CMS-Wave:Governing Equations

Wave-action balance equation with diffraction

Taking into account the effect of an ambient horizontal current or wave behavior, CMS-Wave is based on the steady wave-action balance equation (Mase 2001)

\begin{equation}

 ${\frac {\partial (c_{x}N)}{\partial x}}+{\frac {\partial (c_{y}N)}{\partial y}}+{\frac {\partial (c_{\theta }N)}{\partial \theta }}={\frac {\kappa }{2\sigma }}{\biggl [}(cc_{g}\cos ^{2}\theta N_{y})_{y}-{\frac {cc_{g}}{2}}\cos ^{2}\theta N_{yy}{\biggr ]}-\epsilon _{b}N-S$

\end{equation}

where $N=E(\sigma ,\theta )/\sigma$ is the wave-action density to be solved and is a function of frequency σ and direction θ. E(σ,θ) is spectral wave density representing the wave energy per unit water-surface area per frequency interval. In the presence of an ambient current, the wave-action density is conserved, whereas the spectral wave density is not (Bretherton and Garrett 1968; Whitham 1974). Both wave diffraction and energy dissipation are included in the governing equation. Implementation of the numerical scheme is described elsewhere in the literature (Mase 2001; Mase et al. 2005a). C and Cg are wave celerity and group velocity, respectively; x and y are the horizontal coordinates; Cx, Cy, and Cθ are the characteristic velocity with respect to x, y, and, θ respectively; Ny and Nyy denote the first and second derivatives of N with respect to y, respectively; κ is an empirical parameter representing the intensity of diffraction effect; εb is the parameterization of wave breaking energy dissipation; S denotes additional source Sin and sink Sds (e.g., wind forcing, bottom friction loss, etc.) and nonlinear wave-wave interaction term.

Symbol	Description
$\sigma$	Wave frequency
$N$	Wave action
$E$	Spectral wave density
$c$	Wave celerity
$c_{g}$	Wave group velocity

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