ASPEN – In their ongoing search for alternatives to building dams and reservoirs on Maroon and Castle creeks, Aspen officials continue to explore other options, including an underground reservoir that would store water below the city’s golf course.
Aspen City Council reacted favorably to a presentation in May about an in-situ or underground reservoir beneath the Aspen Golf Club, with one council member saying it was a “great introductory lesson.”
Aspen City Manager Steve Barwick said last week there is plenty more work to be done.
“City Council had a lot of questions regarding the viability, impact and cost of in-situ storage,” Barwick said, “and they have not yet even begun their review of the storage needs.”
Barwick also said recently that the city does not know exactly how much water it needs to store to meet future needs, but the council is set to hear a presentation on the subject at a July 11 work session.
“All of this, this whole notion of how much water do we need and how much water do we need to store, and all of that, has been based upon very preliminary analysis,” Barwick told the Pitkin County Healthy Rivers and Streams Board on March 16. “And now it’s time to tighten up the whole analysis and do a rational set of studies so we can have a rational discussion with the entire valley about what are we going to do here. How much storage do we need, and where do we want to put it?”
Don Deere, a geotechnical engineer who has worked on a long list of water storage projects in Colorado, said during his 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, in which claylike walls are built in trenches around a rock-filled area to hold water.
“Engineering-wise, it’s feasible,” Deere confirmed this week 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.
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.
By comparison, the city 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. For comparison, Ruedi Reservoir holds about 100,000 acre-feet of water. Deere called a 1,200 acre-foot reservoir “a small reservoir.”
A potential 170-foot-tall dam near Ashcroft on Castle Creek would create a reservoir that holds 9,062 acre-feet of water; 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 to maintain its conditional water rights for the two reservoirs on Maroon and Castle creeks, 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 opposers also are questioning whether the city really needs to store nearly 14,000 acre-feet of water.
And as the city tries to answer the “how much” question, they’ve also been looking at the “where” and “how” questions.
A study of the idea of storing water in old silver mines around Aspen was also presented at the May 15 work session by another Deere and Ault engineer, Victor deWolfe III.
He said it likely would be expensive and complicated for the city to use the old mines, especially as 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.
Deere looked at two potential locations for an in-situ reservoir, both on city-owned property, the golf course and the city’s Cozy Point Ranch property at the intersection of Brush Creek Road and Highway 82.
The Cozy Point 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 golf course.
A trench, filled in
Deere told the council 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.
The trench, which would encircle the golf course, would be filled in with a claylike substance (a soil-bentonite mix) that would hold water. Deere said under the right conditions, such a deep trench, sometimes called a slurry wall, 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,” said Deere, who has published a paper on the construction technique.
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.
If the city needed to during a drought, it could then pump the water from the new underground reservoir to its water treatment plant located on a hill behind Aspen Valley Hospital. It’s about a mile from the center of the golf course to the treatment plant.
Councilwoman Anne Mullins asked Deere if the golf course would look the same after an in-situ reservoir was installed.
“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, “it’s out of sight, out of mind.” And he said this week that 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,” Deere said this week.
He said it would require drilling test holes to know more about the feasibility and potential cost, as it would reveal the true depth of the gravel and the condition of the underlying bedrock.
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 next settlement conference in the cases is set for the first week of August.
Editor’s note: Aspen Journalism is collaborating with The Aspen Times, the Glenwood Springs Post Independent, the Vail Daily, and the Summit Daily on the coverage of water and rivers. The Times published this story on Monday, July 3, 2017.