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Holladay officials declared a state of emergency Tuesday, citing an “imminent risk” of localized flooding as warmer temperatures accelerate Big Cottonwood Canyon snowmelt.
The local emergency declaration allows the city to access state and other outside resources should spring runoff cause flooding along Big Cottonwood Creek.
The creek measured at about 340 cubic feet per second at 6 a.m. Tuesday, according to the National Weather Service. That’s similar to about 340 basketballs of water passing by each second.
The creek is forecast to reach 650 cfs on Sunday, surpassing its “action stage” of about 600 cfs. An “action stage” is a threshold that the weather service uses to determine when it should take action to prepare for possible flooding.
Areas adjacent to the creek are expected to experience minor flooding starting at about 1 a.m. Monday, with daily peak flows reaching about 800 cfs from Monday through next Friday, May 26.
“That is due to the heat that we’ve got going there — I think we’re about five to 10 degrees above what the average is,” said Brian McInerney, a hydrologist who consults with Salt Lake City on flooding. “We’re going to be in the in the low 80s into the weekend, and that along with clear skies ... will fuel this rise.”
As of Tuesday afternoon, the highest projected creek flow is 819 cfs, which is forecast for May 25 at 6 a.m.
By comparison, should the creek reach 861 cfs, moderate flooding would likely occur in Murray Park and areas downstream.
Should it reach 893 cfs, major flooding could occur all along the creek, from the Big Cottonwood Canyon outlet to the Jordan River.
An ‘eventful’ few weeks ahead
“While we cannot control the current temperatures and rate of snowmelt from this natural event,” Holladay Mayor Rob Dahle said in a statement Tuesday, “Holladay has invested much time and resources preparing for the spring run-off.”
“I am especially grateful for all the volunteers that have selflessly filled sandbags and helped their neighbors fortify their properties,” Dahle said. “This is an extraordinary situation and the emergency declaration will enable us to get more help when needed.”
The city has closed a portion of Knudsen Park, located at 6290 S. Holladay Blvd., since Big Cottonwood Creek runs through its west boundary. Individuals should avoid the streambank, as the creek’s swift water is frigid with strong currents, officials said.
McInerney said the Mill D North portion of Big Cottonwood Canyon has about 38 inches of snow-water equivalent remaining, which is currently melting at about 1.7 inches per day.
“If we go an inch and a half a day until we melt out 38 inches, we’ll be here through the middle of June,” McInerney said, noting that further up the canyon in Brighton, the snowpack has about 26 inches of snow-water equivalent remaining and is melting at about 1.2 inches per day.
“The North aspect of Big Cottonwood is still frozen up. The south aspect and the west aspects — to some extent on the west — are producing the meltwater we’re seeing now,” he said.
Air temperature doesn’t have as much of an effect on snowmelt as sunlight, McInerney said, because snowmelt is predicated on solar energy melting snow. That’s why the north portion of the canyon — which doesn’t get as much sun — still remains frozen.
“As the sun gets higher in the sky, as we get closer to the summer solstice ... then that’ll start melting water through the snowpack and into the soil,” McInerney said. “We do anticipate quite high flows by mid next week, really across Big and Little [Cottonwood].”
McInerney said the biggest thing people can do right now is to stay away from both Big and Little Cottonwood canyons and creeks.
“It‘s unknown at this point” McInerney said of when the creeks will peak. “And that’s because we can’t forecast weather beyond five days. So if we ramp up the heat once again sometime in June, and that’s a good probability, then we’ll have another peak. But it’s a wait-and-see right now.”
At a Salt Lake County Council meeting Tuesday, Mayor Jenny Wilson said she expects the next six to eight weeks to be “quite eventful.”
Kade Moncur, Salt Lake County’s director of flood control engineering, said county flood control crews are working “normal and extended” hours but will go to 12-hour shifts if needed. Emergency contractors who work on debris removal are also on standby, as are hazard assessment teams.
— Tribune staff writer Blake Apgar contributed to this report.