A three dimensional hydrodynamic model of tides off the north-west coast of Scotland

A three dimensional hydrodynamic model using a fine 1/12 ° × 1/12 ° finite difference grid in the horizontal with a spectral approach in the vertical is used to examine the spatial distributions of the M2 and O1 tidal elevations and currents at the shelf edge. The region chosen for the study is the Malin-Hebrides shelf off the west coast of Scotland, where a set of offshore elevation and current meter data exists. Calculations show that M2 tidal elevations and currents increase rapidly from ocean to shelf having the largest amplitude in the near shore regions. In contrast O1 elevations and currents are intensified at the shelf edge showing spatial changes corresponding to a first mode shelf wave. Computed O1 current ellipses show significant small scale horizontal spatial variability in the shelf edge region. Comparison of computed and observed M2 tidal currents, shows that away from abrupt changes in topography, the model can adequately reproduce the currents, although in regions of rapid change in water depth a finer grid is probably required. Calculations show that with the finite difference grid used here the intensity and the spatial variability of the O1 tidal currents in the shelf edge region is strongly influenced by the magnitude of horizontal eddy viscosity, the exact formulation of which is poorly known.