There are several types of spillways used in dam construction based on their prominent features. The most common types include overflow spillways, which guide water smoothly over an arched crest; side channel spillways, where flow passes over a weir and into a parallel channel; open channel spillways used when there is no space for others; tunnel spillways, where flow is conveyed around the dam within an enclosed channel; and shaft spillways, with a vertical drop shaft connecting an overflow weir to a downstream tunnel. Spillways provide a safe path to release excess reservoir water and prevent dam overtopping and failure.

1. TYPES OF SPILLWAYS
PROMINENT FEATURES

2. SPILLWAYS:-
 A spillways is a structure constructed a or near the dam site to dispose of
surplus water from the reservoir to the channel downstream.
 Spillways are provided for all dams as a safety measure against overtopping
and the consequent damages and failure.
 A spillways acts as a safety valve for the dam, because as soon as the water
level in the reservoir rises above a predetermined level, excess water I
discharged safety to the downstream channel, and the dam is not damaged.

3. TYPES OF SPILLWAY BASED ON MOST
PROMINENT FEATURES
 Free overflow spillway
 Overflow spillway
 Side channel spillway
 Open channel spillway
 Tunnel spillway
 Shaft spillway
 Siphon spillway

4. FREE OVERFLOW SPILLWAY
 A free overflow spillway is a type of spillway in which the control structure
consists of a low- height, narrow crested weir and less vertical.
 The overflowing water may discharge as a free jet clearly away from the d/s
face crest of the spillway.
 If no artificial protection is
provided on the down
stream side of the overflow
section.

5. Overflow spillway:-
 An overflow spillway is the most commonly used spillway. It is widely used
with gravity dams, arch dams and buttress dams.
 An overflow spillway is an improvement upon the free overflow spillway.
 The profile of the spillway is ‘ogee’ or ‘s’. The overflowing water is guided
smoothly over the crest and profile of the spillway so that overflow
water does not break
contact with the spillway
surface.

6. SIDE CHANNEL SPILLWAY
 A side channel spillway is the one in which the flow, after passing over a
weir, is carried away by the channel running essentially parallel to the
crest.
 The side channel spillway is usually constructed in a narrow canyon where
sufficient space is not available for an overflow spillway.
 Side channel spillway is also the best choice where a long overflow crest is
desired in order to limit
the surcharge head and
the abutment are steep.

7. OPEN CHANNEL SPILLWAY
 Open channel spillway is provided in the following situation :
(a) when it is not possible to provide overflow spillway.
(b) when valley is narrow.
(c) when separate site for spillway is not available.
 Generally this type of spillway is provided on earth or rock fill dam, and is
isolated from the main dam.
 The open channel is sometimes of constant width , but usually narrowed for
economy.

8. TUNNEL SPILLWAY
 A tunnel spillway is the one in which a closed channel is used to
convey the discharge around or near a dam.
 The control structure for this spillway may be in the form of an
overflow crest, vertical or inclined orifice entrance and side channel
crest.
 The tunnel is designed to flow partly full and it is not allowed to flow
full because siphonic action may develop due to negative pessure being
developed in the tunnel.

10. SHAFT SPILLWAY
 Shaft spillway is the one which has horizontally positioned lip through
which water enters and then drops through a vertical shaft and then to a
horizontal tunnel which convey water past the dam.
 A shaft spillway consists of three main elements :
(a) overflow control weir
(b) vertical control
(c) closed discharge channel
 The vertical shaft of large structures Is usually made of reinforced concrete
and the horizontal tunnel is tunnelled in rock.
 There are two general types of shaft spillway :
(a) standard crest type
(b) flat crest type

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