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 4.8.2 TAILWATER TECHNIQUES: AERATING WEIRS
4.8.2.1 Description
Advantages of Weirs . The primary advantages of weirs used for aeration is that they are
passive devices that provide excellent aeration, high reliability, with minimal operation and maintenance.
Weirs can also meet a second environmental objective of maintaining minimum flow (wetted area) in the
channel by releasing stored water slowly between generating periods. Weirs designed to aerate
maximum turbine discharges will usually exceed aeration targets at lower turbine discharges. Unless
they are dewatered, weirs will add oxygen in seasons when DO is not needed. Thus, weirs may
produce a significant increment of aeration in excess of the year-round DO target, in contrast to
systems throttled to just meet a flat DO target during the low DO season. Depending on the target DO
levels, this may be bioenergetically significant for downstream fisheries. Unlike hydroplant aeration
methods, weirs are not likely to affect cavitation or turbine efficiency. Potential power impacts from
aerating weirs can be lost revenue during outages required for weir construction, and capacity losses
from reduced turbine heads if the weir is constructed close to the hydroplant. Weirs are attractive to
the public.
Disadvantages of Weirs . The primary disadvantage of weirs is that they can represent a
significant capital investment, especially in deep river situations. Aerating weirs normally require a head
differential of 1 to 2 m (3 to 7 ft) at all turbine discharges; thus they impound a pool of substantial depth
upstream of the weir. Depending on proximity to the upstream dam and the stream gradient, this
impoundment can reduce the effective head on the turbine and increase inundation of adjacent property
along the weir pool during floods. Another disadvantage is that the weir pool is not aerated, leaving a
portion of the tailwater without DO improvement. A weir increases the depth and residence time of
water in the weir pool reach, increasing oxygen depletion there from organic materials in the sediments
and water column and from chemical species released from the upstream reservoir. The public
attraction to weirs as a recreational area can become undesirable if safety concerns are significant.
Weirs may also create a barrier to migration of fish and to human navigation. However, for hydroplant
aeration, this will not be nearly as significant as the barrier presented by the hydropower dam itself,
except for the reach between the weir and upstream dam.
Weir Types. Considerable progress has been made in aerating weirs since the 1990 EPRI
DO guidance (EPRI 1990). Figure 4.8.2 illustrates plan and elevation views of three basic types of
aerating weirs that employ free-fall nappes: linear, labyrinth, and infuser.
The linear weir (Figure 4.8.2a) has been found in the literature (Gameson 1957, Avery and
Novak 1978, Nakasone 1987) to be a good aerator at the lower specific discharges of 0.05 to 0.2
m2/s (0.5 to 2 ft2/s), where specific discharge is defined as flow per unit length of weir crest. At higher
discharges, hydraulic recirculation downstream of the weir intensifies to a hazardous level. Aeration also
reduces at high specific discharge.
4.8-6
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