Arizona officials work to prevent post-fire floods
FLAGSTAFF, Ariz. (CN) — Dave Roth had never seen anything like it.
For the first time in his 38 years in Flagstaff, Arizona, water rushed between nearly every house in his neighborhood, rising to his knees in some parts of the street.
“It was bad,” he said a year later. “We didn’t know what to do.”
The heavy debris flows that careened through Roth’s and other Flagstaff neighborhoods in late summer 2022 came after nearly back-to-back fires that burned tens of thousands of acres in the San Francisco Peaks just north of the city.
Regular fires are a natural adaptation of ponderosa pine forests like the Coconino National Forest, which envelopes Flagstaff. But years of fire mismanagement and climate change have altered the fire regime, creating conditions for hotter-burning fires that leave the ravaged terrain vulnerable to intense mudslides. Now, agencies at all levels of government are working to prevent flood events like last year's.
How fires result in floods and mudslides
Before the federal government managed them, fires burned through northern Arizona forest floors every five to 10 years, clearing out tall grasses and brush but leaving the trees unscathed. The process cleared the way for new growth and prevented natural fires from burning too hot.
Everything changed in 1935 when the U.S. Forest Service implemented the “10 a.m. policy.” As it sounds, the policy mandated that every reported fire be put out by 10 a.m. the next day.
“That got us into the problem that we’re in,” said Matt McGrath, Flagstaff district ranger for the Coconino National Forest. “Fire wasn’t allowed to play its natural role on the landscape, so conditions got all out of whack.”
The Forest Service ended the practice in 1978, opting for controlled burns to manage high-fire areas, but in some places half a century of growth had already lit.
Overgrown forests with more fuel to burn have combined with a heating planet and led to hotter fires than ever recorded, said Kathy Jacobs, director of the University of Arizona’s Center for Climate Adaptation Science and Solutions. When fires burn hot enough to reach the canopies, they quickly change from controlled to catastrophic.
Fires hot enough to burn trees to death create several problems for forests. Without healthy trees, less water is absorbed into the ground, creating more runoff.
Fires of that magnitude also burn the soil, hardening it to absorb even less water. And without trees and other vegetation, nothing can hold the charred soil in place when monsoon rains fall.
“When it pours on that essentially ash field and debris field, you start seeing whole areas sort of sliding down the mountain,” Jacobs said.
A monsoon can drop inches of rain in only a couple hours over a mountainside burn scar: The water digs channels in the earth, eroding loose soil and debris and carrying it downslope, devastating communities that stand in its way.
Flood watch starts before the fire
Flagstaff has seen post-fire floods before. Floods on the east side after the 2010 Schultz Fire were a wake-up call for the community, spurring both the city and Coconino County to commence flood-mitigation efforts that are still evolving today. But proper mitigation begins before the fires ever burn, Flagstaff Fire Captain Dylan Guffey noted.
“If we can reduce the intensity of the fire on these steep slopes, then we won’t lose all the vegetation that holds all that soil in place,” he said.
Guffey noted that the city partners with the Forest Service in the Flagstaff Watershed Protection Project, funded by a $10 million bond approved by voters in 2012 to thin forests from an overgrown state back to a “natural vegetative state." Other contractors work with the city to chop trees and clear the forest floors of debris. To prevent fires from jumping to the treetops, Flagstaff ensures that there are no tree branches lower than 16 to 18 feet from the ground.
More than 12,000 acres of forest land have been treated so far.
“You can see just how many trees and other stuff is on the ground,” Guffey said as he drove his pickup through private property in the forest not treated through the protection program. “That’s what leads to more extreme fire behavior. There’s just that much more material to burn.”
Driving further into the Coconino National Forest, trees are more spread out, and the ground is clear of debris. Towering piles of recently chopped trees lay aside large mechanical harvesters.
After thinning out trees, either selling them as timber or burning them in pile burns, agencies will return to some areas and light prescribed burns, clearing the rest of the dried brush and debris.
While it’s the agencies’ goal to eliminate post-fire floods through the thinning project, it can’t prevent everything. The Tunnel and Pipeline fires that burned across the San Francisco Peaks last summer left nothing to stop heavy mudslides from rushing through Flagstaff neighborhoods, littering the streets with ash and dirt.
When the floods hit
The Pipeline Fire sent floods farther west than the city had seen before, reaching people like Dave Roth for the first time.
