Jan. 10, 2023

Key Milestones Achieved at Chimney Hollow Reservoir in 2022

Chimney Hollow Reservoir construction crews made significant progress in 2022. Work started in August 2021 and is scheduled to continue until August 2025. The project is a collaboration between 12 Northeastern Colorado water providers to improve the reliability of, or make firm, water supplies from the Windy Gap Project. Chimney Hollow Reservoir will be located just west of Carter Lake in Larimer County and will provide dedicated storage to reliably supply 30,000 acre-feet of water each year for future generations. 

Here are some highlights from this year’s work, along with numerical references to the map below. 

Main Dam Foundation Prep

In November 2022, crews completed the main dam foundation rock excavation (No. 1), which marked a huge milestone in reservoir construction after 15 months of work on this component. 

Three-photo collage: left is a photo of the inide of the tunnel, middle is a sunrise shot looking east with the Bald Mountain Interconnect valve vault highlighted, and right is a photo of the man dam with the asphalt core machine placing aspalt.
Hydraulic Asphalt Core

Crews began hydraulic asphalt core placement in October 2022 (No. 2). An asphalt core dam was selected for Chimney Hollow dam because the valley does not provide enough fine-grained clay material in the reservoir footprint to construct a clay core dam, like those in Carter Lake and Horsetooth Reservoir. Asphalt core dams use a special hydraulic asphalt mix, unlike brittle paving asphalt, to create a flexible, very low permeability core. This technology is proven effective, with more than 200 asphalt core dams constructed worldwide having decades of safe operations. 

For the next two years, the asphalt will be placed in 9-inch horizontal increments until the dam reaches a height of about 350 feet. Rockfill and filter/drain construction occur concurrently on each side of the core to complete the embankment construction at any given elevation.  

Bald Mountain Interconnect

One of the most time-sensitive aspects of the Chimney Hollow Reservoir Project was the Bald Mountain Interconnect (No. 3). A temporary shutdown of the Colorado-Big Thompson (C-BT) Project occurred from mid-September through mid-December as crews cut into existing infrastructure to tie in a 126-inch diameter section of steel pipe with a 72-inch diameter steel offtake (known as a wye). Water flowing through the wye structure will be the primary source of water used to fill Chimney Hollow Reservoir.

Larimer County and Saddle Dam Access Roads

On Nov. 15, the Larimer County and saddle dam access roads were completed (No. 4). When the reservoir opens to the public, the Larimer County access road will be the entry road to Chimney Hollow’s future public recreation and open space facilities. The saddle dam road will be used for Northern Water maintenance access only.   

Downstream Tunnel and Valve Chamber

The downstream tunnel portal and excavation of the downstream portion of the inlet/outlet tunnel (No. 5) was completed in October 2022. The downstream portion of the inlet/outlet tunnel is 26 feet in diameter and runs 667 feet from the downstream toe of the dam to the valve chamber at the center of the dam. The 30-foot diameter valve chamber was also excavated to provide room for mechanical equipment installation and maintenance. A 72-inch diameter steel conduit will be placed inside the tunnel to bring water in and out of Chimney Hollow Reservoir. The upstream portion of the inlet/outlet tunnel will be excavated this spring. 

Aerial photo of the Chimney Hollow construction site with labels of various components.