Park County, CO

Tarryall Dam Safety Improvements

Client: State of Colorado, Division of Water Resources
Our Role: Design, Engineering

The Tarryall Dam is situated within the 711-acre Tarryall Reservoir State Wildlife Area near Fairplay, Colorado. Constructed between 1929 and 1931, the dam creates the 175-acre Tarryall Reservoir, a popular site for birding, wildlife observing, boating, camping, and fishing.

The high-hazard dam is a single curvature concrete arch structure standing 70 feet high and 263 feet long. It required measures to mitigate possible abutment and toe erosion identified during a potential failure modes analysis (PFMA) workshop. A PFMA is a systematic evaluation to identify potential ways a dam might fail and prioritize the most critical safety risks.

During the design hydrological event, known as the probable maximum flood (PMF), overtopping of the dam will occur. The PMF is the most significant flood that could conceivably occur at a particular location, and it is used to design critical infrastructure to ensure maximum safety.

The dam’s structure is designed to withstand such an event, except for the identified abutment and toe erosion concerns. If left unaddressed, these issues could lead to an uncontrolled reservoir release. Additionally, expert counsel was needed to address concerns related to joint leakage and areas of deteriorated concrete.

Our dam safety engineers developed a comprehensive solution involving two advanced 3D computational fluid dynamics (CFD) models to simulate flows overtopping the dam through the rock spillway and dual 42-inch steel low-level outlet pipes. These models are critical in informing the design and construction of new structural elements and erosion protection measures.

Based on CFD modeling results, we are implementing detailed analyses and construction measures to protect against potential scour downstream of the dam. A parapet wall is also being constructed to address overtopping flows and prevent erosion at the left abutment. Rehabilitating the dam’s joints enhances its structural integrity and extends its lifespan.

Our team is working with the owner to implement an alternative delivery method, specifically early contractor involvement using the construction manager/general contractor (CM/GC) process. This process condenses the overall project schedule, including the rehabilitation construction, into a single construction season.

Key Features

Two 3D CFD models.
Rehabilitation of critical dam components.
New parapet wall to safeguard against overtopping.
Alternative CM/GC project delivery.

Outcomes

Structural deficiency mitigation and erosion protection measures improve the dam’s safety and stability.
The rehabilitation efforts extend the dam’s operational life, ensuring it will continue to serve the community for future generations.
Maintaining and improving the dam and reservoir preserves and enhances the area’s environmental and recreational value, benefiting wildlife and the local community.