Tennessee Valley Authority
Reservoir releases improvements
Boosting Oxygen Concentrations
The problem
During the summer months, a process called thermal stratification occurs in many deep tributary reservoirs. The water stratifies, or separates, into two layers: a warm surface layer that’s relatively rich in dissolved oxygen and a colder bottom layer. The oxygen in the lower layer is gradually used up as organic material—which is washed into the reservoir when it rains or is discharged from sewage treatment plants, industries, or other sources—settles to the bottom and decays. The two layers of water don’t mix because of the temperature difference, so the oxygen in the lower layer isn’t replaced. By the end of the summer, oxygen supplies near the bottom can be entirely depleted.
Hydroturbine intakes typically draw water from these deeper levels, creating low-oxygen conditions downstream of the dam. This can cause problems for fish and other types of aquatic life, which depend on oxygen as much as creatures living on land do.
View a video on water quality and dissolved-oxygen levels in the TVA reservoir system.
The solutions
Because conditions are different at each dam, TVA uses a wide range of methods to improve dissolved-oxygen concentrations. In some cases, more than one approach is necessary to reach oxygen targets, which vary depending on the type of fishery in the tailwater.
Hub baffles on the underside of a turbine cause air to be drawn into the turbines when they’re running. Air bubbles transfer their oxygen to the water.
• Turbine aeration – Turbine aeration technologies use low-pressure areas to draw air into the turbine as power is being generated. At some dams, TVA has modified the existing turbines to draw air into the water. At other dams, TVA has installed new turbines specifically designed for this purpose.
Surface-water pumps help oxygenate the water below Douglas Dam.
• Surface-water pumps – These pumps, resembling big ceiling fans, push warm, oxygen-rich surface water downward, where it is mixed with low-oxygen bottom water and then drawn in by the turbines during generation. The pumps are positioned above a dam’s turbine intakes and mounted on floats attached to a rail system so they can move up and down as the water level changes.
Injection systems change oxygen from a liquid to a gas and force it through diffuser hoses deep into the reservoir.
• Oxygen injection systems – At some reservoirs, oxygen is injected into the water before it enters the turbine. The system consists of an oxygen tank and evaporators on the bank that are connected to diffusers, perforated hoses suspended above the reservoir bottom upstream of the dam. It’s the same type of hose that’s used on land for irrigation. Gaseous oxygen, instead of water, is pumped into the hose, creating oxygen bubbles that are released into the reservoir along the length of the hose.
The infuser weir at Chatuge Dam consists of a deck made of wooden timbers and steel gratings. Water becomes oxygenated as it falls over the weir.
• Aerating weirs – These are small dams designed to create an effect similar to that of a natural waterfall, adding oxygen to the water as it plunges over the top of the weir walls when turbines are running. Aerating weirs are located a short distance downstream from dams. TVA has designed, built, and tested two different kinds: a long W-shaped structure called a labyrinth weir that creates a zigzag waterfall, and a more compact structure called an infuser weir that uses a slotted deck to create a series of waterfalls. Weirs also serve to maintain minimum flows when hydroturbines are not operating; pipes near the bottom of the weir allow slow drainage of water from the weir pool.
• Low-pressure air blowers – These devices are used to force air into the water flowing through a turbine.