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tongue-and-groove pressure-treated timbers along its upstream face to make it impermeable. The
concrete pad serves as a leveling pad for the structure, making it easier to maintain the weir elevations
across the channel during construction and eliminating shifts in the structure due to changes in the
channel bottom during operation.
The TVA South Holston labyrinth weir, completed in December 1991, was constructed 1.92
km (1.2 mi.) downstream from South Holston dam and is designed to sustain a minimum flow of 2.55
m3/s (90 cfs) between generating periods and to increase river DO content to at least 6 mg/L. It is a
long, W-shaped structure that creates a zigzag waterfall when the weir is overtopped during turbine
operations. The weir can sustain the target minimum flow for up to 18 hours.. The South Holston
labyrinth weir has been meeting or exceeding aeration and minimum flow objectives with minimal
operation and maintenance.
The TVA Chatuge infuser weir, completed in 1992, provides a sustained minimum flow of 1.7
m3/s (60 cfs) during non-generation periods via a 12-hour pulsing interval established at Chatuge hydro
plant to maintain a weir pool. Six low-level pipes penetrating the weir to slowly release the stored
water. Flow through two of these pipes is controlled by an orifice plate attached to the downstream
end of the pipe, and the remaining four are controlled by self-actuating float valves.
Mobile Systems . TVA`s emergency minimum flow system is used as a regional backup to
minimum flow systems for single-turbine projects at Blue Ridge, Chatuge, and Nottely (Schulte and
Harshbarger, 1997). These systems include lifts of from 18 ft at Chatuge to 65 ft at Blue Ridge, and
they provide flows of 30 cfs at each location.
4.8.1.4 Summary
TVA has successfully supplemented minimum flows at over a dozen hydroplants by using
turbine pulsing, small generating units, or reregulation weirs. Because each tailwater is different, there is
no preferred method for all hydroplants. Each tailwater requires study to determine the optimum
method, depending on the desired aquatic restoration goals for the project, the downstream uses, and
the characteristics of the hydroplant and the tailwater.
With the exception of turbine pulsing, augmenting minimum flow requires substantial capital
expenditures, but operational costs are low. Achieving a suitable yet cost-effective solution for all
stakeholders can foster goodwill between the hydropower producer and the public, state agencies, and
special interest groups.
4.8-5

 


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