Geologist Presents New Museum Flood Research in Flagstaff | science and discovery
SEAN GOLIGHTLY Sun Staff Reporter
Geologist Becky Beers waited two years for the 2021 monsoon.
His wait began in the summer of 2019 when the Museum Fire burned 1,961 acres of rugged ponderosa forest above Flagstaff. That year, Beers was working alongside a team of scientists who turned their research to the fire scar. For them, the most interesting part of forest fires was the power they gave to water.
After a fire, there is no vegetation to stop water from spreading across a landscape, accumulating velocity and debris as it erodes the scorched earth. When the museum fire scorched a significant portion of the Spruce Wash watershed — a major drainage that flows directly into Flagstaff neighborhoods and businesses — Beers and his team knew that increased erosion in this area would cause problems. .
“The fire was really concerning as many people were at risk of being affected by flooding and debris flows,” Beers said.
The scar from the museum fire also encompassed an area of critical infrastructure: Elden Lookout Road, the only road leading to a communications tower used by the local emergency dispatch, was vulnerable to erosion. It meandered through the scorch scar above steep hills and watersheds.
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“Floods could potentially destroy this road, especially in areas where the road is just stuck into the side of the mountain,” Beers said. “If we were to lose the road in this type of place, it would be very difficult to repair.”
None of this was new in the summer of 2019. The Flagstaff community was well aware of the risk of flooding following a fire. They had learned some lessons about the flooding caused by fire scars after the Schultz fire in 2010.
The museum fire had not even been extinguished before mitigation efforts were underway. Neighbors piled sandbags in anticipation of flooding. The city erected barriers at the mouths of the washhouses. Impending threats of flooding from the Museum Fire compelled Beers and his team to get to work.
They prepared seismometers and designated areas where they could study debris flows from the fire scar. When a heavy monsoon rain hit the scar of the museum fire, it gave them the opportunity to see firsthand how water and fire mixed in the landscape. If their data was good, they might walk away with a better understanding of fire flood risk and the effectiveness of mitigation efforts. These measurements, in turn, could be invaluable information in a time of rampant wildfires in the West.
That summer, Beers waited among Flagstaff citizens and fellow scientists, prepared for the heavy rains. But they never came – 2019 was Flagstaff’s driest monsoon yet. Despite this, two small rains triggered debris flows in the museum’s drainage basins. Beers’ team successfully measured one. On August 28, somewhere above the city, boulders broke loose 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 about 1/100th the magnitude of a 1.0 earthquake.
“That’s a lot of energy,” Beers said. “If you stood next to the channel in which this happened, the ground would shake.”
After the August 28 event, Beers and his team went up into the hills near Upper Oldham Trail and found a few areas where debris had eroded tree bark. Even with relatively light rains, the affected areas had been cut into falling rocks and roots. It was just the beginning.
After a dry year, Beers’ team extended their research into the next season, hoping that 2020 would provide a more typical monsoon season on which to base their data. They took the time to install trail cameras and other instruments to improve their measurements.
Despite their preparation, the weather disappointed them and 2020 was even drier than the previous year. There has only been one debris flow event all season, but it managed to betray expectations in its own way.
“What was interesting about this event is that it had low rainfall intensity. It didn’t take a lot of rain to trigger this debris flow,” Beers explained. “It was quite alarming.”
The research team waited another year.
When the skies opened on July 13, 2021, it was the moment Beers had been waiting for. Nearly four and a half inches of rain poured within the hour.
“It was amazing, from a geologist’s perspective,” Beers said.
The seismometers shook. In the upper scar of the Museum Fire, several channels of debris left scattered boulders on Elden Lookout Road. Downstream waters crashed against concrete barriers on the outskirts of the city.
“Flood waters overcame the channel at some point and spread through neighborhoods,” Beers said. In town, the flooding was severe.
The rains returned the next day. Again, the seismometers shuddered.
“What was interesting about this event is that it produced debris flows of a similar magnitude with much less precipitation,” Beers said. “We think the reason we saw this trend is that the floors were already saturated from the day before. The glass was empty on July 13, then on July 14 it was three-quarters full. The floor could only accept a lot more water.
Enough water poured out of the saturated grounds to storm East Flagstaff. A viral video from that day showed a Prius being washed away like a toy boat in a raging river on a neighborhood street. Homes were flooded, sustaining millions of dollars in damage, and the rains continued to fall.
Two days later, back-to-back storms hit Flagstaff again and produced two separate debris flows that thankfully had little impact on the city. After that, things remained relatively calm, a month of calm before the August 17 storm.
According to measurements by Beers’ team, the scar of the Museum Fire received nearly five inches of rain in an hour. Flooding and debris flows progressed from the burn scar to downstream drainages. When they went to survey the impact, Beers’ team found flows like four-lane highways of beach ball-sized boulders and sediment. The largest discharge had a volume of 3,000 cubic meters; roughly the same amount of water falls over Niagara Falls every second.
“He buried trees, he buried the trail,” Beers said. “Just amazing.”
That same day, one of his team’s trail cameras managed to capture a channel where the water was carrying boulders five feet in diameter. The images were partly obscured by splatters of mud that coated the lens. They’re lucky the camera wasn’t lost.
“We thought it was somewhere safe: a tall tree on a distant shore,” Beers said. “It wasn’t safe. The tree barely survived.
The August 17 storm caused the most extensive road damage. He clogged culverts, undermined concrete and covered entire sections of Elden Lookout Road. Flooding in Flagstaff reached Killip Elementary School, where it left the halls under two feet of water. Along the entire eastern side, roads were closed to make way for a constant stream of mud and debris. The destruction took weeks to clean up.
As devastating as these 2021 floods were, they provided a long-awaited scientific opportunity. Beers and his team now had data for three very different monsoon seasons with multiple combinations of storm events. With this data, they began to formulate hypotheses that could help southwestern geologists better understand fire-scar flooding.
One of the first things they found was soil hydrology – measurements of the soil’s ability to absorb water before it began to flood.
“We have seen increases [in soil absorbency] every spring, but decreases every fall,” Beers said. The current assumption is that winter freezing and thawing breaks up the soil so that it is more absorbent in the spring.
“The reason we see a decrease in the fall is that the fine particles, ash and silty clay that settle on the landscape, are moved by wind and water in the summer and fill in the small cracks created snow during the winter,” she says. “It seals it up again.” A possible consequence of this dynamic is that early season debris flows may be less severe than late season flows, but it is a bit too early to say for sure.
Beers’ data also showed that when it comes to fire scars, rain, despite the flooding it brings, is the best first step toward healing. She said mulching and other mitigation efforts can help thwart 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 the vegetation returns, that’s when a fire scar can really start to stabilize. For the Museum Fire scar, there is still a long way to go.
“The soils are not yet salvaged in an unburnt state,” Beers said. “We cannot say for sure whether or not there will be future debris flows. It depends on what kind of monsoon we get.
In the more immediate future, there is some hope that the work of Beers and his team may help improve flood warning technology. Prior to the 2022 monsoon season, areas downstream of the museum’s fire scar will be equipped with emergency sirens. While these sirens will work on rain gauges, de Beers’ team research demonstrates that seismometers can be an effective complement to early warning systems.
Ultimately, for researchers like Beers, the scar of the Museum Fire turned out to be both a crisis and an opportunity. The unique multi-year delay of significant rainfall has given scientists the opportunity to be one step ahead of the weather.
“We had all this surveillance before the big floods,” Beers said. “It’s quite rare. Normally you get there after the flood. We have ongoing data collection.