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 techniques are available to estimate the required operational levels to obtain specific goals. Several
example applications of this technique were discussed. Table 4.4.3 summarizes this technique.
Table 4.4.3
Summary of Supplemental Releases For Water Quality
Characteristics
Description
Water quality constituents such as dissolved oxygen (along with related
Targets
concerns), turbidity, temperature, etc.
Use of release structures, regardless of original purpose, to improve
Mode of action
water quality in the reservoir and downstream releases.
Effectiveness
Dependent upon location and elevation of intake of release structures.
Longevity
Days to months
Negative features
Additional releases of water
Costs
Minimal (lost hydropower revenue).
Applicability to reservoirs
Very applicable to projects with multiple release structures.
4.4.4 CONCENTRATION OF FLOW THROUGH ONE GATE
4.4.4.1 Description
Navigation dams are designed primarily to maintain a pool for navigation above the project and
regulate flow for navigation downstream of the project. During the summer and early fall, typically the
low-water period, flows through these structures may be minimal. These low flows may allow some
thermal or chemical stratification to occur in the upstream pool, somewhat similar to a storage reservoir.
Spill operations may not reaerate this poor-quality bottom water, which may lower downstream water
quality.
The operation of a multi-bayed structure, such as a navigation dam, usually requires that all
gates be operated the same for a given release condition to achieve a laterally-uniform flow distribution
downstream. This type of operation is designed to prevent scour and erosion in the stilling basin and
tailrace or to provide the best flow conditions for navigation. While hydraulic conditions may require
uniform operation during high-flow events to minimize scour and erosion downstream, low-flow
conditions may permit the concentrating of flow into a few gates.
During these low-flow periods, the dissolved oxygen may be low due to reservoir-type
conditions in the upstream pool. Therefore, a modification of the gate operation during low-flow
4.4-11
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