A NUMERICAL-MODEL STUDY OF PARAMETERS INFLUENCING TIDAL CURRENTS IN THE IRISH SEA

The mathematical formulation of a mixed finite difference-modal three-dimensional tidal model of the Irish Sea using quadratic bottom friction is briefly presented. By using an expansion of modes in the vertical, a continuous current profile from sea surface to seabed can be computed, and accurate comparisons made with tidal currents measured at a range of positions throughout the water column. The sensitivity of current profiles to various eddy viscosity formulations, and coefficients of bottom friction are examined in detail. Also, the influence of open boundary conditions and water depths upon computed tidal elevations and currents is examined. In common with other earlier published calculations, initial results show that with previous formulations of eddy viscosity and friction, an accurate distribution of sea surface elevations is accompanied with computed currents which exceed observations by the order of 20 cm s-1. However, results from the sensitivity study presented here show that elevations and currents (above the bottom boundary) in good agreement with observations can be obtained with alternative formulations of viscosity and other friction coefficients, together with current input to the radiation condition. However, computed near-bed currents are still larger than those observed, although a slight increase in water depth and bottom friction coefficients corrects this. Calculations clearly show that the computation of near-bed currents, subject to the constraints of obtaining accurate elevation and current distributions is the most sensitive test of the model. Excellent agreement between computed and observed currents and elevations was obtained in a final series of calculations using appropriate parameterizations of eddy viscosity and bottom friction.