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 4.8 TAILWATER TECHNIQUES
4.8.1 MINIMUM FLOW
4.8.1.1 Description
Sustaining minimum flows in tailwater reaches provides 1) continuous wetted channel area to
support benthic organisms; 2) reduced thermal shock for aquatic biota; 3) improved flushing of
otherwise stagnant pools; and in certain cases, 4) improved water velocities and depths for fish
spawning. This description provides an overview of potential mitigative technologies, including turbine
pulsing, small turbine units, reregulating weirs, spilling/sluicing, and mobile systems.
Turbine Pulsing. Turbine pulsing is an operational method that normally results in the lowest
power losses of any of the minimum flow options, and with essentially no capital investment. Turbine
pulsing is the operation of one turbine at a hydropower dam for a short duration at a regular time
interval (e.g., 0.5 hr generation every 4 hrs). Pulsing is performed to provide minimum flow during
times when generation is not otherwise scheduled, such as at night, on weekends, or during filling of the
reservoirs. Pulses typically dampen with a few miles downstream of the powerhouse, depending on
stream hydraulic parameters like width, roughness, and slope. Wear and tear on switching mechanisms
creates the primary O&M expense for turbines used for pulsing, but this can be minimized in multi-unit
plants by rotating service across several turbines.
Small Turbine Units. A small turbine unit addition to provide minimum flow is a capital
intensive, moderate O&M option. The advantage of using a small turbine unit for minimum flow is that
flow is continuous at the release, and power is being generated while achieving the minimum flow target.
An auxiliary mini-penstock is tapped into the most convenient powerhouse feature, such as the main
penstock, sluiceway, surge tank, or turbine bypass piping. The mini-penstock is typically three to four
feet in diameter and routed out of the powerhouse to supply the small generating unit. The small unit
can either be attached to the powerhouse exterior or located on the bank downstream of the
powerhouse.
Reregulation Weirs . Weirs designed for minimum flow are reliable, capital intensive, low-
maintenance structures. Reregulation weirs less expensive than aeration weirs. A weir can reduce
effective turbine head by increasing tailrace depths if the weir is not located sufficiently far downstream
from the powerhouse. Overflow weirs, if not properly designed, can create downstream recirculation
patterns that can pose a safety hazard to unwary river users. Safety is as much a part of weir design as
the minimum flow.
Weirs typically regulate minimum flow during periods of non-generation via slow drainage of the
weir pool through low-level pipes. These pipes can be controlled by self-actuating float-valves to
maintain constant flow as the weir pool fluctuates (Hadjerioua, et. al. 1992 and 1997). The weir pools
4.8-1
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