rompy.swan.components.physics.BRAGG#

pydantic model rompy.swan.components.physics.BRAGG[source]#

Bragg scattering.

BRAGG [ibrag] [nreg] [cutoff]

Using this optional command, the user activates a source term to represent the scattering of waves due to changes in the small-scale bathymetry based on the theory of Ardhuin and Herbers (2002). If this command is not used, SWAN will not account for Bragg scattering.

The underlying process is related to the bed elevation spectrum that describes the random variability of the bathymetry at the scale of the wave length on top of a slowly varying depth. To input this spectrum in the model, two options are available. One option is to read a spectrum from a file. This single bottom spectrum will subsequently be applied in all active grid points. The assumption being made here is that the inputted bottom is gently sloping. Note that the bottom spectrum must be given as a function of the wave number k.

Another option is to compute the spectrum by a Fourier transform from x to k of the bed modulations around a computational grid point. First, one must define a square region with a fixed size around the grid point in order to perform the Fourier transform. The size should correspond to a multiple of the wave length at which refraction is resolved (i.e. consistent with the mild slope assumption). Next, the amplitude modulation of the small-scale bathymetry is obtained by substracting a slowly varying bed level from the inputted high-resolution bathymetric data within this square region. Here, the smooth bed level is achieved using a bilinear fit. During the computation, however, SWAN employs the gently sloping bed as the mean of the original bathymetry within the given square around each computational grid point. Finally, the corresponding bottom spectrum is computed with an FFT.

Notes

The Bragg scattering source term to the action balance equation gives rise to a fairly stiff equation. The best remedy is to run SWAN in the nonstationary mode with a relatively small time step or in the stationary mode with some under relaxation (see command NUM STAT [alfa]).

Examples

In [7]: from rompy.swan.components.physics import BRAGG

In [8]: bragg = BRAGG(nreg=200)

In [9]: print(bragg.render())
BRAGG nreg=200

In [10]: bragg = BRAGG(ibrag=1, nreg=200, cutoff=5.0)

In [11]: print(bragg.render())
BRAGG ibrag=1 nreg=200 cutoff=5.0
Fields:
field cutoff: float | None = None#

Cutoff to the ratio between surface and bottom wave numbers. Note: seethe Scientific/Technical documentation for details (SWAN default: 5.0)

field ibrag: Literal[1, 2, 3] | None = None#

Indicates the computation of Bragg scattering term:

  • 1: source term is calculated per sweep and bottom spectrum is interpolated at the difference wave number a priori (thus requiring storage)

  • 2: source term is calculated per sweep and bottom spectrum is interpolated at the difference wave number per sweep (no storage)

  • 3: source term is calculated per iteration and bottom spectrum is interpolated at the difference wave number per iteration (no storage)

(SWAN default: 1)

field model_type: Literal['bragg', 'BRAGG'] = 'bragg'#

Model type discriminator

field nreg: int [Required]#

Size of square region around computational grid point (centered) for computing the mean depth and, if desired, the bed elevation spectrum. It is expressed in terms of the number of grid points (per direction) of the inputted bottom grid

cmd() → str[source]#

Command file string for this component.

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