With record runoff comes threat of landslides

Earth is already moving at the site of old slides in Wasatch Front neighborhoods.

(Rick Egan | The Salt Lake Tribune) Geologist Ben Erickson of the Utah Geological Survey adjusts the GPS equipment that monitors landslides at the Springhill Geologic Park in North Salt Lake, on Wednesday, April 19, 2023. A creeping landslide destroyed 18 homes here several years ago.

While potential flooding is getting ample attention this spring, Utah officials are almost equally concerned about an influx of landslides that is expected when record-setting snowpacks melt and saturate the ground.

Ominous ground movement is now being observed at various spots in the Wasatch foothills, putting some properties at risk, according to geologist Greg McDonald, a hazards specialist with the Utah Geological Survey, or UGS.

“It’s already begun. We haven’t had time to go investigate all these. We’re in the process of doing that right now,” McDonald said Wednesday while giving reporters a tour of an active landslide area in North Salt Lake.

[Related: Learn about ongoing landslides]

Some 18 homes were demolished at the site on Springhill Drive in 2011, several years after the spot’s geological instability became impossible to ignore.

“It wasn’t obvious,” McDonald said. “It wasn’t a rapid movement, it might have been a few inches a year, enough to damage a house over a few years.”

UGS officials have been monitoring ground movement here for years using GPS instruments. Since 2014, the ground has moved 20 inches, according to Ben Erickson, also a geologist with UGS.

Utah is littered with active slides, such as the one under study on Springhill Drive.

“Even though Utah is a very dry state, we have a lot of landslides. A lot of that is having enough steep topography, enough precipitation and we certainly have enough weak geologic units,” said Rich Giraud, a retired UGS geologist said in a 2021 webcast sponsored by the U.S. Geological Survey.

This spring, Utah should expect larger than average landslide activity, both in the form of new slides and old ones becoming reactivated, according to Jeff Moore, a University of Utah geology professor.

“These kind of slow creeping earth flows are very responsive to water inputs,” Moore said. “And when we have higher-than-average water inputs, we can expect higher-than-average displacement.”

He anticipates the rate of movement on existing slides to measurably increase.

“Similarly, we can expect that some landslides that have been dormant for many years might now reactivate,” Moore said. “We can definitely expect new landslides in areas where we have steep slopes and unconsolidated soils. That’s certainly harder to predict where those are going to occur.”

Following most heavy winters, Utah experiences an uptick in activity, sometimes with tragic results when saturated ground unleashes fast-moving torrents.

For sheer destruction, nothing matches the Thistle slide that occurred in April 1983.

After an extremely wet winter 40 years ago, nearly 20 million cubic yards of earth slid a mile and half in the upper reaches of Spanish Fork Canyon, obliterating U.S. Highway 6 and a railroad. Debris blocked a stream that backed up and inundated the town of Thistle, which had to be abandoned.

At the time, Thistle was considered the costliest landslide in U.S. history.

In contrast to creeping slides like Springhill are the mud flows that arise from rapid snowmelt or heavy rains, particularly on burn areas.

“The hillside would just get so saturated, the soils just mobilized into what we call an earth flow or debris flow depending on what the material is made of,” McDonald said. “They happen very quickly. They deposit into flooding creeks, and that’s what created a lot of the debris flows in Weber and Davis counties [in the 1980s]. Those are a lot harder to predict where they’re going to happen and when.”

The spring of 1983 saw a “perfect storm” of a cold, snowy spring followed by a heat wave.

“All the sudden, everything just heated up in mid-May and the snowmelt was cranking,” McDonald said. “It’s almost like you’re dumping inches and inches of water on a slope. This year, we’re not in that mode yet. We are kind of melting off the snow slowly. What we’re hoping for is that it’s not like a 1983 scenario where it melted off very quickly.”

Utah’s last round of punishing landslides occurred in 2005 and 2006. Several subdivisions, which had been deemed safe during preconstruction geological investigations, proved to occupy unstable ground.

Meltwater from heavy snowpacks reactivated many old slides, like one that invaded Cedar Hills on April 28, 2005, ruining at least one home, according to a 2007 report by the Utah Geological Survey.

A few weeks later on May 12, a boulder broke off a cliff from Y Mountain and dislodged other rocks that tumbled a mile into Provo. A 13-ton rock struck a guest house after falling 2,600 vertical feet.

“No one was home at the time, but the structure was a total loss,” the report said. “Many of the fallen rocks left impact craters (bounce marks) and trails of flattened oak brush on slopes at the base of the cliff and on the slope just above the damaged house.”

Potentially destructive slides have already been observed this spring in Mountain Green, Layton, Fruit Heights, and Salt Lake City’s City Creek and Emigration canyons.

The Springhill neighborhood started showing signs of trouble in the 1990s when cracks opened in the pavement, walls fell out of plumb and fences tilted. These properties were bought out under a federal grant, the homes removed and the site turned into Springhill Geologic Park, permanently off-limits to future development.

A pathway winds through the park, taking visitors above the headscarp, where the hill broke away and the slide started. You can clearly see where the ground has pulled away, leaving a fissure known as a graben in its wake. Mature trees on this ground are tilted toward the hillside, while young ones grow straight up, a sure sign that the earth is in motion.

Looking over this scene, State Geologist Bill Keach highlighted the need to monitor Springhill, one of about 100 slides UGS monitors.

“It’s important that we understand where the hazards are, so that you don’t make a bad decision,” said Keach, who himself lost a home to earth movement in Hurricane. “We can provide really good information to keep us out of those areas that are hazardous. The last thing you want to see is your home falling down.”