A high-resolution (0.5′ north-south by 1.0′ west-east, approximately a 1-km grid) hydrodynamic tidal model of the eastern Irish Sea is used to examine the sensitivity of M2 tidal currents to changes in open boundary data and bottom drag coefficients.
The model uses a finite-difference grid formulation in the horizontal, with an expansion technique in the vertical, giving a continuous current profile from sea surface to sea bed. A slip boundary condition is applied at the sea bed. Eddy viscosity in the model is a function of the flow field.
Comparison of observed and computed tidal elevation and currents show good agreement, although in general, currents are slightly higher than observed.
The effect on tidal current profile of changes in bottom friction coefficient is found to be influenced by the proximity of the open boundary to the region of interest. This suggests, and calculations confirm, that it is not just the local coefficient of bottom friction (i.e., that within the eastern Irish Sea) that influences current profiles in the region, but frictional effects throughout the whole Irish Sea. This illustrates the limitations of limited-area models where the solution can be dominated by the open boundary.
Numerical models are generally run with uniform values of bed friction coefficient, although it is known that in principle this coefficient should change according to the nature of the bed. The effect of spatially varying the bed friction as a function of bed composition is investigated and shown to have a relatively small effect on tidal currents but a significant effect on the bed stress derived from the model.