An official website of the United States government
Abstract A field‐calibrated morphodynamic model (MIKE21) is used to investigate the importance of the dimensions of a rip channel across an outer sand bar to the hydrodynamics and morphological evolution of an inner sand bar and rip channel for a range of initial bathymetries and wave conditions. The model was driven with offshore wave conditions and idealized bathymetry representative of field conditions near Duck, NC, USA during which strong rips and associated channel erosion were observed to occur over an inner bar. Consistent with prior results, the strength of the hydro‐morphological coupling between the two bars depends on the dimensions of the outer‐bar perturbation, as well as the wave forcing. The results suggest that in double‐barred systems, a single moderate‐scale perturbation (O(0.1 m deep, 10 m wide)) in the outer‐bar elevation can lead to the generation of a rip current and associated erosion of a rip channel across the inner bar. The simulations suggest that the magnitude of the inner‐bar rip flow, the depth to which the inner‐bar channel is eroded, and the alongshore position of the inner‐bar rip relative to the outer‐bar perturbation depend on the non‐dimensional outer‐bar channel depth, the transverse rip‐channel slope, and the wave height, period and directional spreading. For deep and narrow outer‐bar channels, the outer‐inner bar coupling is strong. In contrast, for shallow and wide outer‐bar channels, the system may alternate between being coupled and uncoupled with unstable locations of the inner‐bar rip.
more »
« less
Rip current likelihood as a function of incident wave conditions for different bathymetries
The likelihood of rip currents as a function of water depth (tidal level), incident wave height, period, direction, and spectral spreading in both frequency and direction is investigated with a Boussinesq numerical model (FUNWAVE) for alongshore uniform, moderately variable, and strongly variable bathymetry, providing two- dimensional probability distributions of rip-current occurrence along the coast. The simulations suggest that over strongly irregular alongshore bathymetry rip-current likelihood increases with longer wave periods and narrower directional spectra. In contrast, over more uniform alongshore bathymetry, rip current likelihood in- creases with shorter wave periods and broader directional spectra. The simulations suggest that as bathymetric variability increases, the effects of different incident wave fields decreases.
more »
« less
- Award ID(s):
- 2341381
- PAR ID:
- 10668552
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Applied Ocean Research
- Volume:
- 162
- Issue:
- C
- ISSN:
- 0141-1187
- Page Range / eLocation ID:
- 104727
- Subject(s) / Keyword(s):
- Rip currents rip-current likelihood Numerical simulation FUNWAVE Surf currents
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The behavior and predictability of rip currents (strong, wave‐driven offshore‐directed surfzone currents) have been studied for decades. However, few studies have examined the effects of rip channel morphology on the rip generation or have compared morphodynamic models with observations. Here, simulations conducted with the numerical morphodynamic model MIKE21 reproduce observed trends in flows and bathymetric evolution for two channels dredged across a nearshore sandbar and terrace on an ocean beach near Duck, NC, USA. Channel dimensions, wave conditions, and flows differed between the two cases. In one case, a strong rip current was driven by moderate height, near‐normally incident waves over an approximately 1‐m deep channel with relatively little bathymetric evolution. In the other case, no rip was generated by the large, near‐normally incident waves over the shallower (∼0.5 m) channel, and the channel migrated in the direction of the mean flow and eventually filled in. The model simulated the flow directions, the generation (or not) of rip currents, and the morphological evolution of the channels reasonably well. Model simulations were then conducted for different combinations of the two channel geometries and two wave conditions to examine the relative importance of the waves and morphology to the rip current evolution. The different bathymetries were the dominant factor controlling the flow, whereas both the initial morphology and wave conditions were important for channel evolution. In addition, channel dimensions affected the spatial distribution of rip current forcings and the relative importance of terms.more » « less
-
Abstract The cross‐shore transformation of breaking‐wave roller momentum and energy on observed barred surfzone bathymetry is investigated with a two‐phase Reynolds Averaged Navier Stokes model driven with measured incident waves. Modeled wave spectra, wave heights, and wave‐driven increases in the mean water level (setup) agree well with field observations along transects extending from 5‐m water depth to the shoreline. Consistent with prior results the roller forcing contributes 50%–60% to the setup, whereas the advective terms contribute ∼20%, with the contribution of bottom stress largest (up to 20%) for shallow sandbar crest depths. The model simulations suggest that an energy‐flux balance between wave dissipation, roller energy, and roller dissipation is accurate. However, as little as 70% of the modeled wave energy ultimately dissipated by breaking was first transferred from the wave to the roller. Furthermore, of the energy transferred to the roller, 15%–25% is dissipated by turbulence in the water column below the roller, with the majority of energy dissipated in the aerated region or near the roller‐surface interface. The contributions of turbulence to the momentum balance are sensitive to the parameterized turbulent anisotropy, which observations suggest increases with increasing turbulence intensity. Here, modeled turbulent kinetic energy dissipation decreases with increasing depth of the sandbar crest, possibly reflecting a change from plunging (on the steeper offshore slope of the bar) to spilling breakers (over the flatter bar crest and trough). Thus, using a variable roller front slope in the roller‐wave energy flux balance may account for these variations in breaker type.more » « less
-
Abstract A series of idealized numerical simulations is used to examine the generation of mode-one superinertial coastally trapped waves (CTWs). In the first set of simulations, CTWs are resonantly generated when freely propagating mode-one internal tides are incident on the coast such that the angle of incidence of the internal wave causes the projected wavenumber of the tide on the coast to satisfy a triad relationship with the wavenumbers of the bathymetry and the CTW. In the second set of simulations, CTWs are generated by the interaction of the barotropic tide with topography that has the same scales as the CTW. Under resonant conditions, superinertial coastally trapped waves are a leading order coastal process, with alongshore current magnitudes that can be larger than the barotropic or internal tides from which they are generated.more » « less
-
null (Ed.)Abstract Low-frequency currents and eddies transport sediment, pathogens, larvae, and heat along the coast and between the shoreline and deeper water. Here, low-frequency currents (between 0.1 and 4.0 mHz) observed in shallow surfzone waters for 120 days during a wide range of wave conditions are compared with theories for generation by instabilities of alongshore currents, by ocean-wave-induced sea surface modulations, and by a nonlinear transfer of energy from breaking waves to low-frequency motions via a two-dimensional inverse energy cascade. For these data, the low-frequency currents are not strongly correlated with shear of the alongshore current, with the strength of the alongshore current, or with wave-group statistics. In contrast, on many occasions, the low-frequency currents are consistent with an inverse energy cascade from breaking waves. The energy of the low-frequency surfzone currents increases with the directional spread of the wave field, consistent with vorticity injection by short-crested breaking waves, and structure functions increase with spatial lags, consistent with a cascade of energy from few-meter-scale vortices to larger-scale motions. These results include the first field evidence for the inverse energy cascade in the surfzone and suggest that breaking waves and nonlinear energy transfers should be considered when estimating nearshore transport processes across and along the coast.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.apor.2025.104727
