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Sinnett, G, Feddersen F, Lucas AJ, Pawlak G, Terrill E.  2018.  Observations of nonlinear internal wave run-up to the surfzone. Journal of Physical Oceanography. 48:531-554.   10.1175/jpo-d-17-0210.1   AbstractWebsite

The cross-shore evolution of nonlinear internal waves (NLIWs) from 8-m depth to shore was observed by a dense thermistor array and ADCP. Isotherm oscillations spanned much of the water column at a variety of periods. At times, NLIWs propagated into the surfzone, decreasing temperature by approximate to 1 degrees C in 5 min. When stratification was strong, temperature variability was strong and coherent from 18- to 6-m depth at semi-diurnal and harmonic periods. When stratification weakened, temperature variability decreased and was incoherent between 18- and 6-m depth at all frequencies. At 8-m depth, onshore coherently propagating NLIW events had associated rapid temperature drops (Delta T) up to 1.7 degrees C, front velocity between 1.4 and 7.4 cm s(-1), and incidence angles between -5 degrees and 23 degrees. Front position, Delta T, and two-layer equivalent height z(IW) of four events were tracked upslope until propagation terminated. Front position was quadratic in time, and normalized Delta T and z(IW) both decreased, collapsing as a linearly decaying function of normalized cross-shore distance. Front speed and deceleration are consistent with two-layer upslope gravity current scalings. During NLIW rundown, near-surface cooling and near-bottom warming at 8-m depth coincide with a critical gradient Richardson number, indicating shear-driven mixing.

Suanda, SH, Feddersen F, Kumar N.  2017.  The effect of barotropic and baroclinic tides on coastal stratification and mixing. Journal of Geophysical Research-Oceans. 122:10156-10173.   10.1002/2017jc013379   AbstractWebsite

The effects of barotropic and baroclinic tides on subtidal stratification and vertical mixing are examined with high-resolution, three-dimensional numerical simulations of the Central Californian coastal upwelling region. A base simulation with realistic atmospheric and regional-scale boundary forcing but no tides (NT) is compared to two simulations with the addition of predominantly barotropic local tides ( LT) and with combined barotropic and remotely generated, baroclinic tides (WT) with approximate to 100 W m(-1) onshore baroclinic energy flux. During a 10 day period of coastal upwelling when the domain volume-averaged temperature is similar in all three simulations, LT has little difference in subtidal temperature and stratification compared to NT. In contrast, the addition of remote baroclinic tides (WT) reduces the subtidal continental shelf stratification up to 50% relative to NT. Idealized simulations to isolate barotropic and baroclinic effects demonstrate that within a parameter space of typical U.S. West Coast continental shelf slopes, barotropic tidal currents, incident energy flux, and subtidal stratification, the dissipating baroclinic tide destroys stratification an order of magnitude faster than barotropic tides. In WT, the modeled vertical temperature diffusivity at the top (base) of the bottom (surface) boundary layer is increased up to 20 times relative to NT. Therefore, the width of the inner-shelf ( region of surface and bottom boundary layer overlap) is increased approximately 4 times relative to NT. The change in stratification due to dissipating baroclinic tides is comparable to the magnitude of the observed seasonal cycle of stratification.

Kumar, N, Feddersen F.  2017.  A new offshore transport mechanism for shoreline-released tracer induced by transient rip currents and stratification. Geophysical Research Letters. 44:2843-2851.   10.1002/2017gl072611   AbstractWebsite

