Three-dimensional model for tides and surges with vertical eddy viscosity prescribed in two layers—II. Irish Sea with bed friction layer

Summary. Using the theory of Part I of this work, a three-dimensional numerical tidal model of the Irish Sea is formulated with eddy viscosity defined in two layers through the vertical. In the upper layer the viscosity is uniform through the depth; in the lower layer it reduces linearly with increasing depth to a small value at the sea-bed where a no-slip condition is imposed. After experiments with varying frictional levels, the model is verified against observations of tidal elevation throughout the Irish Sea and against observations of current at a position in Liverpool Bay. To a first approximation, the well-known square law of bottom friction (involving depth-mean current) is confirmed, but only when the bottom roughness length is particularly related to the thickness of the lower layer.