From The Sky-Hi Daily News (Lance Maggart):
Ritschard Dam is settling but despite concerns plans for remediation have been halted, with local officials explaining a recent expert review of the dam’s structural viability left them feeling more confident about the long term prospects for the barrier.
Ritschard Dam is an earthen dam north of Kremmling. It was constructed in 1995 and since then officials from the Colorado River District have nervously watched as the structure slowly settled and shifted. While officials have been quick to point out the dam poses no safety risks, and has not been placed under operational restrictions, for the past several years ongoing research and discussions have revolved around what needs to be done to stop the settling process.
During a State of the River meeting held at Mountain Parks Electric in late May officials discussed the current state of the dam and new plans the Colorado River District has to monitor movement in the dam and conduct minor rehabilitation work.
THE RUB
John Currier, Chief Engineer for the Colorado River District, spoke to attendees at the State of the River meeting specifically about Ritschard Dam. “All embankment dams settle,” Currier said. “But designers anticipate how much a dam ‘should’ settle. This dam embankment is 140 feet high. It ought to settle about one foot.”
Currier went on to explain how, in 2009, the River District realized the dam had settled more than one foot by that point, reaching close to 1.5 feet of settlement as well as a slight bow in the center of the dam downstream. The dam continues to settle, at it’s highest points, at a rate of about one inch per year. The center of the dam has, thus far, bowed approximately eight inches downstream.
“We decided we ought to get to the bottom of this and find out why it settled more than anticipated,” Currier said.
Over the intervening years the Colorado River District spent close to $1.5 million on instrumentation and analysis of Ritschard Dam, as officials worked to develop a plan to rehab the structure. Officials discovered the additional settlement of Ritschard was caused by a lack of consolidation in both the upstream and downstream shelves, which hold the true core of the dam.
After officials from the River District determined a cause for the settlement of Ritschard they began considering fixes, which were estimated to cost in the $20 million range.
EXPERT ANALYSIS
“At that time we said, ‘let’s employ an expert peer review panel to kind of look over our shoulders, to look over our engineers shoulders and make sure we are doing the right thing’.” Currier said. “The experts came back and told us, ‘we are not sure you need to do anything’.”
Currier explained the team of experts, made up of former engineers from the Federal Bureau of Reclamation, told the River District that while they may not be happy with the continued settling of the dam the settlement alone doesn’t necessarily pose a significant risk. “They told us, ‘you understand the problem. What is the risk associated with the problem?” Currier said. “We started looking at it probabilistically as opposed to deterministically.”
Officials from the River District then began looking at probabilities, such as the probability of an overtopping flood on Ritschard and the likelihood of a full dam failure. An overtopping flood would require more than 80,000 cubic feet per second (cfs) of water flowing into Wolford Mountain Reservoir from the surrounding area, and even that amount of water would not necessarily overtop the dam, with the emergency spillway filling with excess water. Officials said the chance of such a scenario happening in any given year is roughly one chance in a million.
“The odds of having a dam failure, associated with ongoing movement of the dam; as a practical matter is essentially nonexistent,” Currier said. Currier went on to say that the River District now believes there is no technically demanding reason to conduct a full rehabilitation project on Ritschard.
MOVING FORWARD
The River District still has plans to restore the crest of the dam and bring it back to its original height. They will also continue to monitor Ritschard closely for continued settlement, which is expected. Currier said the River District will periodically reassess their position on Ritschard. “We will go back through a risk analysis process…maybe every five years or so.”
“The last chapter is yet to be written in this book,” Currier said. “But for the time being we have gone back to operating the reservoir full. The dam poses no real hazards to anyone.”