Boosting Oxygen in the Tennessee Valley Tailwaters
TVA cares for the creatures who live below its dams, and has devised several good ways to add dissolved oxygen to the tailwaters to enrich aquatic life.
Not long after TVA built the system of dams throughout the Tennessee Valley, TVA's engineers and biologists learned of a problem. The aquatic populations in the waters downstream of its dams (a.k.a., tailwaters) were suffering and slowly diminishing due to the lack of dissolved oxygen and intermittent drying of riverbeds. All living things need oxygen, even those creatures found beneath the surface of the water—fish, mussels, insects and plants.
Throughout the summer to early winter, the cold deep layer of water upstream of a dam is too dark for plants and algae to grow and produce oxygen through photosynthesis. As a result, this deep layer of water becomes anoxic (without oxygen) due to the oxygen being consumed by bacteria and other bottom-dwelling organisms. As hydroelectricity is produced at a dam, this oxygen-deficient water flows through the dam and ends up downstream—where the problem lies.
In 1991, TVA began making efforts to correct this shortcoming. Five methods for increasing dissolved oxygen in Valley tailwaters are listed below. Since conditions are unique to each dam, different solutions are needed in different places. Dissolved oxygen tailwater targets range from four to six milligrams per liter of water. The following aerating technologies, or a combination of two or more, are used throughout the middle and eastern Tennessee Valley to help meet the dissolved oxygen goals.
Autoventing turbines draw air into the water as it swirls through the scroll case of a dam, rotating the turbine during hydroelectric generation and then increasing the concentration of dissolved oxygen in the tailwater before the water continues downstream.
Air Compressors or Low-Pressure Blowers
Similar to the autoventing turbine, air compressors or low-pressure blowers increase the concentration of dissolved oxygen in hydropower releases by mixing air with water passing by the turbine runner and aerating the flow.
Surface-water pumps are rail mounted on the upstream face of a dam and look and work much like an oversized ceiling fan. This system pushes oxygen-rich surface water downward toward the intakes of the dam. From there, the water flows through the intake, penstock, scrollcase, through the rotating turbine, out the draft tube and finally downstream where it benefits all aquatic life.
This system is installed upstream of a dam, but benefits downstream aquatic life. Miles of perforated hosing (similar to the common garden variety of soaker hosing) have been deployed in nine reservoirs throughout the Valley. Liquid oxygen is converted to a gaseous form and is injected into the perforated hosing, suspended just above the reservoir floor, creating thousands of tiny effervescent-like bubbles. Just like the surface-water pumps, the flow carries this oxygen-rich water through the dam during generation and then downstream.
Aerating weirs are small dams located not far downstream of a larger dam and provide two benefits. First, they maintain a minimum tailwater flow when the larger upstream dam is not generating hydroelectric power. During generation, a pool of water is created between the downstream weir and the upstream dam. When generation ceases, water from the weir pool is slowly released feeding the downstream riverbed with a continuous flow of water—preventing the riverbed from drying and stranding or exposing river creatures. The second benefit of a weir is the aeration of the stream. As water flows over a weir, oxygen is added to the water as it plunges over the weir—just like a natural waterfall.
Studies have shown since these aeration innovations were installed and minimum flows were enhanced, the ecological health of over 300 miles of Tennessee Valley rivers continue to improve and aquatic life is once again thriving.