Dam Tailwaters - Hydro-Environmental Analysis: Freshwater Environments - page 497

18.2.4 e ntraInMent and I MpInGeMent In p uMpback
Peaking hydropower facilities have the advantage that they can generate during peak demand peri-
ods. The energy that they generate is a function of the elevation head and the lows that they can pass
through turbines. Therefore, the water in the reservoir can be viewed as stored energy.
Since the value of that energy varies with the demand, such as over the course of a day, the stored
energy is essentially moved around to make the most effective use of it when the demand (hence
economic value) is greatest, called load balancing. One load balancing strategy is to store releases
in the reservoir tailwaters and then pump that water back into the reservoir when demands are low
(pumpback). This allows for the “reuse” of that water (energy) during periods of peak demand.
The interest in pumpback operations, also referred to as pumped-hydro energy storage (PHES),
is increasing in the United States and around the world, with an additional 76 GW PHES capacity
worldwide expected by 2014. In the United States, the Federal Energy Regulatory Commission
(FERC) has granted 32 preliminary permits as of April 5, 2010 (Ingram 2009; Yang and Jackson
2011), to 25 licensees who are interested in developing new PHES facilities.
One of the issues impacting tailwater storage and pumpback is the entrainment of ish during the
pumpback operations. That is, when the turbines are reversed and water is pumped back into the
upper reservoir, ish may be literally sucked into the turbine and killed by the blades. An example of
an entrainment controversy impacting the operation of a pumpback facility is the Richard B. Russell
(RBR) Reservoir between Georgia and South Carolina. The RBR Dam is a peaking hydropower
facility constructed with four conventional generation units and four reversible pump-turbines,
which began operation in 1985. The pump-turbines were planned for use in generating power and
then to be reversed and used as pumps to move water from the downstream J. Strom Thurmond
Reservoir back to the RBR Dam during periods of low power. The controversy was related to the
estimated entrainment of ish, and subsequent ish kills, during pumpback. As a result, in 1988,
the South Carolina Department of Wildlife and Marine Resources joined by the South Carolina,
Georgia, and National Wildlife Federations, iled an injunction to stop the pumpback operations. It
was not until 2002 that the injunction was lifted by the Federal District Court in Charleston, South
Carolina, and the four reversible pump-turbines were brought on-line for commercial generation.
This followed a series of studies and design alterations to mitigate or reduce ish mortality, including
a sound repulsion system, bar screens at the pumping intakes, and other measures.

18.2.5 r eLeaSe w ater q uaLIty
The water quality of tailwaters is largely inluenced by the water quality characteristics of the
upstream reservoir at the level (depth) from which the water is withdrawn. The depth from which
water is released from a stratiied reservoir can be considered to be one of the most important fac-
tors determining the composition and abundance of tailwater biota (Gore et al. 1990). Water quality
parameters of concern are usually

• Temperature

• Particulate organic matter (POM)

• DO

• Metals (e.g., iron and manganese)

• Methane

• Sulides

• Nutrients

The temperatures immediately below dams and the daily and seasonal variations in those tem-
peratures are primarily determined by the depth of the withdrawal and variations in seasonal strati-
ication. Those temperatures largely impact tailwater chemical and biotic characteristics. Reservoirs