Modeling of Bathymetry-Locked Residual Eddies in Well-Mixed Tidal Channels with Arbitrary Depth Variations

Abstract

Abstract The concept of the in–out-type exchange flow in estuaries only applies to situations with significant freshwater discharge and/or elongated channels with relatively simple variations in depth and coastline along the channel. In waterways with complex bottom topography, the in–out-type exchange flows may be replaced by residual eddies that are locked to bathymetry. This paper develops an analytic model for such tidally induced residual eddies. The model allows arbitrary depth variations in both along- and across-channel directions. The model produces residual eddies locked to bathymetry features, similar to observations in Wassaw Sound using a ship-mounted ADCP. Analysis indicates that for problems in a “short” channel with standing wave characteristics, the residual circulation is significantly influenced by advection. The frictional effect, however, is dominant and the wave propagation effect cannot be uniformly neglected (i.e., it sometimes can still compete or reinforce the effect of advection). The bathymetry function plays an important role in the generation of residual eddies. The wind during a half-dozen field observations does not appear to have significant effect to alter the structure of the flow field. Nor does the tidal range: the structures of the residual eddies do not change with the spring–neap tidal variation of tidal amplitude and remain robust in location. The persistent nature of these residual eddies makes it useful to map them in a specific area in a way similar to a coastal current. Although variabilities are anticipated in response to wind and coastal low-frequency sea level changes, the residual eddies will have significant implications to the flushing of a tidal channel, material transport, and the ecosystem dynamics.