Physics¶

What this shows: the physics options of SWAN available in the PHYSICS component: source-term packages, whitecapping and swell dissipation, breaking, bottom friction, triads and the processes for special cases.

Prerequisites: Tutorial 5: Choosing model settings.

You will learn:

• which source-term packages GEN3 offers, and what they mean
• the options for depth-induced breaking, bottom friction and triads
• how to add set-up, diffraction, obstacles and vegetation, and how to switch processes off

Data used: none.

Each option below links to its section of the SWAN user manual. Physics sensitivity runs SWAN with several packages and compares the results.

Setup¶

In [1]:

from rompy_swan.components.group import PHYSICS
from rompy_swan.components.physics import (
    BREAKING_BKD,
    BREAKING_CONSTANT,
    DIFFRACTION,
    FRICTION_COLLINS,
    FRICTION_JONSWAP,
    FRICTION_MADSEN,
    GEN3,
    OBSTACLE,
    OBSTACLES,
    OFF,
    OFFS,
    QUADRUPL,
    SETUP,
    SSWELL_ZIEGER,
    TRIAD,
    TRIAD_DCTA,
    VEGETATION,
)
from rompy_swan.subcomponents.physics import (
    JANSSEN,
    KOMEN,
    LINE,
    REFL,
    ST6C1,
    TRANSM,
    WESTHUYSEN,
)

from rompy_swan.components.group import PHYSICS from rompy_swan.components.physics import ( BREAKING_BKD, BREAKING_CONSTANT, DIFFRACTION, FRICTION_COLLINS, FRICTION_JONSWAP, FRICTION_MADSEN, GEN3, OBSTACLE, OBSTACLES, OFF, OFFS, QUADRUPL, SETUP, SSWELL_ZIEGER, TRIAD, TRIAD_DCTA, VEGETATION, ) from rompy_swan.subcomponents.physics import ( JANSSEN, KOMEN, LINE, REFL, ST6C1, TRANSM, WESTHUYSEN, )

1. Wind input and whitecapping: GEN3¶

GEN3 switches on third-generation physics. Its source_terms choose the package of wind input and whitecapping formulations:

Package	Formulations	Notes
WESTHUYSEN()	Yan wind input, saturation-based whitecapping	SWAN default since 41.45
KOMEN()	WAM cycle 3	SWAN default before 41.45
JANSSEN()	WAM cycle 4	not recommended by the SWAN team
ST6C1() … ST6C5()	Rogers et al. (2012) with calibrated coefficient sets	pair with SSWELL_ZIEGER

rompy-swan also writes Wu's (1982) drag formulation, DRAG WU, SWAN's default.

In [2]:

for terms in [WESTHUYSEN(), KOMEN(), JANSSEN(), ST6C1()]:
    print(GEN3(source_terms=terms).render())

for terms in [WESTHUYSEN(), KOMEN(), JANSSEN(), ST6C1()]: print(GEN3(source_terms=terms).render())

GEN3 WESTHUYSEN DRAG WU
GEN3 KOMEN DRAG WU
GEN3 JANSSEN DRAG WU
GEN3 ST6 a1sds=4.7e-07 a2sds=6.6e-06 p1sds=4.0 p2sds=4.0 UP HWANG VECTAU U10PROXY windscaling=28.0 AGROW

The ST6 presets fill in the coefficients of each calibration. ST6 is meant to be used with swell dissipation (SSWELL ZIEGER), which the presets do not add:

In [3]:

print(PHYSICS(gen=GEN3(source_terms=ST6C1()), sswell=SSWELL_ZIEGER()).render())

print(PHYSICS(gen=GEN3(source_terms=ST6C1()), sswell=SSWELL_ZIEGER()).render())

GEN3 ST6 a1sds=4.7e-07 a2sds=6.6e-06 p1sds=4.0 p2sds=4.0 UP HWANG VECTAU U10PROXY windscaling=28.0 AGROW

SSWELL ZIEGER

2. Quadruplets¶

Four-wave interactions move energy within the spectrum and are on by default with GEN3. QUADRUPL changes the method (iquad) and coefficients. Without wind, SWAN requires them to be switched off with OFF (see section 6).

In [4]:

print(QUADRUPL(iquad=2, lambd=0.25).render())

print(QUADRUPL(iquad=2, lambd=0.25).render())

QUADRUPL iquad=2 lambda=0.25

3. Depth-induced breaking¶

Breaking is on by default in SWAN, with a constant breaker index.

• BREAKING_CONSTANT(alpha, gamma): Battjes and Janssen (1978), gamma=0.73;
• BREAKING_BKD: Salmon et al. (2015), with a breaker index that depends on the bottom slope and wave steepness, recommended for varying bathymetry.

