Intertidal Motions within Deep Basin of Swan River Estuary

Abstract

The Swan River is typical of the estuaries in southwest Australia. A shallow (5 m) sill near the entrance controls fluid exchange between the estuary and the ocean. The main estuarine basin is relatively deep and wide and is in turn bounded at the upstream end by a shallower (2 m) secondary sill. An understanding of the dynamics associated with the circulation and response of the deep basin over intertidal time scales was required to predict the dispersal of nutrients and provide data to validate numerical models of the system. The dynamics of the deep basin were documented with several C-T-D-DO transects during intensive summer and winter experiments. A variety of internal features including lee waves, undular bores, and basin scale seiching were observed over a tidal cycle. The circulation pattern and the basin response are shown to depend on the interaction between the tidal dynamics, the sill control, and the stratification produced by the gravitational overflow. A one-dimensional, three-layered analytical model was constructed to explain the basin response.