16.2.18. Flash Rips#

Rip currents in the littoral zone are essential for exchanges between the continent and the ocean, as they can extend seaward to the continental shelf. There are two main types: channeled (bathymetric) and transient (flash) rip currents. Flash rips are purely wave-induced processes that can occur on uniform bathymetry along the coast. Over the last decade, the process of generating flash rips by short-crested waves, referred to as the Peregrine process, has become the current consensus in the nearshore community. In this case, the generation of flash rips and associated surfzone eddies requires explicit wave resolution rather than wave-averaged equations (RIP test case). The simulation of flash rips with CROCO is described in Marchesiello et al. [2021] — along with a new species of smaller rip currents, called mini-rips, arising from vertical shear instability of the undertow.

Settings :

The FLASH_RIP test case presents an idealized simulation of flash rips (and mini-rips) generated by wave directional spread over a longshore-uniform bathymetry. It is described in Treillou et al. (2025), as a simplification of the 2009 Imperial Beach experiment IB09 (Hally-Rosendahl and Feddersen 2016), with waves normal to the coast and zero wind stress.

The size of the computational domain is 300 m in both cross-shore and alongshore directions. The bathymetry is a synthethic representation of Imperial Beach. The model is forced by a JONSWAP wave spectrum with parameters : significant wave height Hs = 0.7 m, peak period Tp = 13 s and peak enhancement factor gamma = 3.3. The mean wave angle is zero and and the directional spread is 30 deg. The northern and southern boundaries are periodic (the wave angle around the mean is corrected to preserve periodicity; see Treillou et al. [2024]). The horizontal resolution is 1.5 m (a relatively coarse resolution for wave-resolving simulations) with 8 vertical levels. The timestep is 50 ms with 5 substeps to resolve fast dynamics. The simulation lasts about 15 minutes and has time to spinup and generate a few rip currents and numerous mini-rips.

Input parameters :

Parameters to run FLASH_RIP are in croco.in.Flashrip available in the TEST_CASES directory. A basic Matlab plotting code (plot_flashrip.m) is also available in this directory. Input parameters in croco.in.Flashrip for the wavemaker (see formulation in: Wavemaker) are set under keyword wave_maker :

wave_maker:  amp [m], prd [s], ang [deg], ang spread [deg], freq spread (gamma)
             0.24     13.       0.         30.               3.3

amp: wave amplitude (\(a_m=\sqrt{H_s/8}\))

prd: peak period

ang: mean wave angle

ang spread: directional spread (\(\sigma_{\theta}\))

freq spread: peak enhancement factor of JONSWAP spectrum

Note

The directional spread is essential for generating short-crested waves (and flash rips). The mean crest length should vary as \(\lambda_p/(2 \sin{\sigma_{\theta}})\), with \(\lambda_p\) the peak wavelength.

Note

if WAVE_MAKER_SPECTRUM is not defined (see below), the wavemaker produces a monochromatic wave.

Related CPP options:

FLASH_RIP	Idealized longshore-uniform Imperial Beach (CA)
WAVE_MAKER & NBQ	Wavemaker for wave resolving nonhydrostatic case
WAVE_MAKER_SPECTRUM	Wave spectrum with frequency spread (default: JONSWAP)
WAVE_MAKER_DSPREAD	Add directional spread in wave spectrum
OBC_SPECIFIED_WEST	Specify wave fields at the offshore boundary (not critical)
NO_TRACER & NO_TEMPERATURE	Save time by avoiding calculation of tracer equations

CPP options:

# define FLASH_RIP

# define MPI
# undef  NC4PAR
# define SOLVE3D
# define NEW_S_COORD
# define UV_ADV
# define NO_TRACER
# define NO_TEMPERATURE
# undef  PASSIVE_TRACER
# undef  ANA_TIDES
# define NBQ
# define NBQ_PRECISE
# define LIMIT_BSTRESS
# define WAVE_MAKER
# define WAVE_MAKER_SPECTRUM
# define WAVE_MAKER_DSPREAD
# define UV_HADV_WENO5
# define UV_VADV_WENO5
# define W_HADV_WENO5
# define W_VADV_WENO5
# define GLS_MIXING_3D
# define GLS_KOMEGA
# define NS_PERIODIC
# define OBC_WEST
# define OBC_SPECIFIED_WEST
# define SPONGE
# define FRC_BRY
# define ANA_BRY
# define Z_FRC_BRY
# define M2_FRC_BRY
# define M3_FRC_BRY
# define T_FRC_BRY
# define WET_DRY
# define ANA_GRID
# define ANA_INITIAL
# define ANA_SMFLUX
# define ANA_STFLUX
# define ANA_SSFLUX
# define ANA_SRFLUX
# define ANA_SST
# define ANA_BTFLUX
# define AVERAGES
# define AVERAGES_K
# undef  DIAGNOSTICS_EDDY
# undef  CVTK_DEBUG

Results :

../_images/test_cases_flashrip.png — FLASHRIP results : sea level and wave-mean vorticity#