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bubble dome diffusers were equally spaced along each length of pipe (32 diffusers total). During both
tests, the reservoir was allowed to stratify before the system was started.
Destratification of the entire reservoir occurred in 15 days and significantly reduced the iron and
manganese levels. The reservoir was allowed to begin to restratify afterwards. The effects on turbidity,
algae, iron, manganese, dissolved oxygen, and nutrients were monitored.
A similar system was tested and monitored on the Little Para Reservoir, South Australia
(Croome 1981). Two 15-m-long pipes, each with 20 diffusers (40 total), were located on the face of
the dam near the level of the lowest intake port. Total destratification of the reservoir was
accomplished in approximately 10 days. A smaller compressor (175 L/sec) will be installed
permanently for continuous operation through the stratification season, beginning once the thermocline
has established itself (or the hypolimnion begins to become anoxic).
Bowen (1981) examined a system installed on Prospect Reservoir, Sydney, Australia, to
correct hypolimnetic oxygen deficiency. Rather than formed diffusers, holes were drilled in the pipe to
act as diffusers (this system is similar to the design procedure proposed by Davis 1980). The system
was started after stratification had been established and was stopped after the reservoir had become
isothermal (about 30 days).
4.6.1.5 Summary
Pneumatic destratification can improve the water quality within and released from a reservoir.
This technique uses a rising bubble plume to create vertical mixing. Various forms of pneumatic
destratification systems have been installed with varying degrees of success. In general, linear diffusers
strategically positioned on the lake bottom are supplied by a compressor located on the shore. The air
bubbles released through the diffusers rise through the water column and create an up-welling water
current that spreads out laterally upon reaching the surface. Circulation cells are set up to replace the
upward moving water, and eventually the water column becomes mixed to the level of the diffuser(s).
A method for designing a system was presented, along with references for computational procedures.
Table 4.6.2 summarizes information concerning this technique.
4.6.2 LOCALIZED MIXING
4.6.2.1 Problem Addressed
The thermal stratification that occurs at most reservoir projects can create problems with
release water quality. If the hypolimnion becomes isolated from the surface and turns anoxic, reducing
conditions for iron, manganese, and hydrogen sulfide will prevail. If a hydraulic structure withdraws
water from the hypolimnion, release of these soluble constituents as well as
4.6-6

 


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