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 4000
3500
3000
maximum flow
mean flow
2500
minimum flow
tributar
2000
dam
y
1500
1000
500
0
10
12
14
16
18
20
22
24
26
28
30
32
French Broad River Mile
Figure 4.8.1 Discharge below Douglas Dam with Turbine Pulsing
Small Turbine Units. Small turbines may be less efficient than the larger turbines used for
hydrogeneration, especially if salvaged hardware is used. The advantage of using small units to provide
a minimum flow is that flow is constant near the release, and power is being generated while achieving
the minimum flow target. The operating efficiency of the salvaged hardware installed at TVA's Tims
Ford and Nottely applications is about 60%, whereas, machinery manufactured specifically for small
unit installation at Blue Ridge has an efficiency of about 90%. Addition of a small unit for minimum flow
requires additional consideration for aeration the small unit release.
Reregulation Weirs . TVA's Norris weir shape was developed in physical modeling. The
surface of the weir is porous, and only the upstream face is impervious. Thus, water flows over the
impervious face, then over and through the downstream rockfill portion of the weir, dissipating energy
before the flow meets the weir tailwater. The gradual step-down shape from headwater to tailwater
eliminates a sudden plunge into the weir tailwater, avoiding the dangerous recirculation historically
associated with low-head weirs. Design considerations for locating and sizing reregulation weirs are
similar to that described in aerating weirs used for dual aeration/minimum flow objectives in other
sections of this document.
Spilling/sluicing. Water temperature is a consideration with this option when downstream
fisheries are coldwater fisheries that have arisen from cold turbine releases. Another consideration is
that cavitation potential can be high for low level sluices when minimum flows are discharged at high
4.8-3
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