Inlet/Outlet Works

Getting Water In and Out of Chimney Hollow

Chimney Hollow Reservoir’s location adjacent to C-BT Project infrastructure will enable water to easily be moved in and out of the reservoir when C-BT Project capacity is available per an agreement with the U.S. Bureau of Reclamation.  

The process of getting water to Chimney Hollow Reservoir begins on the West Slope. Just west of Granby sits Windy Gap Reservoir, where water is collected and pumped six miles via pipeline to Lake Granby. From there, existing C-BT Project infrastructure conveys Windy Gap water through Shadow Mountain Reservoir and Grand Lake to the Adams Tunnel, which passes under Rocky Mountain National Park.  

After traveling 13.1 miles through the tunnel to the East Slope and then through four reservoirs and three power plants, water will make its way in and out of the Chimney Hollow Reservoir through the inlet/outlet works, which includes C-BT Project interconnections, the conduit, valve house, the inlet/outlet tunnel and the inlet/outlet tower located within the right (east) abutment. From Chimney Hollow Reservoir, water is released into Flatiron Reservoir where it can be delivered to the 13 participants using the existing C-BT distribution system that spans across northern Colorado. 

Colorado-Big Thompson (C-BT) Interconnects

One of the reasons the Chimney Hollow Reservoir is such a great site is because of its adjacent location to the C-BT system, which allows us to easily move water in and out of Chimney Hollow Reservoir using existing infrastructure. Through an agreement with the U.S. Bureau of Reclamation, the Municipal Subdistrict can allow water to flow into Chimney Hollow Reservoir when there is available capacity in the C-BT Project infrastructure.

In order to fill Chimney Hollow, construction crews tapped into the Bald Mountain Tunnel and installed a 126-inch by 72-inch diameter steel wye. The wye is encased in concrete and affixed with a 72-inch diameter butterfly valve so that Chimney Hollow Reservoir can be isolated from the C-BT Project if necessary.

Delivery of Chimney Hollow water to our southern participants will be done by releasing water from the reservoir into Carter Lake by tapping into the Carter Lake Pressure Conduit with another 72-inch diameter steel wye. Our northern participants will receive water from Chimney Hollow Reservoir as it is released into Flatiron Reservoir and then north through existing delivery canals and pipelines. 

Crews installing the 72-inch butterfly valve at the Bald Mountain Interconnect.
Installation of the first butterfly valve, June 2022
Crews placing the first 40-foot section of the Chimney Hollow Conduit.
First Chimney Hollow Conduit pipe placed, August 2022

Chimney Hollow Conduit  

The Chimney Hollow Conduit consists of about 1-mile of buried 72-inch diameter steel pipe that will bring water from the existing CB-T Project infrastructure into Chimney Hollow Reservoir. The pipe will be buried about 15 to 20 feet deep and will be able to deliver water up to a rate of 400 cubic feet per second into Chimney Hollow Reservoir. The Chimney Hollow Conduit also provides an alternative, backup, way to fill Carter Lake. 

The heaviest section of pipe weighs 33,650 pounds and will be placed just outside of the valve house located at the base of the main dam. As water travels downhill from the Bald Mountain Interconnect, the weight and thickness of the pipe increases to compensate for the rise in pressure as the water flows down by gravity. At the top of the hill, the water pressure will be about 36 pounds per square inch (PSI), whereas at the bottom of the hill (at the onsite valve house), it will be about 550 PSI.   

The Chimney Hollow Conduit construction is expected to be completed in spring 2024. It is a major component of the water conveyance system, which also includes the CB-T Project interconnects, valve house, inlet/outlet tunnel and the inlet/outlet tower. 

Valve House

Pressures and flows between the C-BT Project and Chimney Hollow systems are controlled in the valve house. This large building, approximately 120-by-120-feet and 4 stories tall, will be located at the downstream toe of the main dam. With approximately 50 valves, it is designed to safely control the flow rates and pressures necessary to operate Chimney Hollow among the different pressures found at Pinewood Reservoir, Flatiron Reservoir and Carter Lake.   

The valve house was designed to operate for 12 different operating scenarios. These scenarios include normal filling and water supply releases from Chimney Hollow, but also provides redundancy in the C-BT system by providing an alternative way to fill Carter Lake. Additionally, the valve house contains an emergency release valve to rapidly dewater Chimney Hollow Reservoir in case of an emergency.  

A large overhead gantry crane is included in the design, as well as detachable roof to that the large diameter valves can be maintained in the future. 

Rendering of the valve house.
Rendering of the valve house. Courtesy of Stantec

Inlet/Outlet Tunnel

Water will be brought in and out of Chimney Hollow Reservoir through a 72-inch diameter steel conduit built inside a 2,000-foot-long tunnel under the right (east) abutment of the main dam. A 30-foot diameter tunnel section was excavated between the upstream and downstream tunnels to provide mechanical equipment installation and maintenance.  

The 26-foot diameter downstream portion required about 7 months of excavation, which runs 667-feet to the center of the main dam. Crews headed to the other side of the dam to begin working on the 10-foot diameter upstream portion of the tunnel, which will run about 1,200-feet to the center of the dam, where it will then align with the downstream section to form the full inlet/outlet tunnel. The upstream portion of the tunnel will require approximately 11 months of excavation.   

A 30-foot diameter tunnel section was excavated between the upstream and downstream tunnels to provide room for mechanical equipment installation and maintenance. It will also allow for a grout collar around the tunnel to be installed. Once excavation of the tunnel is complete, tunnel lining crews will install the final reinforced concrete liner, pipeline and mechanical equipment in the tunnel.   

Excavation of the tunnel commenced on April 4, 2022, and, once the reservoir is complete, this infrastructure will play the critical role of filling the reservoir and then draining it to make deliveries to water users.   

Inlet/Outlet Tower 

A submerged reinforced concrete tower, approximately 75-feet tall, will be constructed to provide a ‘dead storage’ space in the bottom of the reservoir. This space will allow sediment and debris to fill in the bottom of the reservoir without clogging up the conveyance infrastructure. The tower also allows operators to draw water from a higher location in the reservoir profile, so that cleaner, oxygenated water can be discharged from Chimney Hollow Reservoir into the downstream water supplies. The tower will contain a slotted intake so that a bulkhead can be lowered from a barge in the reservoir to isolate and maintain the inlet/outlet tunnel. The tower will consist of 5,000 cubic yards of reinforced concrete with large steel-fabricated trash racks. 

Delivery of Chimney Hollow water to southern participants will be achieved through another 96-inch by 72-inch diameter steel wye added to the Carter Lake Pressure Conduit, enabling water to flow from Chimney Hollow Reservoir into Carter Lake. Chimney Hollow Reservoir Project participants located further to the north will receive their Windy Gap Project water via Flatiron Reservoir and existing C-BT Project delivery canals and pipelines.