Building water reliability: New Colorado dam, complex tunnel strengthens water supply

Combatting water supply shortages

Colorado, like much of the US West and other regions around the globe, faces an ever-growing need for water. The Northern Colorado Water Conservancy District (Northern Water) expects water shortfalls in the coming decades. By 2050, the water providers’ combined population will have more than doubled from 2005, to about 825,000 people. Projections show a water shortage in supply of 64,000 acre-feet (approximately 29 billion gallons) in 2030 and up to 110,000 acre-feet by 2050.

Chimney Hollow is the latest project in the state’s long history of moving and storing water. The C-BT Project, which started construction in the 1930s, collects water on the western slope of the Rocky Mountains and conveys it beneath the Continental Divide to provide water and hydropower to northeast Colorado. In Colorado, about 80% of the state’s precipitation falls on the West Slope of the Rocky Mountains, but 80% of the population lives along the East Slope.

The Northern Water’s Municipal Subdistrict is an independent conservancy district formed to plan, finance, build, and operate the Windy Gap project. The Windy Gap project boosted flow through the C-BT system and delivered water starting in 1985. The extra storage provided by the Chimney Hollow reservoir, provides more water reliability. 

Dam safety and water delivery

In addition to providing water to the growing population on the Front Range, much of the work that is to be done at Chimney Hollow was driven by a larger trend of dam safety initiatives in the US. 

As a result of potential hazards associated with the storage of water in reser­voirs, the western US has strict laws governing dams. Potential hazards are addressed through state dam safety programs that strive to improve dam condition and safety. Dam safety at Chimney Hollow was of paramount concern to our design team, the client, the Colorado dam safety regulators, and the review boards. Rightfully so.

To successfully turn the Chimney Hollow reservoir into an essential part of Northern Colorado’s water future, tunnel design is critical. The grouting program and valve cham­ber components of the I/O works were driven by and designed around dam safety priorities.

The tunnel includes four cross sections—the upstream tunnel, the grout chamber, the valve chamber, and the downstream tunnel. A six-foot diameter I/O conduit runs the length of the 2,400-foot tunnel.

Multiple factors impacted the overall tunnel design. These are four of the most important factors:

1. Two roles: During dam construction, the I/O tunnel could serve as a diversion for surface water in the Chimney Hollow Valley. Because the tunnel could act as the diversion during construction, it was designed to be built prior to rockfill placement.
2. Valve chamber: The valve chamber houses the valves, as well as mechanical and electrical equipment, to control water flow within the I/O conduit. The valve chamber is beneath the dam to allow for maintenance access of the conduit. It is as close to the centerline of the dam as possible, providing the greatest length of conduit available for inspection while also taking advantage of the major drop in groundwater pressure head downstream of the main dam centerline. Although the use of the valve chamber increases the complexity of the tunnel design, it greatly improves dam safety. The valve in the chamber provides a way to shut off the conduit in an emergency and redundancy in the control of the con­veyance system.
3. Heavy loads: As the tunnel was designed to be constructed before the rockfill is placed, it is designed for high embankment surcharge loads along with the high hydrostatic loads from the impounded water acting on the lining, which puts extremely high stresses in the lining. The design for the valve cham­ber and the downstream tunnel used a circular cross section to handle the high-loading conditions.
4. Grouting design: The challenging grouting design took many forms based on the project and dam-safety requirements. Eight types of grouting are planned for the I/O tunnel— pre-excavation grouting, feature grouting, collar grouting, backfill grouting, contact grouting, skin grouting, modified contact grouting, and consolidation grouting. The combination of these techniques will create a treated zone around the I/O tunnel to mitigate risks associated with seepage and deformation along the tunnel and beneath the dam. One of the most important grouting methods is the collar grouting per­formed underground to close the dam curtain grouting. Curtain grouting from the surface will be performed prior to the tunnel excavation along the reach of the dam axis penetrated by the tunnel. As the tunnel is excavated, the grout curtain will likely be disturbed by the tunnel blasting excavation techniques and must be closed using collar grouting techniques to maintain its functionality.

Innovative design for a secure water future

In fast-growing Colorado, the time to think about tomorrow’s water supply is today. The Chimney Hollow Reservoir project is a unique combination of one of the state’s tallest dams with one of its most complex tunnel designs. As we look forward to dam construction starting in 2021, Northern Water’s Municipal Subdistrict is taking a critical step in securing the region’s water future.

Our project team is excited to see construction start—and to put our skills to use helping other communities build water resilience.