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Figure 2 -- Several large rocks from the western wall of the Roaring Fork River in Glenwood Springs crashed into the houses below during the early morning causing significant damage, April 2004. Photo credit: Steve Vanderleest, used by permission.

Case Study: Rockfall – Glenwood Springs

2005-07-06 | Dr. John Hopkins

The town of Glenwood Springs in west-central Colorado lies at the confluence of the Roaring Fork and Colorado rivers. The town is tightly constrained by the steep river valleys so land-development pressure is causing residential growth to push into rockfall hazard areas. In West Glenwood, on the west side of the Roaring Fork River, the valley is rimmed with dipping sandstone outcrops of reddish Maroon Formation (Figure 1).

Figure 1 -- Valley rim west of the Roaring Fork River in Glenwood Springs looking north towards the confluence with the Colorado River. Note slumped (tilted) Maroon Formation sandstone blocks  in the exposed rock layer. Some of the rock blocks shown in this picture from 1994 have now fallen/rolled to the valley floor. Photo credit: Jon White for the CGS.
Figure 1 — Valley rim west of the Roaring Fork River in Glenwood Springs looking north towards the confluence with the Colorado River. Note slumped (tilted) Maroon Formation sandstone blocks in the exposed rock layer. Some of the rock blocks shown in this picture from 1994 have now fallen/rolled to the valley floor. Photo credit: Jon White for the CGS.

The sandstone layers are being undercut by the erosion of underlying softer siltstone and shale so that large sandstone blocks are being actively undermined and destabilized. In this area, there have been several large rockfall events from the valley rim; some that have severely damaged homes on the valley floor, 1,100 vertical feet below (Figure 2). Fortunately, there have been no injuries or fatalities. While there has been rockfall mitigation in some locations (Figure 3), the threat remains in other areas.

For more on rockfall issues around the state, see the original RockTalk: Rockfall in Colorado.

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