A Big Wall for Small Fish
by Kathy Gray
The Dalles Chronicle, November 30, 2008
Corps hopes spill wall will keep predators away from young salmon
Smolt have a rough life.
The perilous journey of young salmon from birthplace to sea is fraught with looming barriers and lurking predators. A new project at The Dalles Dam is designed to help them slip past one once they've made it beyond the other.
After weeks of work barges floating in front of the dam spillway, the $45 million fish recovery project broke the surface this week, as workers began placing the massive concrete pillars that will come together like a three-dimensional puzzle to form the wall.
A small crowd of dam workers lined the spillway top Monday to watch the placement of the first pillar using a 300-ton marine derrick, surveying equipment and divers.
The 200-ton, 37-foot-tall block dangled with its base just below the surface as the surveyor lined up its center line with the path the first leg of the 830-foot wall will take.
Below the surface, guide brackets marked where the piece would sit atop the stilling basin, a smooth apron beneath the water that stretches from the base of the spillway.
Young salmon make their way through the dam in one of three ways. About 10 percent go through the powerhouse and turbines. Another 10 percent travel through the blasting water of the ice and trash sluiceway that flows out of the south side of the dam.
The remaining 80 percent are the focus of the spill wall project. They make their way through the spillways from April through August when the dam is spilling water for fish.
"The dams are required to spill 40 percent of the river," explained Lance Helwig, the U.S. Army Corps of Engineers project manager for the spill wall. The biological opinion developed under terms of the Endangered Species Act guides fish protection actions. Getting past predators
Those fish that come through the spillways are the focus of the project. The idea behind it is simple: Fish that come through the fast-flowing waters of the spillway are disoriented afterward and easy prey for the predator fish like northern pikeminnow and smallmouth bass that lurk in the shallows just beyond the dam.
At present, the water from the spillway disperses the little fish toward those shallows. When the spill wall is done, it will direct those young salmon toward a deep, swift-running channel known as a thalway that runs downriver below the surface near the northern riverbank.
Predator fish don't like the fast, deep water, so the idea is that diverting the juveniles to the channel will help them regain their natural defense mechanisms before being faced with predators.
The spill wall is shaped like a giant hockey stick that stretches past the stilling basin and across the natural shelf that extends beyond it, directing water toward the thalway.
Studies performed on a scale model of the dam in Vicksburg, Miss., indicate that guiding juvenile salmon to the deepest, fastest part of the river will likely improve survival rates for yearly chinook and steelhead by 4 percent to 98 percent for fish that travel over the spillway.
"It doesn't sound like much, but 3 or 4 percent makes a big difference when you're talking about tens of millions of fish," Helwig said.
The project should allow The Dalles Dam to reach its survival targets under the biological opinion.
"The spill wall is unique and has broad regional support," Helwig added. "The scientists are all kind of in consensus that this has a high probability of being the solution at The Dalles Dam to make the biological opinion goal."
"We've spent quite a bit of time studing The Dalles and we're confident this may be the solution to get us up to survival targets," Helwig said.
Once the wall is completed in 2010, more fish studies will take place to gauge the success of the project.
The spill wall takes up more than half of the annual $80 million to $90 million budget for fish mitigation that is split between the Portland and Walla Walla Corps of Engineers districts that oversee the dams on the Columbia and its tributaries.
Big work in the water
General Construction Contractors of Poulsbo, Wash., won the contract for the project. The company has worked on a variety of fish mitigation projects on the big Corps dams, including Bonneville, John Day and Ice Harbor.
"We've been working on the Columbia for several decades," said Dan Proctor of General.
Workers have been on site since summer, precasting the heavy pillars that will fit together to make the wall.
The tallest of the pillars is about 37 feet tall and weighs about 200 tons. The anchor tendons 118 feet in length will be drilled deep into the bedrock below to hold each piece of the wall in place.
The pillars are actually leveled in place a few inches above the base on which they sit. Then forms are placed around them and concrete is poured through a round hole that runs the length of the pillar to seal the base. A second pour fills the hole.
Precasting work on the pillars has been under way at the dam since summer, but in-water work had to wait for the six-month window of time from Oct. 1 to April 1, when most of the migratory fish have made their way through the river.
The contractor will build the first 300 feet of wall - the part before the hockey-stick angle - during the current in-water work period. They will continue to cast the pillars on shore until the pieces for the entire wall are completed.
The remainder of the wall will be placed and completed between October 2009 and Apirl 2010, in time for the annual spill season, when the largest number of juvenile fish migrate downstream.
The finished wall will be 10 feet wide and about 830 feet long, with the first 200 feet being 43 feet high and the remainder being between 25 and 30 feet high. It will extend between bays 8 and 9 of the spillway from the river bottom to above the normal level of the tailwater on the downstream side of the spillway.
A diving crew will drill up to 230 rock anchors up to 120 feet into the riverbed below the wall, using ducts cast into the concrete blocks. This will provide the wall's stability and structural strength.
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