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Geologist presents new research from Museum Fire flooding in Flagstaff

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Geologist Becky Beers waited two years for the 2021 monsoon.

Her wait began in summer 2019 when the Museum Fire burned 1,961 acres of steep ponderosa forest above Flagstaff. That year, Beers was working alongside a team of scientists that turned their research to the fire scar. To them, the most interesting part of wildfire was the power it gave to water.

After a fire, there’s no vegetation to stop water from slicking across a landscape, gathering speed and debris as it erodes the scorched earth. When the Museum Fire burned a significant chunk of the Spruce Wash Watershed — a major drainage that flows directly into Flagstaff neighborhoods and businesses — Beers and her team knew that increased erosion in this area would give way to trouble.

“The fire was really concerning because there was a lot of people at risk of being impacted by floods and debris flow,” Beers said.

The Museum Fire scar also encompassed an area of critical infrastructure: Elden Lookout Road, the only route to a communications tower used by local emergency dispatch, was vulnerable to erosion. It snaked through the burn scar over steep hills and drainages.

“Flooding could potentially knock out this road, especially in areas where the road is just plastered onto the side of the mountain,” Beers said. “If we were to lose the road in this type of location, it would be very difficult to repair.”

None of this was news in the summer of 2019. The Flagstaff community was well aware of the flood risks that followed fire. They had learned some lessons about fire scar flooding following the Schultz Fire of 2010.

The Museum Fire hadn’t even been extinguished before mitigation efforts were underway. Neighbors stacked sandbags in anticipation of floodwaters. The city erected barriers at the mouths of washes. The impending threats of Museum Fire flooding compelled Beers and her team to get to work.

They prepared seismometers and designated areas where they could study debris flows from the fire scar. When a strong monsoon rain hit the Museum Fire scar, it would give them a chance to observe firsthand the way water and fire mixed on the landscape. If their data was good, they could come away with a better understanding of fire flooding hazards and the effectiveness of mitigation efforts. These measurements, in turn, could be invaluable information in an era of rampant Western wildfires.

That summer, Beers waited among Flagstaff citizens and fellow scientists, braced for the heavy rains. But they never came -- 2019 was Flagstaff’s driest monsoon to date. Even so, two small rain events triggered debris flows in the Museum Fire drainages. Beers’ team successfully measured one. On Aug. 28, somewhere high above town, boulders dislodged and shook the ground with enough force to be captured by one of their seismometers. According to team seismologist Ryan Porter, the rumbling debris measured in at about 1/100th the magnitude of a 1.0 earthquake.

“This is a lot of energy,” Beers said. “If you were standing next to the channel that this occurred in, the ground would be shaking.”

After the Aug. 28 event, Beers and her team went up into the hills near Upper Oldham Trail and discovered a few areas where debris had scoured the bark from trees. Even with relatively light rain, the affected areas had been carved away into tumbles of stone and root. This was only the beginning.

After a dry year, Beers’ team extended their research into the next season, hoping 2020 would provide a more typical monsoon season to base their data on. They took the time to set up trail cameras and other instruments to improve their measurements.

Despite their preparation, the weather let them down, and 2020 was even drier than the previous year. There was only one debris flow event over the whole season, but it managed to betray expectations in its own way.

“What was interesting about this event is that it had a low rainfall intensity. It didn’t take a lot of rainfall to initiate this debris flow,” Beers explained. “That was pretty alarming.”

The research team waited another year.

When the skies broke open on July 13, 2021, it was the moment Beers had been waiting for. Nearly four and a half inches of rain poured down within the hour.

“It was incredible, from a geologist’s perspective,” Beers said.

The seismometers quaked. In the upper Museum Fire scar, multiple debris channels left boulders strewn across Elden Lookout Road. Downstream waters crashed against concrete jersey barriers at the edge of town.

“The flood waters overcame the channel at one point and spread out through the neighborhoods,” Beers said. In town, flooding was severe.

Rains returned the very next day. Again, the seismometers shuddered.

