band. Nonlinear transfers of energy to high frequencies that occur during
breaking are not represented in the model. Model grid cells where the wave
height is limited by Equation 12 are flagged as actively breaking cells. These
breaking regions can be visualized in SMS.
Wind input. Waves grow through the transfer of momentum from the wind
field to the wave field. The flux of energy, Fin, into the wave field in STWAVE
is given by (Resio 1988a):
ρa
2
u*
Fin = λ
0.85Cm
(13)
ρw
g
where
λ = partitioning coefficient that represents the percentage of total
atmosphere to water momentum transfer that goes directly
into the wave field (0.75)
ρa = density of air
Cm = mean wave celerity
the square root of the drag coefficient, CD = .0012+.000025U)
In deep water, STWAVE provides a total energy growth rate that is consistent
with Hasselmann et al. (1973).
The energy gain to the spectrum is calculated by multiplying the energy flux
by the equivalent time for the wave to travel across a grid cell:
∆x
∆t =
(14)
βC g cosα m
where
∆t = equivalent travel time
∆x = grid spacing
β = factor equal to 0.9 for wind seas
Cg = average group celerity of the spectrum
αm = mean wave direction, relative to the grid
10
Chapter 2 Governing Equations and Numerical Discretization
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