Hydropower - Environmental Impacts of Renewable Energy - page 147

• Flow continuation —Flow continuation is the mechanical capacity to main-

tain flow through the penstock during powerhouse failures.

• Intake failures —Intake failures are all penstock flow disruptions that occur

at the intake structure. Intake failures are less frequent than powerhouse
failures; they usually occur as a result of the accumulation of debris, the
failure of fish screen cleaning equipment, or failure of the dam and associ-
ated gates to deliver water into the intake.

• Cycling —A way to generate power when flow is not enough for continuous

or efficient operation; it is not an attempt to follow load demands. Cycling
will normally occur at low stream flows when the salmonids would be most
vulnerable to fluctuations.

• Multiple turbine operations —If a powerhouse has two or more turbines,

operators can cause abrupt changes in flow when changing the number of
turbines in operation.

• Forebay surges —Forebay surges occur when the powerhouse of some run-

of-the-river plants start generation and are probably caused by a drop in
head at the intake during start-up.

• Reservoir stranding —In large reservoirs, stranding is routinely anticipated

as one of the consequences of drawdowns, and it is sometimes employed as
a method of eradicating undesirable fish.

• Tailwater maintenance and repair activities —All hydropower plants will

eventually require inspection, maintenance, and repair. However, it is often
impossible to inspect or repair the structure or equipment submerged in the
tailwater without completely or substantially disrupting flow to the river.

LOW WATER LEVELS AND EVAPORATION OF RESERVOIRS
They say a picture is worth a thousand words. Well, from the photographs shown in
Figure 4.18 , it should be obvious to even the most casual observer that Lake Mead,
the reservoir formed by the Hoover Dam and the Colorado River, has a very distinc-
tive bathtub ring along the channel cliffs that hold the reservoir water in place until
the water passes through the dam's penstocks to its turbines and then back into the
Colorado River below. The obvious conclusion to be drawn from the photographs is
that Lake Mead is much lower than it has been in the past. This conclusion is correct,
of course. As of October 2010, Lake Mead was only about 40% full. The lake has
dropped 130 ft since 1999 and in 2010 was at 1084 ft, depths not seen since 1956.
Observers, hydrologists, critics, and laypersons (i.e., non-scientists) have differing
opinions as to the causes of Lake Mead's current low water level. Most of these opin-
ions have some merit. For example, the USGS (2013) observed that the bathtub rings
and lower water level in Lake Mead are the result of a “stealth disaster.” Drought is
the culprit. Drought is a stealthy incremental disaster that is much more costly to
the national economy than most people suspect. A prolonged dry spell, lasting over
a decade, is steadily draining the water sources that power Hoover Dam's turbines.
The Southwest and vast stretches of the High Plains region have suffered lengthy
drought conditions for the past several years.