Offshore transport from the shoreline across the inner shelf of early-stage larvae and pathogens is poorly understood yet is critical for understanding larval fate and dilution of polluted shoreline water. With a novel coupling of a transient rip current (TRC) generating surf zone model and an ocean circulation model, we show that transient rip currents ejected onto a stratified inner shelf induce a new, previously unconsidered offshore transport pathway. For incident waves and stratification typical for Southern California in the fall, this mechanism subducts surf zone-origin tracers and transports them at least 800 m offshore at 1.2 km/d analogous to subduction at ocean fronts. This mechanism requires both TRCs and stratification. As TRCs are ubiquitous and the inner shelf is often stratified, this mechanism may have an important role in exporting early-stage larvae, pathogens, or other tracers onto the shelf. Plain Language Summary The principal offshore transport pathway of shoreline-released intertidal larvae and pathogens within 1km of the coast is still a mystery, motivating this study. Here a new offshore transport pathway for shoreline pollution and shoreline-released intertidal invertebrate larvae from the surf zone to 1km offshore is presented. This new and intuitive offshore transport pathway will help provide insight to abundance of offshore larvae and marine population connectivity. This transport pathway is induced by transient rip currents (TRCs, episodic offshore flow from the surf zone) and stratification (temperature difference with depth) and leads to offshore advection of 1.2km/d. The findings are based on a new model that couples TRC generation to a three-dimensional ocean circulation model. Previous models lacked the TRC-generating mechanism and miss this pathway. Without TRCs the offshore transport is only about 5% of that with TRCs. Study findings have implications for marine population connectivity, marine-protected area design, and export of pathogens, contaminants, and nutrients.

Kumar, N, Feddersen F.  2017.  The Effect of Stokes drift and transient rip currents on the inner shelf. Part i: No stratification. Journal of Physical Oceanography. 47:227-241.   10.1175/jpo-d-16-0076.1   AbstractWebsite

This is part one of a two-part study focused on Stokes drift and transient rip current (TRC) effects on the unstratified (this paper) and stratified (see Part II) inner shelf. A TRC-generating, wave-resolving model funwaveC is coupled to the 3D, wave-averaged wave and circulation model Coupled Ocean-Atmosphere-Wave- Sediment Transport (COAWST). Two simulations (R1 and R2) are performed on an unstratified inner shelf and surfzone with typical bathymetry and wave conditions. R1 is a COAWST-only simulation (no TRCs), while R2 has funwaveC-COAWST coupling (with TRCs). R2 and funwaveC vertical vorticity (eddy) statistics are similar, indicating that the model coupling accurately generates TRCs, with TRC-induced eddies out to four surfzone widths offshore. R1 has a two-layered, inner-shelf-to-surfzone-connected, mean Lagrangian circulation, while R2 has separate inner shelf and surfzone circulation cells. The R2, TRC-induced, cross-shore and vertical eddy velocities are stronger than the R1 or R2 mean Lagrangian velocity out to four surfzone widths offshore. The R2, inner-shelf, mean, vertical eddy diffusivity is an order of magnitude larger than R1 out to four surfzone widths offshore. Both R1 and R2 are in a Stokes-Coriolis balance at six surfzone widths offshore, as is R1 at three surfzone widths offshore. For R2, TRC-induced horizontal advection and vertical mixing dominate the cross-shore momentum dynamics at three surfzone widths offshore. The R2 surfzone and inner-shelf cross-shore exchange velocity is 2-10 times larger for R1 because of the TRC-induced stirring. Accurate, unstratified, inner-shelf simulations of pollution, larval, or sediment transport must include transient rip currents. In Part II, the effects of Stokes drift and TRCs on the stratified inner shelf are examined.

Kumar, N, Feddersen F.  2017.  The effect of Stokes drift and transient rip currents on the inner shelf. Part ii: With stratification. Journal of Physical Oceanography. 47:243-260.   10.1175/jpo-d-16-0077.1   AbstractWebsite

