Topographic and boundary effects on steady and unsteady flow through straits

An overview of topographic and boundary effects in flows through straits is presented. The emphasis is on the various types of interaction of uniform and stratified flow with the topography of straits, especially the interaction with sills. The interaction of a single-layer flow in a channel with a sill (obstacle) depends on the Froude number of the flow and the relative height of the sill. Two classification schemes (one steady and one pseudo-steady) are given. For single-layer flow in straits where the effects of rotation are important a further parameter, the ratio of the channel width to a Rossby radius, is needed and a similar classification scheme as for the non-rotating flows can be obtained. Unsteady flows often result in bores (for single-layer flows) and internal bores (in stratified flows) and the behaviour of these is discussed. The flow of stratified flow past sills may generate lee waves, while the increased flow speeds at the sill may allow deep water to be drawn up and over the sill (aspiration). Features of the strait topography away from the sill, including slopes at the side of the channel and also downstream of the sill, channel curvature and channel widening have important effects on the character of the flow and on mixing.