A DISTRIBUTED COLLINEAR TRIAD APPROXIMATION IN SWAN

Waves entering the surf zone initially develop higher harmonics which disappear after two or three wave lengths. The default Lumped Triad Approximation (LTA) in the SWAN wave model generates only (persistent) second harmonics. An alternative approximation (DCTA) is suggested which is based on an analogy with quadruplet wave-wave interactions. It initially generates all (transient) higher harmonics and subsequently a smooth universal spectral tail, in agreement with observations. The DCTA is calibrated with 31 published laboratory cases and then applied to 20 other, one- and two-dimensional cases, including field cases. The conclusion is that using the DCTA instead of the LTA barely affects the significant wave height but improves the mean period and spectral tail of the computed spectra. Further improvements are expected with enhanced high-frequency dissipation.