The Structure of Vertical Current Profiles in a Macrotidal, Partly-Mixed Estuary

Abstract

A local, one-dimensional, depth-dependent model is used in conjunction with a one-dimensional, longitudinal, hydrodynamical model to examine the mechanisms affecting yertical profiles of longitudinal residual current in the macrotidal (tidal range typically exceeds 4 m during spring tides), partly-mixed Tamar Estuary. Residual currents are simulated at a deep (15m) station in the lower reaches, which possesses a small tidal amplitude to depth ratio and a nonzero salinity throughout the tidal cycle, as well as at a shallow station in the upper reaches, which varies in depth from 1 m at low water, when salinity is zero, to 5 m at high water. A slow, up-estuary current dominates the residual circulation just beneath the high-water level at the deeper station. Further down the water column a down-estuary residual current develops which is the near-surface component of a two-layer gravitational circulation. The up-estuary component of this gravitational circulation occurs deeper in the column and extends to the bed at the deep station, whereas at the shallow station it is eventually dominated by a down-estuary current in the bottom 1 m. Simulated residual currents are fairly insensitive to estuary-bed slope and to observed depth variations in longitudinal density gradient. Residual current profiles of the observed form can only be generated by a longitudinal density gradient. The reduction in vertical eddy viscosity by water column stability due to stratification is an essential requirement for producing a strong gravitational circulation of the observed magnitude. Stratification at the shallow station is much higher during the ebb than during the flood and this asymmetry enhances the gravitational circulation in the upper reaches. The formation of residual flows at both stations is illustrated by showing time-series data over a tidal cycle for the simulated current profiles.