Transport and Mixing Processes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 362

associated with pumpback operations is the entrainment and mortality of ish, which has impacted
the operation of some facilities. For example, for the Richard B. Russell Reservoir, between Georgia
and Alabama, the pumpback is from the reservoir tailwaters and has resulted in ish mortality as
ish in the tailwaters become entrained. In order to reduce pumpback mortality, additional ish pro-
tection structures and seasonal and daily limits (e.g., nighttime only) on pumpback operations have
been imposed.
Pumpback operations impact not only the transport in the storage reservoir, but also the transport
in the main reservoir. Since the intake for the turbines is located at an appreciable depth, the return
low usually enters the upper reservoir as a jet (momentum dominates its transport), which entrains
ambient water. Density stratiication in the reservoir eventually results in jet collapse and density
current formation (Roberts and Dortch 1985), which can impact temperature stratiication.

13.3.2 c onVeyance S tructureS and o peratIonS
The size, location, type, and operation of these conveyance structures impact not only the transport
within a reservoir, but also the transport in the downstream tailwaters. An issue, for example, is
that in many stratiied reservoirs, the hypolimnetic water quality degrades following stratiication.
The hypolimnion may become anoxic, or deoxygenated, during the summer months, particularly
in highly productive or recently constructed reservoirs. The resulting hypolimnetic concentrations
of reduced materials represent a deicit that must be overcome before the hypolimnion can again
become oxic. Deoxygenation affects the quality of release water, particularly for those reservoirs
that withdraw water from the hypolimnion, such as J. Percy Priest Reservoir in Tennessee, as illus-
trated in Figure 13.14.

14

12

10

8

6

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2

0

1-Jan-94 20-Feb-94 11-Apr-94 31-May-94 20-Jul-94

Date

8-Sep-94 28-Oct-94 17-Dec-94

Average photic zone 5-ft. depthOutflow bserved 5 ft.

FIGURE 13.14 Predicted and observed dissolved oxygen concentrations in J. Percy Priest Reservoir and
reservoir outlows during 1981. (From Martin, J.L. and Cole, T.M., Water quality modeling of J. Percy Priest
Reservoir using CE-QUAL-W2, U.S. Army Engineer Research and Development Center, Vicksburg, MS,
1999.)