Garnet Mesa Dam Spillway Rehabilitation

Owned and operated by Colorado Parks and Wildlife (CPW), Garnet Mesa Dam is a high hazard dam that impounds Sweitzer Lake. The dam’s existing chute spillway needed replacement due to identified deficiencies including concrete deterioration, leakage through spillway joints, and the potential undermining of the spillway slabs. CPW selected Schnabel with the support of HDR to design a replacement spillway.

To launch the project, the Schnabel team held a 2023 semi-quantitative risk analysis (SQRA) with CPW. The SQRA identified the driving potential failure modes to be loss of the concrete-lined channel and headcutting through the spillway control section.

The project also included a geotechnical exploration (concrete coring of the existing spillway, test borings/pits) and reservoir water sampling for sulfate content. Sulfate attack was believed to be the cause of the concrete’s deterioration.

Schnabel performed hydraulic, structural, and geotechnical analyses to support the design of a two-cycle labyrinth spillway. Based on the SQRA results, the labyrinth will have the capacity to pass the 100,000-year flood. The hydraulic design included computational fluid dynamics modeling and HEC-RAS 2D to evaluate risks related to downstream erosion and provide information for HDR’s design of the instrumentation.

Instrumentation (designed by HDR) will include a reservoir water level monitoring sensor, weather station, cellular modem for transmittal of data, and a reservoir staff gauge. A water level monitoring sensor will also be installed downstream to monitor flows that may indicate excessive seepage or a developing breach. The instrumentation will be connected with an Automated Data Acquisition System designed to provide warning during flood events or other unusual reservoir-level events.