Activation of physical processes

SWAN contains a number of physical processes (see Scientific/Technical documentation) that add or withdraw wave energy to or from the wave field. The processes included are: wind input, whitecapping, bottom friction, depth-induced wave breaking, dissipation due to vegetation, mud, sea ice or turbulence, obstacle transmission, nonlinear wave-wave interactions (quadruplets and triads) and wave-induced set-up. In version 41.41, another source term is introduced called the Bragg scattering, taking into account the scattering of waves by small-scale bottom features.


SWAN can run in several modes, indicating the level of parameterization. SWAN can operate in first-, second- and third-generation mode. The first- and second-generation modes are essentially those of Holthuijsen and De Boer (1988); first-generation with a constant Phillips "constant" of 0.0081 and second-generation with a variable Phillips "constant". An overview of the options is given in Table below.

Table 2.1: Overview of physical processes and generation mode in SWAN.

process

authors

generation

mode

1st

2nd

3rd

Linear wind growth

Cavaleri and Malanotte-Rizzoli (1981)

$\times$

$\times$

(modified)

Cavaleri and Malanotte-Rizzoli (1981)

$\times$

Exponential wind growth

Snyder et al. (1981) (modified)

$\times$

$\times$

Snyder et al. (1981)

$\times$

Janssen (1989, 1991)

$\times$

Yan (1987) (modified)

$\times$

Whitecapping

Holthuijsen and De Boer (1988)

$\times$

$\times$

Komen et al. (1984)

$\times$

Janssen (1991)

$\times$

Alves and Banner (2003)

$\times$

Quadruplets

Hasselmann et al. (1985)

$\times$

Triads

Eldeberky (1996)

$\times$

$\times$

$\times$

Booij et al. (2009)

$\times$

$\times$

$\times$

Depth-induced breaking

Battjes and Janssen (1978)

$\times$

$\times$

$\times$

Bottom friction

JONSWAP (1973)

$\times$

$\times$

$\times$

Collins (1972)

$\times$

$\times$

$\times$

Madsen et al. (1988)

$\times$

$\times$

$\times$

Obstacle transmission

Seelig (1979), d'Angremond (1996)

$\times$

$\times$

$\times$

Wave-induced set-up

$\times$

$\times$

$\times$

Vegetation dissipation

Dalrymple (1984),

$\times$

$\times$

$\times$

Jacobsen et al (2019)

Mud dissipation

Ng (2000)

$\times$

$\times$

$\times$

Sea ice dissipation

Collins and Rogers (2017),

$\times$

$\times$

$\times$

Rogers (2019)

Turbulence dissipation

$\times$

$\times$

$\times$

Bragg scattering

Ardhuin and Herbers (2002)

$\times$

$\times$

$\times$

The processes are activated as follows:

• Wind input is activated by commands GEN1, GEN2 or GEN3^2.1.
• Whitecapping is activated by commands GEN1, GEN2 or GEN3^2.2.
• Quadruplets is activated by command GEN3^2.3.
• Triads is activated by command TRIAD.
• Depth-induced breaking is activated by default^2.4.
• Bottom friction is activated by command FRICTION.
• Obstacle transmission is activated by command OBSTACLE.
• Wave-induced set-up is activated by command SETUP.
• Vegetation dissipation is activated by command VEGETATION.
• Mud dissipation is activated by command MUD.
• Sea ice dissipation is activated by command SICE.
• Turbulence dissipation is activated by command TURBULENCE.
• Bragg scattering is activated by command BRAGG.

For the preliminary SWAN runs, it is strongly advised to use the default values of the model coefficients. First, it should be determined whether or not a certain physical process is relevant to the result. If this cannot be decided by means of a simple hand computation, one can perform a SWAN computation without and with the physical process included in the computations, in the latter case using the standard values chosen in SWAN.


After it has been established that a certain physical process is important, it may be worthwhile to modify coefficients. In the case of wind input one may at first try to vary the wind velocity. Concerning the bottom friction, the best coefficients to vary are the friction coefficients. Switching off the depth-induced breaking term is usually not wise, since this may lead to unacceptably high wave heights near beaches (the computed wave heights 'explode' due to shoaling effects).