Dam Tailwaters - Hydro-Environmental Analysis: Freshwater Environments - page 503

estimated a federal hydropower loss of $109,920,200, and a loss to the nonfederal project of
$41,319,400. Those losses were to be mitigated, in part, by a relief from the federal project debt and
compensation for FERC licensing (USACE 2008). This example illustrates some of the complexi-
ties and costs that may be associated with the reauthorization of projects.

18.4 METHODS TO IMPROVE TAILWATER QUALITY

18.4.1 r eLeaSe M anaGeMent
Release management could involve the timing, magnitude, and duration of releases. The target
could be either to meet some tailwater low or water quality objective or both.
Particularly during periods of stratiication and hypolimnetic DO depletion, the magnitude and
location of the withdrawals can have large impacts on the release temperatures and water quality.
Some reservoirs have the capability of withdrawing from multiple levels (see selective withdrawal,
Chapter 10) so that they can manage the zone from which water is withdrawn from the reservoir in
order to manage the water quality of those releases (e.g., as a function of density and quality vertical
stratiication). Changes in operations such as start-up procedures for turbines can also inluence the
vertical zones of withdrawal from the reservoir and manage water quality.
The timing and duration of the release low also directly impact tailwaters. Minimum lows, in
addition to other beneits, may be used to lush waters from the tailwaters, preventing stagnation
and the resulting oxygen problems. Minimum lows may have many other beneits for tailwater
isheries and downstream users. Flow pulsing, separate from hydropower or lood release, is also a
tool used to manage tailwater quality and biota, such as lushing streambeds, scouring vegetation,
etc. Spillage over structures may also be used to increase oxygen concentrations. The spillages,
however, may also increase problems associated with TDGs and reduce the hydropower capacity.

18.4.1.1 Aerating, Venting Turbines
Turbine venting is a method that is used to increase the oxygen concentration of reservoir releases
from hydropower facilities. Turbine venting introduces or injects oxygen into the low as it passes
through the turbines and is often a relatively economical alternative. The forced injection of oxygen
into turbines has been used to increase the oxygen concentrations of reservoir releases. Perhaps a
more common practice is autoventing. In autoventing, areas of low pressure are used to draw or force
oxygen into the water. Hubs or bafles are often used to reduce the pressure, thereby increasing the
aspiration of oxygen, as illustrated in Figure 18.10 for Norris Dam, Tennessee (including graphical
comparisons indicating improvements in tailwater oxygen concentrations). However, techniques such
as autoventing turbines are very site-speciic and outcomes will vary considerably (USEPA 2006).

18.4.1.2 Gated Conduits
Gated conduits are also used to increase the release of oxygen concentrations. Gated conduits are
hydraulic structures that divert the low of water under the dam and are designed to create turbulent
mixing to enhance oxygen transfer (USEPA 2006). A coniguration is illustrated in Figure 18.11,
where a structure such as a sluice gate is used to create high downstream velocities with reduced
pressures, which are then vented to increase the oxygen concentrations and downstream pressures
(e.g., to prevent cavitation).

18.4.1.3 Oxygen or Air Injection Systems
These systems introduce air or oxygen into the upstream reservoir to improve the oxygen concentra-
tions of releases. They include surface-water pumps, like big fans, that push or pump oxygen-rich
air into the withdrawal zone or inject oxygen directly. Oxygen injection systems usually take liquid
oxygen, convert it into a gas, and pump it through diffusers to the reservoir hypolimnion or directly
into the withdrawal waters prior to them entering the reservoir intake structure.