Inlet/Outlet Works

Getting Water In and Out of Chimney Hollow

Water will be delivered in and out of the Chimney Hollow Reservoir through the inlet/outlet works. The inlet/outlet works include interconnections between the Colorado-Big Thompson (C-BT) Project, the conduit, valve house, the inlet/outlet tunnel and the inlet/outlet tower located within the right (east) abutment.   

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, we 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 are tapping into the Bald Mountain Tunnel and installing a 72-inch diameter steel wye. The wye will be 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. 

Installation of the first butterfly valve, June 2022
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.   

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. The weight and thickness of the pipe decreases as you travel uphill toward the Bald Mountain interconnect because the pressure within the pipeline decreases with elevation. 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 2023. 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, which is a large building, approximately 100-by-100-feet and two stories tall, that will be located at the downstream toe of the main dam. The valve house contains approximately 35 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. Courtesy of Stantec

Inlet/Outlet Tunnel

Crews are constructing a 2,000-foot-long tunnel under the right (east) abutment of the main dam to bring water through a 72-inch diameter steel conduit into and out of the reservoir.  

The 26-foot diameter downstream portion will require about 10 months of excavation, which will run 667-feet to the center of the main dam. Crews will then head 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 40-foot diameter tunnel section will be excavated between the upstream and downstream tunnels to provide 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 in the reservoir. The tower provides a ‘dead storage’ space in the bottom of the reservoir, allowing a place for sediment and debris to fill in the bottom of the reservoir without clogging up the conveyance infrastructure. For instance, if a wildfire burned in the Chimney Hollow valley and sediment and ash ran into the reservoir, those materials would be stored in this ‘dead storage’ space and not impact the long-term operations of the project. 

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 contains a slotted intake so that a bulkhead could be lowered from a barge in Chimney Hollow to isolate and maintain the inlet/outlet tunnel. 

The tower is made up of 5,000 cubic yards of reinforced concrete with large steel-fabricated trash racks.