are typically recharged with a turbine release every 12 to 48 hours if not otherwise generating,
depending on site characteristics. These weirs typically are not large enough to attenuate peaks during
normal turbine operation.
Spilling/Sluicing/Siphons . In most cases it is economically desirable to pass all water for
minimum flow through a project's main turbines. In certain instances where this is not possible (i.e.,
turbines out of service, or other options such as pulsing are not feasible), then spilling, sluicing, or
siphoning the water are potential options. These systems usually require capital outlay to upgrade
equipment sufficiently to handle cavitation and vibration issues.
Mobile Systems . If a minimum flow system relies solely on a single main turbine, then the
minimum flow will not be sustainable during main unit outages. This can occur if a small unit's mini-
penstock taps into main unit hardware, or if pulsing is used at a project that has only one turbine. To
augment minimum flow during such outages, TVA has developed a mobile system (Schulte and
Harshbarger, 1997) that can be deployed to pump or siphon reservoir water over the top of the
spillway. The system consists of electric pumps mounted on rafts, and hoses to carry pumped water to
the dam's spillway. The equipment is stored on trailers, in anticipation of a rapid response that may be
needed to maintain habitat during an outage.
220.127.116.11 Design Considerations
Suitability of certain mitigative techniques varies by project. This section will offer design
considerations based on experiences with these systems.
Turbine Pulsing. Not all tailwaters are good candidates for turbine pulsing (Hauser, 1989).
For hydro projects with large turbines discharging into streams having relatively small natural flows,
turbine pulsing cannot feasibly provide sustained flows small enough to be within the target range.
In the immediate tailrace, turbine pulsing creates flow fluctuations between the leakage flow rate
and one turbine discharge. Shortly downstream, however, the fluctuations dampen such that the
minimum and maximum flows approach a temporal mean. TVA has successfully pulsed certain
hydroprojects in the range of 30 to 60 minutes on intervals ranging from 2 to 6 hours for a variety of
downstream objectives. Figure 4.8.1 shows minimum, mean, and maximum flow profiles below
Douglas Dam for a pulsing pattern of 0.5 hr of one turbine (4000 cfs) every 4 hrs. Use of pulsing at
Douglas Dam to provide 550 cfs cost TVA about one-fourth that of a sluicing option that would have
provided only 300 cfs continuous flow.