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Wave and storm surge models: WAM and COHERENS

The objective within the project of the Belgian Royal Institute of Natural Sciences is to simulate different scenarios in the North Sea and Continental Shelf with the COHERENS model for tides and storm surges, and wave heights with the WAM model. Both models are public domain codes.

COHERENS is an integrated code with modules for physics, sediment transport, biology and particle tracing and has been developed in the frame of European and national projects (Luyten, 2013). The model is operationally used to compute 5 days forecasts of storm surges, tides, temperature and salinity with a spatial resolution ranging from 750 m to 5 km and forced by 6-hourly wind fields and tidal harmonics at the open boundaries. COHERENS has been applied for a study of long-term temperature  ariability in the North Sea (Luyten et al., 2003), a 20-years hindcast of the southern North Sea hydrodynamics (Lacroix et al., 2007) and the assessment of primary impacts of climate change in Belgian coastal waters (Van den Eynde et al., 2009).

The WAM wave model (Günther et al., 1992; WAMDI Group, 1988, Monbaliu et al., 2000;  Bolaños et al., 2011) is a third-generation wave model, where an energy transport equation is solved for each wave component. The wave model calculates for each grid point and for each time step the evolution of the full energy spectrum of the waves, taking into account the generation of the waves by wind, the dissipation of energy by friction with the bottom, by breaking or by white-capping, the propagation of the waves and with non-linear interactions between different wave components. Some shallow water effects, like shoaling and depth and current refraction processes are included in the WAM model. Since there is no restriction in the evolution of the wave spectra, the WAM model can better represent reality, e.g. wave spectra with two peaks, than previous second-generation wave models. The WAM model is implemented on three coupled grids. A coarse WAM grid provides boundary conditions for an intermediate resolution nested WAM model, which is coupled to a local higher resolution model. The coarse grid WAM model extends until 71°N in order to catch the waves that are generated in this area and travel (as swell) towards the Belgian coast. Westward extension is limited since the Southern North Sea is sheltered by the British Isles. The local model has a resolution of 0.022° x 0.033°.