Case Study: Unique Diversion

Multi-use structure provides a permanent water supply from the American River

• By George Slovensky, Alan Stroppini
• Aug 26, 2008

Dam construction in the mid 1960s and seismic, environmental, and cost-sharing concerns in the 1970s created serious problems for the Placer County Water Agency in northern California. The agency retained WHPacific in 2000 to work on the intake to help restore the river and design a permanent water diversion facility. WHPacific is a multidiscipline architectural/engineering firm, 100 percent Native American-owned by NANA Development Corp.

The U.S. Bureau of Reclamation in 1968 began construction of the Auburn Dam on the North Fork of the American River. A 33-foot-diameter tunnel diverted the river around the dam construction. In 1975, a 265-foot high temporary earth fill dam (cofferdam) was completed, which forced the river into the recently completed diversion tunnel. This action dewatered an approximate one-mile reach of the river. The bureau removed a Placer County water intake and pump station and began providing water to the agency with temporary facilities.

That same year, an earthquake near Oroville, Calif., about 50 miles north of the dam site, raised concerns about the seismic nature of the fault systems along the foothills of the Sierra Nevada mountain range. Construction work was temporarily halted as the bureau re-evaluated the site. Environmental and cost-sharing concerns caused further delays. In 1986, record flood flows overtopped and breached the cofferdam depositing an estimated two million cubic yards of rock and debris into the river reach below, dramatically altering it.

Following the failure of the original cofferdam, a much smaller temporary dam was constructed to raise river levels enough to facilitate diversions to the temporary pumping system. This cofferdam was overtopped fairly regularly by high river flows that continued to erode significant amounts of rock and sediment from the dam and deposit them downstream.

By the mid 1990s, Placer County had become one of the fastest growing areas in the nation, and the temporary pumping facilities were no longer adequate to meet year-round demands. The system had limited flow capacity and had to be removed each year to protect it from winter floods. Vulnerability to floods, high costs, and inability to meet demand led the county and the federal government to begin investigating alternatives for a new diversion structure and pumping plant.

WHPacific was part of the project team from a very early stage, helping to evaluate alternatives for restoring the river through the dam site and provide a permanent, reliable diversion facility. The design was complicated by the presence of the massive cofferdam remnant and downstream deposits of debris from its failure. These factors created very aggressive hydraulic conditions on an unstable river bed. Complete removal of the cofferdam remnant and debris from its failure were not financially feasible. WHPacific and the Bureau of Reclamation collaborated on a plan that combined limited re-grading of the cofferdam remnant with grade stabilizing structures in the river.

The firm designed a permanent and reliable water supply intake that would withstand heavy sediment loads and provide the desired capacity including future expansion. This intake also meets environmental requirements related to fish exclusion and bypass flows and allows fish and boat passage through the newly restored river reach. The agency required that the design use tilted, wedge-wire river screens mounted in the bottom of the river in a boat-passable structure.

Colorado State University in Fort Collins and the Bureau of Reclamation's Hydraulics Laboratory in Denver, Colo., built and operated separate physical models to help in the design of the intake structure and river reach rehabilitation. The full-scale, unit width model of the screens constructed by the university was used to evaluate diversion capacity, sediment plugging tendencies, and air-backwash effectiveness. The bureau's much larger scale model of the river reach was used to evaluate options for cofferdam re-grading and overall reach geometry and sediment transport characteristics.

These models were complemented by computer-based numerical modeling using the BriStars sediment transport model and Flow3D, a computer-based three-dimensional hydraulic model. BriStars was used to evaluate both local and reach-scale sediment movement for single event and long-term hydrology. Flow3D was used on a macro-scale to design a boat bypass channel of naturalistic chutes and pools and to track sediment movement to assist in the design of sediment control structures. Flow3D also was used on a micro-scale to evaluate flow patterns and sediment movement through the intake screens.

Construction of the American River Intake Structure and channel were completed in late 2007. The bypass tunnel has been sealed, and the river once again flows through the site. Revegetation will take several years, and evidence of the dam project that started more than 40 years ago will always be visible. In place of the dam now stands a unique multi-use diversion facility that provides a reliable water supply and a bypass for environmental base flows, fish, boat passage, and whitewater recreation. The restored river enhances the appearance of the scarred dam construction site, and new public access facilities have opened the area to recreation.