Study looks at potential for massive flooding in Flagstaff

November 15, 2019 GMT

FLAGSTAFF, Ariz. (AP) — New modeling shows that a rush of water nearing the flow of the Colorado River could run off potential fire-scarred areas and flood parts of Flagstaff.

The chance of massive flooding in the Rio de Flag, a normally dry riverbed, is rare — about 1% each year. But the modeling helps target areas of the surrounding forest that need to be thinned to lessen the risk of catastrophic wildfires, Coconino County officials said.

The county recently released a draft of the study that looks at the potential impact of a 100-year storm following a 14-square-mile (36-square-kilometer) hypothetical wildfire on the southwestern side of the San Francisco Peaks, the Arizona Daily Sun reported.


Information from: Arizona Daily Sun, http://www.azdailysun.com/

Water would hit the closest neighborhoods and fan out through downtown Flagstaff before being contained in a natural channel, said county flood district director Lucinda Andreani.

Flooding would be worse if a wildfire stripped the land of vegetation, she said. The area the county modeled has steep slopes that would shed water quickly, raising the risk to densely populated areas of Flagstaff.

The modeling shows as much as 1,200 cubic feet (34 cubic meters) of water per second could run down the Rio de Flag in a 100-year storm. That figure would increase to more than 9,000 cubic feet (255 cubic meters) per second if the land is scorched by a wildfire.

The average flow of the Colorado River over the last 10 years is 12,700 cubic feet (359 cubic meters) per second at Lees Ferry, about 15 miles (24 kilometers) downstream from the Glen Canyon Dam, according to the U.S. Geological Survey. It fluctuates daily with hydropower needs.

Jay Smith, the county’s forest restoration director, said parts of the Coconino National Forest have been thinned but the work is expensive, and there are some limitations on what tools can be used in wilderness areas where trees tend to be more overgrown.

The mountainous areas generally are cooler, but climate change has increased the risk of wildfires even in those places, county officials said.