As run-off reaches rivers and creeks and flows into Lake Burragorang, Warragamba’s level will rise further.
Two days ago, the dam was 41.8 per cent full, down a third from a year ago. That drop over the past year – one of the driest on record for eastern NSW – has now been recovered in a matter of days, and officials expect it to rise to 70 per cent full or more as a result of the recent rains.
Across the whole network, the increase will be in the order of 30 percentage points. Nepean Dam may spill from the event, WaterNSW said on Sunday.
As of Monday, Nepean, Tallowa and the Fitzroy Falls reservoirs were all at 100 per cent full, with the Prospect and Blue Mountains dams also more than 90 per cent full.
Last year, the government introduced water restrictions for Sydney as dam levels plummeted at a rate faster than during the Millennium Drought, with curbs tightened to level 2 last December. It also said it would speed up plans for the expansion of the desalination plant.
The Herald has contacted Water Minister Melinda Pavey for an update on the status of those water restrictions.
Further sizeable rainfall totals may also land over Sydney’s catchment – much of which was burnt out by this season’s bushfires – as soon as next weekend.
Meteorologists say tropcial cyclone Uesi, which has formed near Vanuatu in the Coral Sea, may track towards the NSW coast.
At this stage, it is not expected to match the “exorbitant” totals of the recent heavy rains, Helen Reid, a Bureau of Meteorology forecaster, said.
Stuart Khan, a professor at the University of NSW’s School of Civil and Environmental Engineering, said the jump in dam levels will likely be larger than a similar increase in 2007.
A key difference compared with the 2007 event, though, is this surge is happening during summer when water in Lake Burragorang is relatively stratified, with little mixing.
“It’s possible that the inflows will run along the top of the dam and not significantly mix with the lower levels,” Professor Khan said, noting WaterNSW is currently extracting Sydney’s drinking water from about 30 metres below the surface.
“Alternatively, if the inflows are very large and intense, they could disrupt the stratification, leading to mixing of top and lower water,” he said.
Any increase in turbidity of the water typically means water authorities have to increase treatment work to maintain drinking water quality.
With so much ash from the recent bushfires available to be washed into the dam, other challenges including the potential for cyanobacterial and algal blooms.
Professor Khan said that authorities will need to watch for impacts on smaller systems, such as the Brogo-Bermagui water supply system in the Bega Valley.
“Those small on-river dams will have much less ability to avoid the contaminated inflows,” he said. “Systems like the Clarence River will also receive major inflows and should expect the water-quality impacts that are normally associated with overland-flow events and flooding.”
Inflows of organic carbon from mud, leaves and bushfire ash could trigger increased biological activity in the waterways with likely drops in dissolved oxygen as a consequence, he added: “I’d expect this to lead to fish kills in some locations.”
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.