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After fires, flood risk elevated as alluvial fan unravels

July 7, 2022

Runoff now unpredictable in Flagstaff drainage basin

Recent monsoon storms over June, 2022’s Pipeline Fire burn scar flooded residential neighborhoods east of Flagstaff, Arizona. Subsequent debris flow has impaired drainage in the Copeland Wash, now buried by boulders and mud. Coconino County plans to restore the watershed and help reestablish predictable drainage patterns.

The county’s flood control district administrator told Arizona Daily Sun that sedimentary processes “naturally fan out and allow water to slow down, spread out, and drop sediment.”

However, “unraveling” processes did not allow the stormwater runoff to naturally spread out as heavy rains fell and rolled off burnt or hydrophobic soils. Instead runoff “channelized” the existing fan, forcing once-settled sediment to flow downstream and overflow culverts into Timberline streets.

Coconino County residents will continue to face elevated flooding risks as “multiple, 6 ft to 8 ft deep channels cut through the fan,” which will send runoff, rock and debris downstream.

Earlier this year, a restored alluvial fan in Spruce Wash increased flood protection below 2019’s Museum Fire burn scar, which incurred flood damage in 2021.

The county has identified “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.” Unfortunately, topography may not provide another site for another alluvial fan in the watershed.

Long-term forest management and flood mitigation projects will require tens of millions of dollars in infrastructure investment. City and county governments are pursuing federal funding as well as state grants, but communities don’t have time as severe weather events become more frequent. Weathering each storm with as little damage as possible is the reality. Sandbags and concrete barriers are part of emergency management tactics.

In Colorado heavy rains over 2020’s Grizzly Creek burn scar led to debris-slides that closed Interstate 70 in Glenwood Canyon multiple times in 2021 and intermittent closures have continued this summer. Flash flood warnings pose risk to travel through the corridor.

Preventing water quality pollutants from entering waterways, via booms and other measures, is neither fast nor cheap. Northern Water conservation district estimates that $20 million already has been spent on wildfire recovery after 2021’s East Troublesome fire “and the work is far from done.”

“We recognized that fire would alter the quantity and quality of the water that would be going into the Colorado-Big Thompson Project … for years to come,” commented Jeff Stahla, Public Information Officer commented to The Colorado Springs Gazette.

Spokesperson for Denver Water Todd Hartman remarked, “There have always been sediment flows in the South Platte River system, but it wasn’t until the 1996 Buffalo Creek fire that the significance of the problem became clear.” More than 25 years later, the water utility is still dealing with the silt and debris from that fire as well as 2002’s Hayman fire.