1. Department of Geology and Geological Engineering Van Tuyl Lecture Series- Spring 2015 4:00-5:00 p.m. in Berthoud Hall Room 241 Thursday, April 9, 2015 Kevin McCoy PhD Candidate, Geology and Geological Engineering Colorado School of Mines “Very Large Landslides in Modern Times: Conceptual Model Development, Slope Stability Analysis, and Multidisciplinary Research Needs for Future Hazard Management” Abstract: On May 25, 2014, an approximately 29 million cubic meter (m 3 ) landslide occurred in a sparsely populated part of western Colorado, resulting in three fatalities. The highly mobile landslide reached velocities on the order of 30 m/sec or more, and travelled several kilometers down-valley. Movement of the slide allowed headward progression of the scarp, causing an approximately 15 million cubic meter block to drop 100 m vertically, forming in a back-tilted graben that impounded a growing lake, creating a new landslide hazard. Rapid analysis of the stability of this block was crucial to disaster management planning, but the size, stratigraphy, and ground-water conditions were not well constrained. Because of the magnitude of the event, and the instability of the fresh landslide deposits, a drilling investigation would be dangerous and nearly technically impossible. Strategies were implemented to constrain likely material strength and groundwater parameters for future stability modeling by using back-calculation to estimate the strength and groundwater conditions necessary for the original landslide to occur. The resulting parameters were then used to model stability of the back-tilted block under immediate post-slide and expected future conditions as the pond filled and groundwater conditions equilibrated. The event in western Colorado occurred just two months after an approximately 7.6 million m 3 landslide occurred north of Seattle, Washington, which travelled more than 1 km burying 40 homes and killing 43 residents. Both of these landslides occurred in areas where deposits of very large landslides have been previously mapped; however, with limited quantitative age-dating, many of these large landslides are assumed to be thousands to as much as tens of thousands of years old based on geomorphic indicators and the conventional wisdom that landslides that large are unlikely in our current climate. The recently observed landslides indicate that such events can indeed occur in our current climate. Limited numerical age-dating recently performed in Washington on large landslide deposits near last year’s landslide indicate that the deposits may be significantly younger than previously assumed. Managing future hazards will require multidisciplinary research to develop a better understanding of historical recurrence intervals for these large long-runout landslides, as well as the physical, hydrologic, and climatic conditions that can trigger them. This talk will discuss conceptual model development and stability analysis for the west Salt Creek landslide, which occurred in western Colorado, and discuss various directions for multidisciplinary research to support improved hazard management in the future.