T180 Landslides and Debris Flows: Understanding Past, Present, and Future Events (Posters)

Tuesday, 11 October 2011
9:00 AM - 6:00 PM
Hall C

THE OURAY AMPHITHEATER LANDSLIDE: A PRELIMINARY ANALYSIS

Paper number: T180-12
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The landscape around Ouray, Colorado, is dominated by a spectacular U-shaped, glacial valley and an extensive amphitheater located south and east of the town. In July 2008, the County of Ouray published their Multi-Hazard Mitigation Plan to better protect the citizens and property of the County from the effects of hazardous events. Numerous potential hazards, including landslides were recognized, as the southeastern part of the town is dominated by a large landslide complex. Although the locations of various potential hazards have been mapped, little research has been undertaken to study processes. We investigated the process-response of the Amphitheater Landslide.

The amphitheater was formed from glacial erosion and an extensive, landslide. A US Forest Service Campground is located in the amphitheater on the landslide. The floor of the amphitheatre is covered with landslide deposits, glacial till, and alluvium. The body of the landslide is surrounded by San Juan tuff. The top of the crown scarp is situated at ~ 3,780 m; the landslide extends 2.25 km to an elevation of ~ 2,740 m and is 1.8 km wide. The narrowest part of the landslide is ~ 965 m. The landslide rests on the Molas Formation, which is composed of weak beds of shale. Evidence suggests that failure of the shale resulted in the landslide.

Preliminary mapping and analysis suggest a series of lobes and intermingled deposits of Molas shale, moraines and alluvium. We hypothesis that the exposed Molas shale experienced stress relief, dilation, and creep, that facilitated strain softening. This strain softening and subsequent saturation resulted in significant loss of shear strength. These processes are responsible for the large-scale landslide. Understanding the process-response characteristics in the Amphitheater will allow the County of Ouray to better plan for hazard mitigation in the future.