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 Table 4.3.1
Summary of Multilevel Selective Withdrawal
Characteristic
Description
To withdraw water from a range of elevations in
Target
the reservoir to obtain the highest quality possible.
Release ports at different elevations in the
Mode of action
reservoir allow access to a wide range of water
quality conditions under stratified conditions.
Effectiveness
Very effective.
Longevity
Years, life of the project.
Additional maintenance and operational costs over
Negative features
a simpler structure.
Costs
Expensive.
Very applicable to reservoirs with a range of
Applicability to reservoirs
conditions that vary vertically in the reservoir.
4.3.2 HYPOLIMNETIC WITHDRAWAL
4.3.2.1 Description
In many reservoirs and lakes, thermal stratification during summer, combined with an oxygen
demand, results in anoxic conditions in the hypolimnion. This anoxic condition allows phosphorus from
internal sinks to be released to the hypolimnion and ultimately contribute to the eutrophication of the
lake. Selective evacuation of the hypolimnion, termed hypolimnetic withdrawal, can remove excess
phosphorus concentrations from the lake and reduce the rate of eutrophication (Figure 4.3.2).
4.3.2.2 Theory
Hypolimnetic withdrawal is a form of selective withdrawal, but with only the release of water
from the hypolimnion. Epilimnetic water, which maintains adequate concentrations of dissolved oxygen,
is retained in the lake. According to Nurnberg (1987), the major objective is the reduction of anoxic
conditions in the hypolimnion which, in turn, will limit the release of phosphorus from the sediments and
reduce the cycling of nutrients to the epilimnion. This can be accomplished by changing the location of
the withdrawal from the epilimnion to the hypolimnion. On an annual basis, the volume of water
released remains unchanged but the
4.3-5
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