Utahns who survived this summer’s fires face the possibility of flooding and devastating mudslides this week — and the danger will continue for three years.
“There’s definitely potential for flooding where there are burn scars,” said National Weather Service meteorologist Christine Kruse.
It’s a particular concern for areas affected by the Pole Creek and Bald Mountain fires, which have burned 102,188 acres and 18,620 acres, respectively. (As of Monday morning, Pole Creek was 81 percent contained; Bald Mountain was 97 percent contained.)
Rain from Tropical Storm Rosa (formerly a hurricane) is forecast to begin Monday night and continue for roughly the next week. The forecast along the Wasatch Front — from Brigham City to south of Nephi — calls for between 1 and 1½ inches, on average.
“To put that in perspective, we’ve had roughly three-quarters of an inch of precipitation since June 1,” Kruse said. The official total at Salt Lake City International Airport from June 1 through Sept. 30 was 0.74 inches.
“We have patches of moderate to high burn severity all along, roughly from Woodland Hills south to Spring Lake,” said Brian McInerney, hydrologist for the weather service’s office in Salt Lake City. “And anytime you have at least moderate severity, you have the potential for debris flows.”
It's not just how much rain falls, but how fast it falls.
“Long-duration, low-intensity rainfall typically does not produce debris flows,” McInerney said. “This storm that’s moving in may be just that. And if that’s the case, then they shouldn’t worry too much.
“However, there's a chance that there's an embedded thunderstorm in this weather system that could bring some short-duration, high-intensity rainfall. That's what we worry about right now.”
The fires not only burned the vegetation but also “cooked the soil.” And the burned vegetation “lays down this kind of film on the hillsides,” making the soils “repel water.” And that, in turn, creates debris flows.
“It’s like having — depending on how much rain — 100 to 500 cement trucks just cutting loose with this wet, muddy debris with logs and rocks at a really fast clip down the hillside,” McInerney said. “And it’ll just take out whatever’s in front of it.”
In Woodland Hills and Elk Ridge, they’re filling sandbags and calling for volunteers. Woodland Hills has issued an emergency declaration. And city officials warn residents to be ready to quickly evacuate.
“We have prepared the best we can,” said Councilwoman Kari Malkovich, describing the threat of mudslides as “this second incident” after the fire.
The city has filled nearly 30,000 sandbags, Malkovich added, handing them out to residents and placing them around property lines. Sandbags can help with flooding, but they’re not going to stop a rock-laden mudslide.
The Utah National Guard is sending 200 soldiers to Utah County, too, to assist with the mitigation efforts at the request of Gov. Gary Herbert.
“Hopefully there is little need for flood response efforts, but we are prepared now to react quickly if called," said Utah National Guard Adjutant General, Maj. Gen. Jeff Burton.
McInerney recently flew around the burned areas and came away with deep concerns.
“It’s pretty daunting when you see the extent of the burn and you just follow it down into the towns,” he said. “It’s a direct shot. If you were to roll a bowling ball from the top, it would roll right through the neighborhoods. And when we talk about debris flows, the physics are frightening at how intense these things can be.”
The danger will continue for three years until the vegetation has regrown to the point that it can hold the soil and the rain.
“And what are the odds that we’re not going to get a thunderstorm over this area in the next three years? It’s quite low,” McInerney said. “Typically, we get at least one debris flow from these moderate to high-intensity burn scars in that three-year period.”
He said the 2012 Seeley Fire in the Manti-La Sal National Forest’s Huntington Canyon produced 22 debris flows.
McInerney said the problem will extend into other areas in coming years.
“With the warming climate, we’re going to see more intense fire behavior, because it’s hotter and drier. And we have more people moving into the valleys with more development higher up in the canyons," he said. “This is a tough mix to deal with.”
In addition to the potential for mudslides, the NWS warned of possible flash flooding in slot canyons and dry washes and localized flooding in urban areas with poor drainage. Rivers are expected to rise, but they’re not projected to flood “at this time.”
—Tribune reporter Courtney Tanner contributed to this story.
