Environmental Engineering Reference
In-Depth Information
Headwater
Dam
Forebay
Generator
Turbine
Penstock
Afterbay
Tailwater
FIGURE 10.28
Typical hydropower section. (Courtesy of the U.S. Bureau of Reclamation.)
10.4.2.2 Intake Structure
Intake structures for embankment dams are typically gated towers, multilevel, uncontrolled two-
way risers, and/or a combination of both (USACE 1980). The lake level at which the intake structure
is placed depends on the design purpose. Dams that store water for irrigation, domestic use, or other
conservation purposes must have the outlet works intake structure low enough to be able to draw the
water down to the bottom of the allocated storage space (USBR 1987; USACE 1997). Similarly, for
hydropower the intake structure is at lower reservoir levels. While there may a “dead” or inactive
storage zone (Figure 10.3), the intake structure is usually placed as low as practicable to allow for
drainage, inspection, and repair.
As will be discussed later, reservoirs commonly thermally stratify for much of the year, resulting
in vertical variations in temperatures and water quality. For example, oxygen is often depleted in
the lower levels of many reservoirs during summer and fall. As a result, for water supply or for the
management of tailwater and downstream systems, it is often necessary to withdraw water at some
level other than at the bottom. Since the “best” water elevation varies seasonally, some reservoirs
are equipped with selective withdrawal structures so that the elevation of the intake can be varied
(Figure 10.29).
10.4.2.3 Conduits
Conduits convey water through, under, or around embankment dams, and may be under pressure
(penstocks), or to hydropower turbines, where the outlet works are used as the penstock for power
plants (USBR 1987). Often, there are several conduits, allowing one to be taken out of service for
maintenance and repair. Conduits that pass water through or under embankment dams are most sus-
ceptible to failure; where possible, tunnels that are not in direct contact with the dam embankment
are used. Conduits may be designed in a variety of shapes. Shape A, illustrated in Figure 10.30, is
most commonly used for pressurized conduits (penstocks), while shapes B and C are commonly
used for nonpressurized conduits, and may be larger and used for downstream access.
10.4.2.4 Control Structure
Outlet works are usually regulated by gates and valves, typically motor operated, hydraulically
operated, or manually operated (USACE 1997), with emergency backup systems. The regulating
gates and valves may be placed at the beginning or at the end of the conduit, or at an intermediate
location (Figure 10.31).
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