Sensitivity of tidal bed stress distributions, near-bed currents, overtides, and tidal residuals to frictional effects in the eastern Irish Sea

Abstract

A three-dimensional nonlinear high-resolution hydrodynamic model of the eastern Irish Sea is used to examine the influence of near-bed viscosity and bottom friction upon the M2 tide and overtides in the region. Initially, bed stress in the model is related to the depth-mean current, the formulation used in a "classic" two-dimensional model. The resulting hybrid "two-dimensional/three-dimensional model" enables changes in current profile due to variations in eddy viscosity to be examined independently of those due to changes in bed stress. Also, results from the model are compared with those from a three-dimensional model with bed stress related to bottom current. Computed tidal elevations and surface currents are found to be fairly insensitive to the parameterization of bottom stress. However, tidal current profiles in the near-bed region and maximum bed stress are found to be sensitive to variations in near-bed viscosity and friction coefficient. Although reducing eddy viscosity in the near-bed region and increasing bottom frictional coefficient makes little difference to tidal elevations and near-surface tidal currents, it is essential to accurately reproduce the bed currents and associated bed stress. Higher harmonics of the tide (the overtides) computed with the model show significant spatial variability in the region, with near-bed currents being influenced by frictional effects. Tidal residual flows also show significant spatial variability with near-bed flows being influenced by friction.