Providing California Hydropower While Improving Salmon Habitat
Tracy B. Vermeyen, P.E.
SACRAMENTO--California's power crisis underscores the value of the clean, renewable energy Reclamation helps to provide the state through its Central Valley Project hydropower plants. Yet the protection of endangered fish and their river habitat, which is affected by dam and power plant operations, also is an important issue in this part of the state.
These competing values posed a major dilemma: the cost efficient operation of Reclamation dams and powerplants require massive amounts of surface reservoir water to be sent through turbines and then released downstream. At Shasta Dam on the Sacramento River, for example, those releases changed stream habitat, often making the water sent downstream too warm for salmon to reproduce.
The dilemma was legally framed by a 1989 listing of the winter-run chinook salmon in the Sacramento River as protected under the state and federal endangered species acts. Then, in 1992, the Federal Government passed the Central Valley Project Improvement Act, which elevated fish and wildlife protection and restoration to a level of equal importance with flood control, irrigation, navigation, and power production.
Since 1987, when Reclamation was initially asked by state and federal resource agencies to help improve the water quality in the Sacramento River, Shasta Dam operators regulated downstream water temperatures by releasing water through the river outlet works. These releases bypassed the power plant, greatly reducing hydroelectric generation. Between 1987 and 1996, according to Reclamation figures, releasing the water through the outlet works instead of the turbines cost about $63 million in replacement power.
In 1989, Reclamation engineers began researching a way to control the temperature of water released from Shasta Lake without sacrificing revenue from power production. As part of the Central Valley Project Improvement Act, Reclamation was then required to construct a selective withdrawal structure at Shasta Dam that would control release water temperatures to improve salmon spawning and rearing habitat.
The dam, completed in 1945, is a 602-foot-tall curved concrete gravity structure that forms Shasta Lake, the largest reservoir in California. Shasta Power Plant, which is used to generate power during periods of peak demand, is located directly below the dam and contains five turbine-generators that can produce 539 megawatts of power.
The dam includes an extensive river outlet works and drum-gated central spillway, featuring intakes at three elevations. Intakes for the five power penstocks (the massive conduits that carry the water from Shasta Lake to the turbines) are located near the right abutment, about 215 feet above the bottom of the reservoir.
Reclamation engineers designed a multilevel intake structure to be installed in front of the existing power penstock intake structure on the face of Shasta Dam. The structure is similar to a successful selective withdrawal structure that Reclamation installed in the 1970s at the 150-MW Flaming Gorge Dam in Utah. That device also was designed to address fishery issues related to year-round cold water releases.
Reclamation also has added selective withdrawal structures to its 428-MW Hungry Horse Dam in Montana, its 198-MW Folsom Dam in California, and at the Lewiston and Whiskeytown dams and reservoirs in the Central Valley Project.
The structure at Shasta, called a temperature control device (TCD), allows Reclamation's Central Valley Project operators to meet federal and state water temperature requirements for salmon habitat in a 60-mile reach of the Sacramento River. The river segment stretches from Shasta Dam to Reclamation's 75-MW Keswick Dam and Reservoir and then on downstream to Red Bluff Diversion Dam.
At Shasta, the intake structure consists of 9,000 tons of structural steel and metal work. Fletcher General Construction (Seattle, Washington) constructed the device. Oregon Iron Works fabricated the structural steel and associated hoists. Divers from Oceaneering International installed the device using underwater construction methods to prevent drawing down the reservoir. Construction cost $64 million; the project, $80 million.
In the device, water withdrawal is controlled by a 250-foot-wide, 300-foot-high shutter structure. The shutter extends about 50 feet upstream from the face of the dam, and is open between units to permit crossflow in front of the existing trashrack structures. Three sets of hoist-operated gates and trashracks, also supplied by Oregon Iron Works, on the front of each shutter unit allow water withdrawal at upper, middle, and low reservoir levels for all five penstocks.
During installation, divers placed individual steel frame segments around existing trashracks on the penstock intakes of Units 1, 3, and 5. After these segments were attached to the dam, divers installed structural members between them to enclose the trashracks for the penstocks of Units 2 and 4. Cladding panels, gates, and trashracks, also supplied by Oregon Iron Works, were installed in front of all five power penstocks.
Another component, a low-level intake, is located to the left and at the base of the shutter structure. Water from the deeper, colder portions of the reservoir near the center and bottom of the dam can be withdrawn from this intake, which is 125 feet wide and 170 feet high. The intake is hydraulically connected to the shutter structure by two gates located on the side of the temperature control device.
Shasta Dam is a popular tourist attraction in northern California. Consequently, the aesthetics of the temperature control device influenced several design decisions. To reduce the device's visual effects from downstream, all of its components were installed below the elevation of the roadway guardrail on top of the dam. Barrier panels, which are similar in appearance to trashracks, were installed to provide a similar profile from the top of the structure down to the middle gate openings. The device was painted gray to allow it to blend with the face of the dam.
Fletcher General workers removed some dam components to provide construction access and to attach the device to the dam. For example, at the dam crest, an outlet gate storage facility and several hundred feet of cantilevered sidewalk were removed. Oceaneering International employed robotic drill trolley systems to perform many of the repetitive tasks, such as drilling holes in the face of the dam in a precise pattern for anchor bolts that would attach the temperature control device. Company divers worked both from the surface and from a saturation diving bell.
To conserve the cold water in Shasta Lake, withdrawals are made from the highest elevation possible while meeting the downstream water temperature targets that were established by the Sacramento River Temperature Task Group. The target is normally 56 degrees Fahrenheit above Red Bluff Diversion Dam.
During the spring, when the temperature of the surface water is coolest, operators release water from the highest level of the temperature control device. During the summer and fall, when surface water has warmed, water is withdrawn through the device from mid- and low-level intakes. Reclamation engineers have analyzed the multilevel intake structure's performance for three years, concluding that the structure is performing as desired.
In 1997, the first year of operation, project operators met temperature targets during the summer, with minimal water bypassing the power plant. However, in mid-September 1997, the structure developed warm water leakage around the middle gates, which required the withdrawal of cooler, deeper water through the low-level bypasses. Despite the leak, reduced bypasses in 1997 allowed Shasta Power Plant to generate an additional 300,000 megawatt-hours of hydropower. In January 1998, seals made by Nachtwey Engineering were installed by Oceaneering International divers, stopping the leaks.
During 1998, because of El Niņo-related weather disturbances, tributaries downstream of Keswick Dam flowed higher and warmer than normal. Consequently, cooler water had to be released from Shasta Dam earlier than usual. However, even with the early releases of cool water, use of the temperature control device allowed project operators to meet water deliveries and temperature requirements throughout 1998 without using the low-level bypasses.
Likewise in 1999, the temperature control device has operated successfully in meeting temperature goals without having to bypass the power plant. Monitoring of the structure's performance continues, with the goal of improving its efficiency.
Tracy Vermeyer is a research hydraulic engineer at the Bureau of Reclamation's Water Resources Research Laboratory in Denver, Colorado. He assisted with the design of the Shasta temperature control device.