Study on low frequency wave penetration in SWAN : Validation of DCTA method

Within the Sea Level Rise Knowledge Program (“Kennisprogramma Zeespiegelstijging” KPZSS), coastal safety assessments are performed to assess the effect of sea level rise of +0.5 m till +5 m on the failure probability of dike segments along the Dutch coast. Use is made of the SWAN model, which is known to underestimate the penetration of North Sea waves into the Wadden Sea towards the coast of Frisia and Groningen. Two potential improvements were considered at the start of this program: inclusion of Bragg scattering and a new three-wave interaction method called DCTA. The DCTA method is able to transfer energy to the low frequency part of the spectrum as well, in contrast to the default LTA method. The focus in this report is on the latter one. The objective of this study is to validate the DCTA method for triad modelling against SWASH results, 2D laboratory measurements (Taman case) and field cases (Eastern Wadden Sea) to see whether its present implementation in SWAN could lead to an improvement of the penetration of low-frequency waves from the North Sea into the Wadden Sea.
Apart from comparing the SWAN results with measurements and SWASH results at various measurement locations, also the evolution along rays of both variance density spectra and triad source terms has been analyzed. For the situation without wind and currents a comparison with SWASH results has been made. Various settings for both the original LTA method and the newly developed DCTA method have been considered. For the DCTA method both the collinear and non-collinear version have been considered. Like the LTA method, in the collinear DCTA method only triads in the same directions are accounted for. In the non-collinear version the interaction between waves from different directions are also considered.
From the analyses we conclude that, from a qualitative point of view, the DCTA method seems to be an improvement over the LTA method. The DCTA method transfers energy to lower frequencies, whereas the LTA method does not. In addition, the shape of the DCTA source term is more realistic, in some cases LTA leads to physically unrealistic peaks in the spectrum.
For the Taman case the variance density computed with the DCTA method at the low frequencies and at the first harmonic does not compare to what is measured or computed with SWASH. The variance density at these frequencies can be increased by decreasing the default value for the critical Ursell number or increasing the scaling factor significantly.

This report summarizes three years of research. This is the followup to Modellering laagfrequente golfenergie met SWAN.
Aangepaste formulering voor een spectrale bronterm voor triads op basis van de DCTA and work on SWAN by the TU Delft.