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Fish Passage

IIHR Engineers Make Dams More Fish-Friendly

A group of six men and one woman stand in a hydraulic model. They are wearing waders.

Pictured in the 1:64-scale Priest Rapids tailrace model are IIHR Director Larry Weber (front, center) and (clockwise from center) Director of Engineering Services Troy Lyons, Staff Engineer Andy Craig, graduate students Antonio Arenas Amado and Ali Reza Firoozfar, Assistant Research Engineer Justin Garvin, and Associate Research Engineer Marcela Politano.

The tricky part of designing a safe passage for fish through a major dam is learning to think like a fish.

Getting fish safely past major hydroelectric dams has been an issue since the early 20th century. Engineers and biologists were initially surprised when the dams had a big impact on fish moving downstream. When fish passed through the turbines, rapid pressure changes left them stressed and susceptible to waiting predators.

Engineers have been working on better ways to get fish safely past the dams ever since. IIHR researchers have been involved in fish passage research since the 1930s, and working with the Grant County Public Utility District (PUD) in the Pacific Northwest for more than 30 years.

The Smoking Gun

Larry Weber, former director of IIHR, was part of a team of IIHR engineers trying to analyze data tracking fish movements near a major hydroelectric dam in the mid-1990s. The researchers saw that some of the fish were swimming right up to the dam and then retreating.

“That’s the smoking gun, right? Why are those fish leaving?” Weber asks.

A breakthrough came soon after when Weber met John Nestler, a fisheries biologist (now retired) with the U.S. Army Corps of Engineers. They realized their work had followed similar paths, but Nestler had focused primarily on the fish’s perspective. “He knew way more about what a fish might like hydrodynamically,” Weber says.

Weber and Nestler began working together, with funding from the U.S. Corps of Engineers. The IIHR team provided flow data developed using computational fluid dynamics (CFD), while Nestler analyzed how fish responded to the flow.

Nestler and his colleague Andy Goodwin also developed a computer model to predict how fish would move in a particular flow environment. For instance, a fish usually swims nose first when it’s in a low-energy flow field and actively trying to reach the ocean. “When the energy level in the flow gets high, or the velocity gets higher, it turns around and swims tail first,” Weber explains. “If it gets bad enough that they want to escape, they’re pointed in the [right] direction.”

This is an aerial photo of water rushing through the fish bypass on the Wanapum Dam.

Wanapum Dam fish bypass

Using new acoustic data, researchers were able to accurately triangulate the positions of the fish and learn more about their behavior near the dam. They found that fish swimming up to a guide wall, even at night, would track along it until they sensed a change in the flow they didn’t like. “They couldn’t see it,” Weber says. “They were responding to the flow field.”

They also noticed that the fish seemed to dislike rapid flow acceleration. “The fish sense that acceleration and they’re avoiding it,” he explains. “They’re going in tail-first, and thinking, ‘Oh, this isn’t right.’”

After retreating, the fish mill around and may go up to the dam to test the waters again. “We have records of fish going up and trying this four or five times,” Weber says. Some fish eventually choose to go through the turbines instead.

Think Like a Fish

The question is, why? Apparently, Weber says, they prefer the smoothness of the flow acceleration up to the turbine. According to Nestler, what matters is the hydrodynamic environment a fish experiences.

Andy Goodwin wrote algorithms that reflected this new knowledge. Their resulting computer model (known as the Numerical Fish Surrogate) was an important breakthrough. “It’s the only model of its kind,” Weber explains. “We continue to apply it at a number of different dams.” These insights led to new fish passage designs with flared entrances to make the flow accelerations more gradual.

As important as the Numerical Fish Surrogate is, Weber says, it needs to be accompanied by good engineering judgment, good biological judgment, and careful laboratory and numerical simulations. “It’s not the only tool in our toolbox,” Weber says.

The fish passage system now in use at the Wanapum Dam in Washington State was developed using this combination of scientific tools and methods. It has helped the utility achieve a 95 percent survival rate for fish passing the dam.

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Last modified on June 11th, 2019
Posted on January 18th, 2011

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