THE HYDRAULICS OF DENSITY CURRENTS OVER ESTUARINE SILLS

It is demonstrated here that density currents, being sensitive to the underlying topographies, are subject to topograhic control in estuaries indented with mild sills. As a density current flows over a ridge, that current shoals downstream but deepens upstream, rendering the upstream flow more subcritical. The Earth's rotation weakens the topographic influence upstream and downstream, and the vertical mixing of momentum and density also weakens the influence. Conventionally, it was thought that the two-layer estuarine circulation characterized by the upward entrainment of a landward undercurrent will be set up following the seaward passage of the density front. In the presence of bottom topographies, however, there should be a final phase of adjustment. During this phase there is a downward entrainment of the flow upstream of sills and a consequent modulation of the estuarine circulation.