This is Part II of a two-part study focused on Stokes drift and transient rip current (TRC) effects on the unstratified (Part I) and stratified (this paper) inner shelf. Part I focuses on funwaveC-Coupled Ocean-Atmosphere- Wave-Sediment Transport (COAWST) coupling and TRC effects on mixing and exchange on an unstratified inner shelf. Here, two simulations (R3 and R4) are performed on a stratified inner shelf and surfzone with typical bathymetry, stratification, and wave conditions. R3 is a COAWST-only simulation (no TRCs), while R4 has funwaveC-COAWST coupling (with TRCs). In R4, TRCs lead to patchy, near-surface cooling, vertical isotherm displacement, and increased water column mixing. For both R3 and R4, the mean Lagrangian circulation has two nearly isolated surfzones and inner-shelf overturning circulation cells, with a stronger, R4, inner-shelf circulation cell. The R4, inner-shelf, vertical velocity variability is 2-3 times stronger than a simulation with TRCs and no stratification. Relative to R3, R4 eddy diffusivity is strongly elevated out to three surfzone widths offshore due to TRCs and TRC-induced density overturns. The R4 inner-shelf stratification is reduced nearshore, and mean isotherms slope more strongly than R3 because of the TRC-enhanced irreversible mixing. At six surfzone widths offshore, both R3 and R4 are in geostrophic balance, explaining the stratified (summertime) observed deviation from Stokes-Coriolis balance. In this region, baroclinic pressure gradients induced by sloping isotherms induce an alongshore geostrophic jet offshore, strongest in R4. In R4, TRCs result in an enhanced (2-10 times) cross-shore exchange velocity across the entire inner shelf, relative to R3. Accurate, stratified, inner-shelf simulations of pollution, larval, or sediment transport must include transient rip currents.

Feddersen, F, Olabarrieta M, Guza RT, Winters D, Raubenheimer B, Elgar S.  2016.  Observations and modeling of a tidal inlet dye tracer plume. Journal of Geophysical Research-Oceans. 121:7819-7844.   10.1002/2016jc011922   AbstractWebsite

A 9 km long tracer plume was created by continuously releasing Rhodamine WT dye for 2.2 h during ebb tide within the southern edge of the main tidal channel at New River Inlet, NC on 7 May 2012, with highly obliquely incident waves and alongshore winds. Over 6 h from release, COAWST (coupled ROMS and SWAN, including wave, wind, and tidal forcing) modeled dye compares well with (aerial hyperspectral and in situ) observed dye concentration. Dye first was transported rapidly seaward along the main channel and partially advected across the ebb-tidal shoal until reaching the offshore edge of the shoal. Dye did not eject offshore in an ebb-tidal jet because the obliquely incident breaking waves retarded the inlet-mouth ebb-tidal flow and forced currents along the ebb shoal. The dye plume largely was confined to <4 m depth. Dye was then transported downcoast in the narrow (few 100 m wide) surfzone of the beach bordering the inlet at 0.3 ms-1 driven by wave breaking. Over 6 h, the dye plume is not significantly affected by buoyancy. Observed dye mass balances close indicating all released dye is accounted for. Modeled and observed dye behaviors are qualitatively similar. The model simulates well the evolution of the dye center of mass, lateral spreading, surface area, and maximum concentration, as well as regional (inlet and ocean) dye mass balances. This indicates that the model represents well the dynamics of the ebb-tidal dye plume. Details of the dye transport pathways across the ebb shoal are modeled poorly perhaps owing to low-resolution and smoothed model bathymetry. Wave forcing effects have a large impact on the dye transport.

Suanda, SH, Kumar N, Miller AJ, Di Lorenzo E, Haas K, Cai DH, Edwards CA, Washburn L, Fewings MR, Torres R, Feddersen F.  2016.  Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA: Observations, simulations, and scalings. Journal of Geophysical Research-Oceans. 121:7455-7475.   10.1002/2016jc011919   AbstractWebsite

In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner-shelf mooring locations throughout the study. A poleward-propagating plume of Southern California Bight water that increases shelf water temperatures by similar to 5 degrees C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface-trapped and slope-controlled dynamics. Plume momentum balances are distinct between the offshore (>30 m depth) region where the plume is surface-trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5 degrees C high, potentially affecting plume propagation distance and persistence.

