<%namespace name="tools" file="tools.tex"/>



Longcrested waves are supported in XBeach by using a value s larger than 1,000. The directional distribution in that case is a spike corresponding to the mean wave direction. Due to the discretization in the wave directional dimension, numerical diffusion in this dimension may occur. In case of a narrow directional distribution, the numerical diffusion may lead to a loss of wave energy through the boundaries of the directional grid.

To limit the numerical directional diffusion, the directional grid may be chosen with large grid cells and thus decreasing the gradients between adjacent grid cells. Directional advection may be turned of entirely when using a single directional grid cell, since refraction is not possible with a single grid cell at all. However, when using oblique longcrested waves in a 2DH model, none of these solutions suffice.

This test shows the numerical directional diffusion in the comparison of a few runs where the angle of wave incidence and the number of directional bins vary. The bathymetry is a linear sloping beach and no morphological change is computed in these runs.

${tools.image('longcrested_refraction/', 'H.eps', 'Wave height')}
${tools.image('longcrested_refraction/', 'thetamean.eps', 'Mean wave direction')}
${tools.image('longcrested_refraction/', 'Fy_mean.eps', 'Time-averaged wave forcing in lateral direction')}
