Colorado's Comprehensive Dam Safety Evaluation process uncovers long history of dam safety concerns and prompts immediate action at Marshall Lake Dam

摘要

Recent past annual dam inspections at Marshall Lake Dam near Boulder, Colorado had been rather mundane and uneventful. From external appearances, the 75 foot (23 meter) tall earth embankment dam was stable and well maintained. However, a review of the movement monument data during the 2017 routine annual dam inspection revealed progressive movement resembling that of a rotational slide. This revelation prompted a detailed analysis of the dam using Colorado's recently developed Comprehensive Dam Safety Evaluation (CDSE). The CDSE is a semi-quantitative risk analysis tool based on the Reclamation and U.S. Army Corps of Engineers risk analysis process. The CDSE process was developed by State of Colorado Dam Safety engineers (Colorado Dam Safety) to consistently identify risk driving failure modes and slowly developing and latent dam safety threats in existing dams while meeting the needs and limited resources available to Colorado Dam Safety. Application of the CDSE process to Marshall Lake Dam revealed a long history of dam safety concerns previously unknown to current dam safety engineers, including 4 significant slope failures that dropped the dam crest up to 17 feet (5 meter) in two separate locations, the presence of an abandoned 30 inch (76 centimeter) outlet pipe extending partway through the dam, and evidence suggesting that coal mine shafts extend fully through the dam foundation. The CDSE process and follow-up slope stability analysis galvanized the need for immediate action despite very little visual evidence of movement that would otherwise have been disregarded. This paper will include an overview of the CDSE process with a focus on the specific application to Marshall Lake Dam.