Suanda, SH, Perez S, Feddersen F.  2016.  Evaluation of a source-function wavemaker for generating random directionally spread waves in the sea-swell band. Coastal Engineering. 114:220-232.   10.1016/j.coastaleng.2016.04.006   AbstractWebsite

A source-function wavemaker for wave-resolving models is evaluated for its capability to reproduce random directionally spread wave fields in the sea-swell band (0.04-0.3 Hz) relevant for realistic nearshore applications. The wavemaker is tested with a range of input wave characteristics defined by the non-dimensional amplitude (a/h), wavenumber (kh), wavemaker width, mean wave angle and directional spread. The (a/h) and kh dependency of modeled results are collapsed with the Ursell number (Ur= (a/h)/(kh)(2)). For monochromatic waves, the wavemaker accurately reproduced the input wave height for Ur<1, with no dependence on non dimensional wavemaker width. For random uni-directional waves, the wavemaker simulated well a Pierson Moskowitz input spectrum. Frequency-integrated statistics are also reproduced with less than 2% difference between modeled to input significant wave height and <10% difference between modeled to input mean frequency for Ur<0.2. For random directionally spread waves, the wavemaker reproduced input frequency dependent and bulk mean wave angle and directional spread to within 4 degrees at Ur<0.12. Lastly, the wavemaker simulated well the spectra, mean wave angle, and directional spread of a bimodal wave field with opposing sea and swell. Based on the Ur<0.12 constraint, a range of dimensional wave height, period, and depth constraints are explored for realistic sea-swell band field application. The wavemaker's ability to generate waves that match the input statistical properties commonly derived from field measurements demonstrates that it can be used effectively in a range of nearshore science and engineering applications. (C) 2016 Elsevier B.V. All rights reserved.

Hally-Rosendahl, K, Feddersen F.  2016.  Modeling surfzone to inner-shelf tracer exchange. Journal of Geophysical Research-Oceans. 121:4007-4025.   10.1002/2015jc011530   AbstractWebsite

A near-shoreline, continuous dye release at an approximately alongshore-uniform beach (IB09 experiment) is simulated with the wave-resolving Boussinesq model funwaveC. The model generates surfzone eddies and transient rip currents but does not resolve inner-shelf vertical variation or stratification. The funwaveC model reproduces well the observed surfzone and inner-shelf dye observations over roughly 350 m cross-shore and 2 km alongshore. Dye is advected alongshore by wave- and wind-driven currents similarly in the observations and model. Near-shoreline mean dye concentration decays downstream as a power law with similar observed (-0.33) and modeled (-0.38) exponents. Observed and modeled cross-shore mean dye profiles are similar, though modeled inner-shelf dye is somewhat elevated. Observed and modeled alongshore dye transports agree, though with compensating surfzone and inner-shelf errors later in the release. For times <3.5 h (before observed and modeled dye advects beyond the model alongshore domain), observed and modeled dye budgets are similar to each other and close to within 10%, and half the observed and modeled dye is exported to the inner-shelf. Later in the release, surfzone and inner-shelf dye masses are under and overpredicted, respectively. Model-data differences may be due to the model's lack of vertical variation, stratification, or tide. The good overall model-data agreement indicates that nearshore tracer transport and dispersion are realistically simulated over 5 h and 2 km alongshore, and that the model transient rip currents accurately induce cross-shore exchange between the surfzone and inner-shelf.

Kumar, N, Feddersen F, Suanda S, Uchiyama Y, McWilliams J.  2016.  Mid- to inner-shelf coupled ROMS-SWAN model-data comparison of currents and temperature: Diurnal and semidiurnal variability. Journal of Physical Oceanography. 46:841-862.   10.1175/jpo-d-15-0103.1   AbstractWebsite

