CS25A - Concurrent Session 25A: Effort Put on the Right Path: Safety Evaluation Risk Assessments & Risk-Informed Decision-Making for Maple Grove Dam in Colorado

Maple Grove Dam is a high hazard potential dam residing in a highly urbanized area, surrounded by residential and commercial development. The dam owner, The Consolidated Mutual Water company (CMWC) and Colorado State Engineer Office (CO SEO) have worked together to review the dam failure risk and evaluate a path forward for both parties. The existing dam spillway contains two crest gates that regulate reservoir levels which can convey approximately 12% of the Probable Maximum Precipitation (PMP) event. A dam rehabilitation will require significant funding, construction, and resources. The goal of this effort was to perform a risk assessment coordinated with the transparent risk-informed decision to show the dam’s dam breach consequences are associated with a low-risk designation and avoid unnecessary, substantial efforts related to dam rehabilitation, developing dam construction plans, and other associated actions. This spillway deficiency led to the development of a risk-based assessment (considering only a dam overtopping failure) using the existing condition of the dam and spillway gate operation.

The risk assessment that was conducted involved utilizing new extreme storm hydrology, existing reservoir routing via the current spillway gates operation, a risk analysis workshop, an incremental consequences analysis, the development of a risk matrix, and documenting results. A critical hydrologic hazard curve was first developed by considering spillway gates operation executed normally and applied with additional hypothetical spillway gate failure scenarios. Each failure scenario was simulated within a developed HEC-RAS 2D model with existing gate standard operation procedures (SOPs) coded within the model for each scenario. A risk analysis workshop was then conducted involving eleven experience engineers and experts to discuss potential failure initiation, progression, and each associated uncertainty, ultimately leading to the development of a failure event tree relating to an overtopping dam failure.

Based on the evaluated probabilities and estimated consequences, the risk level associated with the Maple Grove Dam (due to an overtopping failure) has been reduced from high to low. These results allow the dam owner to receive an exemption from enlarging the existing spillway. Instead, CMWC will pursue other improvements, such as optimizing the operational procedures for the crest gates and improving the existing flood warning system.