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Report details cause, effect of Grand Mesa landslide

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On the evening of May 25, 2014, the longest landslide in Colorado's historical record tumbled down the West Salt Creek drainage on Grand Mesa, overtopped two adjacent ridges, and buried three men: Wes Hawkins, Clancy Nichols and Danny Nichols. The men were investigating irrigation disruptions and safety hazards from smaller slides earlier in the day.

Now, a detailed report of the landslide has been released by the Colorado Geological Survey and the Colorado School of Mines. The report describes the geological history of the location, what investigators believe actually happened during the slide, current conditions in the slide area, potential hazards, and recommendations for future risk reduction. You can download the report for free from http://coloradogeologicalsurvey.org/.

According to the report, melting snow and intense rainfall likely played a role in lubricating existing weaknesses in the underlying rock to send a "rapid series of cascading rock avalanche surges of chaotic rubble" down the slope. The toe of the slide, which just missed an active gas well, is 2.8 miles from the top, where a 1⁄2 mile wide block rotated and slipped free of the northern flank of Grand Mesa. At its deepest point the pile of debris is 123 feet deep.

The slide is located about 6 miles southeast of the town of Collbran, which sits along Plateau Creek downstream from its confluence with Salt Creek. Impacts to the surrounding community, in addition to the loss of life, include irrigation disruptions, loss of grazing land, and the ongoing threat of additional slide movement and/or the sudden release of water from a pond that has formed at the top of the slide.

There was a tense moment during snowmelt last spring, when the water level in the pond at the head of the slide began rising quickly, and there were fears that it could overtop. Collbran residents were warned that they may need to evacuate, but then the water levels gradually diminished again.

The slide area is being intensively monitored, and so far there haven't been any major movements beyond the settling of the debris field.

The pond at the top remains a concern, however. Several different scenarios could cause the pond to spill suddenly, sending large amounts of water and debris downslope and downstream. These include further slumping of the block of material that created the natural dam holding the water in place; exposed bedrock above the slide falling into the pond and causing a "mini-tsunami," and simple overtopping that then erodes the bank holding the pond in place.

Alternatively, it's possible that the slide will remain mostly stable and the water will find its way out less dramatically, gradually re-establishing the stream channel. Water is already beginning to percolate through the slide and emerge at the toe.

The report notes that it is very likely that similar slides have occurred in this area in the past, and that the existing inventory of landslides from published maps doesn't capture the full story. New technology has made it possible to identify many more old slides.

The report recommends conducting a comprehensive landslide risk assessment of the greater Plateau Creek Valley and other landslide-prone areas in Mesa County, as well as limiting development in hazardous areas.

This is part of a series of articles coordinated by the Water Center at Colorado Mesa University in cooperation with the Colorado and Gunnison Basin Roundtables to raise awareness about water needs, uses and policies in our region. To learn more, go to www.coloradomesa.edu/WaterCenter. You can also find the Water Center on Facebook at https://www.facebook.com/WaterCenter.CMU or on Twitter at https://twitter.com/WaterCenterCMU.

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Grand Mesa, Landslide
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