First came the fires, denuding millions of acres of forest in eastern Australia. Now comes the rain, more than 12 inches in just 48 hours over this past weekend in some areas of New South Wales.

That sequence, severe bushfires followed by torrential rain, is bringing a third cataclysm — landslides and large-scale erosion.

Here's why. Without leafy trees to offer protection, the water falls directly on hard earth. It pools and rolls, gathering into a torrent. If the hill the rain falls on is just the wrong steepness, the soil just the wrong hardness, the downpour just a little too intense... whoosh.

"Essentially it's a flash flood that's full of rocks and logs," says Charlie Showers, a groundwater geologist for the Department of Environment, Land, Water and Planning in the Australian state of Victoria. Showers is part of a team of government scientists and analysts who are called in after severe bushfires to figure out where so-called debris flows are most likely, and warn local authorities.

But identifying potential landslide areas can be difficult, because there are so many potential places to look. This season's fires have been very large — millions of hectares have burned in New South Wales and Victoria — and heavy rain is relatively common this time of year in the areas that burned, which means there's a lot of land with a lot of potential risk for dangerous erosion.

Over the weekend, Showers and his team finished studying the landslide potential after a fire that burned through part of Mount Buffalo National Park in a mountainous part of Victoria that is popular with outdoorsy tourists. The fire started at the end of December, and burned ferociously for weeks, eating up more than 100,000 hectares, or about 380 square miles.

"Normally a 100,000 hectare fire would be a very, very large fire," he explained, "but in the context of this year it's actually going to be the smallest of the fires that we're deployed to."

Lessons from California

To narrow down which burned gullies present the highest risk during rainstorms, Showers is using relatively new scientific research conducted by soil and hydrology experts at the University of Melbourne.

"In the U.S., you guys have been doing [research] for decades and decades, and we had just a handful of little studies," explains Gary Sheridan, a soil scientist at the University of Melbourne.

Sheridan has spent the past 15 years or so studying what causes debris flows after fires, and he has discovered that southeast Australia has a lot in common with parts of California, where post-fire debris flows are also common. In 2018, more than 20 people died when recently burned hillsides came loose and buried homes in Montecito, Calif.

Sheridan found that soil type, slope steepness and fire severity all factored in to whether a certain hill will give way. He also found that the rainstorms that trigger dangerous debris flows are relatively common.

"You know people would talk about 'freak mud-flows' and 'freak landslides' after a fire, and it was always the idea that these things were extraordinary events," he says. "But the kind of rainfall that triggers them off, although it's intense and very short, it's actually quite a common rainfall event. It's the sort of rainfall event you could expect once every two years, or once every five years."

When large, hilly areas burn the likelihood that a rain storm will trigger a debris flow somewhere in the burned region can be all be certain. It's just a matter of where.

Into the field

"We're very reliant on their research to help us understand which parts of the fireground are at risk," says Showers, referring to the work Sheridan and his colleagues have conducted.

He pulls up a topographical map of the burned area on his computer at an incident command center in the town of Ovens, a few miles from the edge of the area burned by the so-called Abbeyard-Yarrabulla fire in January, and checks a box on the left side of the screen which reveals red and orange dots between the contour lines.

The red dots are in the gullies: they are the areas where debris flows are most likely to happen when thunderstorms come through and drop a lot of rain in a short amount of time, as had happened one day earlier. Orange dots mean there's a medium likelihood.

Showers and his team factor in what is downstream of potential debris flows — for example, schools, roads or water supplies — and use that information to pinpoint the areas of most concern.

One thing they look for is rivers downstream of unstable hillsides.

Some small towns near this year's fires in Victoria have had their water supplies disrupted because debris flows — some of them deep in the forest — sent so much sediment into rivers that it overwhelmed water treatment systems. In some cases, local water authorities were forced to truck water into mountain communities until the rivers ran clean again, Showers says.

Research shows that as fires and severe rain get more likely, driven in part by climate change, the risk posed by debris flows in Australia will only get worse.

In response, the water authority that serves Australia's second-largest city, Melbourne, has upgraded a major reservoir to help prevent debris flows from contaminating the water with sediment. It was an investment that would have been unimaginable just 15 years earlier, when scientists were still unsure how and why post-fire debris flows occur.

So far, this year's fires have not burned the forests upstream of the city's reservoirs.

"You know Melbourne is quite vulnerable to contamination of its water supply from bushfire," says Sheridan. "We've kinda dodged a bullet in recent years."

Some towns have also upgraded their water systems to guard against debris flow contamination. The town of Bright, which is home to a little over 2,000 people and suffered water contamination after fires in 2006 and 2007, now routinely pumps clean river water into a reservoir as a backup in case the river is contaminated with sediment after a fire in the future. The town of Wangaratta added groundwater pumps to its drinking water system to protect the nearly 30,000 residents there against post-fire water contamination.

Showers and Sheridan say watching the latest research influence how communities plan for the future is gratifying. "It's crucial for us to understand these [debris flow] systems from a public safety point of view," says Showers, "but also from a water quality point of view."

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