“What was interesting about this event is that it produced similar magnitude debris flows with significantly less rainfall,” Beers said.“We're thinking that the reason we saw this trend is because the soils were already saturated from the previous day. The glass was empty on July 13, and then on July 14, it was three-quarters full. The soil could only accept so much more water.”

Enough water spilled off saturated soils to take Flagstaff’s eastside by storm. A viral video from that day showed a Prius being carried away like a toy boat in a river raging through a neighborhood street. Homes were inundated, suffering millions of dollars of damage, and the rains kept coming.

Two days later, back-to-back storms hammered Flagstaff again and produced two separate debris flows that fortunately had little impact in town. After that, things stayed relatively quiet, a month of calm before the storm of Aug. 17.

According to measurements from Beers' team, the Museum Fire scar took on almost five inches of rain in an hour. Flooding and debris flows advanced from the burn scar to the drainages downstream. When they went to survey the impact, Beers’ team found flows like four-lane highways of beach-ball sized boulders and sediment. The largest flow had a volume of 3,000 cubic meters; about the same as the amount of water that tumbles over Niagara Falls each second.

“It buried trees, it buried the trail,” Beers said. “Just incredible.”

On the same day, one of her team’s trail cameras managed to capture a channel where water carried boulders five feet across. The footage was partly obscured by splashes of mud that coated the lens. They’re lucky the camera was not lost.

“We thought it was in a safe spot: high tree on a far bank,” Beers said. “It was not safe. The tree barely made it.”

The Aug. 17 storm caused the most significant road damage. It clogged culverts, undercut concrete and covered whole sections of Elden Lookout Road. Flooding in Flagstaff reached as far as Killip Elementary School, where it put hallways under two feet of water. Across the entire east side, roads closed to make way for steady streams of mud and debris. The destruction took weeks to clean up.

Devastating as these 2021 floods were, they provided a long awaited scientific opportunity. Beers and her team now had data from across three very different monsoon seasons with multiple combinations of storm events. With that data, they began to put together some hypotheses that may help geologists across the Southwest better understand fire scar flooding.

One of the first things they found has to do with soil hydrology — measurements of how well the soil absorbs water before it begins to flood.

“We saw increases [in soil absorbency] each spring, but decreases each fall,” Beers said. The current hypothesis is that winter freezing and thawing breaks up the soil so that it’s more absorbent in the spring.

“The reason we’re seeing a decrease in the fall is that the fine particles, the ash and silt clay that’s laying on the landscape, gets moved around by wind and water in the summer and fills in the little cracks that the snow created during the winter,” she said. “It seals it off again.” A possible consequence of this dynamic is that early season debris flows may be less severe than late season flows, but it’s a little too early to say with certainty.

Beers’ data also showed that when it comes to fire scars, rain, despite the flooding it brings, is the best first step to recovery. She said mulching and other mitigation efforts can help stymie erosion, but nothing is as good as nature’s design.

“We want vegetation,” Beers said. “We finally got that in 2021 because we finally got some rain.”

Once vegetation comes back, that’s when a fire scar can really start to stabilize. For the Museum Fire scar, there is still a ways to go.

“The soils are not yet recovered to an unburned state,” Beers said. “We can't quite say for sure whether there will be future debris flows or not. It depends on the type of monsoon we get.”

In the more immediate future, there is some hope that the work of Beers and her team may help augment flood warning technology. Before the 2022 monsoon season, areas downstream of the Museum Fire scar will be outfitted with emergency sirens. While these sirens will operate on rain gauges, the research of Beers’ team demonstrates that seismometers may be an effective supplement to early warning systems.  

Ultimately, for researchers like Beers, the Museum Fire scar has proven to be both a crisis and an opportunity. The unique multi-year delay of significant rains gave scientists the chance to be one step ahead of the weather.

“We had all this monitoring prior to the big flooding events,” Beers said. “That’s pretty rare. Normally you get there after the flooding has happened. We have continuous data collection.”

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