Transport and Mixing Processes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 363

Artiicial reaeration can be used to increase dissolved oxygen concentrations in the hypolim-
nion, typically using either air injection or oxygen injection. The quantity of air injected is often
large and the increased vertical mixing that results may destroy the stratiication within a lake or
reservoir (Figure 13.11). There are many cases where increased vertical mixing is not desirable. For
such cases, dissolved oxygen rather than air is often injected. The quantity of gas required is much
reduced so that oxygen injection may often leave the stratiication intact. The reader is referred to
Gächter and Imboden (1985) and Pastorok et al. (1981) for additional information.
The transport within a reservoir and the characteristics (e.g., temperature and oxygen concen-
trations) of reservoir releases are impacted by the characteristics of the conveyance structure, the
magnitude of the outlow, and the density stratiication (Martin and McCutcheon 1999). The vertical
zones from which water is withdrawn are referred to as the withdrawal distribution. In the absence of
stratiication, the withdrawal distribution may be essentially uniform over the depth of the reservoir.
However, under stratiied conditions, the withdrawal may be limited to a smaller zone within the epi-
limnion or the hypolimnion or both (Figures 13.15 and 13.16). The withdrawal distribution depends on
the degree of stratiication, the conveyance structure, and the energy associated with the withdrawal.
Two examples of representative withdrawal distributions (computed using the SELECT model;
Davis et al. 1987; Schneider et al. 2004) are illustrated in Figures 13.15 and 13.16, along with the
vertical temperatures and oxygen concentrations. For the irst example, the withdrawal is from a
port 150 ft. above the bottom of the reservoir and located near the thermocline (Figure 13.15). The
normalized withdrawal distribution is a relatively narrow, nearly Gaussian, distribution centered
on the port location and extending between 123 and 171 ft. This distribution would result in a cur-
rent along the thermocline. For this stratiication pattern, the estimated release temperature (from
SELECT) would be 21.7°C and an oxygen concentration of 5.7 mg L -1 . In the second example, the
port is located 15 ft. from the bottom of the reservoir and the rate of the withdrawal is increased by a
factor of 10 (Figure 13.16). The withdrawal zone is larger than the irst example, and the lower limit
is restricted by the bottom of the reservoir. The estimated release temperature is colder (16.5°C) and
the oxygen concentration is lower (3.5 mg L -1 ).

Withdrawal results

250

200

150

100

50

0

0 5 10 15 20 25 30 35

Do (mg L -1 )

TEMP (C) Normalized withdrawal

FIGURE 13.15 A representative withdrawal pattern for a point source (port) withdrawal from a stratiied
reservoir; Case A, with the withdrawal from a port at an elevation of 150 ft.