Bottom Stress and Free Oscillations

Abstract

Three hydrodynamical models (2-D, 3-D, and 2.5-D) applicable to storm surge simulation are briefly described. A test problem (enclosed rectangular basin, uniform wind) is used to compare the results of the 2-D and 3-D models. A satisfactory overall agreement is observed between the patterns of surface elevation and depth-mean currents. However, it appears that long gravity waves propagate at different speeds in the two models. Part of the discrepancy can be ascribed to the formulation of the bottom boundary condition, i.e., to the modeling of the bottom stress. In models that resolve the vertical structure of the flow, the divergence of the bottom stress has two contributions. The first one reduces the “effective depth” of the water column, thereby increasing the periods of the free modes. The second contribution attenuates the amplitude of the oscillations.