Accurately representing diurnal and semidiurnal internal variability is necessary to investigate inner-shelf to midshelf exchange processes. Here, a coupled Regional Ocean Model System (ROMS)-Simulating Waves Nearshore (SWAN) model is compared to observed diurnal and semidiurnal internal tidal variability on the mid and inner shelf (26-8 m water depth) near San Pedro Bay, California. Modeled mean stratification is about one-half of that observed. Modeled and observed baroclinic velocity rotary spectra are similar in the diurnal and semidiurnal band. Modeled and observed temperature spectra have similar diurnal and semidiurnal band structure, although the modeled is weaker. The observed and modeled diurnal and semidiurnal baroclinic velocity- and temperature-dominant vertical structures are similar and consistent with mode-one internal motions. Both observed and modeled diurnal baroclinic kinetic energy are strongly correlated to diurnal wind forcing and enhanced by subtidal vorticity-induced reduction in the inertial frequency. The mid- and inner-shelf modeled diurnal depth-integrated heat budget is a balance between advective heat flux divergence and temperature time derivative. Temperature-velocity phase indicates progressive semidiurnal internal tide on the midshelf and largely standing internal tide on the inner shelf in both observed and modeled. The ratio of observed to modeled inferred phase speed is consistent with the observed to modeled stratification. The San Pedro Bay modeled semidiurnal internal tide has significant spatial variability, variable incident wave angles, and multiple local generation sites. Overall, the coupled ROMS-SWAN model represents well the complex diurnal and semidiurnal internal variability from the mid to the inner shelf.

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.

Suanda, SH, Feddersen F.  2015.  A self-similar scaling for cross-shelf exchange driven by transient rip currents. Geophysical Research Letters. 42:5427-5434.   10.1002/2015GL063944   Abstract

Transient rip currents, episodic offshore flows from the surf zone to the inner shelf, present a recreational beach hazard and exchange material across the nearshore ocean. The magnitude and offshore extent of transient rip-current-induced exchange and its relative importance to other inner shelf exchange processes are poorly understood. Here 120 model simulations with random, normally incident, directionally spread waves spanning a range of beach slopes and wave conditions show that the transient rip current exchange velocity is self-similar. The nondimensional exchange velocity, surf zone flushing time, and cross-shore decay length scale are scaled by beach slope and wave properties, depending strongly on wave directional spread. Transient rip-current-driven exchange can be compared to other cross-shelf exchange processes. For example, transient rip-current-driven exchange is stronger than wave-induced Stokes-drift-driven exchange up to six surf zone widths from shore.

Kumar, N, Feddersen F, Uchiyama Y, McWilliams J, O'Reilly W.  2015.  Midshelf to surfzone coupled ROMS-SWAN model data comparison of waves, currents, and temperature: Diagnosis of subtidal forcings and response. Journal of Physical Oceanography. 45:1464-1490.   10.1175/jpo-d-14-0151.1   AbstractWebsite

A coupled wave and circulation model that includes tide, wind, buoyancy, and wave processes is necessary to investigate tracer exchange in the shelf region. Here, a coupled Regional Ocean Model System (ROMS)-Simulating Waves Nearshore (SWAN) model, resolving midshelf to the surfzone region of the San Pedro Bay, California, is compared to observations from the 2006 Huntington Beach experiment. Waves are well modeled, and surfzone cross- and alongshore velocities are reasonably well modeled. Modeled and observed rotary velocity spectra compare well in subtidal and tidal bands, and temperature spectra compare well in the subtidal band. Observed and modeled mid- and inner-shelf subtidal velocity ellipses and temperature variability determined from the first vertical complex EOF (cEOF) mode have similar vertical structure. Although the modeled subtidal velocity vertical shear and stratification are weaker than observed, the ratio of stratification to shear is similar, suggesting model vertical mixing is consistent with observations. On fortnightly and longer time scales, the surface heat flux and advective heat flux divergence largely balance on the inner shelf and surfzone. The surfzone and inner-shelf alongshore currents separated by 220 m are unrelated. Both modeled and observed subtidal alongshelf current and temperature are cross-shelf coherent seaward of the surfzone. Wind forcing explains 50% of the observed and modeled inner-shelf alongshore current variability. The observed and modeled inner-shelf alongshelf nonuniformities in depth-averaged alongshore velocities are similar. Inferred, inner-shelf, wave-induced, cross-shore exchange is more important than on the U.S. East Coast. Overall, the coupled ROMS-SWAN model represents well the waves and subtidal circulation dynamics from the midshelf to the surfzone.

