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 subsequent breakup of stratification in the fall. Concurrent nutrient profile data are also required to
determine the contribution of nutrients from the hypolimnion.
The effects on the nutrient budget of releasing a significant portion of the hypolimnion should be
estimated. Several numerical techniques and simulation models are available which may assist in this
process. If the numerical studies predict little or no change in nutrients, then further analysis is
unnecessary and alternative techniques must be investigated. If, however, there is a reduction of
epilimnetic nutrient concentration without significantly impacting total lake volume, then a hypolimnetic
withdrawal analysis should be conducted.
If a hypolimnetic outlet exists in the release structure, the numerical water quality models can be
used to determine the maximum release flow and yet minimize the release of epilimnetic water. If a new
hypolimnetic port is needed, its elevation, as well as the flow rate, can be determined. This analysis
should be conducted over the entire stratification season and will likely require the use of a one- or
two-dimensional water quality model.
4.3.2.4 Review of Applications
This technique has been used on a number of lakes that are relatively small, with the largest
being 78,570 acre-ft (97 million cubic meters) (Nurnberg, Hartley, and Davis 1987). Although smaller
reservoirs may be suited for this technique, application to large reservoirs has not been attempted.
Results of hypolimnetic withdrawal at a number of lakes indicated that hypolimnetic withdrawal is an
effective technique for the reduction of nutrients in the epilimnion; however, impacts on anoxia in the
hypolimnion are inconclusive. A summary of hypolimnetic withdrawal projects with relevant hydrologic
and morphometric information is given in Nurnberg (1987).
4.3.2.5 Summary
Hypolimnetic withdrawal is an application of selective withdrawal designed to remove anoxic
hypolimnetic water from a reservoir. The anoxic conditions allow nutrients to be released into the
epilimnion, contributing to eutrophic conditions. By releasing hypolimnetic water instead of epilimnetic
water, a reduction in the internal nutrient cycling can be achieved. This technique uses the stratification
of the reservoir to maximize releases from the hypolimnion to reduce its volume. Although this technique
may be applicable to smaller reservoirs, it has not been implemented at large reservoirs. Pertinent
information is summarized in Table 4.3.2.
4.3-7
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