Geologists' report on Riverdale landslide details a dangerous situation

Thursday , December 14, 2017 - 5:00 AM

MARK SAAL, Standard-Examiner Staff

RIVERDALE — When a section of the bluff overlooking the Weber River gave way on Nov. 19, eventually requiring the evacuation of three homes, it was anything but your typical Utah landslide.

The Utah Geological Survey reports that the Spring Creek Road landslide — so named for the road at the base of the bluff — was a “water-rich, quickly moving debris slide.” Within seconds, a 650-foot section of the bluff gave way, sending between 4 and 7 feet of mud and debris onto pasture land below.

By contrast, most landslides in Utah are of the slow-moving variety.

RELATED: 3 Riverdale homes under mandatory evacuation as land continues to crumble below

“Most we see along the (Wasatch) Front are slow-moving landslides,” said Gregg Beukelman, a landslide hazards geologist with the Utah Geological Survey. A typical one can move anywhere from a foot per year to 100 feet in 15 minutes.

“That would be on the border of a fast-moving landslide,” he said of the latter. “So this was anything but a slow-moving landslide.”

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Beukelman and other geologists with the Utah Geological Survey recently sent a 12-page letter to Riverdale City with their observations and recommendations for the slide area going forward. The UGS made five field visits to the slide site between Nov. 21 and Dec. 5.

The area around the Nov. 19 landslide continues to be extremely dangerous, geologists said.

“The hazard posed by the 2017 Spring Creek Drive landslide should not be underestimated,” geologists state in the letter, blaming “easily erodible” silty sand soils in the upper two-thirds of the bluff that can quickly transform into a fast-moving mud slurry or debris flow.

“Based on eyewitness accounts by others, these movement episodes lasted 10 to 20 seconds, which would prevent anyone within the landslide boundary, or on part of the failed material, the time to escape, likely leading to death or very serious injury,” according to the report.

Geologists believe the top portion of the bluff is an overly steepened slope that is unstable, and future landslide movement along the scarp is “likely.” And, once the initial landslide occurred, the area became even more susceptible to future movement.

Story continues below photo gallery.

The three evacuated homes on 600 West are at the greatest risk, but geologists also say more movement may cause the landslide to enlarge across the slope, affecting additional lots.

On Nov. 30, UGS installed seven movement monitoring points above the bluff. Beukelman says thus far they’ve seen no conclusive evidence of landslide movement between Nov. 30 and Dec. 5. However, he stresses that it’s still early in the monitoring process, and this doesn’t necessarily mean landslide activity has stopped.

“It could be a very dangerous slide,” Beukelman said. “The upper half of that is nearly a vertical face, and things don’t want to lay vertically.”

Riverdale City Administrator Rodger Worthen said his city is closely watching the slide area.

“Right now it continues to be a process of monitoring and gathering information, then we can make decisions,” he said. “Up to this point it’s been a public safety response to the slide. Now we’re looking at ‘What’s its potential?’”

To that end, Worthen says the city is in the process of acquiring a work proposal for a geotechnical survey of the slide. The survey won’t be cheap, but Worthen says it’s necessary to know what they’re dealing with.

A geotechnical investigation — to understand the geologic materials and groundwater conditions affecting the slide area — was one of nine recommendations offered in the UGS report to the city. Also among the UGS suggestions are the following:

• Continued monitoring of water, sewer and storm drain lines to ensure utilities aren’t contributing water to the slide area.

• Additional fencing and signage along the sides of the landslide “at least one lot width away,” prohibiting access to the immediate landslide area.

• The possible installation of inclined horizontal drains to reduce groundwater and flow along the slope.

• Stabilization of the slope using a variety of methods, such as a “soil-nail launcher and resulting ground surface treatments.” A soil-nail launcher, according to Beukelman, basically shoots long pieces of rebar into the side of a slope in an effort to stabilize it.

• Preventing surface water from flowing near or over the bluff edge. Roof downspouts on structures should be piped away from the bluff and into storm drains.

• Eliminating excessive landscape irrigation. “People should know that excessive irrigation can easily cause a neighbor near or on a slope to lose their home from a landslide,” according to the report.

