While potential flooding is getting a lot of attention this spring, Utah officials are almost as concerned about an influx of landslides expected as record-breaking snowpack melts and saturates the ground.
Menacing ground movement is now being observed at various locations in the Wasatch foothills, putting some properties at risk, according to geologist Greg McDonald, a hazard specialist with the Utah Geological Survey (UGS).
“It has already started. We didn’t have time to examine all of this. We’re in the process of doing that,” McDonald said Wednesday while giving reporters a tour of an active landslide area in North Salt Lake.
[Related: Learn about ongoing landslides]
About 18 homes were demolished on the Springhill Drive site in 2011, several years after the site’s geological instability became unavoidable.
“It wasn’t obvious,” McDonald said. “It wasn’t a quick move, it was maybe a few inches a year, enough to damage a house over a couple of years.”
UGS officials have been monitoring ground movements with GPS instruments here for years. The ground has moved 20 inches since 2014, according to Ben Erickson, also a geologist at UGS.
Utah is littered with active slides, like the one on Springhill Drive reviewed here.
“Even though Utah is a very dry state, we have a lot of landslides. A lot of it has enough steep topography, enough precipitation, and we certainly have enough weak geologic units,” Rich Giraud, a retired UGS geologist, said in a 2021 webcast sponsored by the US Geological Survey.
According to Jeff Moore, a geology professor at the University of Utah, Utah should expect above-average landslide activity this spring, with both new and old landslides being reactivated.
“These types of slow-creeping earth currents are very responsive to water intrusion,” Moore said. “And if we have above-average water input, we can expect above-average displacement.”
He assumes that the movement speed on existing slides will increase measurably.
“Similarly, we can expect that some landslides that have been dormant for many years may now be reactivated,” Moore said. “We can definitely expect new landslides in areas where we have steep slopes and loose soil. It is certainly more difficult to predict where these will occur.”
After most severe winters, Utah experiences a surge in activity, sometimes with tragic results when saturated soil unleashes fast-flowing streams.
In terms of sheer destruction, nothing quite compares to the thistle slide of April 1983.
After an extremely wet winter 40 years ago, nearly 20 million cubic yards of soil slid a mile and a half into the headwaters of Spanish Fork Canyon, destroying US Highway 6 and a railroad line. Debris blocked a creek, which dammed and flooded the town of Thistle, which had to be abandoned.
At the time, Thistle was considered the costliest landslide in US history.
Unlike creeping landslides like Springhill, the mudflows created by rapid snowmelt or heavy rains are particularly common in burn areas.
“The slope would just become so saturated that the soils would just mobilize into what we call an earth flow or a debris flow, depending on what the material is made of,” McDonald said. “They happen very quickly. They deposit in flooded creeks and this has caused many debris flows in Weber and Davis counties [in the 1980s]. These are much more difficult to predict where and when they will take place.”
The spring of 1983 saw a “perfect storm” of a cold, snowy spring followed by a heat wave.
“Suddenly in mid-May everything started to heat up and the snowmelt kicked in,” McDonald said. “It’s almost like dumping inch by inch water down a slope. We’re not in that mode yet this year. We’re slowly melting the snow. What we’re hoping is that it’s not like a 1983 scenario where it melted away very quickly.”
Utah’s last round of punishing landslides occurred in 2005 and 2006. Several subdivisions that had been deemed safe during geological surveys prior to construction proved to have unstable ground.
Meltwater from heavy snowpack reactivated many old landslides, like one that invaded Cedar Hills on April 28, 2005 and destroyed at least one home, according to a 2007 report by the Utah Geological Survey.
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A few weeks later, on May 12, a boulder broke off a cliff on Mount Y, dislodging other rocks that plunged a mile into Provo. A 13-ton rock hit a guest house after falling 2,600 meters.
“No one was home at the time, but the structure was a total loss,” the report said. “Many of the fallen stones left impact craters (crack marks) and traces of flattened oak tufts on the slopes at the base of the cliff and on the slope directly above the damaged house.”
Potentially destructive landslides were observed earlier this spring at Mountain Green, Layton, Fruit Heights, and City Creek and Emigration Canyons in Salt Lake City.
The Springhill neighborhood began to show signs of trouble in the 1990s, as sidewalk cracks opened, walls fell out of alignment, and fences toppled. These properties were purchased under a federal grant, the homes removed and the site converted into the Springhill Geologic Park which is permanently closed to future development.
A path meanders through the park, taking visitors over the headscarp where the hill broke off and the slide began. You can clearly see where the ground has retreated leaving a crack known as a ditch. Older trees on this soil lean towards the slope while young ones grow straight up, a sure sign that the earth is in motion.
Looking at this scene, State Geologist Bill Keach emphasized the need to monitor Springhill, one of about 100 slides UGS monitors.
“It’s important that we understand where the dangers lie so you don’t make a bad decision,” said Keach, who himself lost a home-to-earth movement to Hurricane. “We can provide really good intelligence to stay away from these dangerous areas. The last thing you want to see is your house falling down.”