|On the high-resolution simulation of the dynamic interaction between current and waves in coastal waters: an application to the Southern North Sea|
Osuna Cañedo, J.P. (2002). On the high-resolution simulation of the dynamic interaction between current and waves in coastal waters: an application to the Southern North Sea. PhD Thesis. Katholieke Universiteit Leuven (KUL): Leuven. XXVI, 196 pp.
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VLIZ: Open Repository 74804 [ OMA ]
|Document type: Dissertation|
Coastal waters; Dynamical oceanography; Resolution; Simulation; Wave-current interaction; ANE, Belgium [Marine Regions]; ANE, North Sea, Southern Bight [Marine Regions]; Marine
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The study about wave-current interaction is usually carried out through the use of coupled systems that consider the interaction processes in one way (waves modified by currents). During the PROMISE project, a two-way coupled model was tested and disseminated for the North Sea shelf region. The applicability of such a study was restricted to offshore areas as the used spatial resolution was relatively coarse. In this study, the implementation of an improved version of that system was used in order to reach the coastal area by means of successive nested grids. Attention is focussed on the effect of using a high resolution for the study of the wave evolution in coastal areas, specifically the Belgian coastal area. A systematic approach is used to take the original coupling system to coastal areas. First, the performance of the coupled system in an ideal basin, whose dimensions resemble the North Sea basin, was assessed. Results of this experiment indicate that tides and surges induce oscillations of Hs and Tm02 by 5% and 15%, respectively, with respect to the typical values computed from an uncoupled version. The modulations of Hs depend on the depth (they increase in shallower areas), while the TM02 oscillations are more dependent on the sea state and the current field. The spectral shape is also modified. lt was observed that considering a wave dependent sea surface stress in the hydrodynamic model under variable wind conditions increases the computed surge effect. Then, after the implementation of a nesting procedure to the hydrodynamic model, the coupled system was applied to the North Sea region. During the analysis period, an underestimation of wave parameters and surge effect was observed, mainly as a result of the misrepresentation of the wind field. The effect of coupling on Hs and TM02 is about 3% and more than 20%, respectively. The nesting procedure (which is equivalent to increase the spatial resolution of the simulation) tends to improve the qualitative agreement between computed wave parameters and measurements. The same trend is observed for the computed surge effect. Spectral characteristics, like directional spreading and energy distribution, are also improved when nesting is included. In the fine grid, which covers the Belgian coastal area, the radiation stress effect was quantified and was found to be as important as considering the effect of a wave dependent surface stress in the fully coupled system. The order of magnitude of the residual current (with respect the uncoupled version) produced by this latter effect is about 10cm/s. Concerning the specification of an open boundary conditions in the grid that covers the Belgian coastal region, it is found that the wave evolution in this area is more dependent on the consideration of the coupling between waves and hydrodynamics than on the source of boundary conditions (i.e., either obtained from a coupled or an uncoupled coarser model). Finally, the effect of considering a high spatial and spectral resolution in the wave evolution is studied. This experiment was assessed for the Belgian coastal area implementation. Comparison between measured and computed time series of Hs and TM02 at two stations indicate that the use of a higher spatial resolution is important to account for the right energy dissipation rate. The spectral energy distribution and directional spreading is clearly improved when a higher spatial resolution is included. Also important is to consider the coupling effect on the spatial distribution of the energy. Difference maps of Hs indicate that the time varying watercolumn induces changes in the order of 10% compared to the Hs computed from the uncoupled implementation. Most of these variations are found at the sand banks region, and the sign of the differences depend on the phase of the elevation field. The use of a high directional resolution has a comparable effect as considering the coupling. Again, the main effect is observed at the sand banks and at the entrance of the Scheldt river when waves and currents are collinear. The Hs differences pattern is dependent on the wave regime and almost independent on the phase of the hydrodynamic fields.