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Living with Dams: Know Your Risk | April 2012
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Water normally passes through the main
spillway or outlet works; it should pass over
an auxiliary spillway only during periods of
high reservoir levels and high water inflow.
All embankment and most concrete
dams have some seepage. However, it
is important to control the seepage to
prevent internal erosion and instability.
Proper dam construction and maintenance
and monitoring of seepage provide this
control.
Release of Water
Intentional releases of water from dams are
confined to outlet works and spillways. A
dam typically has a principal or mechanical
spillway and a drawdown facility.
Additionally, some dams are equipped
with auxiliary spillways to safely pass
extreme floods.
Even when operated as designed, many
dams will pass huge volumes of flood
water into downstream areas.
OUTLET WORKS
—In addition to
spillways designed to prevent overtopping
of dams, dams contain outlet works that
allow water to be drawn continuously, or
as needed, from the reservoir, and provide
a way to draw down the reservoir for repair
or safety concerns. Water withdrawn may
be discharged into the river below the
dam, run through generators to provide
hydroelectric power, or used for irrigation.
Dam outlets usually consist of pipes, box
culverts or tunnels with intake inverts near
minimum reservoir level. Such outlets are
provided with gates or valves to regulate
the flow rate.
SPILLWAYS
—The most common type of
spillway is the free overflow spillway. This
spillway may be located over or through
the dam or an abutment. To permit
maximum use of storage volume, movable
gates are sometimes installed above the
spillway crest to control discharge. Many
smaller dams have a pipe and riser spillway,
used to carry most flows, and a vegetated
earth or rockcut spillway through an
abutment to carry infrequent high flood
flows. In dams such as those on the
Mississippi River, flood discharges are of
such magnitude that the spillway occupies
the entire width of the dam and the
overall structure appears as a succession
of vertical piers supporting movable gates.
High arch-type dams in rock canyons
usually have downstream faces too steep
for an overflow spillway. In Hoover Dam
on the Colorado River, for example, a shaft
spillway is used. In shaft spillways, a vertical
shaft upstream from the dam drains water
from the reservoir when the water level
becomes high enough to enter the shaft
or riser; the vertical shaft connects to a
horizontal conduit through the dam or
abutment into the river below.
Source: ODNR Division of Water
Crest
Vertical
filter
Phreatic
surface
Blanket drain
Riprap
slope
protection
Cutoff
Core
Cutoff
trench
Seepage
Drain inlet
Trashrack
Freeboard
Anti-Seepage coller
Sheet piling
cutoff
Foundation
Sti l l ing basin
Out let
Toe drain
Toe of dam
B e r m
Embankment
Groin area
Pipe conduit
Drain
gate
Riser
OUTLET WORKS
INSPECTION
SPILLWAY