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 4.6 IN-RESERVOIR TECHNIQUES: MIXERS
4.6.1 DESTRATIFICATION
4.6.1.1 Problem Addressed
The thermal stratification that develops in most reservoirs during the summer months can isolate
the hypolimnion from the epilimnion and block the transfer of oxygen from the surface to the bottom
waters. This can result in anoxic conditions in the lower levels of the reservoir, which can expand as
biological and chemical activity continues to deplete the dissolved oxygen (DO). Reservoir
destratification is an enhancement technique in which hypolimnetic and epilimnetic water are mixed to
eliminate or prevent stratification. If total reservoir destratification is not practical or desired, a system
can be designed to affect a specific region of the lake, such as in front of intake structures to improve
the water quality of the releases. Although destratification can be achieved with mechanical pumps,
pneumatic systems have been the most commonly used systems in large reservoirs.
Destratification will maintain near-uniform mixed conditions through the water column. These
conditions will impact in-reservoir and release water quality. For example, algal blooms can be
reduced by limiting the amount of sunlight the algae receive (by circulating the algae to below the photic
zone). Initial start-up of the system can increase nutrient loading and DO depletion if sediment is
resuspended and exposed to upwelling water currents. Destratification of the reservoir will tend to
reduce the habitat available for cold water fisheries, but increase the habitat for warm water fisheries. It
will warm the release waters, which may impact the users downstream. Lorenzen and Fast (1977) out-
line some of the additional benefits and consequences for reservoir destratification.
4.6.1.2 Theory
Destratification of a specific portion of the lake or reservoir can be accomplished by the
introduction of a diffused bubble plume in the water column. The rising bubbles induce a flow pattern
by entraining hypolimnetic water transporting it to the surface, where it moves out laterally. Additional
water moves in to replace the upward flow, and a circulation cell is created (Figure 4.6.1). The size
and volume of these cells are usually determined by the physical boundaries of the reservoir, the
strength of the generation source, and the configuration of the diffusers.
4.6.1.3 Design Procedures
Johnson (1984) provides a good overview of the various types of aeration/destratification
systems in use and how they operate. Also provided are some guidelines for system selection based
upon the lake characteristics and the goals of the system to be installed.
4.6-1
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