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Draft study of post fire flooding impacts to the Rio de Flag

A new county study modeling post fire flooding in the Rio de Flag shows such an event could send a slimier amount of water that runs down the Colorado River past Less Ferry through the middle of Flagstaff.

A draft version of the study was finished earlier this year and modeled the impacts of a 100-year storm on the Rio de Flag if it followed a large wildfire.

Storms of such size are rare. A 100-year storm has a 1% chance of occurring every year, and the chances of one happening after a large wildfire is even less likely.

But Flood District Director Lucinda Andreani said it is important to model as the flood district develops a forest restoration plan, identifying high impact sections of the forest that the district may want to target for restoration efforts in the future.

Joe Loverich with JE Fuller Hydrology & Geomorphology worked on the efforts to model the event.

The model is based on a potential 9,000-acre fire on the southwestern side of the San Francisco Peaks, encompassing areas of the forest north of Baderville and around the Arizona Snowbowl Ski Resort, Loverich said.

That burn area would be smaller than the Schultz Fire but larger than the Museum Fire, Loverich said. But unlike those two burn areas, they modeled this fire upstream from larger and higher population sections of Flagstaff.

Then they modeled what could occur if a storm dropped 4 inches of rain north of the city in only six hours.

Without a wildfire, their model shows as much as 1,200 cubic feet of water per second could be running down the Rio de Flag.

But the model shows a wildfire could increase the amount of flooding to over 9,000 cubic feet per second just under Cheshire and 7,500 near the Frances Short Pond.

“What happens when you put the additional water in is that it fans out in Cheshire, it fans out in Coconino Estates, downtown,” Loverich said.

After passing through downtown and the Southside, floodwaters would be far more contained in the natural channel, lessening its impact.

Part of the reason Loverich said they believe post-fire flooding could be so substantial is the state of the forest their fictional fire burned.

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Using data from the Four Forest Restoration Initiative (4FRI), Loverich said they were able to guess at how intense a fire in that area could become based on how overgrown those sections of the forest are.

Generally, hotter fires fed by overgrown forests change the underlying soils in a way that increases the amount of flooding. As such, 35% of the fire's area was modeled to have burned with high intensity and 26% with moderate intensity.

Similarly, a substantial portion of the burn area they modeled also contains steep slopes that shed more water, creating larger floods.

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Loverich admitted the model depicts an event that may be somewhat unlikely, especially in using such a large storm. But Loverich said the model can still be helpful in illustrating what post-fire flooding could look like given lesser storms and even different burn scars.

And it tells them sections of the forest on the southwest side of the Peaks may be a high priority as they look at future forest restoration efforts, Andreani said.

One challenge for forest restoration in that area, however, is the wilderness area, Andreani said. Many of the most overgrown sections of the forest in the area are within the wilderness area's boundary.

Andreani added that on top of much of the forest being overgrown, there are also areas with layers of dead trees across the floor of the forest, particularly dead and downed aspen. And such environments contributed to the intensity of the Schultz Fire when it burned, Andreani said.

County Forest Restoration Director Jay Smith said there have been some efforts to thin in the wilderness area before, but they are few and expensive.

Federal law disallows the use of machines, including tools like chainsaws, within the wilderness area. It is possible the Forest Service could allow helicopters to help thin as they would not be touching down in the wilderness area, but any action to that end may be far in the future, Smith said.

Luckily, while very overgrown, Smith said the wilderness area is also at higher elevation, which tends to mean cooler temperatures and a somewhat wetter environment. Smith said climate change could increase the risk of fire in those elevations.

Outside of the wilderness boundary, much of the forest in the area is included in the 4FRI project and was sold to a bidder in 2015, Smith said. But as with much of the rest of the 4FRI project, thinning has not yet occurred. 

Updated for correction at 5 p.m. on November 14. 

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Adrian Skabelund can be reached at the office at askabelund@azdailysun.com, by phone at (928) 556-2261 or on Twitter @AdrianSkabelund.

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