Sinnett, G, Feddersen F.  2014.  The surf zone heat budget: The effect of wave heating. Geophysical Research Letters. 41:7217-7226.   10.1002/2014gl061398   AbstractWebsite

Surf zone incident wave energy flux is dissipated by wave breaking which through viscosity generates heat. This effect is not present in shelf heat budgets and has not previously been considered. Pier-based observations of water temperature in 1-4m depth, meteorology, and waves are used to test a surf zone heat budget, which closes on diurnal and longer time scales. Wave energy flux is the second most variable term with mean contribution one fourth of the mean short-wave radiation. The heat budget residual has semidiurnal and higher-frequency variability and net cooling. Cross-shore advective heat flux driven by internal wave events, rip currents, and undertow contribute to this residual variability and net cooling. In locations with large waves, steeper beaches, or less solar radiation, the ratio of wave energy flux to short-wave radiation may be >1.

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.

Clark, DB, Lenain L, Feddersen F, Boss E, Guza RT.  2014.  Aerial imaging of fluorescent dye in the near shore. Journal of Atmospheric and Oceanic Technology. 31:1410-1421.   10.1175/jtech-d-13-00230.1   AbstractWebsite

Aerial images are used to quantify the concentration of fluorescent Rhodamine water tracing (WT) dye in turbid and optically deep water. Tracer releases near the shoreline of an ocean beach and near a tidal inlet were observed with a two-band multispectral camera and a pushbroom hyperspectral imager, respectively. The aerial observations are compared with near-surface in situ measurements. The ratio of upwelling radiance near the Rhodamine WT excitation and emission peaks varies linearly with the in situ dye concentrations for concentrations <20ppb (r(2) = 0.70 and r(2) = 0.85-0.88 at the beach and inlet, respectively). The linear relationship allows for relative tracer concentration estimates without in situ calibration. The O(1 m) image pixels resolve complex flow structures on the inner shelf that transport and mix tracer.

Feddersen, F.  2014.  The generation of surfzone eddies in a strong alongshore current. Journal of Physical Oceanography. 44:600-617.   10.1175/jpo-d-13-051.1   AbstractWebsite

The surfzone contains energetic two-dimensional horizontal eddies with length scale larger than the water depth. Yet, the dominant eddy generation mechanism is not understood. The wave-resolving model funwaveC is used to simulate surfzone eddies in four case examples, from the SandyDuck field experiment, that had alongshore uniform bathymetry. The funwaveC model is initialized with the observed bathymetry and the incident wave field in 8-m depth and reproduces the observed cross-shore structure of significant wave height and mean alongshore current. Within the surfzone, the wave-resolving funwaveC-modeled E(f, k(y)) spectra and the bulk (frequency and k(y) integrated) rotational velocities are consistent with the observations below the sea-swell band (<0.05 Hz), demonstrating that the model can be used to diagnose surfzone eddy generation mechanisms. In the mean-squared perturbation vorticity budget, the breaking wave vorticity forcing term is orders of magnitude larger than the shear instability generation term. Thus, surfzone eddies (vorticity) generally are not generated through a shear instability, with possible exceptions for very narrow banded in frequency and direction and highly obliquely large incident waves. The alongshore wavenumber spectra of breaking wave vorticity forcing is broad with the majority (>80%) of vorticity forcing occurring at short alongshore scales <20 m. However, the alongshore wavenumber spectra of vorticity is red, which may be due to a 2D turbulence inverse energy cascade bringing energy to longer wavelengths or may result from an amplified vorticity response to direct forcing at smaller k(y).

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.

