Water Journal : Water Journal April 2012
catchment management technical features 114 APRIL 2012 water Following the 2009 fires Ashwatch was re-established to collect water quality data on 12 priority river reaches (28 sites) (see Figure 4). The data was valuable and the program engaged the community in collecting the data to understand the impacts of poor water quality. The project was headed up by the Goulburn Broken Waterwatch initiative.(www.gbwaterwatch.org.au). Analysis of Event The potential for catastrophic impacts on water quality following the 2009 fire was minimised due to the lack of high intensity rainfall in the first 12 months after the fires. There were approximately 20 rainfall events recorded in the two years after the fires, where water quality was affected in the fire-affected tributaries and, in some cases, the Goulburn River. Most rainfall events were less than 30mm and resulted in only temporary peaks in fire-affected tributaries. There were two exceptions to this -- the first was in late September 2009 after 15mm to 45mm fell across the catchment and turbidity peaked at 270 NTU in the Goulburn River at Tabilk (downstream of the fire-affected area); the second event occurred at the start of January 2010. The mid-Goulburn catchment received significant rainfall on 1 and 2 January 2010 with rainfall ranging between 52mm at Kilmore to 88mm in Yea. As a result, flash flooding and the mobilisation of sediments occurred in both fire-affected and other tributaries. The Goulburn River to the Goulburn Weir and beyond was impacted by this event with high turbidity and salinity, and low dissolved oxygen levels recorded. Daily flow data shows the tributaries peaking either on 2 or 3 January. The exact time and magnitude of flow peaks in the tributaries cannot be established as there is only instantaneous daily flow data available. Daily flow data shows at least a twofold increase in all fire-affected tributaries (see Table 2). The Goulburn River at Goulburn Weir did not peak until 4 January, where flows increased from around 3000ML/day to 8000ML/Day. Releases from Lake Eildon were decreased to compensate for the increased flows in the mid-Goulburn catchment. Releases were decreased from around 2500ML/day to 500ML/day for around five days. At the time of this rainfall event there were six online turbidity sensors installed in the mid-Goulburn catchment; Acheron River at Taggerty, Yea River at Castella, Yea River at Yea Pump Station, Goulburn River at Trawool, Goulburn River at Tabilk and Goulburn Weir. Additionally, spot readings were taken on three occasions under the AshWatch program. Turbidity in the Acheron River peaked at approximately 1,000 NTU on 1 January, which was around the same time as the first peak inflows. Turbidity remained elevated for 10 days at or above 50 NTU. Since then, concentrations have mainly been around 10 NTU, with the exception of the influence of a minor rainfall event on 8 January 2010. AshWatch data was also consistent with these findings, as 60 NTU was measured on 2 January at Acheron River near the Goulburn River confluence. Turbidity in the Yea River at both sites exceeded 500 NTU on 2 January and at Yea the turbidity consistently remained above 20 NTU until 10 January. At the AshWatch site on the Yea River at Devlins Bridge, turbidity was found to be over the detection limit of the equipment (1,000 NTU). Other significant turbidity Table 2. Summary of flows in the Mid-Goulburn Catchment. Waterway December 2009 average flow (ML/Day) Maximum instantaneous flow* ML/Day (logged in SPM) Date of maximum flow* Rubicon R 156 364 2/01/2010 Acheron R 451 910 2/01/2010 Yea R 53 173 3/01/2010 King Parrot Crk 37 402 2/01/2010 Sunday Crk 7 246 3/01/2010 Goulburn R @ Seymour 2,763 11,983 3/01/2010 Goulburn R @ Goulburn Weir 2,780 8,085 4/01/2010 * Instantaneous daily flow data, therefore daily maximum may be higher. Figure 4. Water quality data (results) through Ashwatch.
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