A three-dimensional hydrodynamic model of the Irish Sea into which surface tidal elevations can be assimilated is used to examine the influence of blending elevation data upon tidal currents and energy flux in the region. The model uses a sigma coordinate in the vertical, with discretization accomplished using finite difference grids in all dimensions. An alternating direction implicit method is used to integrate the hydrodynamic equations through time, and an elevation specified open boundary condition is applied. Vertical eddy viscosity is computed using a Prandtl mixing length submodel. In an initial series of calculations the influence of the coefficient of bottom friction upon tidal elevations and currents is examined with a view to determining an optimal value, in the sense that the model can reproduce the extensive data set of elevations and currents in the region. In subsequent calculations the influence of assimilating elevation data for a range of bottom friction coefficients and weighting factors upon computed elevations and currents is examined. Calculations show the importance of using an optimal friction coefficient in a three-dimensional model in order to reproduce surface elevation and current profiles. Although the assimilation of elevation data can improve computed elevations even when the bottom friction coefficient was not optimal, it can lead to significant physically unrealistic flows with an associated energy flux in the region of the blending point, which can lead to a deterioration in the accuracy of the computed currents in the region of the blending point. ¿ 2001 American Geophysical Union