Neighbors did everything they could to protect their property, spending hours a day filling and layering sandbags around their homes.
Roth and his neighbors broke down a fence behind their houses to build an easement that would guide water into the Rio de Flag just past their neighborhood.
Despite their efforts, the sole storm drain on the street quickly clogged with debris, pooling up water multiple times throughout the summer.
“We all have rubber boots now,” said Deb Thompson, a 36-year resident. The city used her backyard as an easement, channeling water just past her house, protected only by a yellow plastic tube meant to act as a barrier between the water and patio.
“We sure were worried," Thompson said, pointing to the ditch in her yard. “We’d never experienced this. We didn't know how much the city was going to divert, even though they said they would not divert more than they could handle. But who really knows?”
While the water came and went within hours each time it rained, debris remained.
“Mud is still around,” Roth said, picking it up with three fingers. “It’s not going away."
He said the ash from fires mixing with dirt causes it to cake onto everything. After a year of spraying down his driveway, it lingers.
Chase Kempf, who, like other neighbors, replaced his old wooden fence with a brick wall to protect from future floods, said the land on the side of his house is nearly a foot higher than before due to sediment dropping as the water gushed past.
In the aftermath, the city built an easement through the neighborhood to better channel water into the river.
It also built three large retention basins to the north designed to detain up to 18 million gallons of water. The basins aren’t designed to hold the water forever but to slow the flow and re-release it at a controlled pace. The basins work in tandem with the Schultz Creek Watershed Project, a collaboration between the Forest Service and Coconino County.
A response to a symptom
Coconino County has worked to reduce post-fire floods since the 2010 Schultz Fire, using advanced engineering and mathematical modeling to determine the sediment source and how to best reduce erosion.
When rain falls on charred land, it carves out channels in the ground, carrying more sediment downstream.
By identifying how the channels form and carry sediment, the county can decrease the amount of sediment making it downstream, said Sean Golightly, community relations manager for the Coconino County Flood Control District. Eager to make the most of its resources, the district tapped the engineering firm Natural Channel Designs to study erosion rates and sediment-carrying capacities of channels.
“If you put these channels into good conditions, that can reduce your sediment loads by about 200% in some cases,” said Allen Haden, an ecologist for Natural Channel Designs.
The erosion rate depends on the shape, dimensions and stability of a channel’s bank. Deep channels with tight curves, for example, put more stress on the banks, increasing the erosion rate. By calculating a channel’s bank recession rate, measured in tenths of feet per year, and multiplying it by the height and length of the bank, the company can conclude how much sediment comes from a given channel, measured in tons per year.
Similarly, the sediment-carrying capacity is measured based on the shape of a channel. Engineers multiply the slope of the channel by the depth to get shear stress, measured in pounds per square inch of pressure. The higher a channel’s shear stress, the more and larger material it can carry.
Once the engineers determine where most of the sediment comes from and how it is carried down the mountain, they are ready to begin carving out channels and rebuilding eroded alluvial fans — naturally occurring systems that resemble a river delta and spread water over wide areas to drop sediment.
After spreading across alluvial fans, the water is channeled through a series of check dams built along the watershed to drop even more sediment.
Next, the water reaches the concrete channels of the city’s retention basins, which allow it to move slowly and safely through the community.
“The hope is that by the time water is leaving these basins here and getting into the city of Flagstaff, it’s mostly free of sediment and debris,” Golightly said.
The county got to work almost immediately after the Pipeline Fire floods, beginning construction on the dams in August 2022 after receiving a $2.4 million grant from the Natural Resources Conservation Service.
But flood response costs a lot more than a few million.
“We will spend about $90 million over the next few years just building long term mitigation from the Pipeline Fire,” Golightly said. “What it would cost to thin those forests is maybe a third, if that.
“This is a response to a symptom,” he added. “A response to flooding, which is a symptom of fire, which is a symptom of forest management. Instead of shelling out tens of millions of dollars on these flood responses, we need to be investing more into supporting some good forest management.”
The Forest Service has invested more than $12 million into Flagstaff’s forest-thinning project. Over the next few years, the Flagstaff Watershed Protection Program will receive an additional $7 million from the Department of Defense and $9 million from the Arizona Department of Forestry and Fire Management.
While Flagstaff and Coconino County have taken steps toward mitigation efforts, deep burn scars, eroded channels and charred soils remain. With monsoons set to hit the region any day now, time will tell if the steps taken thus far will protect the community in the future.