Environmental Engineering Reference
In-Depth Information
Subdermal emphysema in the mouth of
a rainbow trout
Subdermal emphysema on the head of
a rainbow trout
Severe exopthalmia in a juvenile
rainbow trout
Intracorporeal bubbles in the lamella of
a rainbow trout
FIGURE 18.6
Examples of the impacts of gas bubble disease in rainbow trout. (From Fidler, L.E. and
Miller, S.B., British Columbia water quality guidelines for dissolved gas supersaturation, Canada Department
of Fisheries and Oceans, Environment Canada, Canada, 1994.)
the states of Oregon and Washington have both listed basically the entire lower Columbia River as
being impaired due to TDG exceeding their state standards (McGrath et al. 2006). That is, the states
have listed multiple reaches as being impaired due to TDG on their federal Clean Water Act 303(d).
A TDG total maximum daily load (TMDL) was developed for the lower Columbia River in 2002.
The spill of water over the spillway can be voluntary or not. Uncontrolled releases may occur
during high-low events. Spillway release may also occur due to dam maintenance (such as the
maintenance of turbines for hydropower facilities). In some cases, voluntary spills also occur, such
as to facilitate the downstream movement of migratory ish, such as salmon.
A variety of methods have been developed to reduce or mitigate the impact of gas bubble disease
in reservoir tailwaters. These include (Richardson and Baca 1974):
•
Reducing the spill volume and modifying spillways to reduce the degree of air entrainment
to stilling basin depths.
•
Increasing the water low through powerhouses, thereby reducing the volume passing over
the spillway.
•
Using spillway delectors to reduce plunging.
•
Transporting salmonids around the dams.
A variety of other alternatives have been developed, such as low water ports, spillway bypasses,
and others, many of which are described in feasibility and abatement plans, such as in the
“Alternatives for TDG abatement at Grand Coulee Dam, feasibility design report” (Frizell and
Cohen 2000).
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