The Influence of Eddy Viscosity Formulation, Bottom Topography, and Wind Wave Effects upon the Circulation of a Shallow Bay

Abstract

A three-dimensional hydrodynamic model taking account of enhanced bed stress due to wave-current interaction is used to examine the wind-induced circulation and flushing time of an idealized homogeneous bay or estuary. The model uses a spectral approach in the vertical and, hence, computes a continuous current profile from sea surface to seabed. Calculations show that in the case of an estuary of nearly constant cross-sectional depth with an along-estuary wind, the intensity of differential flow in the vertical is critical in determining the flushing time. In this case, the three-dimensional circulation and, hence, flushing time is sensitive to the formulation of vertical eddy viscosity, and in the case of a tidal estuary its variation over the spring/neap cycle and with wind intensity must be included. For an estuary with a deep central channel, horizontal rather than vertical circulations dominate, the flow is more uniform in the vertical, and, hence, the circulation is fairly insensitive to eddy viscosity formulation. The inclusion of wave-current interaction effects in the shallow water regions at the head and sides of the estuary enhances the bed stress in these areas and affects the three-dimensional flow field. In any long-term simulation in which the wind field and the wave climate changes, the model shows a residual flow in shallow water due to variations in wave-current interaction, which will affect the flushing time.