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 Table 4.6.2
Summary of Pneumatic Destratification
Characteristic
Description
Reservoir conditions that are initiated or aggravated by thermal stratification
or that may be improved by mixing (i.e. anoxic conditions, nutrient cycling,
Targets
chemical resuspension, algal blooms, etc.).
A continuous bubble stream is used to create recirculation cells in the
Mode of action
reservoir to mix or circulate hypolimnetic and epilimnetic waters.
Effectiveness
Very effective.
Longevity
Months to years.
low DO could create water quality problems downstream. Localized mixing is a technique that is
being used as a means to improve the release quality of some projects which exhibit these
characteristics. It can also be used to improve the water quality in localized areas in the reservoir by
mixing epilimnetic with hypolimnetic waters.
4.6.2.2 Theory
Localized mixing is a technique that provides enough mixing in a local area to reduce, if not
eliminate, the thermal stratification. This is usually accomplished by pumping surface water down into
the hypolimnion to achieve a locally uniform vertical temperature profile (Figure 4.6.2). The jet from
the pump provides the energy needed to disrupt the stratification and causes entrainment of the
epilimnetic waters in the release discharge to enhance the water quality.
This technique, as indicated by its name, is used to mix only a small or localized area of the
reservoir. Therefore, only the area of concern as related to the localized mixing objective is impacted.
In the case of improvement of release quality, the intended impacts are related only to the release
volume, not the entire reservoir. If this technique were used to improve the water quality of a small
cove or marina, only the area under consideration (such as the cove or marina) enters into the design
methodology. This also minimizes costs of the system.
4.6.2.3 Design Methodology
The design of a localized mixing system will usually involve determination of the desired water
quality to be achieved in the release. A general procedure for design of a system was given by Holland
(1983). The procedure described below is modified from Holland (1983) to include recent
developments.
4.6-7
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