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Giant metallic nets will be strung across several creeks in the mountain forests from where most of Melbourne’s water is sourced to save the city’s supply from bushfire contamination that could render it undrinkable for up to a year.
Melbourne Water has been warned that a significant fire is likely to occur around the city’s water catchments in coming years and when it does, burnt debris could pollute the supply for three to 12 months, forcing millions of people to boil tap water to make it safe to drink.
The Upper Yarra Reservoir, east of Melbourne, is vulnerable to contamination from a major bushfire. Credit: Joe Armao
More than 60 per cent of Melbourne’s water is sourced from a chain of reservoirs in the mountain ranges east of Melbourne, which are off-limits to the public to keep the water supply pristine.
The catchments have long provided Melburnians with some of the purest tap water in the country, says professor Stuart Khan, head of the School of Civil Engineering at the University of Sydney and an expert on water infrastructure.
“One of the great things about Melbourne – it’s a great thing and a vulnerability – is that you have that really pristine catchment, which is all forested, which means that there’s actually not very much water treatment in Melbourne,” Khan said.
But Melbourne Water has received expert advice that a major fire is increasingly likely to burn in catchment areas and that when it does, debris, including ash, soil and rock, could pollute those reservoirs.
Bushfire is the biggest future threat to Melbourne’s water supply, and climate change is raising the risk of a severe fire occurring, the advice says.
Fire burnt 6300 hectares of remote forest near the portal where water is released from the giant Thomson Dam to the Upper Yarra Reservoir in the Black Summer fires of 2019-20, and researchers calculated that 1200 tonnes of sediment was at risk of flowing into Melbourne’s water supply at the time.
“The Catchment Management and Optimisation Project identified that a significant bushfire event in the Upper Yarra is likely to occur in the next 30 years, which could result in the water supply being off-line for up to 12 months due to debris contaminating the water storages,” Melbourne Water said in documents submitted to the Department of Climate Change, Energy, the Environment and Water (DEECA).
Melbourne Water’s proposed solution is to put debris flow nets in 35 sites where water begins to stream into the Upper Yarra Reservoir.
The reservoir is located about 100 kilometres east of Melbourne and has a capacity of more than 200,000 megalitres.
It is a key part of the “Silvan system”, a chain of four reservoirs that provide more than 60 per cent of the city’s water supply.
Melbourne Water hopes to begin the project late next year, but must get approval from DEECA before it can proceed.
It said in its application that studies had found that establishing debris flow nets in the Upper Yarra catchment after a fire “would help reduce turbidity following rainfall events and the potential for the Silvan system to be off-line and boiled water notices being required.”
The nets would be installed in two phases. In the first phase, technicians would be helicoptered into the inaccessible terrain to install a series of eight-metre-deep anchor points spanning multiple creek channels upstream of the Upper Yarra Reservoir. The nets would only be installed after a fire and before any heavy rainfalls.
Australian National University distinguished professor David Lindenmayer co-authored a book on Melbourne’s water catchments with Jim Viggers, who looked after the catchments for 25 years.
Lindenmayer says successive Victorian governments “have kicked a major own goal by allowing logging in some of their most important catchments” for many years.
Native forest logging was banned in Victoria last year. Tributaries of the Yarra, including the Muddy, Salvation and McMahons creeks, were logged before the ban in a state-run program that has increased the likelihood of a severe fire, according to research by Lindenmayer and others.
“Logged and regenerated forests are incredibly flammable for at least 40 years after the forest was regenerated,” Lindenmayer said.
Backburning in Melbourne’s water catchments increases the risk of a severe fire, expert David Lindenmayer says.Credit: Jason South
He also urged authorities to put an end to back-burning operations in the catchments for the same reason.
“For those tall wet forests – mountain ash, alpine ash – which [are] the most high-yielding catchments, the last thing you want to be doing is prescribed burning,” he said.
Sean Hanrahan, an executive manager at Melbourne Water, said bushfire risks to the water supply had been managed alongside Forest Fire Management Victoria for 150 years.
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“The debris barrier project helps maintain safe, uninterrupted supply in the months following a major bushfire. Paired with logs, these barriers stop contamination post-fire,” Hanrahan said.
Khan said the nets were a sensible solution tailored to the unique vulnerability of Melbourne’s water supply.
“If you just look at it from a technical perspective, holding debris back in the catchment after a fire is definitely a sensible strategy to try and manage what could be a very significant risk to Melbourne’s water supply,” he said.
Melbourne Water has never issued a boiled water alert after a fire. The last such alert was issued in 2020 after a power outage at Silvan dam.
Sydneysiders came close to having to boil their drinking water in 2021 after a one-in-100-year flood caused the Warragamba Dam to spill over. But in 1998, they had three months of boiled water notices due to a cryptosporidium outbreak.
Last month, households in Frankston South were told to boil their water for three days after South East Water blamed the issue on a faulty valve.
People were told to boil water before drinking, mixing cold drinks, preparing food, making ice, brushing teeth and gargling. The advice was even extended to giving cooled, boiled water or bottled water to pets.
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