Russell W. Ray, senior associate editor, Hydro Review
July 01, 2010 | 0 Comments
It is a hulking mass of concrete, built to perform a delicate task.
"The wall's stability and structural integrity comes from the 178 anchors, averaging 120 feet in length. To withstand the dynamic forces of the river's flow, these rope anchors will put compression loads into the wall to hold it firmly to the river bottom." Pat Duyck, project manager, U.S. Army Corps of Engineers.
It took 17 months and more than $45 million to build. It is 830 feet long and more than 40 feet tall.
More than 1,000 truckloads of high-quality concrete were used to build the large spillwall, the latest improvement at 1,800-MW The Dalles Dam hydroelectric project on the Columbia River.
The spillwall is designed to steer young salmon to a safer part of the river, away from ambushing predatory fish and birds. The spillwall, which was completed in March 2010, is expected to boost survival rates for yearly chinook and steelhead by 4 percent.
“The wall appears to be doing what we anticipated it would do,” said Gary Fredricks, a fisheries biologist with the National Oceanic and Atmospheric Administration. “The flow is doing what we expected.”
The structure could help The Dalles Dam, overseen by the U.S. Army Corps of Engineers, meet or exceed salmon survival goals established by a federal plan, known as a Biological Opinion, for protecting salmon and steelhead listed under the Endangered Species Act.
Shaped like a giant hockey stick, the spillwall creates an escape route for endangered juvenile salmon by guiding them to the deepest, fastest, and safest part of the river downstream from the dam.
About 80 percent of migrating juvenile salmon pass through the dam’s spillway. The rest go through the powerhouse turbines or trash sluiceway.
Before the new spillwall was built, the spillway’s fast-flowing waters would propel young salmon toward the shallows, where they were easy prey for smallmouth bass and other predatory fish. Still, the survival rate was above 90 percent, just shy of the targets set by the Biological Opinion. The new spillwall is expected help the dam reach those standards.
“Every percentage of survival improvement is very difficult to get when you’re already over 90 percent,” Fredricks said. “We’ve been working on these mitigation issues for decades now, and we’re getting down to the bottom of the barrel in terms of easy things to do.”
Several studies on salmon survival at The Dalles Dam will be performed to measure the project’s success, Fredricks said. Some studies are now underway. Fish with acoustic tags have been released upstream above John Day Dam and are being tracked by biologists.
“We are monitoring the survival of those fish all the way down to the lower part of the Columbia River,” Fredricks said.
Examining the options
The spillwall project was federally funded under the Columbia River Fish Mitigation Program. A $45 million construction contract was awarded to General Construction Co., a subsidiary of Kiewit Corp.
In 2004, the Corps built a 100-foot spillwall between bays 6 and 7 to disrupt a lateral flow that seemed to steer the salmon toward waiting predators. The lateral flow was eliminated and fish mortality and injury decreased. But salmon survival rates were still low due to predatory fish and birds in shallow waters.
A study of fish survival at the dam showed that more direct conveyance from the spillway to a deep, swift-running channel known as a thalweg would likely improve survival rates for juvenile salmon.
The modeling, design, and construction of the new spillwall between bays 8 and 9 were performed by a team of fisheries biologists and engineers who worked closely with state and federal agencies and tribal representatives at the Corps’ Engineering Research and Development Center (ERDC) in Vicksburg, Miss.
“All the right people worked together right there at ERDC on the biological, technical, and structural elements of the wall’s design,” said Bob Wertheimer, the Corps’ lead fisheries biologist on the project. “All options or ideas to increase fish survival were considered.
The team considered an extension of the 100-foot spillwall to the thalweg. But modeling showed the extension would likely cause an unacceptable increase in total dissolved gas, which can be harmful to fish. The new spillwall is not expected to cause significant changes in total dissolved gas.
The new spillwall runs straight downriver from the middle of the 20-bay spillway and bends near the end to ensure salmon reach the thalweg.
The team built physical and computer models to test their concept. Colored dye tests confirmed that a wall built between bays 8 and 9 with a slight bend at the end could lead to significant fish passage improvements.
“We did very extensive model investigations of this before we actually started pouring concrete,” Fredricks said. “We used our knowledge of predator behavior and habitat to outline high risk predation areas.”
A challenging construction
The spillwall plan was reviewed in technical committees that included representatives of the Corps, NOAA Fisheries Service, the Bonneville Power Administration, the U.S. Fish and Wildlife Service, Idaho, Oregon, and Washington fish-management entities, the Columbia River Inter-Tribal Fish Commission, and the Northwest Power and Conservation Council.
Construction began in October 2008 and involved synchronizing complicated and potentially dangerous operations.
The primary juvenile fish passage period at The Dalles Dam is April through November. To limit the effects of construction on the annual migration of juvenile and adult salmon, in-water work was limited to October through March.
A team of divers and a large support crew maneuvered barges, cranes, and metal forms to build large concrete leveling slabs on the uneven basalt river bottom downstream of the spillway apron.
“With the amount of equipment, people, barges, and boats, there was little margin for error,” said Rick Benoit, dive coordinator and dive safety officer. “Safety awareness and accident prevention are keys here, and everyone has a role.”
Contractors cast 43 10-foot-wide concrete blocks, some weighing up to 200 tons. Another team of divers kept the slabs free of debris to ensure a level surface for the blocks. Crews then placed concrete underneath each block and filled the hollow cores to ensure full load transfer to the underlying surfaces.
Anchoring the wall deep into the bedrock came next, which was vital to the project’s success. Workers lowered drilling equipment through four precast, vertical ducts in each block and drilled through the leveling slabs and into the bedrock. Wire ropes were then strung through the ducts and slabs and grouted into the riverbed.
“The wall’s stability and structural integrity comes from the 178 anchors, averaging 120 feet in length,” said Pat Duyck, project manager. “To withstand the dynamic forces of the river’s flow, these rope anchors will put compression loads into the wall to hold it firmly to the river bottom.”
Each anchor is locked off to the top of the wall at 1 million to 1.5 million pounds of force. That means up to 6 million pounds of force is being used to hold each block in place. Nearly 200 anchors were used, with each anchor penetrating up to 120 feet into the bedrock.
“The tension on those anchors is about the same amount of weight as a Boeing 747,” Fredricks said.
The first 200 feet of the spillwall is 43 feet high and the rest of the wall is between 25 feet- and 40 feet high, rising 15 feet above the normal level of the tailrace.
“I was amazed at what our engineers were able to come up with,” Fredricks said. “We knew what we wanted to see, but to make it happen was pretty ingenious.”
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