Open Boundary and Frictional Influences in 3D Tidal Models

Abstract

Both 2D and 3D hydrodynamic models of the Irish Sea are used to examine the influence of the open boundary condition (OBC), bottom friction, and vertical eddy viscosity upon tidal currents in this region. Three eddy viscosity formulations, namely, a two-equation turbulence model, a simple flow-dependent eddy viscosity model, and a constant eddy viscosity of 1,000 cm²/s, are considered. In an initial series of 2D calculations the influence of OBC and bottom friction coefficient (BFC) upon tidal elevations is examined with a view to determining an optimal value related to different OBCs, in the sense that the model can reproduce the extensive data set of elevations in the region. In subsequent 3D calculations in which bottom friction is related to the depth-mean current, the influence of vertical eddy viscosity upon computed currents is examined using identical BFCs to those used in 2D model. These calculations clearly show the performance of different vertical eddy viscosity formulations in a 3D model. Calculations are also carried out using a single point model in the vertical. Results from the calculations and comparisons with data show that the optimal BFC can vary depending upon the vertical eddy viscosity formulation used in the calculation. Computed elevations and currents from a 3D model (in which bottom friction is computed from bottom current) are compared with observations and show a similar agreement for each viscosity parameterization provided the optimal BFC for this parameterization is used. These calculations suggest that to rigorously test a range of turbulence models in 3D calculations, in addition to accurate current measurements in the bottom boundary layer, it is then necessary to determine the bed types and forms and also to derive accurate tidal input to the model along its open boundaries.