Simulations of the Surf Zone Eddy Field and Cross‐Shore Exchange on a Nonidealized Bathymetry

Abstract

The nearshore ocean spans the cross-shore region from the shoreline through the surf zone out to the inner shelf and is characterized dynamically by the importance of waves and wave-driven flows. Recent work indicates that exchange between surf zone and inner shelf is predominantly due to transient and morphologic rip currents (Brown et al., 2015;Hally-Rosendahl & Feddersen, 2016), which are offshore-oriented flows generated in the surf zone that extend out beyond the breaker line. Quantifying the hydrodynamic processes controlling surf zone mixing and cross-shore exchange via rip currents in different environmental conditions is necessary in order to predict the movement and dispersal of biological, chemical, and other types of tracers that have implications for both nearshore ecology (

Modeled Three‐Dimensional Currents and Eddies on an Alongshore‐Variable Barred Beach

“…The χ 2 is sensitive to the cross-shore extent over which it is computed and increases for smaller surf zone widths. The χ 2 of the bathymetry observed on October 14 is ∼5 times larger than previous experiments with alongshore bathymetric variations described as small to moderate in Duck, NC and central California (Ruessink et al, 2001;Feddersen & Guza, 2003;O'Dea et al, 2021), and is >25 times larger than observed on a beach described as alongshore uniform (Spydell & Feddersen, 2009). On October 14 (November 16), the average shoreline position was approximately x = 108 m (110 m), and a single bar was located near x = 235 m (215 m) with an alongshore-variable trough near x = 175 m (170 m).…”

“…The cross-shore variability of eddy length scales has been investigated with models that account for the generation of eddies from short-crested wave breaking (Kumar & Feddersen, 2017;O'Dea et al, 2021). However, the vertical variability of eddy length scales is not known and has not been investigated using a three-dimensional phase-resolving model.…”

“…The length scales of horizontal eddies and their dependence on the incident wave forcing, coupled with the role of surfzone bathymetry, is a focus of recent studies (Feddersen, 2014;Kumar & Feddersen, 2017;O'Dea et al, 2021). Here, for simulations with varying offshore directional spread (σ θ ) on an alongshore inhomogeonous bathymetry, the vorticity variance,   2 (the area under alongshore wavenumber spectra of depth-averaged vertical vorticity separated for inverse length scales (wavenumbers divided by 2π, L −1 = k y /2π) on either side of 0.01 m −1 , S ωω , in Figure 7) at small length scales (L < 100 m) increases with directional spread in all cross-shore regions (Figure 10a, circles, squares, triangles, and linear fit with slopes = 3.2, 6.0, and 0.6 × 10 −6 m −1 s −2 , respectively), consistent with results from previous modeling studies using a BAKER ET AL.…”

“…Here, for simulations with varying offshore directional spread (σ θ ) on an alongshore inhomogeonous bathymetry, the vorticity variance,   2 (the area under alongshore wavenumber spectra of depth-averaged vertical vorticity separated for inverse length scales (wavenumbers divided by 2π, L −1 = k y /2π) on either side of 0.01 m −1 , S ωω , in Figure 7) at small length scales (L < 100 m) increases with directional spread in all cross-shore regions (Figure 10a, circles, squares, triangles, and linear fit with slopes = 3.2, 6.0, and 0.6 × 10 −6 m −1 s −2 , respectively), consistent with results from previous modeling studies using a BAKER ET AL. depth-integrated wave-resolving Boussinesq model with alongshore-uniform bathymetry (Spydell & Feddersen, 2009;Suanda & Feddersen, 2015) and alongshore-variable bathymetry (O'Dea et al, 2021). Multiple surfzone processes may contribute to the vorticity variance at small length scales, including generation by directionally spread short-crested breaking waves (e.g., σ θ = 28.6° in S3a).…”

“…As a result of alongshore gradients in wave dissipation, short-crested breaking waves generate vorticity with length scales (10)  m (Clark et al, 2012;Peregrine, 1998), which are hypothesized to coalesce to larger scales (100)  m (Elgar & Raubenheimer, 2020;Spydell & Feddersen, 2009) due to an inverse energy cascade (Boffetta & Ecke, 2012;Kraichnan, 1967). The length scales associated with vortical motions generated by both shear instabilities and wave breaking have been explored for depth-integrated velocities on alongshore-uniform beaches (Feddersen, 2014;Feddersen et al, 2011;Kumar & Feddersen, 2017;Spydell & Feddersen, 2009) and an alongshore-varying beach (O'Dea et al, 2021), but have not been explored for depth-varying velocities on beaches with complex bathymetry.…”