Environmental Engineering Reference
In-Depth Information
Each lake and reservoir has a capacity for sustaining good water quality under a certain nutrient loading
to avoid eutrophication. Although many studies have been done (Vollenweider, 1968 Pearl, 1988), more
work is needed to find a method for estimation of the critical nutrient loadings to avoid eutrophication.
7.1.3 Water Quality Released from Reservoirs
Despite the range of parameters which control water quality, the thermal and chemical regimes of rivers
will be moderated as a result of upstream impoundment: annual variations will be reduced, short-time
extremes will be virtually eliminated and seasonal maxima and minima will be delayed. Long-retention
reservoirs are often characterized by phyptoplankton assimilation of nitrate-nitrogen, and by the anaerobic
metabolism of nitrate by bacteria in the hypolimnion (Petts, 1984). Nitrate loadings are reduced, whilst
loads of ferrous iron can reach unusually high levels during stratification; the reduction of the insoluble
ferric form of Fe to the soluble ferrous form is caused by high concentrations of CO 2 , low pH, and the
presence of organics within the anoxic hypolimnion (Hannan and Broz, 1976). In some cases, the effects
of a reservoir may be limited to only 1 kilometer below the dam but often the effects have been transmitted
for tens of kilometers downstream.
Where reservoir-discharge quality differs markedly from that of the natural discharges, thermal and
chemical gradients may be established along the river and the downstream extent of any such gradient
will reflect the relative discharges from the reservoir and tributary sources. For thermal gradients to be
established, reservoir releases must be of sufficient magnitude to overcome gradients, produced by
hypolimnial releases, have been reported below Cheesman Dam, South Platte River, Colorado, USA (Ward,
1974). The reservoir exerts a regulating effect upon temperatures immediately below the dam, so that the
primary natural peaks, expected during August and April, are not experienced. Hypolimnial releases
during reservoir stratification cause a thermal gradient to form between March and September, and
temperatures increase downstream most rapidly in April and August. In October, uniform temperatures
reflect the release of mixed water due to the autumn overturn.
The location of the outflow facility will determine the quality of releases from stratified reservoirs,
because withdrawal will occur from a relatively narrow layer. Churchill and Nicholas (1967) showed that
outflows from Cherokee Reservoir, Hoston River, USA, were withdrawn from a fairly narrow layer in
the pool, located at the level of the intakes. However, within density-stratified reservoirs, vertical
movements are suppressed whilst horizontal movements are enhanced (Wunderlich, 1971). Water will be
drawn from all layers in the first moments of a release (Elder and Wunderlich, 1968), but once a steady
rate of outflow has been achieved, a withdrawal layer of restricted vertical dimensions develops near the
intake elevation and 'continuity' is satisfied by the establishment of secondary currents. Water from
relatively narrow layer of approximately constant density will be withdrawn, so that the water quality of
the outflow will vary considerably.
Reservoirs normally reach a condition of stability after between 5 and 10 years. After the initial effects
produced by the submergence of vegetation and soils have disappeared, the water downstream from a
nonpolluted reservoir improves to the point where only the yearly increment of leaves, dead algae, and
other natural situations may arise to produce the short-time release of low-quality water. Low latitude
reservoirs, in particular, can exert a pronounced cooling effect on streams via hypoliminial release (Pearson
and Franklin, 1968). Oxygen depletion within the regulated Catawbe River, South Carolina, USA, was
caused by the first reservoir release after a period of stable low-flows (Ingols, 1959). Reduced dissolved-
oxygen concentrations occur because of the lower reaeration of the greater depth of water. Seasonal-pulses
are climatically induced and may be caused by a major flood inflow which stirs the bottom sediments,
releasing large quantities of orthophosphate into the water (Hannan and Broz, 1976), or by sudden
destratification, which can produce extreme deoxygenation and the release of toxic water (Arumugam
and Furtado, 1980).
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