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Reservoir releases improvements

Boosting Oxygen Concentrations

The problem

During the summer, 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 by 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 is not replaced. By the end of the summer, oxygen supplies near the bottom of the reservoir 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.

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.

Aerating Turbines

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Hub baffles on the underside of a turbine cause air to be drawn into the water when the turbine is running. Air bubbles transfer their oxygen to the water.

Aerating turbine technology uses low-pressure areas to draw air into the water 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

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Surface-water pumps help oxygenate the water below Douglas Dam.

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 intakes and mounted on floats attached to a rail system so they can move up and down as the water level changes.

Oxygen injection systems

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Injection systems change oxygen from a liquid to a gas and force it through diffuser hoses deep into the reservoir.

At some reservoirs, oxygen is injected into the water before it enters the dam's intake. The system consists of an oxygen tank and evaporators on the bank that are connected to perforated hosing suspended above the reservoir floor upstream of the dam. It’s the same type of hosing that’s used in gardens for irrigation. Gaseous oxygen, instead of water, is pumped through the hosing, creating oxygen bubbles that are released into the reservoir along the length of the hosing.

Aerating weirs

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The infuser weir below Chatuge Dam consists of decking made of metal grates. As water flows across the metal decking and falls through the small openings, it becomes oxygenated.

These are small dams designed to mimic a natural waterfall, adding oxygen to the water as it plunges over the top of the weir walls. 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 waterfall, and a more compact structure called an infuser weir that uses a slotted decking 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 dam.

 

Tributary dams

To see what TVA is doing about the problems of low oxygen concentrations and low flows at tributary dams, choose from the list below.

Main-river dams

View information on improvements at main-river dams.

           
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