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Soaking rain, flash floods in California could pose a risk of mudslides near wildfire-scorched areas

By Ashley Williams, AccuWeather staff writer
November 30, 2018, 3:42:32 PM EST

Following recent California wildfires that have obliterated tens of thousands of structures and killed at least 85 people, survivors and residents within the proximity of fire-scorched areas have to deal with yet another potentially deadly threat.

Water poses the next major threat to areas that have been burned by rampant blazes.

“Trees and other vegetation have burned away, and where the fire was intense, soils can become hydrophobic,” said Dr. Chris Renschler, a University at Buffalo expert in integrated watershed management and disaster response. Renschler is an associate professor of geography in the University at Buffalo’s College of Arts and Sciences and director of the university’s Landscape-based Environmental System Analysis and Modeling (LESAM).

2018 Calif. mudslides - AP Image

Debris from the Jan. 9 mudslide sits on the side of the Montecito Creek as it flows Thursday, March 22, 2018. (AP Photo/Daniel Dreifuss)

Water will roll right off of hydrophobic soil instead of soaking into the ground.

“This means the soil absorbs much less water than before, if any, [which] can lead to increased surface runoff, creating a higher risk of flash floods and landslides that can endanger both people and property,” Renschler said, noting that people living downstream of burned areas need to be well aware of this risk, as increased runoff during rainstorms can continue to cause problems even after the fire is extinguished.

“Even with less extreme runoff after a storm, ash with all kinds of contaminants — all these toxins — are being rushed down into streams and rivers. From an ecological perspective, streams will be impacted," Renschler said.

The United States Geological Survey has stated that 0.3 of an inch of rain lasting for longer than half an hour has triggered debris flows in the past, according to Renschler, and any rainfall of 0.4 of an inch or more per hour will definitely pose a risk of debris flows.

“One of the biggest problems now is that these fires are happening with a frequency and a scale that we haven’t seen before,” Renschler said. “The sheer size of these burned areas changes the conditions of the landscape in terms of its land cover and rainfall-runoff behavior.”

The risk of mudslides depends on the burn intensity of the fire, according to Renschler.

“If you have a very high fuel load and a very intense fire, then the hydrophobicity can be significantly higher than in areas where the fire just went through with a relatively low intensity,” he told AccuWeather.

Earlier in 2018, mudslides in Southern California took lives and injured many others living downstream from areas devastated by the Thomas Fire.

Wildfires have been known to dramatically alter geography for years afterward, said Clark Woodward, founder of RedZone, a wildfire disaster response software for the public safety, insurance and risk management markets.

“[This] greatly increases the potential for debris flows, landslides, flash flooding and other catastrophes affecting local residents,” Woodward said. “These events have the greatest potential to occur within one to two years of the wildfire event due to surface erosion, root decay and a lack of water absorption into the soil.”

When looking at mudslide risk predictions for the recent Woolsey and Camp fires, the two wildfires exhibit significantly different landslide risk potential, according to Woodward.

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“The lighter shrub type fuel, which tends to burn and completely leave the hillsides bare, and the steep geography in the Malibu area [both] increase the possibility of the Woolsey burn scar causing damaging landslides similar to those which impacted Montecito after the Thomas Fire in 2017,” Woodward said.

“Conversely, the Camp Fire burned through much heavier fuels – note that many of the trees around Paradise are still green – and the town of Paradise is located atop a ridge with canyons descending on either side of town, limiting the potential for debris flows or mudslides,” he added.

A question that often arises after wildfire and landslide events is “how are we going to solve this?,” said Renschler, who designed a software now regularly used by Burn Area Emergency Response Teams, helping them test different kinds of mitigation measures to burn areas and assess how they can reduce the risk of runoff, soil erosion and sediment transport in impacted areas.

Renschler noted that so much effort and money is put toward fighting fires that the investment of reducing the risk of wildfires is highly underfunded, adding that focus needs to shift toward fuel load reduction efforts.

“I hope that this event, with all the trouble and suffering that occurred, that there is finally a change to address this issue systematically and understand that fire is a part of nature, and one works with the fire in terms of reducing the fuel load,” he said.

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