Sediment transport in the King River, Tasmania

Supervising Scientist, 1997

Supervising Scientist Report 120
Locher H
ISSN 1325-1554
ISBN 0 642 24320 4

Executive summary

The King River system on the west coast of Tasmania has received a total of 97 million tonnes of fine-grained sediments ('tailings') from the Mount Lyell Copper Mine in Queenstown. These sediments were discharged over the period 1916 to 1994, in addition to 1.4 million tonnes of smelter slag and an estimated 10 million tonnes of topsoil. This report presents the results to date of the King River Sediment Study, a PhD study on the response of the river system to this artificially high sediment load.

Of the total tonnage of sediments discharged into the river system, an estimated 3.4 million tonnes are in sediment banks and a maximum of 10 million tonnes are in the river bed in the last 8 km of the King River, raising the river bed by as much as 9 m. About 100 million m3 are stored in a delta at the mouth of the King River where it meets Macquarie Harbour. Reduced peak flows from a power scheme in the upper King River commencing in 1992 curtailed the further growth of the sediment banks, but caused an increased rate of deposition in the bed close to the river mouth. The processes of deposition and scour of fresh tailings on the sediment banks were very dynamic while the mine was discharging its tailings, due to the regular fluctuations in water level from the power station operations.

Suspended sediment concentrations while the mine discharged its tailings typically ranged from 10,000 mg/L in the Queen River (into which the tailings were discharged) to 500 mg/L in the lower King River under the power station's efficient operating load. Suspended sediment concentrations were very uniform across the channel cross-section, as was particle size (median grain size 7-8 µm). Concentrations rose as much as two orders of magnitude (100-10,000 mg/L) due to the initial flush of water with the power station coming on line, creating a wave of sediment clearly traceable as it propagated downstream. Since the mine has ceased to discharge tailings, suspended sediment concentrations are dramatically lower, in the range of 10-20 mg/L, and no longer uniform across the channel cross-section.

Ongoing work in this study involves comparison of pre- and post-mine closure suspended sediment and bed load transport rates, monitoring of changes in hydraulic geometry and sediment bank stability, and application of flow and sediment transport models to the King River.