Oak Creek could save “millions” off the projected $14 million price tag for fixes at Sheriff Reservoir after updated engineering on the project showed the town’s water source needs a much smaller spillway than originally thought.

While the town previously believed the new spillway needed to be 300 feet wide, the updated work shows it only needs to be about 60 feet wide, according Steve Jamieson, an engineer with W. W. Wheeler that has been consulting for the town on the project. That is still twice the size of the existing spillway.

“We’re going to proceed forward with it with the final design for a much smaller spillway,” Jamieson said. “The result of all this really is, I think, it has saved the town millions of dollars in your final spillway rehabilitation costs.”

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Jamieson said more work will be done to determine how much savings the town will see by building the smaller spillway. Town Administrator David Torgler said while the updated work has provided the town positive news, it also revealed another hazard with the reservoir related to earthquakes that was previously unknown.

The recent work resulted from a Comprehensive Dam Safety Evaluation, which looked at ways the dam could fail during normal loading, flood loading and earthquake loading.

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The highest risk found was due to a gate failure, something that Jamieson said isn’t surprising as the town works to replace the original head gate on the nearly 70-year-old dam. Oak Creek has gone through a bid process for this work twice, but each effort failed to find a contractor the town could afford.

A gate failure wouldn’t lead to loss of life, the analysis showed, but it would compromise the town’s water source, making the impact significant.

The new risk identified is called a “liquefaction failure,” and it is related of the area’s seismic activity. While noticeable earthquakes are not common in Routt County, they are not unheard of. Since 2000, Routt County has seen approximately two-dozen earthquakes, with the largest being a 3.5 magnitude event about 10 miles northwest of Oak Creek in 2011, according to the U.S. Geological Survey.

The risk with this, Jamieson explained, is that the foundation of the dam could fail during a large seismic event.

“That means if there are saturated soils in the foundation, they kind of turn to quicksand (during an earthquake), and you can have a slope failure and the dam can fail,” Jamieson said. “This liquefaction issue was a surprise.”

He proposed the town move forward with a geophysical study this summer to further explore the seriousness of this risk, as confidence in this finding is considered poor. The study would cost about $30,000. While Jamieson hopes it could find there isn’t a significant risk and the issue doesn’t need to be addressed, the study could show the opposite.

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“We’re hopeful that it will not require additional support to the dam structure,” Torgler said.

Another risk identified would be overtopping of the dam during a significant flood event, which is where the spillway comes in.

A 2021 study by Wheeler is what identified the spillway needing to be 300 feet wide, but Jamieson said they were able to reassess this with recently updated hydrology methods. This change allowed the spillway to be designed to accommodate a flow of 5,400 cubic feet per second rather than 22,000 cfs.

Even then, a weather event that produced 5,400 cfs would be referred to as a 10-million-year storm, which state dam engineers agreed was more than adequate, Jamieson said. When recalculated to accommodate a 100,000-year storm, the spillway only needs to accommodate flows up to 2,700 cfs, allowing it to be just 60 feet wide.

Jamieson said this change also means they probably don’t have to add any improvements downstream of the dam, potentially saving more money.

After the two failed bid processes, Oak Creek will change strategies for the improvements, going with a construction manager-general contractor method rather than a typical request for proposals. Once a contractor is brought on, the contractor will work to get the town a guaranteed maximum price, which could help secure grant funding for the project.

Jamieson said he hopes to recommend a contractor to the town board in February with work to install the new head gate happening this summer. The town has already purchased the head gate equipment; Oak Creek just needs someone to install it. Nine contractors have expressed interest in the project, from Colorado and other states.

Spillway work would be done in the summer of 2024, Jamieson said.

“This is a much more attractive approach than $14 million,” said Oak Creek trustee Bernie Gagne.