River district frets over flawed dam above Kremmling
The Colorado River District board of directors was told last week it is time to address the steady settling and movement occurring in Ritschard Dam, which holds back 66,000 acre-feet of water to form Wolford Mountain Reservoir, 5 miles upstream of Kremmling.
“The continued movement of the dam at Wolford Mountain Reservoir is the most important issue currently facing the River District,” states a Jan. 8 memo from John Currier, the chief engineer at the district.
Though the dam is far from being at risk of failure, if that happened at some point in the future with a full reservoir behind it, there is the potential for 600,000 cubic feet per second to run down Muddy Creek toward Kremmling, said Jim Pokrandt, communications manager for the district. Eighteen hours later, the massive surge of water would reach Glenwood Springs.
The board discussed the situation at the dam during a regular quarterly meeting in Glenwood Springs.
“We see this continued downward movement of about an inch a year, unabated, with no evidence that it is really going to slow down,” Currier told the River District board. “It is a significant concern and is something we need to address.”
Ritschard Dam is 122 feet tall and 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 now 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 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 and start transporting material and widening the cracks, and then the dam could eventually be at risk of failing.
“If we lose the integrity of that core, at that point the threat we’re under is essentially having the dam completely fail,” Dan Birch, the deputy general manager of the river district, told his board of directors.
“And the scenario that happens when the dam fails is a lot worse than any flood that Mother Nature could create, and we’re talking just tremendous amounts of devastation and tremendous potential for loss of life,” Birch said.
“It is a very serious situation (that) as an organization we are, have been, and will continue to take very seriously,” Birch said.
Bill McCormick, the chief of dam safety for the state of Colorado, was briefed on the dam’s situation on Jan. 14.
“Everyone involved (including the downstream public and the emergency management community) are now more aware of the dam’s operation than they ever have been before,” McCormick said via email on Jan. 22.
“The extensive investigations and analysis to date show that the dam remains strong and stable but that if left unabated, the movements could ultimately (in decades) create greater damage to the dam structure,” McCormick said. “The safety of the dam is being managed through the river district’s changes in normal operations, which include maintaining a reduced reservoir level and increased surveillance including real-time monitoring.”
The dam was last inspected by the state in November, and there are no restrictions yet in place from the state that would force the river district to lower the water level in the reservoir. The district did, however, voluntarily lower the water level by 10 feet in 2014 to take pressure off the dam.
Dick Davidson, a senior engineer and dam expert with AECOM, told the River Board it was his professional opinion that it was time to move toward a solution.
“We are not in an emergency, by any stretch, but we have definitely stressed our core beyond where we like it to be,” Davidson said. “So let’s figure out what we need to do. Let’s look at our options.”
One option is to spend up to $30 million rebuilding the dam.
Another potential option is to store significantly less water in Wolford Reservoir, which is about two-thirds the size of Ruedi Reservoir, in order to take pressure off the dam. However, that may buy the river district some time but not solve the problem.
A refined set of options is now expected to be brought back to the river district board at both its April and July board meetings.
Aspen Journalism is collaborating with The Aspen Times and the Glenwood Springs Post Independent on coverage of rivers and water. More at http://www.aspenjournalism.org.
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