spillway,types of spillways, Design principles of Ogee spillways ,Spillway gates. Energy Dissipaters and Stilling Basins Significance of Jump Height Curve and Tail Water Rating Curve, USBR and Indian types of Stilling Basins.

1. IRRIGATION AND HYDRAULIC
STRUCTURES
Unit-3
Spillways
By
MOODNARESH

2. INTRODUCTION
• A passageway through which surplus water escapes
from a reservoir, lake, or the like.
• Discharges the flood water to downstream of river
without overtopping the dam.
• A structure which will not let the water rise above the
maximum reservoir level.
• A spillway is essentially a safety valve for a dam.
Location of Spillway
• Within the body of the dam
• At one end of dam.
• Entirely away from it, independently in a saddle.

4. Requirements of Spillway
Provide structural stability to dam under all conditions.
Able to pass the designed flood without raising the
water level above H.F.L. Safe disposal of water without
toe erosion. Should have an efficient operation and
should be economical.
Types of Spillways
• Straight drop spillway
• Overflow spillway
• Chute spillway
• Side channel spillway
• Shaft spillway
• Siphon spillway

5. Straight drop Spillway
• In this type of spillway, the water freely drops
down from the crest.
• It is a low weir and simple vertical fall type
structures.
• The water falls freely from the crest under the
action of gravity.
• To prevent scouring at the downstream, an
auxiliary dam of Artificial pool is to be constructed
at the place of fall of water.

7. Overflow spillway
• It is generally known as Ogee spillway.
• It represents the S-shape curve so, it is called ogee
spillway.
• It is an improved form of straight drop spillway.
• It is mainly used in gravity dams.
• It has got the advantage over other spillways for its
high discharging efficiency.

9. Chute spillway
• It is often called as trough or open channel spillway.
• For earthen and rockfill dams, spillway is to be
constructed separately away from the main valley.
• Chute Spillway is the simplest type of a spillway
which can be easily provided independently and at
low costs.
• It is lighter and adaptable to any type of
foundations.

11. Side channel spillway
• The flow in this spillway is turned 90 ̊ after passing
the crest such that the flow is parallel to the weir
crest.
• Best suitable for non rigid dams like earthen dams.
• It is preferred where space is not available for
providing sufficient crest width for chute spillway.
• The discharge carrier may be an open channel type
or a conduit type.

13. Shaft spillway
• The water from the reservoir enters into a vertical
shaft which conveys this water into a horizontal
tunnel which finally discharges the water into the
river downstream.
• This type of spillway is preferred where the space is
not available for providing the above type of
spillways If the inlet leg is provided in shape of a
funnel, it is called Morning Glory Spillway.
• It has maximum discharge even at low heads.

15. Siphon Spillway
• It works on the principle of siphonic action.
• It consists of a siphon pipe whose inlet leg is kept just below the normal pool
level and an air vent kept at normal pool level is connected to the crown of
siphon.
• When the water raises the pool level, siphonic action starts automatically and
the water discharges to downstream side.
• When the water level falls below the pool level, air is entered through air vent
and the discharging of water stops.
There are two types of siphon spillways:
1. Saddle siphon spillway
2. Volute siphon spillway
1. Saddle siphon spillway: Saddle siphon spillway can be constructed in
many designs but basic concept for all of them is the same. One type
of saddle siphons is shown in Figure below. The diagram consists of a
U-shaped siphon pipe whose D/S end remains submerged in tail
water. An air inlet is provided near the top of the bend.

16. Figure 1 Figure 2

17. Ogee Spillway
(Design Principles)

19. Energy Dissipation
Hydraulic Jump
• When flood discharge passes over the spillway
crest, it has high potential energy which gets
converted into kinetic energy as it glides along it.
• This high energy has to be dissipated otherwise it
would cause erosion at the downstream toe.

20. Energy Dissipation
• When flood discharge passes over the spillway crest, it has high
potential energy which gets converted into kinetic energy as it glides
along it
• Water flowing over a spillway acquires a lot of kinetic energy
because of the conversion of the potential energy into kinetic energy.
• If the water flowing with such a velocity is discharged into kinetic
energy.
• If the water flowing with such a high velocity is discharged into
river it will
scour the river bed.
• If the scouris not properly controlled, it may
extend backward and may endanger the spillway and
the dam.
• In order to protect the channel bed against scour, the kinetic energy
of the water should be dissipated before it is discharged into the d/s
channel.
• For the dissipation of the excessive kinetic energy of water, the
following two methods are commonly adopted.
1.By developing a hydraulic jump

21. • Hydraulic jump
• Hydraulic jump is the sudden rise of water that takes place when the flow changes
from supercritical flow state to the subcritical state.
• When a stream of water moving with a high velocity and low depth strikes another
stream of water moving with low velocity and high depth, a sudden rise in the surface
of water place.
• This phenomenon is called hydraulic jump.
• This is generally accompanied by a large scale turbulence,
dissipating most of the kinetic energy of supercritical flow.
• Such a phenomenon may occur in a canal below a regulating sluice, at the bottom of
the spillway, or at a place where a steep channel slope turns flat.

