Snowy River Recovery - Snowy flow response monitoring and modelling: Physical response to the spring 2010 environmental flow release to the Snowy River estuary
Over the period 2-12 November 2010 a spring environmental flow release to the Snowy River via Jindabyne Dam was secured by the NSW, Victorian and Commonwealth Governments. An additional 16.6 gigalitres were released, with a maximum discharge rate of 3,080 ML/d released over four days. The objectives of the release were primarily focussed on the upper freshwater reaches of the Snowy River and intended to scour the riverbed sediments and benthic algae as the bed of the river is in poor condition and remains one of the major limiting factors for recovery.
The 2010 Spring environmental release provided an excellent opportunity to gain a better understanding of the physical changes to the Snowy River estuary, a part of the system not well understood. Additionally, the study allows calibration of the existing Snowy River estuary model. A data collection program was devised and conducted by E. McLean and J. Hinwood, with partial support from the West Gippsland Catchment Management Authority, Victoria and the NSW Office of Water.
The aims of the estuary study were to assess the salinity change and recovery and define any geomorphic changes to the entrance channel of the Snowy River. The data collection was conducted on four field trips, each of a few days duration, at approximately two week intervals in November and December 2010. Tide and salinity loggers were deployed over the period of measurement. Salinity profiles were taken at a high tide on each trip. Detailed bathymetric soundings were made of the Snowy River entrance channel using survey grade eco-sounders.
This report presents preliminary results and interpretations of the data. The principal results were the recording of the washout of salt water from the upper estuary, followed by its slow return. The washout has the potential to deflocculate bank silt deposits, raising water turbidity and weakening the banks, but has the benefit that overbank flows would be fresh and would not contribute to soil salination. The area of the estuary with the best environment for recreational fish species was reduced during the release, as it is during natural fresh events.
The geomorphic results were less clear owing to the occurrence of two fresh flows of comparable magnitude to the environmental release, one just before and one just after the environmental release. The peak flow caused very minor erosion to the channel while the recovery to a smaller channel was prevented by the subsequent freshes. It appears that the channel cross section was close to that which would be in equilibrium with the peak environmental flow.
The data set is well suited to validating computer models of the estuary, and this study will be used in the near future to improve the validation of the two simple models which have already been used to demonstrate the feasibility of modelling the response of the estuary to transient flows. Once fully validated such models would form the core of a decision support tool to aid in management of the estuary.