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 retrofit of a plastic curtain to serve as a submerged weir and allow the release of warmer surface water.
It was also to be designed such that it could be raised to permit the release of bottom water (CH2M
Hill 1977). Although evaluation of this technique indicated that release quality could be met, some
concern over potential blockage of hydropower intakes if the curtain broke free from moorings has
delayed implementation of this technique.
4.3.4.5 Summary
The submerged skimming weir is a technique that will modify,-the withdrawal zone of the
release structure, thereby modifying the release quality during the stratified periods. This technique has
been used at large CE projects for maintaining release quality at peaking hydropower projects that
maintain a relatively stable pool elevation. A design approach using numerical models to simulate the
effects of hydrological and meteorological conditions on the reservoir was identified. Several projects
for which weirs have been evaluated and/or installed were discussed. Summary information on this
technique is presented in Table 4.3.4.
Table 4.3.4
Summary of Submerged Skimming Weir
Characteristic
Description
Improvement of the release water quality by restricting the withdrawal to
Target
the epilimnion of the reservoir.
A submerged weir in the vicinity of the intake is used to block the
Mode of action
withdrawal of hypolimnetic water and allow the release of epilimnetic water.
Effectiveness
Effective at given flow rates.
Longevity
Years.
Negative features
May strengthen the thermocline in the main part of the lake.
Limits the transport of sediments thr6ugh the project.
May increase the anoxic conditions (and related concerns) in the
hypolimnion.
Costs
Expensive (construction).
Applicability to
Very applicable, even to reservoirs with selective withdrawal structures.
reservoirs
4.3-14
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