Water district awards dam contract for Chimney Hollow

BERTHOUD — Barnard Construction Inc., a Bozeman, Montana, company, will build the Chimney Hollow dam under contract with the Northern Water Municipal Subdistrict. The board of the subdistrict selected the firm on Friday.

Barnard will enter into a $485.4 million contract that calls for the construction of a 355-foot-tall asphalt-core dam in the valley west of Carter Lake in southern Larimer County. The board voted unanimously to approve the contract during its meeting held Thursday at the Northern Colorado Water Conservancy District offices in Berthoud. 

When completed, the dam will establish Chimney Hollow Reservoir, the 90,000 acre-foot water storage component of the Windy Gap Firming Project. The reservoir will provide a portion of the water supplies needed in the future for 11 water providers and a utility. 

“This contract marks an important milestone for the future of the water providers participating in the Windy Gap Firming Project,” Northern Water General Manager Brad Wind said in a written statement. “Construction of Chimney Hollow dam and reservoir will offer assurance to water managers that they will have the water their residents need for the next generations as well as fulfill key elements of the Colorado Water Plan.” 

The participants in the Windy Gap Firming Project are paying for the construction. 

“The need for this project for every one of the 12 participants is undeniable,” said Larry Howard, manager of water resources for the city of Loveland. He praised the detailed and comprehensive process taken to choose a contractor for the project. 

For the participants, the selection of a contractor marks an important milestone in the development of future water supplies in Northern Colorado. Planning for the reservoir began in 2003, culminating with a successful federal Record of Decision on the project in May 2017. That decision is being challenged in court. 

Barnard Construction Inc. has a long history of major water infrastructure projects. They include work such as the Keeyask Generating Station, a series of dams and dikes that will generate hydropower for 

communities in Manitoba, Canada, and a new reservoir in Central Florida to help protect water quality in the St. Lucie Estuary. 

Unlike the dams that hold back Horsetooth Reservoir and Carter Lake, which have impermeable clay cores at their centers, Chimney Hollow Dam will have a core made of flexible asphalt. While such asphalt-core dams are common in Europe, Chimney Hollow dam will be only the second in the United States to have that feature. Barnard will employ a subcontractor with experience in building asphalt core dams. 

The outer shell fill material for the dam will be quarried from the property that will house the reservoir, and that quarry will be below the water line when the reservoir is filled. 

Construction could begin as soon as May and is estimated to take about four years. The reservoir will fill as the hydrology allows. When water levels reach a point at which recreation is feasible at the reservoir, Larimer County will manage the water recreation as well as the trails above the shoreline that will connect to a larger network of trails throughout the area. 

In addition to the main Chimney Hollow dam, Barnard crews will build a 40-foot-tall saddle dam at the south end of the valley, and other associated facilities including access roads, outlet works, and a valve house. 

Although construction of Chimney Hollow will occur in Larimer County, its largest environmental improvement effort will be built in Grand County on the west side of the Continental Divide. The $18 million Colorado River Connectivity Channel will create a new channel for the Colorado River that skirts around the Windy Gap Dam and connects the ecosystems above and below Windy Gap Reservoir. Other environmental enhancements are planned along areas that were damaged in the 2013 Front Range Flood, as well as improvements to the wastewater treatment plant serving the Fraser River Valley. 

Participants in the Windy Gap Firming Project include Broomfield, Longmont, Loveland, Greeley, Erie, Little Thompson Water District, Superior, Louisville, Lafayette, Fort Lupton, Central Weld County Water District and the Platte River Power Authority.