Diffraction is included using the phase-decoupled refraction-diffraction approximation proposed by Holthuijsen et al. (2003). For more details see the scientific manual.
The approximation is based on the mild-slope equation for refraction and diffraction, omitting phase information. It does therefore not permit coherent wave fields in the computational domain as in deterministic phase-resolving models such as Boussinesq type models.
For instationary calculations the inclusion of diffraction can cause oscillations in the numerical solution in areas with very fine resolution and/or large ratio between element sizes. For quasi-stationary calculations the inclusion of diffraction can cause convergence problems. To reduce these problems a smoothing is introduced for the discrete values of the square root of the directional spectral energy density, Ai,l = A(xi,yi,sl), which is used in the calculation of the diffraction parameter . This smoothing is done according to
(6.8)
Here k is the number of smoothing steps and a is the smoothing factor. The smooth approximation, A* , is calculated by first calculating the vertex values using the pseudo-Laplacian procedure proposed by Holmes and Connell (1989) and then calculating the cell-centred values by averaging the vertex values corresponding to each element. By default one filtering step is performed with a smoothing factor of a = 1. Note, the smoothing is only used in the calculation of the diffraction parameter. Increasing the smoothing (increasing the number of smoothing steps) with reduce the oscillation/convergence problem, but will also has the effect that the diffraction effect will be reduced.
