Surface release can be accomplished with an overflow spillway (usually for uncontrolled releases
from floods), Tainter gates (or other types for controlled releases of floods or supplemental spillage), or
intake structures with surface withdrawal capabilities (Figure 1.2.12 and Figure 1.2.13). Bottom
releases are accomplished with gates conduits near the base of the dam (sluice gates) (Figure 1.2.12)
or an intake structure with bottom withdrawal capabilities (Figure 1.2.14). Mixed releases are usually
accomplished with an intake structure with ports at several elevations for selective withdrawal (Figure
1.2.14) or are the result of a large withdrawal zone during stratification at hydropower projects with
large penstock openings (Figure 1.2.15). Other mechanisms for mixed releases include using
combinations of available release features. For example, bottom releases through sluice gates can be
augmented with a minimal spill from flood gates to increase aeration. Many hydropower projects also
operate a small unit for local power needs and the withdrawal for this unit may not coincide with the
withdrawal for the main hydropower units or releases during nongeneration. This type of operation
would result in mixed releases. Effects on water quality as a function of release regimes may be
evaluated with knowledge of water quality processes as briefly described in preceding sections and can
be predicted for selected constituents with models.
Pool fluctuations (changes in surface elevation including the tailwater region) are also determined
by reservoir operations and can impact uses such as fish and wildlife habitat and recreation. Generally
a water control plan is developed for each project and is defined by project authorization (purposes for
which the project was constructed) and an area-capacity curve indicating available water supply at a
given elevation (Figure 1.2.16). Operations and water allocation also affect the retention time (defined
as the volume divided by the discharge) of the reservoir water which in turn effects the water quality in
the reservoir and its release.


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