Slide risks uncertain because of limited mapping efforts
May 27, 2014
Garfield County shares some of the same geologic characteristics as the remote part of neighboring northeast Mesa County where a massive landslide covered an area 3 miles long and a half mile wide and left three people missing, but the risks are not fully assessed.
Unlike the Mesa County area, which was known to have a history of slide activity dating from ancient times up through the mid-1980s, similar formations have not been fully mapped out, according to Karen Berry, state geologist with the Colorado Geological Survey.
"In a lot of these areas we don't really know what the landslide susceptibility is, because a lot of it hasn't been mapped," Berry said Tuesday as the damage from the slide that occurred on the northern flank of the Grand Mesa was still being assessed.
Authorities on Tuesday called off the search for three men who disappeared in the slide area. They had gone to check on problems with an irrigation ditch caused by an initial, smaller slide. Mesa County Sheriff Stan Hilkey said the slide remained too unstable to continue looking for the men.
Berry said mapping is key to determining where the potential is for large landslides or debris flow activity to occur. The mapping helps geologists recommend precautions when it comes to land development in those areas.
No structures were lost in the slide that occurred Sunday, though it did stop just short of a natural gas well pad, prompting the operator to shut down three wells and to monitor other wells in the area for any problems.
"In the 1970s and '80s there was quite a bit of funding and legislation to do mapping, but that degree of funding isn't available now," Berry said. "We do continue to map these critical areas, it's just at a slower pace."
The Salt Creek area east of Collbran where Sunday's slide occurred is defined by weak shale and sandstone formations, which became saturated with spring runoff and heavy rains. Eventually, the entire hillside gave way, Berry explained.
"These events are most prevalent in late spring and early summer when you have a lot of ground water, but we haven't seen a large event like this for several decades," she said. "In this case, we had an entire failure of the bedrock surface, which liquefied and turned into a rapid slide."
Similar rock formations can be found on either side of Interstate 70 in south-central Garfield County along the Mesa County line.
"The closest area people can identify with is in De Beque Canyon, where you have the same geologic units and frequent slide activity," Berry said.
A formation above Glenwood Springs that is prone to surface saturation and occasional mud flows, the alluvial fan beneath the northern flank of Red Mountain, is different, she said.
"The activity you see there is more debris-flow related," she said, adding that such events are usually triggered by a single rain event. "You can get slope failures, but it's shallower and more concentrated along the drainages."
The Salt Creek slide was due to saturation over time as the heavy snowpack on the Grand Mesa melted, and originated much deeper, Berry said.
Geologists do take into consideration geologic hazards including slide-prone areas when any type of surface development or resource extraction activity, such as natural gas drilling, is proposed, Berry said.
The gas industry also accounts for geologic activity when it comes to determining where to locate well pads, pipelines and other facilities, said David Ludlam, executive director for the West Slope Colorado Oil and Gas Association.
However, a major event such as the one that occurred Sunday is hard to gauge and account for, he said.
Three wells operated by Oxy Petroleum were shut off remotely as a precaution when the slide came close to one of its well pads, though none of the wells was damaged, Ludlam reported on Monday.
"What has happened since then is because the slide is still taking on water and additional instabilities are likely, our producers are working with the USGS and state regulators to develop a shut-in plan," he said.
Any wells that might be affected by additional slide activity can be taken off line from remote monitoring sites operated by the companies, he said.
In the case of Sunday's mudslide, a collector pipeline serving wells in the area was also depressurized, Ludlam said.
Berry said there are often warning signs if a hillside has become saturated and is about to give way.
The sound of cracking trees or excessive rockfall activity is one sign, she said, "and sometimes you'll see muddy water coming from seeps that hadn't been there before."
"Those are some signs, and you just have to pay attention to them," Berry said.