Colorado Landslide Prompts Flood Hazard Evaluation by State Dam Safety Officials

In May of 2008, local emergency management officials contacted the Dam Safety Branch of the Colorado Division of Water Resources regarding a landslide on the East Fork of the San Juan River upstream of the City of Pagosa Springs. Emergency management personnel recognized that the slide had the potential to form a landslide dam on the East Fork, which would pose a significant flooding hazard to downstream populations including the City of Pagosa Springs. In response to the request for assistance, State Dam Safety Engineers prepared a Flood Hazard Study to provide emergency management personnel with a basis for assessing the threat as the situation continues to develop. The landslide in question is an old slide, which accelerated dramatically in May of 2008. As a result of the accelerated movement, a natural gas pipeline was ruptured, alerting authorities to the changing conditions. Upon recognition of the recent developments, monitoring of the landslide was initiated, and the mass was found to be progressing into the East Fork of the San Juan River at a rate of 4 feet per day. Fortunately, the sediment carrying capacity of the river was sufficient to erode the deposited material and the formation of a landslide dam did not occur. The rapid increase in creep rate did, however, prompt concerns over the potential flooding hazard that would result in the event of the formation of a landslide dam and subsequent breaching. Recognizing the unpredictable nature of landslides, Colorado Dam Safety Officials considered a broad spectrum of dam sizes for assessment in the Flood Hazard Study. Engineers considered peak breach discharges ranging from 15,000 cfs, which approximates the 100-year flood in Pagosa Springs, to 150,000 cfs, the expected discharge associated with a 150 foot high landslide dam. Dam Safety Engineers and GIS specialists collaborated to develop hydraulic models and floodplains for multiple breach scenarios thereby providing emergency management personnel with a tool to assess the flood hazard for all foreseeable conditions associated with the formation of a landslide dam at this location. The landslide location is approximately 15 miles upstream of the City of Pagosa Springs, a considerable reach length to be considered for hydraulic modeling. To expedite the process of hydraulic modeling and floodplain mapping for the multiple profiles considered, engineers worked with GIS specialists using HEC-GeoRAS. The software provides an interface between HEC-RAS and ArcGIS, allowing for the automated development of HEC-RAS geometry and floodplain mapping. Using this approach, State Dam Safety Officials were able to quickly develop a Flood Hazard Study with an unlimited shelf-life to assist emergency management personnel in assessing the ongoing situation. 7 pp., 7 references.