Abstract
This paper describes a laboratorium data set of wave propagation over steep slopes with large angles of incidence, and the model-data comparison with linear and nonlinear wave models (SWAN and TRITON). The reason for this study is that the SWAN model showed an underestimation of the observed low frequency wave energy at nearshore sites in the Eastern Wadden Sea tidal inlet. Van Vledder (2010) showed that potential inaccuracies in modeling refraction over steep slopes of the tidal channel may result in observed discrepancies. The paper addresses the set up of the laboratory experiment, the novel measurement techniques and an analysis of the observed differences between models and data. The results show that there is not one single mechanism that explains the differences, but a combination of mechanisms, and perhaps other phenomena such as reflection play role.
Highlights
BackgroundThe safety of the Dutch primary sea and flood defenses must be assessed every six years (2011, 2017, etc.)
Approach Since the major research question was identified as the wave propagation, and refraction, it was decided to study the refraction of long waves over steep slopes and large angles in the 50mX50m wave basin of Deltares
In order to explain the observed differences between SWAN computations and the measurements in the eastern Wadden Sea area, where the low frequency energy, in a hindcast study of the storm of 8th and 9th November 2007 was underestimated, an extensive physical model study was carried out
Summary
The safety of the Dutch primary sea and flood defenses must be assessed every six years (2011, 2017, etc.). The measured spectrum (black line) is compared to SWAN results with the default settings (blue line), and to SWAN results at this location with adjusted bottom friction coefficient (Cf,jon=0.038 m2/s3 instead of 0.067m2/s3) With this calibration, the amount of lowfrequency wave energy in the computation is increased. The right panel, shows the measured spectrum compared to SWAN results with the default settings (blue line) and SWAN results with reduced bottom friction and an artificial limiter for low-frequency wave refraction (red line). Approach Since the major research question was identified as the wave propagation, and refraction, it was decided to study the refraction of long waves over steep slopes and large angles (resembling refraction over the shoals towards the tidal flats in the estuary) in the 50mX50m wave basin of Deltares For this purpose a physical model was constructed and the physical model results were compared to the SWAN model results. In order to understand the differences between SWAN and the measurements, a more extensive study, by using a Boussinesq model, TRITON, Borsboom et al (2000), the laboratory experiments were numerically re-generated, and some sensitivity analysis was carried out, which provided more insight in the physics
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