The Colorado River Water Conservation District’s board of directors has approved a contract with an engineering firm to address problems with a dam that are turning out to be worse than previously thought.
At its second quarterly meeting, held in April, the River District board agreed to pay $323,840 to HDR Engineering to further study the movement and potential cracking at the district-owned Ritschard Dam. The dam forms the 66,000-acre-foot Wolford Mountain Reservoir across Muddy Creek, about 5 miles north of Kremmling in Grand County. Muddy Creek is a tributary of the Colorado River.
River District staff, aware since 2008 that the dam is settling and moving more than expected, have been monitoring the situation. However, a 2020 Comprehensive Dam Safety Evaluation prepared in December by HDR Engineering for the state’s Dam Safety section of the Division of Water Resources found that the risk of internal erosion of the dam due to cracking had increased from a 2016 evaluation. That year’s evaluation estimated the chances of a dam failure at 1 in a billion in any given year; the 2020 report found a 1.5-in-10,000 chance of a dam failure.
A crack causing internal erosion is the primary driver of the risk of dam failure.
“It is currently expected that the core will crack at some point, if it has not already done so,” the report reads. “Although a deep crack through the core would represent a severe defect and a serious dam safety incident that significantly compromises the dam’s ability to store water, formation of a crack does not necessarily mean the dam would breach.”
Ritschard Dam has an impermeable clay core that is covered on the upstream and downstream sides with rockfill. Because the rock-fill is poorly compacted, the dam’s outer shells are still moving, especially on the downstream side. The 122-foot-tall dam was built for the River District in 1995 by D.H. Blattner and Sons of Minnesota. The cost was $42 million.
According to the report, normal reservoir operations that involve cycles of drawdown and refill appear to have a detrimental effect on the deformations. Even if the dam does not breach, there could still be very serious incidents that necessitate emergency actions and downstream evacuations or a reservoir restriction.
“The probability of a serious dam safety incident will only increase over time as deformations continue, and therefore there is urgency in taking actions,” the report reads.
The report said risk of dam failure at Ritschard is about the same as the historical failure rate for dams built prior to modern dam safety. Generally, a dam designed and constructed in the 1990s should have a much lower risk of failure than the historical rate.
The River District has been monitoring the dam with instruments, but HDR Engineering will help identify areas that could benefit from additional inclinometers, which measure slope angle, and piezometers, which measure underground water pressure.
“First and foremost, the River District puts public health and safety as our number one priority always, and every action we take is with public safety in mind,” said River District chief of operations Audrey Turner, who is acting as spokesperson on Ritschard Dam matters. “The River District has and will continue to increase our monitoring and emergency preparedness at the dam as recommended by the report.”
The River District also will use a LiDAR survey program — which utilizes lasers for remote sensing — to track and visualize dam deformation, stockpile emergency materials onsite such as gravel and riprap and is planning an exercise for the fall that will improve the community’s emergency preparedness.
It’s still unclear whether or when the dam will need to be rehabilitated; that’s what adding more monitoring instruments may help the district figure out.
“There are still some other areas of exploration and additional information before any decision is made toward the rehabilitation of the dam,” Turner said.
Runoff season operations
The dam deformation won’t stop the reservoir from filling this runoff season. The plan is to still fill the reservoir, as long as the drought conditions allow. Turner said the River District also will still be able to fulfill all of its contract demands for water later in the summer.
Dam Safety has signed off on the district’s operation plan, which calls for letting Denver Water make releases from Wolford instead of from other Western Slope reservoirs in order to get the reservoir level down to 10 feet below the crest as soon as possible after it fills with runoff.
“The plan looks well thought out, and we appreciate the proactive steps taken to continue to monitor conditions and toward emergency preparedness,” Bill McCormick, chief of Colorado’s Dam Safety Branch, said in an email to the River District.
Denver Water leases 40% of the water in Wolford. After 2020, the Front Range water provider was supposed to have become the owner of that water. But the deal is off, at least temporarily, while the dam’s problems are studied.
“The River District and Denver Water have temporarily postponed that transfer of ownership to allow the parties to conduct further study related to the risk-assessment recommendations,” Turner said.
Denver Water spokesman Todd Hartman said the two entities mutually decided to extend the lease temporarily while they determine the next steps.
“We were supportive of the 2020 risk assessment and shared some costs of that process along with the expertise and guidance of our engineering team,” Hartman said in an email. “We’ve continued to consult on the path ahead and will remain engaged in helping to develop and guide the upcoming engineering study.”