KREMMLING – Denver Water has taken steps to distance itself from a recent risk assessment of Ritschard Dam, which forms Wolford Reservoir five miles north of Kremmling.
The 21-year-old dam has been found to be moving slightly and settling more than normal and a risk analysis workshop was held in February by a group of experts assembled by the Colorado River District, which owns the dam and reservoir.
After the risk workshop, John Currier, the chief engineer for the River District, wrote a memo to the district’s board saying “a key conclusion” of a consultant review board is that “the dam is safe” and “there is no need for immediate action.”
Currier also wrote in the April 7 memo that the “key parties and participants” in the February risk workshop “included 1) the State Dam Safety Branch, 2) Denver Water, 3) our consultant review board, 4) our engineer, AECOM and 5) River District staff.”
But on May 2, Robert Mahoney, the director of engineering for Denver Water, sent Currier a letter critical of his April memo.
“In the memorandum, you characterize Denver Water as a ‘risk estimator’ and an active participant during the workshop,” Mahoney wrote. “Denver Water takes exception to these characterizations. At no time did Denver Water participate in the workshop as a risk estimator, nor was it ever invited to participate as a risk estimator on the panel.”
Mahoney also said that Denver Water “disagrees with characterizations in the memorandum implying that Denver Water was an active participant and that we concluded and agreed with the findings of the risk estimators. Denver Water’s role in attending the workshop was that of a concerned observer.”
Currier included the letter from Denver Water in a July 7 memo to the River District board. The memo and the letter were made public this week when the public agenda was released for the district’s July 20 board meeting in Glenwood Springs.
Mahoney raised other concerns in his letter as well.
“Based on our observations, the workshop and your memorandum only addressed the probability of a dam failure consequence,” Mahoney wrote. “While the probability of a dam failure appears low, dam failure is not the only potential adverse impact of concern to Denver Water. The probability of cracking in the core of the dam, which could reduce storage capacity, has a much greater range of uncertainty.”
Denver Water currently leases 40 percent of the water in Wolford Reservoir from the River District.
The reservoir can store 66,000 acre-feet of water and on July 14 the dam was holding back 65,240 acre-feet.
When its lease expires at the end of 2020, Denver Water is slated to become a part owner of the water in the reservoir.
“The River District will convey ownership, use and control of 40 percent of storage space and water right in Wolford Reservoir to Denver Water,” according to Jimmy Luthye, a communications specialist with Denver Water who checked Friday on the status of Denver Water’s stake in the facility.
As Jim Pokrandt, director of community affairs at the Colorado River District put it on Friday, “Denver Water currently holds a 40 percent lease. After 2019, it will be a 40 percent owner.”
Dam has issues
In his letter, Mahoney also suggested that developing a plan to fix the dam would be “beneficial.”
“According to Mr. Dick Davidson (of AECOM), cracking of the core has a 50 percent annual probability of occurrence starting in 20 years (the time criteria set for the workshop and probability estimation),” Mahoney wrote. “Given this uncertainty, it would be beneficial to develop plans now to remediate Ritschard Dam in the event of a crack.
“Further, based on the information presented at the workshop, Denver Water does not agree that Ritschard Dam is functioning as designed because no dam is designed to function with the degree of movement observed at Ritschard Dam to date.
“As addressed in the April 27, 2016 letter from Bill McCormick, chief of the Dam Safety Branch of the state engineer’s office, Ritschard Dam is in the category ‘of dams with significant issues’ and is on ‘an abnormal trend,'” Mahoney wrote.
Ritschard Dam is 122 feet tall, 1,910 feet wide, and sits across Muddy Creek, which flows into the Colorado River east of Gore Canyon. It was built for the River District in 1995 at a cost of $42 million by D.H. Blattner and Sons of Minnesota.
The dam has an impermeable clay core that is covered on both the upstream and downstream sides with rock fill, including shale rock excavated on site during construction.
In 2008, engineers working for the river district noticed the dam had settled downward by a foot-and-a-half, instead of the expected normal settling of one foot.
They decided to install monitoring equipment, including inclinometers, which measure slope angles.
