CALIBRATION OF MODELING PARAMETERS FOR A LARGE LANDSLIDE, WEST SALT CREEK, MESA COUNTY, COLORADO

On June 1, 2014, a 29 million cubic meter landslide occurred in a sparsely populated area in western Colorado. Tragically, three men were killed when checking irrigation ditches disrupted by earlier movement of the slide. The landslide reached high 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 failure scarp, and a block roughly 15 million cubic meters in size dropped 100m vertically. This formed a back-tilted block that impounded a growing lake, creating a new landslide hazard. Rapid analysis of the stability of this block was crucial, but the size, stratigraphy, and ground-water conditions were not well constrained. Drilling investigation would be dangerous and nearly technically impossible. Therefore, strategies were implemented to narrow the range of unknown values using back-calculation on pre-slide topography. These values were then used for forward modeling of the current stability of the back-tilted block. Back-calculation modeling was iterated by varying strength and ground-water parameters while analyzing multiple cross-sections to reduce uncertainty. Forward modeling analyzed a range of lake levels to evaluate conditions anticipated during Spring snowmelt runoff next year. The results show that even when dealing with large landslides in inhomogeneous materials, enough constraints can be incorporated to support decision-making and to evaluate the effects of changing water conditions.