What is a spillway?
A spillway is an integral part of a Dam(large wall built to hold back water
usually across rivers).A spillway is usually used to remove water from a
reservoir to prevent overflow and to release pressure on a dam from
increasing quantities of water. It is normally composed of three major
The approach facility admits flow to the spillway.
The discharging conduit evacuates the flow from the approach facility to
an outlet structure.
The outlet structure (tailwater channel) dissipates the excessive energy
of the flow from the discharging conduits and conveys tranquil flow to
Types of spillways:
There are two main types of spillways:
Controlled & Uncontrolled spillways
A controlled spillway has mechanical structures or gates to regulate the rate
of flow of water from the reservoir. This design allows nearly the full height of
the dam to be used for water storage year-round, and flood waters can be
released as required by opening one or more gates.
An uncontrolled spillway, in contrast, does not have gates; when the water
rises above the lip or crest of the spillway it begins to be released from the
reservoir. The rate of discharge is controlled only by the depth of water within
the reservoir. All of the storage volume in the reservoir above the spillway
crest can be used only for the temporary storage of floodwater, and cannot be
used as water supply storage because it is normally empty.
In an intermediate type, normal level regulation of the reservoir is controlled
by the mechanical gates. If inflow to the reservoir exceeds the gate's capacity,
an artificial channel called either an auxiliary or emergency spillway that is
blocked by a fuse plug dike will operate. The fuse plug is designed to over-top
and wash out in case of a large flood, greater than the discharge capacity of
the spillway gates. Although it may take many months to restore the fuse plug
and channel after such an operation, the total damage and cost to repair is less
than if the main water-retaining structures had been overtopped. The fuse
plug concept is used where it would be very costly to build a spillway with
capacity for the probable maximum flood.
This report will focus on types of spillways based on their shapes and
structures and main working, since the same type of spillway can be classified
under controlled and uncontrolled.
Further classification of Spillways
Straight Drop Spillway (fig. 1a & 1b):
Water flows over a relatively thin spillway crest and falls freely to the
Usually appropriate for thin dams having almost vertical downstream
This type of spillways may be economical for low heads as compared
with overflow spillways (discussed next) because of saving in concrete.
Not recommended for high heads because of structural instability
Figure 1b(top right) shows a trough(d50)
which breaks the force of the overfall
Figure 1a(left) shows straight drop or free
overfall spillway of an arch dam.
Overflow spillway (fig. 2):
Overflow spillways are also called ogee-shaped (S-shaped) spillways.
This type of spillways allows the passage of the flood wave over its crest
(which is S-shaped).
Can be classified under controlled or uncontrolled.
Widely used on Gravity dams, Arch dams, and Buttress dams.
Figure 2 contains a dashed line showing the
distinct S shape of an overflow spillway.
Chute Spillways (fig. 3):
Chute spillways are common and basic in design as they transfer excess
water from behind the dam down a smooth decline into the river below.
The spillway’s slope and its’ sides are lined with concrete.
In case of having sufficient stiff foundation conditions at the spillway
location, a chute spillway may be used instead of overflow spillway
due to economic consideration.
Chute spillways can be ingrained with a baffle of concrete blocks but
usually have a 'flip lip' and/or dissipator basin which creates hydraulic
jump, protecting the toe of the dam from erosion.
Figure 3 Ataturk dam with a stepped-chute
Side channel spillways (fig.4a & 4b):
If a sufficient crest length is not available for an overflow or chute
spillways in narrow valleys, excess water is removed from the reservoir
through a side channel spillway.
The side channel through which water is discharged can also be lined
with concrete to prevent erosion and subsequent sedimentation in
dams on the course of the river.
Figure 4a (above) and 4b (below) shows view of a side channel.
Large black arrow in 4b shows the side channel.
Shaft spillway (fig. 5a & 5b):
A shaft spillway, as its name suggests, discharges excess water from a
reservoir through a shaft that is constructed near the crest of the Dam
with height less than that of the crest.
The shaft spillway is constructed when the types of spillways mentioned
before in this section of the report cannot be constructed due to a lack of
When the shaft is completely submerged, further increase in head will
not result in appreciable increase in discharge.
This type of spillway is not suitable for large capacity and deep
reservoirs because of stability problems.
Special designs are required to handle cavitation damage at the
transition between shaft and tunnel.
Repair and maintenance of shaft spillways are difficult.
Figure 5a (left) shows a bell mouthed shaft spillway and 5b
(right) shows cross-section of the reservoir.
Siphon spillway (fig. 6):
A siphon spillway is similar to a shaft spillway but instead is
incorporated into the dam as shown in figure 6.
The presence of a siphon spillway weakens a dam at certain points, so
the dam has to be reinforced at these weak points incurring extra cost.
Maintenance of this spillway is very difficult.
Siphon spillways comprise usually of five components, which include an
inlet, an upper leg, a throat or control section, a lower leg and an outlet.
Figure 6 contains a dashed line showing the flow of water
through a siphon spillway.
Labyrinth Spillways (fig. 7a & 7b):
A labyrinth spillway is composed of a crest formed by series of this
staggered walls such that a given discharge can pass under a small head
because of the large spillway length afforded.
Flow conditions around these structures are highly complicated.
Intensive physical model studies are required to check their
Figure 7a (above) shows a 3d view of a labyrinth spillway and 7b
(below) shows a plan and sectional view of a labyrinth spillway.
Cascade Spillways (fig. 8):
Cascade or stepped spillway has recently been used as alternative to the
conventional overflow spillways for small to medium discharges.
The spillway is composed of series of steps where excessive energy of
the flow is dissipated.
Shorter stilling basin is required compared to the conventional overflow
The spillway face requires higher sidewalls due to the increased
turbulence over the steps.
Details of the performance of such structures needed to be investigated
through hydraulic mode studies.
Figure 8 shows flow patterns over steps or cascades of the
spillway shown. This is a sectional view of the reservoir.