North Michigan Creek Dam Rehabilitation

North Michigan Creek Dam, nestled in the jagged peaks and thick, wooded areas of State Forest State Park, is a hidden gem in northern Colorado. The creek’s waters overlie glacial deposits, attracting anglers keen on catching kokanee salmon, Arctic char, and rainbow trout. Built in 1963 by Colorado Parks and Wildlife (CPW), the 75-foot-high and 400-foot-long embankment dam sits at an elevation of nearly 9,000 feet and has a storage capacity of more than 1,100 acre-feet at normal pool.

CPW, which owns over 120 dams, regularly monitors, inspects, and improves its assets. During a routine dam safety inspection of the North Michigan Creek Dam, CPW found that seepage levels from the right abutment exceeded acceptable limits, thus prompting a declaration of emergency conditions and a restriction of the reservoir’s water level. The situation underscored the urgent need for a comprehensive rehabilitation strategy to ensure the dam’s integrity and functionality.

GFT provided design engineering services for Phase 1 of the rehabilitation program to address the dam’s seepage issues and enhance its structural resilience.

Phase 1 design began with updating the dam breach analysis, as required by the state’s dam safety program. Working closely with CPW engineers and state park staff, GFT completed hydrologic analyses using the then-newly released 2018 Colorado-New Mexico Regional Extreme Precipitation Study data. Estimating probable maximum precipitation is critical to designing spillways that protect dams against extreme weather events.

Historically, the dam had been classified as a low-hazard potential dam, meaning minor damage to non-critical infrastructure could be expected in the event of dam failure. However, an updated dam breach analysis by GFT indicated that several roads and access to private residences would suffer damage, increasing the dam’s hazard potential. As a result, the dam was reclassified as a significant-hazard potential structure. Deficiencies were also identified in the existing concrete chute spillway and earthen embankment.

Rehabilitation measures included:

• Replacing the concrete-lined spillway, which sat on an erodible foundation.
• Replacing the intake structure using a slide gate and operator grade beam.
• Replacing the pedestrian bridge.
• Installing a cured-in-place pipe lining and filter diaphragm for the low-level outlet pipe.
• Improving the outlet channel.
• Installing a right abutment seepage collection system, including instrumentation for monitoring.

Recovered materials, such as demolished concrete from the spillway and intake structure, were recycled to reduce environmental impacts and used to develop a fish habitat within the reservoir.

Mid-project, CPW opted to convert the project delivery from design-bid-build to construction manager/general contractor (CM/GC), making this the first-ever application of alternative delivery for a Colorado Department of Natural Resources construction project. The CM/GC delivery enabled the team to streamline the design-construction approach into a construction package that could be completed in a single season. The project schedule and budget were maintained despite the changes in the delivery method.

GFT conducted a potential failure modes analysis (PFMA) focused on the proposed solutions of the dam rehabilitation design. After an initial potential failure mode screening, the credible failure modes were carried forward to a PFMA workshop facilitated by GFT. This risk-based approach led to a more complete understanding of the downstream consequences and risks associated with specific potential dam deficiencies and helped prioritize analyses, actions, and alternatives that reduced the highest risks.

During Phase 2, GFT provided quality assurance during construction to ensure adherence to design specifications. As engineer of record, we provided full-time, on-site quality assurance, managed submittals, proactively resolved construction issues, and developed as-constructed drawings.