23. JHC vs TWC
Jump Height Curve (JHC):
• It is a curve representing post jump depth (d2) with
discharge (q)

24. Tail Water Curve (TWC):
• It is a curve representing tail water depth (D) with discharge (q)
• Tail water rating curve The tail water rating curve (TWRC) gives the
relation between the tail water depth y2' (i.e. the actual water
depth in the river on the downstream) as ordinate and the
discharge intensity q as abscissa [Fig. 2 (b)]. The actual tail water
depth corresponding to any discharge intensity q depends upon
the hydraulic characteristics of the river downstream. The values
of y2' corresponding to different values of q are obtained by actual
stream gauging. If there is a suitable control somewhere
downstream of the spillway where the depth of water and
discharge can be accurately measured, the tail water depth y2' at
the spillway can also be determined by backwater computation.
While plotting the tail-water rating curve, an allowance for
channel retrogression, which is likely to occur, must be made.

25. Refer text book
(B.C.PUNMIA) for more
explanation REGARDING
JHC vs TWC

27. STILLING
BASINS
CHAPTER - 3

28. STILLING
BASINS
• Stilling basins are external energy dissipators placed
at the outlet of a culvert, chute or rundown.
• These basins are characterized by some combination of
chute blocks, baffle blocks and sills designed to trigger
a
hydraulic jump in combination with a required tail
water condition.
• With the required tail water, velocity leaving a properly
designed stilling basin is equal to the velocity in
the receiving channel.
• While various stilling basin designs ODOT practice is to
use the St. Anthony Falls (SAF) stilling basin, which can
operate over a range of approach flow Froude
numbers from 1.7 to 17

29. TYPES OF STILLING BASINS
• [A] U.S.B.R. Stilling basin
1. Type – 1 Basin
2. Type – 2 Basin
3. Type – 3 Basin
• [B] Indian Standard Basin
1. Horizontal Apron Type -1
2. Horizontal Apron Type -2
3. Sloping Apron Type -3
4. Sloping Apron Type -4

30. U.S.B.R. Stilling
basin

31. U.S.S.R. Type -1
Stilling Basin

32. U.S.S.R. Type -2
Stilling Basin

33. U.S.S.R. Type -3
Stilling Basin

34. Indian Standard
Stilling Basin

35. I.S. Type -1
Basin

36. I.S. Type -2
Basin

37. I.S. Type -3 and Type -
4 Basin

38. SPILLWAY GATE
Spillway gate, also called stop gates, are adjustable
gates used to control water flow in reservoir, river,
stream systems.
→They also acts as barrier for the storage of
additional water.
→By these the height of the dam will be more such
that land acquisition will be more.
→ Gates can be provided to all types of spillways

39. TYPES OF SPILLWAY GATES
1. DRIPPING SHUTTERS OR PERMENENT FLASH
BOARDS
2. STOP LOGS AND NEEDLES
3. RADIAL GATE OR TAINTER GATES
4. DRUM GATES
5. VERTICAL LIFT GATES OR RECTANGLE GATES

40. DRIPPING SHUTTERS OR PERMENENT
FLASH BOARDS
• Consists of wooden panels of usually 1.0-1.25m HIGH
• Hinged at bottom and are supported against water
pressure by structs. The shutters falls flat on the crest
when the downstream supporting structs are tripped.
Hence they are not suitable for curved crests.
• These shutters can be raised or lowered from an over
head cable way or a bridge.
• Various automatic gates which drop themselves have
been designed these days.

41. STOP LOGS
• They consists of wooden beams or planks placed
one upon the other and spanning in the grooves
b/w the spillway piers .
• →they can be romed either by hand or with
hoisting mechanism. It takes lot of time for
removing them, if they become jammed in slots.
• →Leakage bw logs is a big problem hence they are
used in minor works.

42. NEEDLES
• Needles are wooden logs kept side by side with
their lower ends resting in a keyway on the spillway
and upper ends supported by a bridge.
• It is difficult to handle these at the time of flow
hence these are not used on any major works.
• They are sometimes used for emergency bulk
heads, where they need not be replaced until the
flow has stopped.

44. RADIAL GATE OR TAINTER GATES
• Radial gates are rotary gates consisting of cylindrical
sections.
• They may rotate vertically or horizontally. Tainter
gates are a vertical design that rotates up to allow
water to pass underneath.
• Low friction trunnion bearings, along with a face
shape that balances hydrostatic forces, allow this
design to close under its own weight as a safety
feature.

46. Drum Gates
• Drum gates are hollow gate sections that float on
water. They are pinned to rotate up or down. Water
is allowed into or out of the flotation chamber to
adjust the dam's crest height.
• These are desirable to longer spans in order of 40 or
so and medium heights of 10 or so.