At the Whiskeytown Reservoir
By Bob Gee, Greg Morris, and Stanford W. Slifer
In California’s Central Valley, geomembrane temperature curtains provide an example of how geosynthetic materials can help stabilize environmental conditions along water diversion systems.
Located in the Whiskeytown Reservoir, about 10 miles west of Redding in Northern California’s Shasta County, one of these curtains makes it possible to selectively withdraw cold water needed for successful salmon spawning and divert it to the Sacramento River run, an area where chinook salmon are considered an endangered species. Originally installed in 1993, the curtain was replaced in 2011 with a newer model estimated to last at least 15 years.
The Whiskeytown Reservoir is just one part of a complex water reclamation system known as the Central Valley Project.
Located in the Shasta and Trinity River Division, the Whiskeytown Reservoir holds water that is on its way east toward the Sacramento River. The water takes a circuitous route, passing through several different tunnels, reservoirs, dams, and power plants before it is discharged into the upper reaches of the Sacramento River (Figure 1).
Prior to the construction of three flexible temperature-control curtains in 1993, water diverted from the Trinity River would increase 5 to 7 C (10 to 13 F) in temperature before it reached the Sacramento River.1 This increase raised the temperature above the 12 C (53.6 F) maximum temperature limit imposed on the river by the National Marine Fisheries Service. Salmon eggs and larvae require lower temperatures of 5.5 to 13.3 C (42 to 56 F) to live and any increase in water temperature can endanger their survival.
The geosynthetic curtains, installed in 1993 and located in various positions along the water division, were intended to lower the water’s temperature before it was diverted back toward the Sacramento River and the chinook salmon’s spawning grounds. Historically, the curtains have lowered water temperatures by 2 to 3C degrees, but the poor condition of the curtain located in the Whiskeytown Reservoir required a change last year.2
The original curtains were made of Hypalon®—a synthetic rubber known for its long life span and capable of surviving harsh environments. According to Bob Gee, a mechanical engineer with the Bureau of Reclamation, after 17 years of service this curtain in Whiskeytown was riddled with holes. “The holes were caused by the curtain rubbing against chains linking buoys holding the top of the curtain to anchors on the lake bottom,” Gee said.3
“It just had deteriorated,” said Brian Person, manager for the Bureau of Reclamation’s Northern California office. “It was time for a replacement.”
The old curtain was removed in the fall of 2010 and the new curtain was installed and made operational in 2011.
Redesigned using a 60-mil, reinforced polypropylene geomembrane, commonly installed in wastewater lagoons, industrial waste ponds, or fish hatcheries, it took almost 260,000ft2 to complete this job. Because of the deterioration of the Hypalon curtain, the Bureau of Reclamation decided to use polypropylene instead, hoping that it would better withstand the rubbing against the chains.
Welding the panels together was also difficult. According to Gee, just finding an open area ashore large enough for assembly of the curtain was difficult. “Working with the National Park Service, we were able to utilize a parking lot at a beach prior to the summer recreation season,” Gee said. “It measures 2,400 feet in length and reaches depths of approximately 100 feet. The curtain was hot-spliced on the reservoir shoreline and assembled as one continuous sheet of polypropylene.”
To help save space, the fabricated curtain was floated out into the water even as the panels were being completed. A conveyor system was used to help clamp the curtain into 20-foot-long metal floats and deploy it into the reservoir without dragging it along the ground. While some of the original floating tanks were reusable, many needed to be replaced or repaired. Other existing components, such as lake anchors, chains, cables, and surface stabilizing tanks were inspected prior to installation and some were also reused.
Once finished, the curtain was anchored to the bottom of the reservoir by 800-lb weights. Designed to float vertically, the curtain stretches 2,400ft from shoreline to shoreline.
How it works
Located in the headwaters of the Whiskeytown Reservoir is what is known as a “plunge zone.”4 Cold water diverted from the upstream Lewiston Reservoir enters Whiskeytown after it is discharged through the Carr Powerhouse. The cold water plunges beneath the surface water and travels downstream, producing stratified water flow. Another geosynthetic curtain, located near the discharge point, prevents the cold water from mixing with the warmer water in the reservoir and keeps it moving toward the lower end of the reservoir.
When it reaches the base of the reservoir, the water passes under the recently replaced curtain and through an intake. Because the reservoir is thermally stratified and the curtain is designed to leave a 30-ft gap between the bottom of the curtain and the bottom of the lake, it is possible to draw in only the cold water. After the water moves through the intake, it continues on through a tunnel and on toward the Sacramento River.
The $3 million Whiskeytown geomembrane curtain replacement project was completed in June 2011, a month ahead of schedule.
According to Gee, the new 2,400-foot-long curtain should last at least 15 years. Historically, the curtain has reduced water temperatures by 2-3C degrees during late summer and early fall to facilitate salmon spawning. The new curtain is expected to yield similar results and be more durable than the previous one.
The success of the thermal curtains installed in the Shasta and Trinity River Division in 1993 and the recent replacement of the Whiskeytown curtain are a prime example of how geosynthetics can help rectify environmental problems caused by water reclamation systems. The Whiskeytown curtain makes it possible for power plants and salmon to exist within the same water system. As more demands are placed on our natural resources, more innovative solutions such this one are required.