West Salt Creek Landslide
On 25 May 2014 the longest landslide in Colorado’s historical record occurred in west-central Colorado, six miles southeast of the small town of Collbran in Mesa County. Three local men perished during the catastrophic event. The toe of the landslide came within 200 ft of active gas-production wellheads and loss of irrigation ditches and water impacted local ranches and residents. The landslide complex is referred to as the West Salt Creek landslide (WSCL) by local authorities. The fast-moving (40-85 mph – 60-130 kmph), high-mobility landslide was caused by an initial rotational slide of a half-mile-wide block of Eocene Green River Formation. The resultant rock failures, rockmass disaggregation, and mostly valley-constrained rock avalanche, dropped approximately 2,100 ft (640 m) in elevation while extending 2.8 mi (4.5 km) from head scarp to toe; 2.2 mi (3.5 km) as a rapid series of cascading rock avalanche surges of chaotic rubble composed of fragmented to pulverized rock, vegetation, topsoil, and lesser amounts of colluvium and mud. Local seismometers recorded a magnitude 2.8 earthquake from the event with a seismic wave train duration of slightly less than three minutes. Within those few minutes, WSCL covered almost a square mile of the West Salt Creek valley and the net volume displacement, calculated by digital elevation model (DEM) difference modeling, was 38 million yd3 (29 million m3). Though, its very high mobility and surface morphology normally indicate a flow-like mechanism, the rock avalanche deposits were surprisingly dry with no visible mud or water seeps when examined 24 hours after the event.
Calculated thicknesses of the rock avalanche deposits were as high as 125 ft (38 m) at the valley floor. Although precise pre-event conditions are largely unknown, geomorphic analyses of aerial photography, digital terrain models, and review of earlier landslide hazard mapping (Soule, 1988) indicate the WSCL occurred at the same location of a similar prehistoric landslide from the recent geologic past (Holocene, <11,700 years Before Present). Much of the surrounding area shows evidence of previous landslides. The failure was likely triggered by spring seasonal snowmelt in conjunction with two days of rainfall that infiltrated rock joints, fractures, and shear surfaces, increasing the pore pressure in the already disturbed and weakened marlstone and shale rock.
Currently, part of prehistoric landslide block still looms above the valley floor. The oversteepened block remnant, with an estimated net volume of 65 million yd3 (50 million m3), is back-rotated approximately fifteen degrees (15°), forming a large depression at the base of the headscarp. Runoff from a large tributary area now flows into the depression, creating a sag pond that a year later (June 2015) is almost at the spill-over elevation. Water from the pond is also continuously percolating into the landslide mass. Many ponds formed in the rock avalanche debris field and a stream from these ponds began to flow from the landslide toe in early 2015. The increased pore pressure from this infiltration poses long-term threats for additional slope instability, and mud/debris flows caused by catastrophic release of the impounded water (similar to a dam embankment failure). In May 2015, as the pond was rising, a natural outflow pipe formed at the base of the landslide block that is flowing significant amounts of water.
Geologists from the CGS and USGS conducted a landslide investigation, including field mapping. The landslide and surrounding area is being actively monitored for additional movement and changes in surface water levels by Mesa County and the USGS. The purpose of this report is to describe the geologic setting, the ground conditions that existed prior to the WSCL event, the mechanisms of the landslide failure, descriptions of potential future hazards, and guidelines for future land use in the vicinity.
Download the full B-55 The West Salt Creek Landslide: A Catastrophic Rockslide and Rock/Debris Avalanche in Mesa County, Colorado report — there are a few paper copies left or as a free PDF download.