Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 401

and mechanical pumping. The maintenance of the reservoir storage in a nearly isothermal condition would
permit continued water circulation and, therefore, an available supply of dissolved oxygen throughout the
entire water-mass. Anaerobic conditions would be eliminated, along with many of the problems, which
accompany them.
Artificial destratification and the mixing of epilimnial and hypolimnial waters during summer may often
be desirable in order to obtain an improved and more uniform water quality within and, consequently,
downstream from reservoirs. Indeed, the Quality Control in Reservoirs Committee in the USA recommend
artificial destratification to water suppliers who are experiencing any water-quality deterioration in their
reservoirs resulting from anaerobic conditions in the hypolimnion caused by thermal stratification (American
Water Works Association, 1971).
Several investigators have attempted artificial destratification by injecting diffused air into a reservoir's
hypolimnion. The mixing process, through interchange of heat from warmer to colder water, causes the
water-mass to become nearly isothermal. Early attempts often failed to achieve sufficient circulation (e.g.,
Derby, 1956; Schmitz and Hasler, 1958). In Sweden, Lake Langsjon experienced severe stratification,
with a hypolimnion devoid of oxygen (Heath, 1961); but compressed-air injections effectively eliminated
stratification. Similar benefits of forced circulation have been reported by Riddick (1957) for the Ossining
Reservoir, New York, U.S. and for Lake Wohiford, a subtropical reservoir in southern California (Ford,
1963). In the latter case, stratification was completely eliminated throughout the lake after 7 days of
operation. During which air injections lasted for 9 hours on each of the first 6 days and for 24 hours on
the last.
An air diffusion system has been successfully applied to the destratification of a medium-sized lake-
Allatoona Lake, Georgia, USA, which, under normal conditions, begins to stratify in mid-March and
achieves complete stability in mid-July, with overturn in early October (Rogers et al, 1973). In late summer,
a maximum temperature difference of 17 ć is established between the surface and bottom of the lake,
and dissolved oxygen may become exhausted below the upper 20% of the depth. During the summer of
1968, air was continuously supplied to the hypolimnion, and the reservoir was maintained in the destratified
condition, with adequate dissolved-oxygen concentrations throughout the Lake. Improvement in the
quality of reservoir releases is particularly noticeable if comparisons are made between the specific flow
conditions. Under high flows, dissolved oxygen was slightly less during early summer in Allatoona Lake,
but significantly higher during August and September under conditions of artificial hypolimnion aeration.
Moreover, during low-flow conditions, levels of dissolved oxygen were generally maintained above 4
mg/L, whereas prior to air injection, concentrations of less than this value were experienced for most of
the summer and temperatures were elevated by a maximum of 8ÛC. Although air injection can be applied
to a considerable area and a large amount of water can be set in motion, it is a relatively inefficient
means of water transfer because it involves intermediate energy conversion for air compression.
Pumping cold water from the bottom of a water-body and discharging it at the surface has been
suggested as an alternative method of control to air injection (Irwin et al., 1966). Symons et al. (1965)
applied mechanical pumping to destratify a small lake, increasing the temperature and dissolved-oxygen
concentrations in the lower layers; manganese and sulphide concentrations were reduced to zero and the
ammonia nitrogen concentration resulting from anaerobic decomposition, was also reduced.
Pumping can be particularly useful if the operation is begun before a lake becomes stratified (Garton et
al., 1976). An axial-flow pump was used in midsummer to transfer water at 0.674 m 3 /s from the oxygen-rich
epilimnion to the hypolimnion. Within 2 weeks the procedure had completely destratified water temperature
in the lake, but a longer period of time was required to destratify dissolved-oxygen levels, as shown in
Fig. 7.6. Significantly, the lake warmed uniformly at a particular depth, regardless of distance from the
pump. Initially the dissolved-oxygen concentration of the surface water was markedly reduced, because