Beukelman says while it isn’t feasible to reduce the amount of water in the area from naturally occurring springs and other sources, it is relatively easy to reduce groundwater from excessive landscape irrigation.

Geologists also recommend communication and coordination between Riverdale and Washington Terrace, as the watershed contributing to this landslide groundwater issue extends upslope far beyond Riverdale city limits.

And the soil in this area is part of the problem, according to Beukelman. Roughly the top two-thirds of the 200-foot bluff is “basically sand you’d find in a sandbox,” which allows water to pass through very easily. Underneath that is a layer of clay soil, which doesn’t let water pass through it. So all the groundwater anywhere near that bluff — from natural springs to stormwater runoff to lawn irrigation to leaky water lines — filters down through the sand and collects atop that clay surface, looking for a place to go.

The fact that there is so much water in the area is particularly troubling to geologists. Beukelman says the first thing they noticed in the aftermath of the slide was the amount of water in the slope. 

“Anything that’s adding water to the system is a bad thing,” Beukelman said. “The question becomes, ‘Where does that water come from?’ And the reason you ask that is so you can answer the question of what you can do to minimize that water.”

One reader contacted the Standard-Examiner to report they saw what they considered a sizable leak in a Washington Terrace water line on the day of the landslide. But Steve Harris, director of public works for that city, disputed that report.

“The resident who called that in didn’t know what she was talking about,” Harris said. “She saw that water there, and it’s been there for years, depending on the season. It’s a long-standing issue. But (the leak) was a long ways away and had nothing to do with the landslide.”

Harris said the city has long had a “slow, off-and-on” leak in a water line on 5200 South at 200 West, which he said is a good half-mile from the slide site.

“When I mean slow, it’s probably — well, for sure it’s less than a half-gallon a minute,” he said. “Not even that. Probably 10 to 15 gallons a day.”

Washington Terrace City Manager Tom Hanson said in the aftermath of the landslide he got a call from Riverdale City asking him to check for any water leaks in their municipal system. Hanson said they found no appreciable leaks.

“The amount of water it would take to make that kind of landslide happen is much greater than what we had leaking,” he said.

Hanson, like the geologists, points out that there are plenty of sources contributing to the groundwater in that area.

“When you look at that aquifer, you’d better start at the top of Ogden Peak and work your way west,” he said. “It’s all groundwater — that whole area is listed as sensitive land.”

Hanson points out there are several elevated terrace levels at the city’s Rohmer Park, and all are the result of ancient sloughs in the area. Hanson simply doesn’t trust the stability of that entire bluff’s edge.

“Frankly, that’s part of why we’re moving our shop from Rohmer Park to a new public works facility just south of the library,” he said.

Worthen says the city has been in touch with various government agencies — including the Natural Resources Conservation Service and the local arm of the Federal Emergency Management Agency — to see about some sort of financial assistance for the three homeowners displaced by the slide taking parts of their backyards.

However, those government wheels turn slowly, and Worthen guesses any possible help is a year or two down the road.

Worthen also hinted there might be a possibility — albeit remote — that one or two of the homes could eventually be occupied again.

“I don’t want to give a false sense of hope,” he said. “But perhaps, depending on the geotechnical report, there is that potential. But that’s going to be sometime down the road.”

In the aftermath of the Nov. 19 landslide, Worthen said his city is also looking at adopting changes to its hilltop ordinance. The current law only covers new construction proposed in and around hillside areas. Changes to the ordinance would govern activities for existing residents at the tops and bottoms of steep slopes.

Worthen said there are multiple potential sources of water on the bluff, and they’ll be looking at all of them in considering changes to the hilltop ordinance. For example, some residents have swimming pools. When they drain those pools, Worthen asks, where does the water go?

“It could be a leak, it could be watering grass, it could be how roof structures divert water,” he said. “The bottom line is, all of these activities can affect the hillside, and we’ll look at all of that.”

Beukelman suspects monitoring of the slide area will continue for the foreseeable future.

Contact Mark Saal at 801-625-4272, or msaal@standard.net. Follow him on Twitter at @Saalman. Friend him on Facebook at Facebook.com/MarkSaal.

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