Storage levels at Sydney’s Warragamba Dam have jumped from 43% to 61% after a weekend of heavy rain.
Over the past four days, about 392 mm of rain fell over Sydney. This was the wettest period for the city since 414.2 mm fell during four days in February 1990, according to the Bureau of Meteorology (BOM).
The downpour caused flooding, road closures and power outages across New South Wales (NSW), including in areas like Lake Conjola on the South Coast, which was dealing with bushfires just six weeks ago.
On Wednesday night, there were 62 active fires around NSW and the ACT. By 7pm Thursday, there were just 42.
“Today we were over the moon to see rain arrive across many parts of NSW, with decent fall in the state’s north,” the RFS said in a Facebook post on Thursday.
“Fingers crossed we see this rainfall remain steady and reach the firegrounds in southern NSW over the weekend.”
Unlike the spot showers a few weeks ago, Rural Fire Service spokeswoman Angela Burford said the significant rainfall was what the RFS had been waiting for.
“We got significant rainfall up north – as we speak it’s still very heavy near the coast, which is obviously very favourable for us,” she said.
“In saying that, one of the flow-on effects is the potential for flooding, so we’re now working quite closely with the SES to prepare for flooding in northern NSW.”
But the much-needed downpour also has the potential to impact the state’s drinking water catchments.
At a workshop organised by the NSW Water Directorate, University of NSW Global Water Institute, WSAA and AWA last week, representatives from utilities and local councils were joined by academics and other industry figures to discuss the challenges of supplying drinking water in bushfire-affected areas.
Arran Canning, Water Quality Specialist at Water Futures, presented a fact sheet on bushfires and the risks to drinking water quality, which he developed with Water Futures colleague Dan Deere and Kelly Hill from Water Research Australia.
“For months and years afterwards, [bushfires] change the water balance and landscape,” the trio wrote.
“Rainfall events post-fire can have a significant effect on water quality; from increased rates of erosion, increased sediments and turbidity, and the introduction of a range of chemicals and precursors into the water supply.”
The risks to water quality differ depending on whether the supply comes from a dam or river, and whether the fire was a low- or high-intensity burn.
“In a dam, water flows in and can stay there for some time,” Canning said.
“You can get issues with increased suspended solids and nutrients, and it can take a couple of years for the dam to get back to how it was.”
For example, Canberra’s Cotter fire in 2003 saw turbidity increase thirtyfold. It took two years to get back to base levels.
“We’re getting a lot of rain at the moment on the east coast, so utilities managing those areas would be worrying because the risk of ash and material on the ground will start washing into catchments,” Canning said.
This is in contrast to a river supply, where a rain event might lead to a spike in turbidity, solids and nutrients, but these will be flushed through.
WaterNSW prepared for the recent rain by installing silt curtains at Warragamba Dam to prevent organic material entering the storage.
It is also releasing water from 30 m below the surface as a precaution.
WaterNSW CEO David Harris said some debris was likely to be transported into the storage, but the organisation was well placed to manage the risk.
He said scientists were monitoring water quality in real time and that the organisation was in contact with Sydney Water and NSW Health.
“Over the years our people have pioneered highly sophisticated models that predict what’s coming into the storage so we can respond operationally to that information,” Harris said.
“This includes elevated monitoring by our expert scientists using state-of-the-art vertical profilers and other instrumentation; we can draw water from different depths in the storage.
“Furthermore, we can source water from off-dam storages elsewhere in the supply network if required. In short, we have many levers available to manage water quality.”
Canning said there is a gap in resources between large organisations and smaller water authorities, which might only have a few people managing the whole supply, but both groups can be prepared.
“Larger utilities tend to have really good monitoring in their catchments, they have models of their dams, they can run these models to predict what the effects could be,” he said.
“That said, we also heard from some of the bigger utilities that it was challenging to get things like booms set up really quickly.”
He said it was important for water authorities to undertake risk assessments in order to understand their specific challenges.
“It’s about understanding your catchment and what risks are posed so you know what preparation to do,” Canning said.
“For example, some councils and water utilities have bushfire management programs where they try and reduce the amount of vegetation through prescribed burning to reduce the severity of the fires.
“You can also look at what you can do from a treatment point of view and what you can have on standby, such as silt curtains and floating booms.”
This preparation will become increasingly important as Australia sees more frequent extreme weather events such as bushfires and floods.
“In Australia we always do well and make progress out of crises,” he said.
“Now we’ve seen these fires, utilities will be looking at options for treatment but also what they can do in the catchment.
“That was one of the main outcomes of the workshop: getting contingency planning in place so when it happens next time people will be even more prepared.”