Rippy, MA, Franks PJS, Feddersen F, Guza RT, Moore DF.  2013.  Factors controlling variability in nearshore fecal pollution: The effects of mortality. Marine Pollution Bulletin. 66:191-198.   10.1016/j.marpolbul.2012.09.003   AbstractWebsite

A suite of physical-biological models was used to explore the importance of mortality and fluid dynamics in controlling concentrations of fecal indicator bacteria (FIB) at Huntington Beach, CA. An advection-diffusion (AD) model provided a baseline to assess improvements in model skill with the inclusion of mortality. Six forms of mortality were modeled. All mortality models performed better than the AD model, especially at offshore sampling stations, where model skill increased from <0.18 to >0.50 (Escherichia colt) or <-0.14 to >0.30 (Enterococcus). Models including cross-shore variable mortality rates reproduced FIB decay accurately (p < 0.05) at more stations than models without. This finding is consistent with analyses that revealed cross-shore variability in Enterococcus species composition and solar dose response. No best model was identified for Entero coccus, as all models including cross-shore variable mortality performed similarly. The best model for E. colt included solar-dependent and cross-shore variable mortality. (C) 2012 Elsevier Ltd. All rights reserved.

Rippy, MA, Franks PJS, Feddersen F, Guza RT, Moore DF.  2013.  Physical dynamics controlling variability in nearshore fecal pollution: Fecal indicator bacteria as passive particles. Marine Pollution Bulletin. 66:151-157.   10.1016/j.marpolbul.2012.09.030   AbstractWebsite

We present results from a 5-h field program (HB06) that took place at California's Huntington State Beach. We assessed the importance of physical dynamics in controlling fecal indicator bacteria (FIB) concentrations during HB06 using an individual based model including alongshore advection and cross-shore variable horizontal diffusion. The model was parameterized with physical (waves and currents) and bacterial (Escherichia coli and Enterococcus) observations made during HB06. The model captured surfzone FIB dynamics well (average surfzone model skill: 0.84 {E. coli) and 0.52 {Enterococcus)), but fell short of capturing offshore FIB dynamics. Our analyses support the hypothesis that surfzone FIB variability during HB06 was a consequence of southward advection and diffusion of a patch of FIB originating north of the study area. Offshore FIB may have originated from a different, southern, source. Mortality may account for some of the offshore variability not explained by the physical model. (C) 2012 Elsevier Ltd. All rights reserved.

Rippy, MA, Franks PJS, Feddersen F, Guza RT, Warrick JA.  2013.  Beach nourishment impacts on bacteriological water quality and phytoplankton bloom dynamics. Environmental Science & Technology. 47:6146-6154.   10.1021/es400572k   AbstractWebsite

A beach nourishment with approximately 1/3 fine-grained sediment (fines; particle diameter <63 mu m) by mass was performed at Southern California's Border Fields State Park (BFSP). The nourishment was found to briefly (<1 day) increase concentrations of surf-zone fecal indicator bacteria (FIB) above single-sample public health standards [104 most probable number (MPN).(100 mL)(-1)] but had no effect on phytoplankton. Contamination was constrained to the nourishment site: waters 300 m north or south of the nourishment were always below single-sample and geometric mean [<= 35 MPN.(100 mL)(-1)] standards. Nourishment fines were identified as a source of the fecal indicator Enterococcus; correlations between fines and enterococci were significant (p < 0.01), and generalized linear model analysis identified fines as the single best predictor of enterococci. Microcosm experiments and field sampling suggest that the short surf-zone residence times observed for enterococci (e-folding time 4 h) resulted from both rapid, postplacement FIB inactivation and mixing/transport by waves and alongshore currents. Nourishment fines were phosphate-rich/nitrogen-poor and were not correlated with surf-zone phytoplankton concentrations, which may have been nitrogen-limited.

