As officials in Aspen, Colo. consider alternatives to building dams and reservoirs on local creeks, the concept of an “in-situ” reservoir under the city’s golf course has attracted interest. An engineer says the course has the right combination of bedrock and terraced gravels required for an in-situ reservoir, for which claylike walls are built in trenches around a rock-filled area to hold water.
As part of an ongoing search for alternatives to building dams and reservoirs on local creeks, city officials in Aspen, Colo., continue to explore other options, including an underground reservoir that would store water below the city’s golf course, The Aspen Times reported.
The Aspen City Council reacted favorably to a presentation in May about an “in-situ” or underground reservoir beneath the Aspen Golf Club, The Times reported, with one council member saying it was a “great introductory lesson.”
But there is more work to be done, Aspen City Manager Steve Barwick said. “City Council had a lot of questions regarding the viability, impact and cost of in-situ storage,” Barwick told The Times, “and they have not yet even begun their review of the [city’s water] storage needs.”
The council was set to hear another presentation on the subject at a July 11 work session, The Times reported,
Don Deere, a geotechnical engineer who has worked on a long list of water storage projects in Colorado, said during a presentation to the council in May that the city’s golf course has the right combination of bedrock and “terraced gravels” required for an in-situ reservoir, for which claylike walls are built in trenches around a rock-filled area to hold water, The Times reported.
“Engineering-wise, it’s feasible,” Deere later confirmed to The Times in a phone interview. “You’ve got to drill it to know for sure if the site’s going to work, but there are some favorable aspects to that site, for sure.”
The 148-acre public golf course is located between lower Castle and Maroon creeks and sits on top of about 75 feet of gravel and river rock left by retreating Ice Age glaciers, said Deere, who is chairman of the civil engineering firm Deere and Ault Consultants, Inc. in Longmont, Colo.
An in-situ reservoir under the golf course could hold about 1,200 acre-feet of water, Deere said, which the city could then pump back up to its water-treatment plant if needed.
Deere looked at two potential locations for an in-situ reservoir, both on city-owned properties: the golf course and the city’s Cozy Point Ranch property.
The Cozy Point location has a better combination of gravel and bedrock for an in-situ reservoir than the golf course, Deere said, but most of the focus at the work session was on the golf course site, in part because the city currently delivers water from Castle and Maroon creeks to irrigate the course.
A reservoir under the golf course could be built by using a long-armed excavator to dig a 3-foot-wide trench around the course through the estimated 75 feet of gravel and river rock down to a solid layer of bedrock, Deere told the council.
The trench, which would encircle the golf course, would be filled in with a claylike substance (a soil-bentonite mix) that would hold water. Under the right conditions, Deere said, such a deep trench can be dug and filled back in with the claylike material at the rate of about 100 feet a day.
“In a couple of months, on a typical site, I can have a completely lined vessel,” he said.
City-owned water from Castle and Maroon creeks could then be delivered to new and existing ponds on the golf course and allowed to slowly infiltrate into the spaces between the loose rock left in the vessel, The Times reported.
If needed during drought conditions, the city could then pump the water from the new underground reservoir to its water treatment-plant, which is located about a mile from the center of the golf course.
Councilwoman Anne Mullins asked Deere if the golf course would look the same after an in-situ reservoir was installed, The Times reported.
“I think we’d need to add some ponds, so there would be more water hazards when we’re done,” Deere said. But other than that, he added, “it’s out of sight, out of mind.” And after revegetation, no one would even know the in-situ reservoir was there.
As a general rule, Deere said it costs about $10,000 per acre-foot-of water stored to build an in-situ reservoir — if favorable soil conditions allow for the standard use of an excavator. But if conditions such as deeper gravel or harder bedrock require a crane and a platform to be used instead, the cost can go up by a multiple of five or six, he said.
Conceptually then, the construction costs of a 1,200 acre-foot in-situ reservoir could range from $12 million to $72 million, according to Deere.
“But we haven’t done a site-specific cost estimate for the golf course,” he needed.
Test holes would need to be drilled to know more about the feasibility and potential cost, Deere said, as they would reveal the true depth of the gravel and the condition of the underlying bedrock.
The city of Aspen currently has a 10 acre-foot reservoir at its water-treatment plant, which it says amounts to about a day’s use of water for the city’s water system, The Times reported. For comparison, Ruedi Reservoir, a major water source for the area, holds about 100,000 acre-feet of water. Deere called a 1,200 acre-foot reservoir, such as was being considered for the Aspen GC site, “a small reservoir.”
A potential 170-foot tall dam near Ashcroft, Colo. on Castle Creek would create a reservoir that holds 9,062 acre-feet of water, The Times reported, and a 155-foot dam on Maroon Creek near the Bells would hold 4,567 acre-feet.
The city applied to Division 5 water court in October 2016 to maintain its conditional water rights for the two reservoirs on Maroon and Castle creeks, The Times reported, and is facing opposition from 10 parties, including the U.S. Forest Service and Pitkin County.
Much of the opposition is because of the locations of the potential dams and reservoirs, both of which would inundate portions of the Maroon Bells-Snowmass Wilderness. Some of those opposing the projects also question whether the city really needs to store nearly 14,000 acre-feet of water, The Times reported.
And as Aspen officials try to answer the “how much” question, they’ve also been looking at the “where” and “how” questions, The Times reported.
A study of the idea of storing water in old silver mines around Aspen was also presented at a May 15 work session by another Deere and Ault engineer, Victor deWolfe III. It likely would be expensive and complicated for the city to use the old mines, deWolfe said, especially because it would be difficult to maintain control of the water in the complex maze of old shafts and tunnels.
The in-situ option, by comparison, sounded more feasible, The Times reported.
In May, city officials told the parties in the water court cases that it expected to finish its study of in-situ storage by July, The Times reported. The next settlement conference in the cases is set for the first week of August.
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