A High-resolution, Three-dimensional Model of the M2, M4M6, S2, N2, K1and O1Tides in the Eastern Irish Sea

The spatial distribution of the M2, S2, N2, M4, M6, K1and O1tidal elevations and currents in the eastern Irish Sea is examined using a high-resolution, three-dimensional model of the region. The model uses a functional representation of the current in the vertical and can resolve the near-bed, high shear bottom boundary layer. Comparison of computed and observed tidal elevations and currents shows that the grid is sufficiently fine (of order 1 km) and the near-bed region sufficiently well resolved to enable tidal elevations and currents, particularly near-bed currents, to be accurately reproduced. In order to accurately reproduce the near-bed currents, its is essential to reduce eddy viscosity close to the bed, and enhance the bottom friction coefficient above that used in two-dimensional models. Calculations of the M2, M4and M6tidal currents, with and without the other tidal constituents, show that it is essential to include the other constituents if tidal elevations and currents are to be determined with a high level of accuracy, with the M6tide being significantly influenced by the presence of the other constituents. The presence of other tidal constituents not only increases bottom friction (in an analogous manner to a two-dimensional model), but because the model involves a flow-dependent viscosity, they increase its magnitude which affects current structure. The calculation of parameters such as the maximum bed stress, which is important in sediment transport, can be severely underestimated if the other tidal constituents are not included.