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‘Unraveled’ watershed causes massive rockslide through neighborhood near Flagstaff

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Last Friday, post-fire flooding beneath the Pipeline Fire burn scar drastically changed the landscape when it carried countless tons of mud, stone and debris off the forest and into the Timberline neighborhood east of Flagstaff. This radical movement of earth was most visible in the Copeland Wash, where the tumult replaced the drainage with a field of boulders that crossed roads and extended into the neighborhood.

County crews are still actively clearing roads following the incident, and plans to restore the watershed that loosed this rockslide are already underway.

The boulder field seen at Copeland Wash is the result of the watershed’s natural alluvial fan “unraveling,” explained Coconino County deputy manager Lucinda Andreani. Above the Timberline neighborhood, there was a place in the Copeland Wash that naturally fanned out and allowed water to slow down, spread out, and drop sediment.

But during the extreme flows of Friday’s flood events, this fan could not contain the water coming off the burn scar. Instead of spreading out, the water eroded into and “channelized” the fan, turning it a source of new sediment — and boulders up to five feet in diameter — to be washed downstream.

“Now we have multiple, six-foot-deep to eight-foot-deep channels cut through the fan,” Andreani said. “And it's going to keep sourcing all this rock and debris until we can get it restored.”

Restoring alluvial fans is a large part of Coconino County’s long-term strategy to protect homes against post-fire flooding. In many cases, it’s proven to be a successful tactic. Just this year, the county has observed the effectiveness of alluvial fan restoration in Spruce Wash beneath the Museum Fire burn scar, which flooded catastrophically in 2021.

“We've had now two pretty good rainfall events at Museum, and we've had no water enter the city,” Andreani said. “We haven't had to go out and clear roads or do anything. So far, it's working really well.”

Following the Pipeline Fire, Andreani said there are at least six watersheds where fan restoration work is needed to get ahead of "unraveling": Schultz Creek, Government Tank, Paintbrush, Peaceful, Wupatki Trails and Copeland. In some cases, this work is merely expansion or maintenance of existing mitigation, as is the case for the alluvial fan that was restored on the Wupatki Trails watershed following the Schultz Fire flooding.

“It's way too small for the level of water we're getting now,” Andreani said. The Wupatki Trails fan has been inundated with an estimated seven feet of sediment during this year’s rain events, forcing the county to manually remove and clear out sediment to maintain the fan’s function.

Expanding an alluvial fan is not as simple as identifying the need. The topography of the landscape must be conducive to restoration of a fan, or else “it isn’t going to sustain, you’ll have to maintain it constantly,” Andreani said. Fortunately in the case of Wupatki Trails, she said that there is potential for place another fan farther up the watershed.

“It's going to be a real challenge,” she said “But we're going to be proposing that.”

Flooding Unravells Alluvial Fan Flood Mitigation

A house off of North Copeland Lane is surrounded by mud, silt, boulders and dead trees Tuesday afternoon after heavy rains unraveled a natural alluvial fan above the Copeland Watershed on Friday and sent a torrent of debris-filled water down the mountain and into the neighborhood.

In other places, such as Government Tank, Peaceful and Copeland, there is no preexisting mitigation. Andreani said the county and its engineers have identified areas that would be receptive to alluvial fan restoration in these area. But they may not be able to get to them soon enough, as is the case with the two fans identified in the Government Tank Watershed.

“One has unraveled and one is probably going to unravel unfortunately this season,” Andreani said. “And that's why we're now seeing even more [sediment] than what the modelling predicted.”

As a long-term goal, alluvial fan restoration is one part of a three-part strategy that also includes channel systems to safely convey waters through neighborhoods and culvert improvements to manage floodwaters where they interact with the highways. Fortunately, the county is not starting from scratch — in many affected areas these systems are in place thanks to efforts following the Schultz Fire flooding. The Pipeline Fire has expanded and exacerbated the need.

Andreani expects that the long-term mitigation projects necessitated by the Pipeline Fire will come with a $20-$30 million price tag that rivals the mitigation put in place after the Schultz Fire. The county has already begun submitting requests for funding, the bulk of which is expected to come from National Resource Conservation Services (NRCS), a division of the U.S. Department of Agriculture. Andreani said her department is prepared to submit for NRCS funding by August 11.

Smaller portions of funding may come from the state, the Forest Service and FEMA, the latter of which “can take years,” Andreani said. “And we don’t have years.” She is committed to moving these projects forward as aggressively as possible.

“An aggressive timeline would be to have these completed in two years,” she said. “Three years is probably more realistic.”

In the meantime, getting through this monsoon season safely and with as little damage as possible is the priority. Emergency measures are centered around the placement of sandbags and concrete barriers to protect homes. Andreani estimates that four miles of concrete barrier have been placed since the Pipeline Fire.

One Timberline resident, who asked to remain anonymous, reported that the county has been good about implementing emergency measures where needed.

“I totally appreciate the speed that they got this stuff together,” he said, referring to the wall of concrete barriers that now stands between his home and the Copeland wash rockslide. He said it was only two days from Friday’s event that the county installed concrete barrier to protect his home from future unraveling — a measure he’s glad to have. The memory of Friday’s flood is still fresh with him.

“We were on the front porch, and water was just gushing, gushing, gushing,” he said. “Rocks hitting the pillar, boom, boom, boom. And then it was just huge, crazy boulders that we couldn't see, because they were underwater.”

While clearly necessary, Andreani stressed that all flood mitigation was secondary to a need for proper forest management to prevent further fires like the Pipeline Fire. During a recent meeting with regional forest managers, she said that the county Flood Control District appealed directly to the need for proactive management.

“This fall, we need boots on the ground, getting restoration on the west side of the peaks,” Andreani said. “It’s got to happen. Because this fire, if had burned west, you can’t imagine the catastrophe we would have in Flagstaff. All of Flagstaff would be under sandbags for ten years.”

Sean Golightly can be reached at sgolightly@azdailysun.com.
 
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