Fish leap ladders, thanks to 2D CAD

DN Staff

November 4, 2002

4 Min Read
Fish leap ladders, thanks to 2D CAD

When migrating salmon commute between their feeding grounds in the ocean and their breeding grounds in freshwater rivers, they have one big problem-dams.

"There are two ideas with fish passage: to get upstream they need a ladder, and to get downstream you need to corral them, divert them around the turbines, then dump them out; it's basically a water park ride," says Greg Savage, a water resource engineer and environmental hydrology specialist.

Using simple software tools, Savage designed a fishway that could handle both challenges-no CAE stress analysis or 3D CAD, just Mathcad for his equations and a simple 2D CAD package.

The Oregon Dept. of Fish and Wildlife was trying to make life a little easier for salmon in the Willamette River. So they asked Savage and his engineering team at Bookman-Edmonston (Sacramento, CA) to renovate 1960s-era fish ladders in a dam.

Sounds straightforward, but there were three extra challenges: a 50-ft-tall waterfall, a power plant, and Pacific Ocean tides. Because of those variables, water levels varied greatly. The average flow year round is 30,000 ft3 /sec, but can fall to 10,000 in a drought, or rise to 150,000 in a flood. That can be a problem at both extremes: fish depend on a river's steady flow to find their way upstream. But when flow is too fast, turbulence disorients the fish. So Savage had to create a constant water discharge from the bottom of the ladders: 7 ft3 /sec, plus or minus 1 ft3 /sec.

He used Mathcad software (from Mathsoft, Cambridge, MA) to model the hydraulics. Savage calculated that to create a clean flow for the fish to follow, water had to drain from the fishway through a weir "shaped like a green onion stalk; it's narrow, then it tapers near the bulb, and the bottom is bulbous." Indeed, the result is shaped like a giant hourglass, 23 ft tall, varying from 1.5 to 6 ft wide, with vertical slots. Compared to the old method, hoist-lowered iron or concrete gates, the new weir could work in nearly any flood condition.

From there, the water flows into a sequence of 40 pools, each the size of a small bedroom, Savage says. The scale is enormous: a single pool is 8 x 12 ft2, with 6-ft walls and a 30-ft ceiling. And every pool is a foot lower than the last, so altogether the fish can climb 43 ft in their rush upstream to spawn (mate) and die. The three new ladders also let scientists count and identify species as they pass through.

"I'm kind of a Mathcad junkie," Savage says. "I absolutely love it, because it prints out equations as you'd see them on paper, so you can see your work. I ran it on a PC, then I roughed in the design of the slotted weir. It's nice to iteratively optimize the shape 'til I get the performance right; I know when I need to neck it down here, or flare it out there."

He exported the final Mathcad values to an Excel spreadsheet, then tied that to his CAD model, built with SmartSketch from Intergraph (Madison, AL). Instead of doing stress analysis, he worked with a structural engineer to specify details like bolt and rebar size. The structure itself was 18-inch thick concrete formed in sections in a series of monolithic pours.

"I had the joy of climbing through an entire fishway all the way through," Savage says. "It's kind of like walking into a cathedral, except for the rotting fish, tree debris, and silt."

Above the ladder, the dam itself is just 6 ft high, wrapped around the top of the half-mile, horseshoe-shaped waterfall. A power plant, paper mill, and navigation lock complicate the design. But for Savage, that was part of the thrill.

"The complexity of this project was fun," Savage says. "The construction had to happen so quickly; we had a six-week window so we could be open in time for the spring Chinook run and then the fall Chinook run."

Workers finished the job in 2000, but not before running into a final challenge-they had to design and install a sea lion gate, because the marine mammals would swim into the fish ladder and eat adult salmon. Savage's team had a great reward: they'd soon see the young smolts begin to head downstream: wild spring Chinook, wild fall Chinook, hatchery steelhead, hatchery spring Chinook, wild steelhead, and wild coho.

"It was one of those once-in-a-lifetime projects, and it was a gas," Savage says.


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