J. Groeneweg

Improvements in spectral wave modeling in tidal inlet seas

The performance of the spectral wind wave model SWAN in tidal inlet seas was assessed on the basis of extensive wave measurements conducted in the Amelander Zeegat tidal inlet and the Dutch Eastern Wadden Sea, as well as relevant data from other inlets, lakes, estuaries and beaches. We found that the 2006 default SWAN model (version 40.51), the starting point of the investigation, performed reasonably well for measured storm conditions, but three aspects required further attention. First, over the near-horizontal tidal flats, the computed ratio of integral wave height over water depth showed an apparent upper limit using the default depth-limited wave breaking formulation and breaker parameter, resulting in an underprediction of wave heights. This problem has been largely solved using a new breaker formulation. The second aspect concerns wave-current interaction, specifically the wave age effect on waves generated in ambient current, and a proposed enhanced dissipation in negative current gradients. Third, the variance density of lower-frequency wind waves from the North Sea penetrating through the inlets into the Wadden Sea was underpredicted. This was improved by reducing the bottom friction dissipation relative to that of the default model. After a combined calibration, these improvements have resulted in a relative bias reduction in Hm0 from -3% to -1%, in Tm-1,0 from -7% to -3%, and in Tm01 from -6% to -2%, and consistent reductions in scatter, compared to the 2006 default model.

RELIABILITY OF SWAN AT THE PETTEN SEA DEFENCE

In this report the reliability of the wave prediction model SWAN and the reliability of the hydraulic boundary conditions at the Petten Sea Defence have been investigated by hindcasting four or five instants from five storms, i.e. three in January 1995, one in February 2002 and one in October 2002. In 1999 the hydraulic boundary conditions have been determined with SWAN, version 30.62 and have been assimilated in the RAND2001 database. Alkyon & WL | Delft Hydraulics (2002) have proposed an advanced method for hindcasting measured storm events with the SWAN model. In this study a first application of this generic hindcasting method has been presented. The results will be compared with the results obtained with the more simplified, standard hindcasting method that has been used in 1999. The reliability of the hydraulic boundary conditions contained in RAND2001 has been investigated by comparing the SWAN results for the standard and advanced hindcasting method with measurements. Since 1999 SWAN has developed significantly. The present standard version 40.11 contains improvements in physical formulations, handling of the boundary and preand postprocessing. After version 40.11 only research versions have been developed, each containing changes in one aspect. One of them is 40.16. Also with SWAN 40.16 computations have been carried out applying the standard and advanced hindcasting method. The SWAN results have been compared with measurements by means of scatter plots and statistical parameters. The reliability of SWAN has been investigated by comparing the scatter plots and statistical parameters for all SWAN versions. The comparison has been made based on a subdivision in locations, storm days and classes (current following and opposing wind direction, depth or not-depth limited situations and presence of low-frequency energy). The major conclusions that have been drawn from this study are the following. The RAND2001 data set is not necessarily reliable, since the computational results at MP6 are questionable. Furthermore, the advanced hindcasting approach generally leads to improved results in comparison with the ‘standard’ approach. The inclusion of current effects and the use of a more recent bottom topography improves the results the most. Finally, the performance of SWAN 40.16 is similar to SWAN 30.62. REFERENCES: Proposal no. MCI03614/H4197/JG, d.d. 30 October 2002 Contract no. RKZ-1244, d.d. 26 November 2002 Revised contract no. RKZ-1244A, d.d. 15 april 2003 VER. ORIGINATOR DATE REMARKS REVIEW APPROVED BY 1 J. Groeneweg et al. February 7, 2003 draft phase1 A.R. van Dongeren W.M.K. Tilmans 2 J. Groeneweg et al. April 28, 2003 draft M.R.A. van Gent W.M.K. Tilmans 3 J. Groeneweg, G.Ph. van June 19, 2003 final M.R.A. van Gent W.M.K. Tilmans Vledder, D. Hurdle, N. Doorn, C. Kuiper PROJECT IDENTIFICATION: H4197 / A1044