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 Table 4.3.3
Summary of Temperature Control Weirs and Curtains
Characteristic
Description
To modify withdrawal characteristics to skim
surface water or skim cool hypolimnetic water for
Targets
release. To create a cool-water refuge for cold-
water fisheries.
An underwater weir or curtain is constructed to
Mode of action
skim either surface or hypolimnetic water,
depending upon operational objectives.
Effectiveness
Depends on stratification.
Longevity
Years.
May limit the lateral exchange of waters between
Negative features
the main lake and the cold-water refuge.
May act as a sediment trap.
Limits the migration patterns of certain species of
fish.
Costs
Moderate to high (site and construction specific).
Very applicable, but feasibility and effectiveness
Applicability to reservoirs
are site specific.
ports) will result in a large withdrawal zone that usually extends surface to bottom. To increase the
amount of epilimnetic water in large releases, a submerged skimming weir can be designed and
installed. This structure is designed to extend to th6 top of the reservoir thermocline and prevent the
withdrawal of the hypolimnion.
This technique should be used only at projects with minimal lake surface elevation fluctuation
during the period of hypolimnetic anoxia, since the effectiveness of the weir will diminish as a function of
the drawdown on the reservoir. In addition, a weir can strengthen the thermocline in the main portion
of the lake. This can affect a number of related concerns, such as anoxic conditions in the hypolimnion
and nutrient cycling. A weir may also act as a sediment trap, which may or may not be beneficial.
4.3-11
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