Abstract
Over the last five years a research program has been carried out to assess the performance of the spectral wave model SWAN in the Wadden Sea so that it may be used for the transformation of offshore wave conditions to wave boundary conditions near the sea defenses (dikes and dunes). The assessment was done on the basis of extensive wave measurements conducted in Ameland inlet and the Dutch Eastern Wadden Sea, as well as relevant data from lakes and estuaries. After a first round of assessment, we found that SWAN performed reasonably well for storm conditions but three aspects required further attention. Firstly, focusing on the main channel, SWAN formulations needed to be modified in order to eliminate overprediction of the significant wave height in opposing currents. Secondly, the primary spectral peak of North Sea waves penetrating into the inlet was underpredicted. Best results were obtained when the refraction of low-frequency waves was limited and the bottom friction coefficient was set at a lower value than the current default for wind seas. Thirdly, over the tidal flats the computed ratio of integral wave height over water depth showed an apparent upper limit using the conventional Battjes and Janssen (1978) depth-limited wave breaking formulation, because the wave growth over finite depth is hampered by the present formulation of depth-induced wave breaking. The problem has been solved using a new breaker formulation. All these improvements have lead to a wave transformation model with which reliable wave conditions in the Wadden Sea and related complex areas can be determined.
Highlights
A significant part of the Netherlands lies below sea level and is protected from flooding by dunes and dikes
In order to compute these Hydraulic Boundary Conditions (HBCs) at the toe of the dikes and dunes, offshore wave statistics are transformed using the spectral wind wave model SWAN (Booij et al, 1999) which is widely used for the computation of wave fields over shelf seas, in coastal areas and in shallow lakes
(2001) showed that SWAN seems to underestimate the penetration of low-frequency storm waves from the North Sea into the tidal inlet of Norderney in the German part of the Wadden Sea
Summary
A significant part of the Netherlands lies below sea level and is protected from flooding by dunes and dikes. Until 2006 this procedure could only be applied for sea defences along the west coast of the Netherlands, including the uninterrupted Holland coast and the Scheldt estuaries in the southern part of the Netherlands It was not applied in the complex Wadden Sea tidal inlet system where there were uncertainties regarding the quality of the model’s performance. In 2006 an additional set of measurement devices was deployed in the eastern Wadden Sea. In 2006, the Dutch Public Works Department (Rijkswaterstaat) awarded Deltares a commission to assess the performance of the wave transformation model SWAN as part of a larger five-year project called SBW (Strengths and Loads of Sea Defenses). In 2006, the Dutch Public Works Department (Rijkswaterstaat) awarded Deltares a commission to assess the performance of the wave transformation model SWAN as part of a larger five-year project called SBW (Strengths and Loads of Sea Defenses) This paper will discuss the method with which the project was executed and show the most important findings, which will be of interest to the broader community of wave modellers
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