and plunge pool geometries. Obstructions in the plunge pool can dramatically alter flow patterns and
18.104.22.168 Design and Construction Considerations
Location. The weir should be located close to the upstream dam to maximize the length of
improved tailwater but far enough to minimize undesirable reduction in the effective turbine head.
Combining the aeration and minimum flow requires a location far enough downstream of the dam to
provide adequate storage behind the weir to sustain a minimum flow between generating periods.
Suitable foundation material for weir location is often found at locations of rock outcropping, where
dewatering requirements during construction can be minimized. A labyrinth can require ten times the
flat channel area as an infuser, making site selection more difficult where the exposed rock is irregular or
not level. For efficient aeration, a sufficiently deep plunge pool must be available at all turbine flows,
and it is often more economical to achieve the proper depth naturally via site selection rather than
artificially constructing a plunge pool. Sites should avoid sediment or pollution sources in the weir pool.
Materials . Concrete is considered more durable and has a longer life expectancy than wood
but is more costly, due largely to forming costs. Once constructed, concrete weirs are not as easily
changed as a weir made of wood and concrete. TVA's South Holston weir, for example, uses tongue-
and-groove pressure-treated timber walls supported in a stoplog arrangement by slotted concrete piers.
The piers were constructed with reusable slip forms, and the wall sections were designed for easy
replacement, should the need arise.
Weirs in Combination with other Aeration Systems . When used in combination with other
aeration systems, weirs are more suitable as base aeration systems rather than topping systems,
because aeration potential is fixed at construction by the water level difference across the weir. When
used in conjunction with other technologies, it may be advantageous to pair a weir with a topping
system rather than with another base system. Attempting to combine multiple aeration systems, such as
a weir and turbine venting, can reveal site-specific incompatibilities. The backwater induced by the weir
may adversely impact the aeration performance of the venting, which relies on a high turbine elevation
relative to tailwater elevation to develop low pressures for natural aspiration. Also, DO depletion in a
deep weir pool could act to negate part of the DO improvement added by an upstream method.
Although much literature has been published on the aeration characteristics of conventional
weirs, few existing weirs were designed specifically for aeration. The Table 4.8.1 is a list of existing
weirs designed specifically for aeration.