Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 400

Supersaturated water can be released into the river downstream from the epilimnion. The supersaturation
of air gases can also be caused by the passage of water through turbines (Dominy, 1973) or by over-dam
spillage (Beiningen and Ebel, 1970).
7.1.4 Water Quality Control
The impoundment of a river causes two major changes in the character of the water, both of which have a
marked effect upon the water quality of the releases: firstly, the creation of a reservoir greatly increases
the travel-time of the water through the system; and secondly, stratification may occur. Density stratification
can have severe consequences for downstream water quality, particularly if the reservoir releases are
large in relation to the cumulative volume of tributary discharge. The regulation of the physico-chemical
quality of river-flows may prove beneficial, but reduced dissolved-oxygen concentrations, in particular,
can be detrimental. The control of the quality of water released from impoundments, to maximize the
inherent benefits and minimize the inherent detriments, is in all cases advisable and in many cases
essential. Numerous operational techniques and structures of different design have been used to control
the quality of reservoir releases and to minimize the adverse effects of reservoir stratification. These may
be classified as selective withdrawal techniques and destratification techniques, and both have been
recommended to maintain minimum-quality standards in the outflows.
7.1.4.1 Selective Withdrawal Techniques
Multiple-level intake points to facilitate selective withdrawal provides the simplest method of controlling
water quality. Deoxygenated hypolimnial water, for example, can be drawn off slowly and blended with
highly oxygenated water from the epilimnion. Furthermore, large discharges from outlets can generate
mixing currents within the lake, which inhibit the development of anoxic conditions. The effectiveness of
valve releases is well demonstrated by Gore (1978). Operational requirements of the Tongue River
Reservoir Dam, Montana, USA, resulted in the control gates of the dam remaining open for most of the
spring and summer of 1975. The continual high release of water prevented thermal stratification and the
associated development of hypolimnial waters, so that the water quality of the release was near pre-
impoundment conditions. Below the dam, discharges were observed to have temperatures only negligibly
cooler than at the mouth of the river, approximately 150 km downstream, and the diurnal and monthly
thermal fluctuations approached those expected for unregulated streams. The use of selected-depth
release may, therefore, prove to be beneficial.
Selected releases of water from the epilimnion, through surface outlets, during times of stratification
will provide high-quality discharges until the lake overturns, when the benefit may be lost. Releases from
a number of outlets would dilute the hypolimnial waters over a longer period, and provide less extreme
water quality conditions downstream. However, selective withdrawal has been considered by some authors
(e.g. Brooks and Koh, 1969; Fruh and Clay, 1973) to provide only a limited solution for highly stratified
lakes within which mixing is not induced by the releases themselves. Indeed, water-quality problems during
summer below Stanley Reservoir, River Cauvery, India, related to two outflow sources which continued
to flow as separate and distinct streams for about 2 km below the dam (Ganapati, 1973). Water released
from the high-level sluice was relatively warm, well oxygenated, and contained low concentrations of
phosphates, silicates, and nitrates. Tailrace water from turbines abstracted at a depth of 30 m (up to 3ÛC
cooler, containing 50% less dissolved oxygen, lower pH and high phosphates, silicates, and nitrates) was
released simultaneously but mixing and hence dilution did not occur. After the autumn overturn, uniform
releases were discharged from both sources.
7.1.4.2 Artificial Destratification
Two approaches to artificially destratifying reservoirs have been commonly applied, namely air injection