Surf zone diffusivity on a rip‐channeled beach

Brown, Jeff; MacMahan, Jamie; Reniers, Ad; Thornton, E. B.

doi:10.1029/2008jc005158

Cited by 48 publications

(59 citation statements)

References 52 publications

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“…Inside the surf zone, this behavior was attributed to the presence of a shoreline boundary (Spydell and Feddersen 2012); however, a nonmonotonic k xx could also be due to weaker diffusivity seaward of the surf zone. This supports the findings that, in general, the asymptotic values of diffusivity obtained outside the surf zone (k (Spydell et al 2007;Brown et al 2009;Spydell et al 2009). Additionally, the time to approach k ' xx and k ' yy is longer outside the surf zone (.1000 s) compared to k ' xx and k ' yy of less than 1000 s found in previous work inside the surf zone Spydell et al 2009), indicating differences in Lagrangian time scales inside and outside the surf zone (Spydell and Feddersen 2012).…”

Section: September 2015 B R O W N E T a Lsupporting

confidence: 80%

Field Observations of Surf Zone–Inner Shelf Exchange on a Rip-Channeled Beach

Brown

MacMahan

Reniers

et al. 2015

Journal of Physical Oceanography

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Cross-shore exchange between the surf zone and the inner shelf is investigated using Lagrangian and Eulerian field measurements of rip current flows on a rip-channeled beach in Sand City, California. Surface drifters released on the inner shelf during weak wind conditions moved seaward due to rip current pulses and then returned shoreward in an arcing pattern, reentering the surf zone over shoals. The cross-shore velocities of the seaward-and shoreward-moving drifters were approximately equal in magnitude and decreased as a function of distance offshore. The drifters carried seaward by the rip current had maximum cross-shore velocities as they exited the surf zone and then decelerated as they moved offshore. The drifters moving shoreward accelerated as they approached the surfzone boundary with maximum cross-shore velocities as they reentered the surf zone over shoals. It was found that Stokes drift was not solely responsible for the onshore transport across the surfzone boundary. The cross-shore diffusivity on the inner shelf was greatest during observations of locally contained cross-shore exchange. These field observations provide evidence that the cross-shore exchange between the surf zone and inner shelf on a rip-channeled beach is due to wave-driven rip current circulations and results in surface material being contained within the nearshore region.

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Section: September 2015 B R O W N E T a Lsupporting

confidence: 80%

Field Observations of Surf Zone–Inner Shelf Exchange on a Rip-Channeled Beach

Brown

MacMahan

Reniers

et al. 2015

Journal of Physical Oceanography

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“…First, in section 2 we give a brief overview of RCEX results (Brown et al 2009;MacMahan et al 2010b) where observations of a cellular rip current system were made. In section 3 and 4, we discuss numerical modeling of the hydrodynamics on the rip-channeled bathymetry subjected to monochromatic wave forcing using a phaseresolving nonlinear shallow water model, with an explicit treatment for wave breaking due to Kennedy et al (2000).…”

Section: Introductionmentioning

confidence: 99%

Unforced Oscillation of Rip-Current Vortex Cells

Geiman

Kirby

2013

Journal of Physical Oceanography

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A numerical simulation of a monochromatic surface gravity wave-driven flow over an alongshore quasiperiodic rip-channeled beach using the wave-resolving model Funwave is used to investigate coherent, very low-frequency (VLF) motions with characteristic frequencies f , 4.0 mHz inside of the surf zone generated by wave breaking. These oscillations of the nearshore cellular vorticity pattern occur for shore-normal waves over a wide range of amplitudes of the incident wave field and occur despite the wave forcing being essentially constant. The oscillations occur at the lower end of the VLF spectrum or around f p 5 0.55 mHz. For small incident wave amplitudes, an equilibrium state consisting of a staggered counterrotating vortex array generates a net weak alongshore current that is also seen in drifter trajectories observed in the field. Using a simpler pseudospectral vorticity model of a single dipole generated by a smooth, stationary in time forcing function s b , this study shows show that the Strouhal number of the vortex shedding process responsible for the oscillation is dependent on the circulation strength of the vortices in the dipole, as well as the bottom friction parameter. This process includes the pinching off, advection, and eventual regeneration of the vortex in the dipole. A simple scaling argument shows good agreement with the frequencies observed in the simulations.

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“…Yet, despite the detriment of contaminated coastal water to our health and economy, understanding of nearshore tracer transport and mixing remains relatively poor. Several field experiments have tracked Lagrangian surface drifters on alongshore-uniform beaches [e.g., Spydell et al, 2007Spydell et al, , 2014 and rip-channeled beaches [e.g., Brown et al, 2009;MacMahan et al, 2010;Brown et al, 2015] to investigate dispersion in the nearshore. Similarly, fluorescent dye [e.g., Harris et al, 1963;Inman et al, 1971;Grant et al, 2005;Clark et al, 2010] has also been used to explore nearshore mixing.…”

Section: Introductionmentioning

confidence: 99%

Surfzone to inner‐shelf exchange estimated from dye tracer balances

et al. 2015

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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 ( , best fit slope ) using a bulk exchange velocity and mean surfzone to inner‐shelf dye concentration difference. The best fit cross‐shore exchange velocity is similar to a temperature‐derived exchange velocity on another day with similar wave conditions. The 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.

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Surf zone diffusivity on a rip‐channeled beach

Cited by 48 publications

References 52 publications

Field Observations of Surf Zone–Inner Shelf Exchange on a Rip-Channeled Beach

Field Observations of Surf Zone–Inner Shelf Exchange on a Rip-Channeled Beach

Unforced Oscillation of Rip-Current Vortex Cells

Surfzone to inner‐shelf exchange estimated from dye tracer balances

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