Taming Total Dissolved Gas
When a human diver comes to the surface from deep water too fast, the change in water pressure can cause a painful and sometimes fatal condition known as the
bends. A similar condition, gas bubble disease, can harm or kill migrating fish as they pass through large hydroelectric dams.
Marcela Politano, an IIHR associate research engineer, is conducting research that contributes to systems that help fish pass safely through these dams. A native of Argentina, Politano is also an adjunct assistant professor of civil and environmental engineering at the University of Iowa.
Preserving Water Quality
Politano is leading an IIHR research team working with the Idaho Power Company, which operates hydroelectric dams on the Columbia and Snake rivers. These two rivers are among the largest sources of hydropower in the United States. “Hydropower is the most important renewable source of energy,” Politano says. Hydroelectric power does not produce greenhouse gas emissions; the electricity is generally available as needed; and the reservoirs can be used for numerous purposes, including recreation.
But hydroelectric power can have negative impacts as well. “Hydroelectric projects have altered the natural habitat,” Politano says. One environmental concern is total dissolved gas (TDG), or the amount of gas present in water. Elevated TDG can occur immediately downstream of the dam—known as the tailrace—and farther
downriver. Elevated TDG harms many aquatic species, including salmon. Fish exposed to water with elevated TDG can develop gas bubble disease.
Recent work on the Hells Canyon Dam is designed to help the utility company meet state and federal regulations for water quality standards, including TDG.
Politano’s research focuses on numerical modeling of TDG. Water flowing over a dam can become supersaturated with gas as the water plunges to extreme depths below the dam. “The severity of the effect depends on the level of TDG and exposure time,” she explains.
Politano developed the first two-phase numerical model to represent the complex physics of a dam’s tailrace. Her model can also evaluate technologies designed to reduce TDG and protect fish. For instance, spillway flow deflectors designed at IIHR redirect spill water to form a surface jet that prevents bubbles deep in the tailrace.
The numerical model allows testing of these technologies before construction begins.
Graduate students Antonio Arenas Amado and Michael Carbone contribute to many aspects of the project. Arenas says he has learned about teamwork from the research. “This project was characterized by constant interaction with the sponsor,” he says. He also jointly wrote several reports and a conference paper on the Hells Canyon Dam TDG simulations with Politano and Kelvin Anderson of Idaho Power. They presented their work at the 34th IAHR World Congress.
Politano’s work is part of a long-term IIHR effort to develop a reliable numerical model to predict TDG in tailraces, complementing experimental studies. “Numerical simulations are becoming an indispensable tool,” Politano says.