Tidally induced jet in Koombana Bay, Western Australia

Abstract

A study is made of a coastal tidal jet, based on a field program together with numerical and analytical modelling of the tidal discharge and jet dynamics. A new criterion is demonstrated for bottom attachment of low-aspect-ratio buoyant jets. The slightly buoyant jet is attached to the seabed over the initial 2 km of its trajectory, which lies in shallow coastal waters of less than 10 m depth. The jet is about 200 m in width and so its ratio of depth to half-width (aspect ratio) is much lower than for previously reported bottom-attached jets. The longitudinal retardation of the axial speed of the jet is due to bottom friction and entrainment. The jet widens only slowly with distance along its trajectory because entrainment is limited to its sides and is compensated by bathymetric deepening. The jet attaches to the coastline by turning, without loss of speed, to move parallel to the shore. The coastal attachment width is found to be a simple function of the ratio of the jet discharge velocity to the speed of the prevailing alongshore current.