This is what an ideal spring runoff in Utah would look like to avoid flooding
Para empty este artículo en español, haz clic aqui.
What ends up in the mountains has to come down into the valleys.
Utahns across the state are bracing for a huge spring runoff after historic, record-breaking snow cover blanketed mountains across Utah. State and local officials are bracing for the worst as the US Department of Agriculture expects this year’s snowpack to hold more than 27 inches of water.
For comparison, Utah had about 11 inches of water last winter and just over 24 inches in 1983 when runoff caused widespread flooding in the spring.
Water managers and emergency officials across Utah agree: A slow-warming spring is key to avoiding flooding.
Ideally, temperatures in the mountains would warm to above freezing during the day – allowing snow to melt into valleys, rivers and reservoirs – before falling below freezing at night. The night’s frost would slow runoff and allow for a longer release of water over time.
A worst-case scenario would be temperatures suddenly jumping to summer levels in late spring. This situation played out in 1983 when floods turned State Street into a makeshift river and mudslides engulfed a city, a highway and a railroad in Spanish Fork Canyon, in addition to causing landslides in Davis and Sanpete counties.
Salt Lake County emergency management director Clint Mecham said for ideal spring runoff, rain and snow would also need to stop falling. No matter how much preparation can be done, there are only a limited number of safety precautions that can be taken.
“We’re really kind of dependent on Mother Nature, how she decides to get us into these summer months,” Mecham told The Salt Lake Tribune. “Are we going to have a nice gradual warm up with cool evenings, or is it going to be hot all at once?”
But there’s also a downside to snowpack slowly melting, says Jordan Clayton, director of the Utah Snow Survey.
“If snowmelt spreads out over a really long period of time, we lose a percentage of that water compared to a short period of time,” Clayton said. “If you melt all that snow in a short amount of time, your risk of flooding is much higher, so obviously flooding is a problem.”
Utah’s Prognosis? Expect it to stay cold
Current forecasts show that the weather will not feel like spring in the foreseeable future.
Linda Cheng, a weather forecaster with the Salt Lake National Weather Service, said forecasts show another storm rolling through Utah beginning Sunday night and possibly lasting through the middle of the week.
“This will be another nationwide storm with cold temperatures in most valley floors except for the St. George area,” Cheng said.
Cheng added that forecasts show temperatures will warm slightly in the coming days, but the weather is not expected to warm to typical spring temperatures.
Outflow in southern Utah
Washington County has already seen the Santa Clara and Virgin rivers reach flood stage, according to Jason Whipple, the county’s director of emergency services. Central Utah has also seen rivers above flood stage, with flooding along the Sevier River recorded earlier this spring.
Whipple said Washington County’s runoff in recent weeks has caused damage to hiking trails and some remote dirt roads, but no damage to critical infrastructure. He added debris from rivers that have overflowed onto golf courses that line the two main rivers.
An ideal spring runoff in the South is the same as anywhere else in the state: nice and slow. Whipple said gradually warming temperatures would melt snow from the bottom up — meaning snow at lower elevations would melt into rivers before transitioning to mid-altitude snow and then high altitude.
“If we could get it to warm up slowly, that would be perfect,” Whipple said.
The less than ideal runoff scenario would be immediate onset of high temperatures. An even worse situation would be if rain fell on top of existing snow, “especially at mid-level and higher levels, then we’re going to get a lot of runoff very, very quickly,” Whipple said.
Snow has remained on the valley floor in parts of northern Utah, particularly on the back side of the Wasatch — in places like the Ogden Valley and Morgan County.
So has Cache County, despite some warm days lately. The snow on the surrounding mountains hasn’t melted at all, according to Cache Water District manager Nathan Daugs. He told The Tribune in March that basements in Logan have started to flood, which he says will continue as the drain drags on.
He is also hoping for an end to the rain.
“If we can shut off the moisture for a while during warming and start breaking some of that snowmelt without adding the additional rain in the valley — that would be the ideal situation for flooding purposes,” Daugs said.
There are only so many waterholes in a place like Cache Valley, with only the Cutler Reservoir to the west and the Porcupine and Hyrum reservoirs to the south to soak up the runoff.
“Places like Cache County are hopefully expecting a slow warm-up, because if everything goes very fast, all the snow in the valley isn’t going anywhere but in people’s yards,” said Jordan Clayton, Utah Snow Survey Supervisor.
Elsewhere in Utah
As in other areas of the state, much of northern Utah’s spring runoff is still trapped at higher elevations. Weber Basin Water Conservancy District general manager Scott Paxman said the snow at higher elevations hasn’t moved.
Paxman reckons snow could fall in mid-April and peak discharges probably won’t occur until mid-May. Regardless, the district has drained water from its reservoirs to make room for the upcoming runoff, no matter how soon it arrives.
Like other water managers, Paxman said, “Those warmer (then) cooler cycles really help bring it down, and do it in manageable amounts so it doesn’t all come down at once.”
Mecham, Salt Lake’s emergency management director, said in normal years, the area’s runoff begins in late April or early May. He added that the areas of concern during the runoff will be the county’s many creeks, as Salt Lake officials have previously said the Big Cottonwood and Little Cottonwood creeks are special spots to focus on.
“We’re looking at literally a record year for snow-water equivalent and snowpack in the mountains above Salt Lake County,” he said.
“There’s a lot of water up there, and at some point it has to come down again.”
https://www.sltrib.com/news/environment/2023/04/03/heres-what-an-ideal-spring-runoff/ This is what an ideal spring runoff in Utah would look like to avoid flooding