
L11. Diffraction behind a gap in a breakwater
The purpose of this test is to validate the diffraction approximation in Swan.
The approximation is described by Holthuijsen, Herman and Booij (2003).
The results are compared with observations reported by Yu et al.(2000).
Two tests have been used, both with a gap width equal to 4 times the wave length (2 m);
one test was carried out with regular incoming waves,
the other with unidirectional irregular waves.
A domain with horizontal bottom is considered.
In order to simulate the unidirectional incoming waves
at the Southern boundary an incident wave field is assumed with narrow directional distribution ([ms]=500)
and a Jonswap spectrum over frequencies with peak period of 1.2 s.
The wave field propagates in N direction.
7 cases are considered:
 L11a
 Swan without diffraction
 L11b
 Swan with standard diffration
 L11c
 diffraction with enhanced smoothing
 L11d
 diffraction with strong smoothing
 L11e
 standard diffraction on a grid, rotated over i90 degrees, in order to
test the correct implementation
 L11f
 standard diffraction on a grid, with singularity approximation
 L11g
 as L11f, but with simplified representation of the breakwater
Graphs are shown for cases a, b and c, along lines parallel to the breakwater at various distances
behind the breakwater: 0 m (i.e. in the gap), 3 m, 6 m and 9 m.
 L.H. Holthuijsen, A. Herman, N. Booij (2003)
 Phasedecoupled refractiondiffraction in a spectral wave model,
Coastal Engineering, vol.49, PP.291305
 Yu, Y.X., S.X., Li, Y.S., Onyx, Wai, W.H. (2000)

 Refraction and diffraction of random waves through breakwater,
Ocean Engng, vol. 27, pp.489509