Engineers for the River District have since installed an increasingly sophisticated array of monitoring devices. And they’ve verified that the dam has settled over 2 feet downward.
The dam has also moved horizontally, by 8 inches, at a location about 40 to 50 feet from the top of the dam.
Mike May, an engineer with AECOM, told the river district board in January 2015 that because of “poorly compacted rock fill,” the dam’s rocky outer shells are still moving, especially the downstream shell, and that the clay core of the dam, which is somewhat elastic, is also moving.
While the dam does not have “a global stability problem,” May said the concern is that if enough movement occurs, it could cause cracks in the clay core.
Water could then find its way into those cracks, start transporting material and widening the cracks, and the dam could eventually be at risk of failing.
McCormick, in his April 27 letter referenced by Mahoney, also included Denver Water as part of the risk assessment group.
“At the conclusion of the meeting it was the opinion of the participants, CRD, AECOM, risk analysis consultants John Smart and Larry Von Thun, Colorado Dam Safety and Denver Water that the risk of sudden failure of the dam by any of the failure modes analyzed was remote,” McCormick wrote. “It was also agreed that given that determination the need to continue to actively pursue physical modifications to the dam was not warranted at this time.”
McCormick also said that the results of the risk analysis session “now allow the Colorado River District and Colorado Dam Safety Branch to return to more normal reservoir operations with confidence that public safety is not being compromised.”
However, McCormick also noted that “due to the remaining uncertainty of the deformation behavior we agreed that Ritschard can only be classified as ‘conditionally satisfactory’ and that continuing action with respect to monitoring and observations is required by Colorado River District to operate the reservoir as planned.”
In his letter, McCormick cited a presentation at the risk workshop by Dr. Gavin Hunter, a professor at the University of New South Wales who has researched deformation behavior in 130 embankment dams.
Gavin compared the magnitude of the settlement observed at Ritschard dam with other dams in his data set.
“The displacement observed at Ritschard exceeds the majority of the dams studied, with only half the available data,” McCormick wrote. “Dr. Hunter describes this as ‘an abnormal trend.'”
McCormick also noted that Gavin’s research on the amount of settlement at Ritschard put it in the “region of dams with significant issues” category.
As such, McCormick said the River District should develop a plan for remediation work on the dam.
“We would encourage the Colorado River District to fully appreciate the abnormal and as yet not fully understood behavior of Ritschard dam and put an appropriate timeline on the ‘foreseeable future,’ McCormick wrote. “Based on the analyses done to date one could reasonably anticipate that remediation work will be necessary at some point in the future. We strongly encourage the Colorado River District to continue to plan for such remediation to avoid undue pressure on the operation of that facility as might be caused by a sudden change in the dam’s performance.”
McCormick said Friday that a workshop to develop an “action plan” has been set for the third week of August.
In his April 4 memo, Currier of the River District had written that the district’s analysis indicated that risk of failure of the dam from deformation was 1 in 100 million, while the risk of the dam failing by a “probable maximum flood” causing overtopping – a standard measure of risk – was one in a million.
He also explained why monitoring the dam’s movement was a better approach than trying to stop the dam from moving.
“With the dam failure risk so low, even with twice the current deformation, the estimators concluded that there is really no compelling technical or health, safety and welfare reason to embark on a remediation plan,” Currier wrote. “In fact, from a ‘do no harm’ perspective continued monitoring is equally if not more preferable to active remediation.
“While remediation might put to rest some nagging uneasiness about on-going deformation and when it might end, there is no absolute certainty that it would or should allay that uneasiness.
“In essence, remediation might replace one known uncertainty with a new, unknown, uncertainty. All dam owners are faced with some level of future uncertainty, we just happen to be keenly aware of it by virtue of extensive monitoring and investigation,” Currier wrote.
And in his July 7 memo sent to the River District board, Currier said an additional inclinometer has recently been installed at the toe of the dam to track movement, and that he would be sending Mahoney of Denver Water “a short response clarifying a few matters and inviting Denver’s continued involvement and expertise in the deformation issues.”