Rogers, JS, Monismith SG, Feddersen F, Storlazzi CD.  2013.  Hydrodynamics of spur and groove formations on a coral reef. Journal of Geophysical Research-Oceans. 118:3059-3073. AbstractWebsite

Spur and groove (SAG) formations are found on the fore reefs of many coral reefs worldwide. Although these formations are primarily present in wave-dominated environments, their effect on wave-driven hydrodynamics is not well understood. A two-dimensional, depth-averaged, phase-resolving nonlinear Boussinesq model (funwaveC) was used to model hydrodynamics on a simplified SAG system. The modeling results show that the SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells. The mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient (PG) and nonlinear wave (NLW) terms in the momentum balance. Variations in model parameters suggest the strongest factors affecting circulation include spur-normal waves, increased wave height, weak alongshore currents, increased spur height, and decreased bottom drag. The modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of NLW, PG, and bottom stress terms. Model results indicate that the SAG formations efficiently drive circulation cells when the alongshore SAG wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle.

Spydell, MS, Feddersen F.  2012.  A Lagrangian stochastic model of surf zone drifter dispersion. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007701   AbstractWebsite

Drifter-derived cross-shore and alongshore surf zone diffusivities were previously estimated on an alongshore uniform beach over 1000 s for five Huntington Beach, California, 2006 (HB06) experiment release days. The cross-shore diffusivity K-x had a nonmonotonic time dependence, potentially due to the shoreline or to weaker diffusivity seaward of the surf zone. The alongshore diffusivities K-y were qualitatively consistent with shear dispersion but differed from the classic Taylor laminar theory. Here, modeled and analytic diffusivities for the five release days are derived from a Lagrangian stochastic model (LSM) that uses the drifter-derived bulk (cross-shore averaged) velocity variance and cross-shore-dependent mean alongshore current. The LSM modeled and analytic cross-shore diffusivities are nonmonotonic due to the shoreline and strongly suggest that the observed cross-shore diffusivity is shoreline affected. The LSM typically reproduce well the observed Kx with Lagrangian time scale between 75 and 200 s, consistent with surf zone eddy time scales. HB06 drifter trajectories were too short to observe the analytic long-time Kx limit, and weaker diffusivity seaward of the surf zone may be important at longer times (>1000 s). On all release days, the LSM model and analytic alongshore diffusivity reproduce well the observed Ky with alongshore Lagrangian time scales between 95 and 155 s. The isolated shear-induced diffusivity is very well represented by an analytic theory which incorporates a nonzero Lagrangian time scale. Many of the stochastic model parameters can be specified a priori with reasonable assumptions to predict surf zone dispersion of an initial value problem pollution spill.

Feddersen, F.  2012.  Observations of the Surf-Zone Turbulent Dissipation Rate. Journal of Physical Oceanography. 42:386-399.   10.1175/jpo-d-11-082.1   AbstractWebsite

The contributions of surface (breaking wave) boundary layer (SBL) and bottom (velocity shear) boundary layer (BBL) processes to surf-zone turbulence is studied here. The turbulent dissipation rate is an element of, estimated on a 160-m-long cross-shore instrumented array, was an order of magnitude larger within the surf zone relative to seaward of the surf zone. The observed is an element of covaried across the array with changing incident wave height, tide level, and alongshore current. The cross-shore-integrated depth times is an element of was correlated with, but was only 1% of, the incident wave energy flux, indicating that surf-zone water-column turbulence is driven directly (turbulence injected by wave breaking) or indirectly (by forcing alongshore currents) by waves and that the bulk of is an element of occurs in the upper water column. This small fraction is consistent with laboratory studies. The surf-zone-scaled (or Froude-scaled) is an element of is similar to previous field observations, albeit somewhat smaller than laboratory observations. A breaking-wave is an element of scaling is applicable in the midwater column at certain locations, indicating a vertical diffusion of turbulence and is an element of balance. However, observations at different cross-shore locations do not collapse, which is consistent with a cross-shore lag between wave energy gradients and the surface turbulence flux. With strong alongshore currents, a BBL-scaled is an element of indicates that shear production is a significant turbulence source within the surf zone, particularly in the lower water column. Similarly for large currents at one location, the dissipation to shear production ratio approaches one. Both dissipation scalings depend upon wave energy flux gradients. The ratio of BBL to SBL is an element of has complex dependencies but is larger for a deeper part of the surf zone and more obliquely incident waves.