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Colorado is building the nation’s largest dam since 2000, at a time when dams are more likely to go down than up.
Colorado general contractor Barnard Construction and Swiss engineering firm Walo are building Chimney Hollow Reservoir along the Front Range of the Southern Rockies, with the goal of providing residents north of Colorado more reliable access to water. The reservoir, a major component of the Windy Gap Firming project, will store up to 90,000 acre feet of water, according to Barnard, the volume of about 45,000 Olympic swimming pools.
The dam and reservoir will supplement the water supply to six Front Range communities of about 825,000 people, said Jeff Stahla, public information officer for Northern Water, which owns the project.
When complete, the dam will be 350 feet tall, according to Northern Water, making it the third largest dam in the state and one of the tallest in Colorado’s Front Range. The dam is only the second asphalt core dam to be built in the US, although this construction technique has been popular in Europe since the 60s.
The stalled dam is finally moving forward
This is the largest dam to be built in the United States since Seven Oaks Dam, a 550-foot structure built in California in 2000. Currently, dams are more likely to be removed, with 80 falls in 2023, according to American Rivers. Indeed, there is there are few places left to build new dams in the US, according to High Country News.
Some of Colorado’s Front Range communities have not had their own water storage. Instead, their water has been stored in another tank, and when there is no space for their water, they have no access to it, a problem the project aims to solve.
The road to the construction of the dam and reservoir has not always been smooth sailing. The project began the permitting process in 2003, but faced lawsuits from environmental groups that wanted to prevent it from transferring water from the Colorado River. Finally Northern Water settled with those groups for $15 million.
The project began in August 2021 and is expected to be completed in the summer of 2025. It is expected to cost $370 million for the dam itself, with another $150 million for work at the source of the ‘water on the west side of the continental divide. Stahla said. Northern Water ratepayers who will get this water are funding the project.
Dam reaching new heights
Typically, dams on the Colorado Front Range have been made with clay, but there is not enough fine-grained clay material in the reservoir’s footprint, Stahla said. So, instead, it has an asphalt core.
The Chimney Hollow Dam will resemble other dams with a shell filled with rock and gravel, but inside will be a hydraulic asphalt mix. This is not like asphalt on the road, however. It’s not brittle, but it has a very high bitumen with a very small aggregate that “gives that watertight ability and a little bit of flexibility when it’s in the center of the dam,” Stahla said.
Although it is only the second asphalt dam in the US, the other is the Silver Basin Dam and Reservoir in Arizona — There are more than 200 asphalt core dams in the world. European engineers began using this technique to build reservoirs in the Alps, where clay is also missing. This experience is one of the reasons why Walo is part of the team.
After construction, the reservoir will take about three years to fill with water, according to Northern Water.
“By building this reservoir, you’re consolidating the performance of that Windy Gap water so you can count on it year after year,” Stahla said.