In [5]:

print(BREAKING_CONSTANT(alpha=1.0, gamma=0.73).render())
print(BREAKING_BKD(alpha=1.0, gamma0=0.54, a1=7.59, a2=-8.06, a3=8.09).render())

print(BREAKING_CONSTANT(alpha=1.0, gamma=0.73).render()) print(BREAKING_BKD(alpha=1.0, gamma0=0.54, a1=7.59, a2=-8.06, a3=8.09).render())

BREAKING CONSTANT alpha=1.0 gamma=0.73
BREAKING BKD alpha=1.0 gamma0=0.54 a1=7.59 a2=-8.06 a3=8.09

4. Bottom friction¶

Bottom friction is off unless it is set:

• FRICTION_JONSWAP(cfjon): empirical, cfjon=0.038 m²/s³ (0.019 for sandy beaches with swell);
• FRICTION_COLLINS(cfw): drag law with a friction factor;
• FRICTION_MADSEN(kn): eddy-viscosity model with a roughness length kn.

In [6]:

for friction in [FRICTION_JONSWAP(cfjon=0.038), FRICTION_COLLINS(cfw=0.015), FRICTION_MADSEN(kn=0.05)]:
    print(friction.render())

for friction in [FRICTION_JONSWAP(cfjon=0.038), FRICTION_COLLINS(cfw=0.015), FRICTION_MADSEN(kn=0.05)]: print(friction.render())

FRICTION JONSWAP CONSTANT cfjon=0.038
FRICTION COLLINS cfw=0.015
FRICTION MADSEN kn=0.05

A spatially varying friction coefficient is read from an input grid (var="friction", see Input grids).

5. Triads¶

Three-wave interactions transfer energy to higher frequencies in shallow water. They are off unless set. TRIAD uses SWAN's default method (DCTA since 41.45); TRIAD_DCTA exposes its coefficients.

In [7]:

print(TRIAD().render())
print(TRIAD_DCTA(trfac=4.4).render())

print(TRIAD().render()) print(TRIAD_DCTA(trfac=4.4).render())

TRIAD
TRIAD DCTA trfac=4.4 COLL

6. Special processes¶

Component	Process
SETUP()	wave-induced set-up (only on open coasts, not in closed basins)
DIFFRACTION()	phase-decoupled diffraction approximation
OBSTACLES([OBSTACLE(...)])	transmission and reflection at breakwaters and dams
VEGETATION(...)	dissipation by vegetation (heights, diameters, densities per layer)
OFFS([OFF(physics=...)])	switch off a default process: windgrowth, quadrupl, wcapping, breaking, refrac, fshift, bndchk

In [8]:

breakwater = OBSTACLE(
    transmission=TRANSM(trcoef=0.3),
    reflection=REFL(reflc=0.5),
    line=LINE(xp=[115.735, 115.745], yp=[-32.065, -32.055]),
)
physics = PHYSICS(
    gen=GEN3(),
    breaking=BREAKING_BKD(alpha=1.0, gamma0=0.54, a1=7.59, a2=-8.06, a3=8.09),
    friction=FRICTION_JONSWAP(cfjon=0.038),
    triad=TRIAD(),
    setup=SETUP(),
    diffraction=DIFFRACTION(),
    obstacle=OBSTACLES(obstacles=[breakwater]),
    vegetation=VEGETATION(iveg=1, height=[0.5], diamtr=[0.01], drag=[1.0], nstems=[400]),
    deactivate=OFFS(offs=[OFF(physics="refrac")]),
)
print(physics.render())

breakwater = OBSTACLE( transmission=TRANSM(trcoef=0.3), reflection=REFL(reflc=0.5), line=LINE(xp=[115.735, 115.745], yp=[-32.065, -32.055]), ) physics = PHYSICS( gen=GEN3(), breaking=BREAKING_BKD(alpha=1.0, gamma0=0.54, a1=7.59, a2=-8.06, a3=8.09), friction=FRICTION_JONSWAP(cfjon=0.038), triad=TRIAD(), setup=SETUP(), diffraction=DIFFRACTION(), obstacle=OBSTACLES(obstacles=[breakwater]), vegetation=VEGETATION(iveg=1, height=[0.5], diamtr=[0.01], drag=[1.0], nstems=[400]), deactivate=OFFS(offs=[OFF(physics="refrac")]), ) print(physics.render())

GEN3 WESTHUYSEN DRAG WU

BREAKING BKD alpha=1.0 gamma0=0.54 a1=7.59 a2=-8.06 a3=8.09

FRICTION JONSWAP CONSTANT cfjon=0.038

TRIAD

VEGETATION iveg=1 height=0.5 diamtr=0.01 nstems=400 drag=1.0

OBSTACLE TRANSM trcoef=0.3 REFL reflc=0.5 LINE 115.735 -32.065 115.745 -32.055

SETUP

DIFFRACTION

OFF REFRAC

This combination is only to show the syntax; switching refraction off, for example, only makes sense for tests.

Summary¶

• GEN3(source_terms=...) chooses the wind input and whitecapping package; WESTHUYSEN is SWAN's current default.
• Breaking is on by default; friction and triads must be switched on for coastal models.
• Set-up, diffraction, obstacles and vegetation are added when needed, and OFF removes default processes.