Lake Powell, on the Colorado River, is the second largest manmade reservoir in the US. More than two million people a year vacation there in the Glen Canyon National Recreation Area, which is managed by the National Park Service.
The Navajo Generating Station is located three miles (4.8 kilometers) south of Lake Powell, near Page, Arizona. The station provides electricity to customers in Arizona, Nevada, and California, as well as power to pump about 1.5 million acre feet per year of Colorado River water to central and southern Arizona.
About 30,000 gallons (113,000 liters) per minute is drawn from Lake Powell to cool the plant and operate its pollution control scrubbers. The water is raised through five closely-spaced inclined shafts drilled from the top of the sandstone shoreline. These shafts, built in 1972 and 1973, are about 300 feet (91 meters) long and inclined at 26 degrees from vertical.
When drought conditions lowered the level of Lake Powell, it was decided that new intakes more than 400 feet (122 meters) deep were needed to ensure the station can continue to draw the water it needs. The new intakes would have to remain within the boundaries of the utility’s pump station lease, and within the easement negotiated with the National Park Service.
In 2004, the Salt River Project, which operates the generating station, retained Mott MacDonald to assess several drilling and tunneling approaches for the new intakes.
We recommended five new inclined shafts, each 43 inches (1.1 meters) in diameter and 500 feet (152 meters) deep. In order to stay within the necessary boundaries, the shafts were required to hit an extremely small breakout target 250 feet (76 meters) underwater.
Sophisticated drilling equipment and techniques were used to maintain the shaft trajectory and limit deviation to less than 1%. Breakout locations were examined using an underwater remotely operated vehicle (ROV) to ensure correct placement.
Over a five-year period, Mott MacDonald provided services including these:
- Preliminary and final design
- Alignment definition
- Interpretation of underwater survey and geotechnical site investigation results
- Computational fluid dynamics (CFD) modeling of the shafts and pumping system
- Bid document preparation
- Review of contractor submittals
- Inspection of shaft drilling and installation of steel liner
The inclined intake shafts were selected as the most environmentally friendly solution with the lowest technical risks. Mott MacDonald’s design helped ensure a continued supply of cooling water to a power station that is vital to the region, while protecting the natural environment of one of the country’s most popular destinations.
To comply with environmental requirements, drilling fluids introduced to the lake were kept to a minimum. Grout was not allowed into the lake under any circumstances.