LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 277

overturn and be continuous. Lake destratification by
aeration mechanisms during summer oxygen depletion
may sometimes be very damaging, since when the low-
oxygen hypolimnetic water is brought to the surface, it
may result in fish kills.
Operational data on the performance of mechanical
mixers for destratification applications depends on indi-
vidual mixer characteristics and should be sought from
equipment manufacturers prior to deployment. Studies
by Hill et al. (2008) indicate that mechanical mixers that
are most efficient in any particular case are character-
ized by a Richardson number, Ri 0 , on the order of 0.1,
where

the hypolimnion. Hypolimnetic aerators require a large
hypolimnion to work properly and are generally inef-
fective in shallow lakes and reservoirs. Costs depend on
the amount of compressed air needed, which is a func-
tion, in turn, of the area of the hypolimnion, the rate of
oxygen consumption in the lake, and the degree of
thermal stratification.

7.6.2.4 Oxygen Injection. Studies have shown that in
some lakes that are used for water supply, it is some-
times more cost-effective and practical to inject pure
oxygen into the hypolimnion, as opposed to air injec-
tions or aerating the hypolimnion via airlift systems.

7.6.2.5 Pump-and-Baffle Aeration System. Using
this method, oxygen-poor water is extracted from the
nearshore area of the lake, pumped to the top of a chute
located on shore, and then allowed to cascade over a set
of baffles (constructed of wooden boards). The turbu-
lence created as the water passes over the baffles helps
to reaerate the water. The reoxygenated water is then
returned to a different part of the lake, away from the
intake area, creating a zone of oxygen-rich water. Gen-
erally, approximately 10% of the lake's volume should
be aerated. Pump-and-baffle systems have several major
advantages relative to other aeration techniques that
are commonly used to aerate ice-covered lakes. In par-
ticular, when properly operated, only a small area of the
lake's ice cover is opened. Open areas and thin ice are
safety hazards for which the operator of an aeration
system is liable. All of the major pieces of equipment
are on shore. In addition, the chute can be mounted on
a trailer and moved from one lake to another or to dif-
ferent areas of the lake as needed. Generally, to prevent
winterkill, aeration will be required for about 2 months,
depending on winter conditions. By monitoring DO
levels in the lake, the system can be operated only
during those times when needed. Pump-and-baffle
systems have been built by lake associations and can
also be purchased as a unit from manufacturers.

g H
R ω

Ri 0 = (7.95)

2 2

where g ′ is the effective gravity (= g Δ ρ / ρ ) (LT −2 ), Δ ρ is
the density difference between the water above and
below the mixer (ML −3 ), ρ is the average density (ML −3 ),
g is the acceleration due to gravity (LT −2 ), H is the height
of the impeller (L), R is the radius of the impeller (L),
and ω is the rotational speed of the impeller (T −1 ). Equa-
tion (7.95) is used to guide the selection of impeller
(mixer) dimensions and rotational speed for given strat-
ification conditions.

7.6.2.2 Water Fountains. Water fountains pump
water from the surface layers lakes into the air, and, in
the process, aerate lake waters. Water fountains have a
limited effect in aerating lake water since they tend to
pump water from the upper layers of the lake that are
already well aerated. Several designs of water foun-
tains are available, and several of the more popular
designs are shown in Figure 7.17. In selecting a water
fountain for a particular lake, aesthetics are usually
the primary motivation, and aeration a secondary
benefit.

7.6.2.3 Hypolimnetic Aeration. Hypolimnetic aera-
tion is used in cases of high hypolimnetic oxygen defi-
cits, taste and odor problems, and increased concentrations
of manganese and iron. Hypolimnetic aeration systems
must deal with the more difficult problem of aerating
waters at greater depths. Where the objective is to estab-
lish or maintain a coldwater fishery, hypolimnetic aera-
tion must be achieved without disturbing the lakes
thermal stratification. For most applications, an airlift
device is used to bring cold hypolimnetic water to the
surface, the water is aerated by contact with the atmo-
sphere, gases such as methane, hydrogen sulfide, and
carbon dioxide, which may accumulate under anaerobic
conditions, are lost, and then the water is returned to

7.6.2.6 Snow Removal to Increase Light Penetra-
tion. Snow removal from the lake surface to increase
light penetration and photosynthesis (oxygen genera-
tion) under the ice is a low-tech, low-cost alternative to
aerators that may be sufficient to prevent winterkill in
lakes with marginal levels of DO. Snow is a much more
effective absorber of light than is ice. While 85% of the
available light will penetrate 12.5 cm (5 in.) of clear ice,
5 cm (2 in.) of snow over 7.5 cm (3 in.) of ice will block
out almost all light. Even thin layers of snow can
greatly decrease light penetration, decreasing primary
productivity, and thus leading to oxygen depletion and
winterkill.