The hydrology of the upper Swan River Estuary with focus on an artificial destratification trial

摘要

A trial to artificially destratify part of the water column of the Swan–Canning Estuary took place over four weeks in 1997. Destratification was attempted with bubble plumes created by pumping compressed air through a porous pipe near the bed at a location in the upper estuary. The purpose of the trial was to determine whether the resultant bubble ‘curtain’ could mix the water column, thereby altering the extent of upstream and downstream mixing and the vertical structures of salinity, temperature, dissolved oxygen and turbidity. The destratification trial was monitored using three techniques: vertical profiles taken on several days at selected stations along the estuary, fine scale profiles in the vicinity of the bubble curtain on one day, and continuous water column profiles taken 1 km upstream of the curtain. The seasonal upstream propagation of brackish water underneath a residual freshwater discharge was affected by the magnitude of freshwater discharge and tidal elevations. Superimposed upon diurnal and semi-diurnal tides were non-tidal water level changes induced mostly by variations in barometric pressure. These caused large changes in the salt wedge position. Destratification by the bubble curtain was compromised by these large oscillations, which limited the exposure of the salt wedge region to the mixing action of the bubble curtain. Complete vertical mixing of the water column was observed up to 30 m either side of the curtain on 30 October. Disruptions to the density stratification were not evident beyond 350 m of the curtain. In the immediate vicinity (∼30 m) of the curtain, deficits of dissolved oxygen in bottom waters were generally reduced or obliterated and a naturally turbid plume of water near the bottom of the estuary was mixed through the water column. The effects of the bubble curtain on dissolved oxygen, turbidity and temperature were, however, similar to those for salinity, with mixing confined to around 30 m from the curtain and no effect observed further than 350 m away. The upper Swan River estuary, although relatively narrow and strongly vertically stratified, is unsuited to destratification using bubble plumes. Cycling of spring and neap tides and non-tidal water level changes, together with freshwater inflows to the estuary, strongly limit the longitudinal extent of mixing by bubble curtains. Copyright © 2001 John Wiley & Sons, Ltd.