eulerian observations of rotational, low-frequency (10(-4) to 10(-2) Hz) surfzone eddies are compared. Surface drifters were tracked for a few hours on each of 11 days at two alongshore uniform beaches. A cross-shore array of near-bottom current meters extended from near the shoreline to seaward of the surfzone (typically 100 m wide in these moderate wave conditions). Lagrangian and Eulerian mean alongshore velocities V are similar, with a midsurfzone maximum. Cross-shore dependent Lagrangian (sigma(L)) and Eulerian (sigma(E)) rotational eddy velocities, estimated from low-pass filtered drifter and current meter velocities, respectively, also generally agree. Cross-shore rotational velocities have a midsurfzone maximum whereas alongshore rotational velocities are distributed more broadly. Daily estimates of the Lagrangian time scale, the time for drifter velocities to decorrelate, vary between 40 and 300 s, with alongshore time scales greater than cross-shore time scales. The ratio of Lagrangian to apparent Eulerian current meter decorrelation times T-L/T-A varies considerably, between about 0.5 and 3. Consistent with theory, some of the T-L/T-A variation is ascribable to alongshore advection and T-L/T-A is proportional to V/sigma, which ranges between about 0.6 and 2.5. Estimates of T-L/T-A vary between days with similar V/sigma suggesting that surfzone Lagrangian particle dynamics vary between days, spanning the range from "fixed-float'' to "frozen-field'' [Lumpkin et al., 2002], although conclusions are limited by the statistical sampling errors in both T-L/T-A and V/sigma.

Omand, MM, Leichter JJ, Franks PJS, Guza RT, Lucas AJ, Feddersen F.  2011.  Physical and biological processes underlying the sudden surface appearance of a red tide in the nearshore. Limnology and Oceanography. 56:787-801.   10.4319/lo.2011.56.3.0787   AbstractWebsite

The sudden appearance at the surface of an alongshore-parallel band of red tide near Huntington Beach, California, is described in high spatial and temporal resolution using novel instrumentation including a global positioning system-tracked jet-ski. The scale of the surface chlorophyll a (Chl a) band was small (similar to 200 m cross-shore) and ephemeral (3 h) compared with the subsurface extent of the red tide (similar to 2 km, > 7 d). The red tide was dominated by the regionally common dinoflagellate Lingulodinium polyedrum (F. Stein) and had developed as a subsurface Chl a layer during the 7 d prior to the surface appearance. A few hours before the surface appearance, a subsurface patch of elevated Chl a (Chl a > 30 mu g L(-1)) was observed in 13-m total depth in the trough of a shoreward-propagating internal wave, consistent with dinoflagellate vertical swimming interacting with the internal wave-driven convergence. Internal wave-breaking-induced vertical mixing in similar to 8-m water depth vertically spread the Chl a patch to the surface, creating the alongshore surface band similar to 500 m from shore. Both the subsurface Chl a patch and the surface Chl a band were prevented from entering the surf-zone by a density barrier of warm water adjacent to the beach. These high-resolution observations emphasize the role of nearshore physical dynamics in controlling the duration and intensity of red tide exposure to coastal habitats.

Clark, DB, Feddersen F, Omand MM, Guza RT.  2009.  Measuring Fluorescent Dye in the Bubbly and Sediment-Laden Surfzone. Water Air and Soil Pollution. 204:103-115.   10.1007/s11270-009-0030-z   AbstractWebsite

Decisions about recreational beach closures would be enhanced if better estimates of surfzone contaminant transport and dilution were available. In situ methods for measuring fluorescent Rhodamine WT dye tracer in the surfzone are presented, increasing the temporal and spatial resolution over previous surfzone techniques. Bubbles and sand suspended by breaking waves in the surfzone interfere with in situ optical fluorometer dye measurements, increasing the lower bound for dye detection (a parts per thousand 1 ppb) and reducing (quenching) measured dye concentrations. Simultaneous turbidity measurements are used to estimate the level of bubble and sand interference and correct dye estimates. After correction, root-mean-square dye concentration errors are estimated to be < 5% of dye concentration magnitude, thus demonstrating the viability of in situ surfzone fluorescent dye measurements. The surfzone techniques developed here may be applicable to other environments with high bubble and sand concentrations (e.g., cascading rivers and streams).

Spydell, MS, Feddersen F, Guza RT.  2009.  Observations of drifter dispersion in the surfzone: The effect of sheared alongshore currents. Journal of Geophysical Research-Oceans. 114   10.1029/2009jc005328   AbstractWebsite

Surfzone dispersion is characterized with single-particle Lagrangian statistics of GPS-tracked drifters deployed on 5 days at Huntington Beach, California. Incident wave heights varied weakly between days, and stationary rip currents did not occur. Generally, the time-dependent bulk surfzone cross-shore diffusivity kappa(xx) was similar on all days, reaching a local maxima of approximately 1.5 m(2) s(-1) between 160 and 310 s, before decreasing to about 1 m(2) s(-1) at 1000 s. The alongshore diffusivity kappa(yy) increased monotonically to 1000 s and was variable between the 5 days. For times greater than 30 s, the alongshore diffusivity is greater than the cross-shore diffusivity, consistent with previous observations. The observed diffusivities are fit to analytic functional forms, from which asymptotic diffusivities and Lagrangian timescales are determined. The asymptotic alongshore diffusivity (kappa) over cap (infinity)(yy) varies between 4 and 19 m(2) s(-1), and this variation is related to the variation in the maximum of the mean alongshore current (v) over bar (m), broadly consistent with a shear dispersion scaling (kappa) over cap (infinity)(yy) similar to (v) over bar (2)(m). Cross-shore variation in dispersion processes, lumped together in the bulk kappa, is apparent in the non-Gaussian probability distribution function of drifter displacements at intermediate times (30 s). Both biased and unbiased diffusivity sampling errors depend on the number and length of drifter trajectories and limit aspects of the analysis.

Apotsos, A, Raubenheimer B, Elgar S, Guza RT.  2008.  Testing and calibrating parametric wave transformation models on natural beaches. Coastal Engineering. 55:224-235.   10.1016/j.coastaleng.2007.10.002   AbstractWebsite

To provide coastal engineers and scientists with a detailed inter-comparison of widely used parametric wave transformation models, several models are tested and calibrated with extensive observations from six field experiments on barred and unbarred beaches. Using previously calibrated ("default") values of a free parameter T, all models predict the observations reasonably well (median root-mean-square wave height errors are between 10% and 20%) at all field sites. Model errors can be reduced by roughly 50% by tuning T for each data record. No tuned or default model provides the best predictions for all data records or at all experiments. Tuned T differ for the different models and experiments, but in all cases T increases as the hyperbolic tangent of the deep-water wave height, H.. Data from two experiments are used to estimate empirical, universal curves for T based on Ho. Using the new parameterization, all models have similar accuracy, and usually show increased skill relative to using default T. (c) 2007 Elsevier B.V. All rights reserved.

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.