Wave diffraction approximation

[aronroland]

Is your feature request related to a problem? Please describe.
In the presence of strong local gradients in the solution the group velocities depend on the solution itself. Different approximation have been suggested in literature (e.g. Liau et al. 2011, DOI: 10.1016/j.coastaleng.2011.01.002.

Describe the solution you'd like
A better physical description of wave propagation in the presence of strong spatial gradients in the solution.

Describe alternatives you've considered
A better alternative would be the implementation of Toledo et al. 2012 (DOI: 10.1098/rspa.2011.0377, however there it is shown that wave action is not conserved in the presence of strong bottom and currents gradients. This leads to a new balance equation, which is beyond the present scope of work.

Additional context
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