Fixing ‘moving’ dam near Kremmling could cost $15M
It could cost $15 million to dig up and recompact the rocks on the downstream side of the dam that creates Wolford Reservoir, north of Kremmling, in order to stop the dam from moving slightly, but steadily.
“It is a pretty significant surgery of the dam,” John Currier, chief engineer at the Colorado River District, told the district board July 22.
Ritschard Dam was built for the river district in 1995 by D.H. Blattner and Sons of Minnesota for $42 million. The dam is 122 feet tall and 1,910 feet wide.
The $15 million estimate to rehabilitate the dam includes a 35 percent contingency factor and is still preliminary, Currier stressed.
The project would include removing the top 25 feet of the dam and then stepping down the downstream face of the dam in layers to get to three “bad acting” zones of poorly compacted rock, some of which are 90 feet inside the dam.
Currier said there are “no near-term safety concerns” regarding the dam’s current ability to hold back 66,000 acre-feet of water from Muddy Creek, which flows into the upper Colorado River east of Gore Canyon.
“We’re not in any crisis,” Currier said. “We’re just ready to move this forward.”
Engineers with the district noticed in 2008 that the dam had settled vertically by a foot-and-half instead of just 1 foot, as expected.
The dam has now settled 2 feet, at the rate of about an inch a year. And it has also moved horizontally, by about 8 inches, at a spot 40 to 50 feet below the crest of the dam.
The dam’s impermeable clay core is held in place by rock shells on both the upstream and downstream sides of the dam.
TROUBLE AT SOME POINT
The upstream rock shell appears to have settled appropriately in place, perhaps because of varying degrees of water pressure on its face as reservoir levels have fluctuated.
But the downstream shell is still moving.
“At some point that movement will compromise the ability of the core of the dam to hold back the water,” Currier said.
Currier said construction-sequence photos indicate the movement appears to be related to how fill material above and below haul roads was compacted during construction.
The plan is to dig into sections of the shell, remove the poorly compacted rock, and then recompact those zones, mainly using the same rocks.
“The bad-acting layers may be more a function of how the material was placed, not the material itself,” Currier said.
Currier said a consulting engineer at the firm of AECOM had observed that the construction work is essentially “just a big dirt job,” albeit one that will require complicated sequencing and careful on-site supervision by experienced engineers.
Over the last six years staff and consulting engineers have taken a variety of steps to investigate the situation at Ritschard Dam.
$1.5 MILLION SPENT
They’ve installed inclinometers, established an expert review panel, developed modeling and conducted lab tests on the core material to establish that the dam did not present an immediate safety problem.
The river district has now spent close to $1.5 million on instrumentation and analysis, Currier said, and recent work on a range of alternatives has given engineers enough information to move from the “what” stage to the “how” stage.
“We have a very good understanding of what solutions might, or might not work, and thus we can we have a great deal more confidence in our solution,” Currier said. “We’re confident that structural rehab is required.”
Currier said that simply storing less water in order to take pressure off the dam won’t solve the long-term problem.
In a memo to the board, Currier wrote “operating at reduced levels slows the deformation rate but does not stop the deformation.”
The river district has been operating the reservoir at 10 feet below normal levels since 2014 as a standard precautionary measure.
Another option looked at was installing a series of concrete columns down through the downstream shell of the dam in order to stiffen it, but Currier said it was ruled out due to higher costs and doubts about its effectiveness.
There is also the opportunity to increase the amount of water the reservoir can hold by increasing the height of the existing spillway, but Currier advised it was better to first just fix the dam to avoid a “permitting quagmire” by trying to also expand the reservoir’s capacity.
Water in the dam would likely have to be lowered or completely drained during the project in order to take enough pressure off the dam. Those lower water levels could cause ripples in regional water-supply operations, especially in a dry year.
But Dan Birch, the district’s assistant general manager, said water-supply concerns would not be the tail that wags the dog of the rehabilitation project and related safety concerns.
However, it was also noted that the project would take 220 days over two construction seasons and would likely have an affect on four years of water operations in all.
“It could have some impact on water operations for one year pre- and one year post-construction,” Currier said.
Currier and other engineers plan on continuing their analysis of the proposed solution, including meeting with the state dam safety engineer in August and continuing to ask a panel of experts to peer review the plans.
A refined proposal will be presented to the River District board in September as part of the district’s annual budget meetings.
Aspen Journalism is collaborating with the Glenwood Springs Post Independent and The Aspen Times on coverage of rivers and water. More at http://www.aspenjournalism.org.
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The two rest areas in Glenwood Canyon reopened last week for the first time since the Grizzly Creek Fire began in August.