Research reveals new details of fatal event, potential risks
The size of the landslide that took three lives this spring on the north side of Grand Mesa was remarkable in the relatively recent history of the mountain, the U.S. Geological Survey said.
A research project headed by Jeff Coe, a research geologist who has studied landslides elsewhere in Colorado and around the world, found that the slide moved in at least five phases when it ran down from the top of the mesa at a speed of about 50 mph on May 25.
Three men — Wesley Hawkins, Clancy Nichols and his son, Dan Nichols — died while they were working in the slide’s path.
“If we look over geologic time, there have been bigger slides,” said Jonathan Godt, landslide hazards program coordinator for the Geological Survey office in Golden, “but in human time, this slide was really big.”
Dirt and debris clinging to the basalt cap of the mesa was already soggy from melting snow when it was pelted with rain falling at a rate of about eight-tenths of an inch per hour for half an hour, setting off a 40-million-cubic yard avalanche of mud, rock and debris down an existing rockslide for more than three miles, overtopping two ridges as it swept down.
“The chronological sequence of movement was complex, with at least 5 phases: a debris flow; the catastrophic, high-energy rock avalanche, including the rotational rock slide; movement of the hummock-rich, central core of the avalanche deposit; a second debris flow; and ongoing movement of the upper central core,” the abstract says of the slide.
Early movement was noted between 9:30 a.m. and 2 p.m., when residents reported that their irrigation was disrupted and saw trees falling on the face of the old rockslide, the abstract says.
“Seismic data indicate that catastrophic movement began at about 3:44 p.m. and lasted about 3 minutes,” the abstract said.
A “large, back-rotated, rock-slide block remains at the top of the avalanche,” holding back a lake, the abstract says.
There is some concern about the stability of the lake, though it’s unlikely that water will cascade over it this year, Godt said.
Continuing threats include the catastrophic failure of the rock-slide block, a large failure upslope from the headscarp, a rapid release of water from the lake, and rapid or slow movement of the avalanche deposit, the abstract said.
The slide itself is relatively stable, having settled into a flatter area than the steep slopes from which it tumbled away, Godt said.
Researchers hope that continuing study of the slide will yield clues that can prevent tragic losses such as the loss of the three men, who were believed to be working on clearing an irrigation ditch when they were swept under.
Hawkins, manager of the Collbran Conservancy District, was remembered Friday by the Colorado Water Congress.
Coe will present an abstract of the research in October at a meeting of the Geological Society of America in Vancouver.