| Description |
Rock avalanches are high-magnitude, low-frequency mass wasting events characterized by high mobility and fluid-like runout motion. Yet, little information is typically available to describe the hazard posed by these events because of their scarcity. Geologic records thus provide key data regarding rock avalanche metrics, such as size, timing, and dynamics needed to characterize the hazard. In this study, we present a detailed case history analysis of the Devils Castle rock avalanche located near the town of Alta in the Wasatch Mountains of Utah. The deposit is approximately 1.5 km in length with a Fahrboeschung angle of 14°. Through topographic reconstruction, we calculate a deposit volume of 1.7 million cubic meters, with a maximum thickness of 25 m and an average thickness of 7 m. We estimate a bracketed age of occurrence as 11,000 to 16,000 years old from limiting radiometric and cosmogenic exposure ages. The Devils Castle headwall is complex with no obvious evidence to indicate the precise source location and geometry. Therefore, we reconstructed two plausible source areas and ran 3D numerical runout simulations for each. Results agree well with mapped deposit boundaries for both source scenarios. However, the east source model better represents material and dynamic characteristics of the deposit (e.g., lithology, superelevation) observed in the field. The rock avalanche is located near the seismically active Wasatch fault zone, and we identified five additional, similar events in the region highlighting the extent of the potential hazard. Individual case history analyses such as this allow us to better understand the processes and controls of similar large-scale mass movements, and ultimately evaluate the risk posed by these extreme events. |
