Topics: 1. Types of Spillways 2. Design Principles of Ogee Spillway 3. Spillway Gates 4. Energy Dissipaters 5. Significance of Jump Height Curve & Tail Water Curve 6. Stilling Basins

1. SPILLWAYS
(WATER RESOURCES ENGINEERING – II)
UNIT – IV
Rambabu Palaka, Assistant ProfessorBVRIT

2. Learning Objectives
1. Types of Spillways
2. Design Principles of Ogee Spillway
3. Spillway Gates
4. Energy Dissipaters
5. Significance of Jump Height Curve & Tail
Water Curve
6. Stilling Basins

4. Introduction
A Spillway is the overflow portion of dam, over
which surplus discharge flows from the
reservoir to the downstream.
For safety, spillways should have sufficient
capacity to discharge floods, likely to occur
during the lifetime of the dam.

5. Location of Spillway
1.Spillways may be provided within the body of
the dam
2.Spillways may sometimes be provided at one
side or both sides of the dam
3.Sometimes by-pass spillway is provided
which is completely separate from the dam

6. Types of Spillways
1.Free Overfall or Straight Drop Spillway
2.Ogee or Overflow Spillway
3.Side Channel Spillway
4.Chute or Open Channel or Trough Spillway
5.Conduit or Tunnel Spillway
6.Drop Inlet or Shaft or Morning Glory Spillway
7.Siphon Spillway

7. 1. Water flows over a relatively thin spillway crest and falls freely
to the downstream.
2. Usually appropriate for thin dams having almost vertical
downstream faces.
3. This type of spillways may be economical for low heads as
compared with overflow spillways because of saving in
concrete.
4. Not recommended for high heads because of structural
instability problems.
Suitable Artificial Protection should
be provided to avoid SCOUR
Free Overfall or Straight Drop Spillway

8. Overflow spillways also called ogee-shaped (S-shaped) spillways.
This type of spillways allows the passage of the flood wave over
its crest.
Widely used in Gravity dams, Arch dams, and Buttress dams
Overflow spillways:
 Controlled (gated, guided)
 Uncontrolled (un-gated, free)
Almost all recently constructed dams are installed with crest gates
to store more water in the reservoir
Overflow or Ogee Spillway

9. Side Channel Spillway
Discharge characteristics of a side channel spillway are similar to
those of an ordinary overflow spillway and are dependent on the
selected profile of the weir crest.
Although the side channel is not hydraulically efficient, nor
inexpensive, it has advantages which make it adoptable to
spillways where a long overflow crest is required in order to limit
the afflux (surcharge held to cause flow) and the abutments are
steep.

10. Chute Spillway
Chute spillways are used in flow ways where water is to be
lowered from one level to another and where it is desirable to
avoid a stilling basin.
These are mostly used with earth dams
Advantages:
 It can be provided on any type of foundations.
 Simplicity of design.
 However this type of spillway should not be provided where too
many bends are to be given as per topography.

11. Conduit or Tunnel Spillway
Where a closed channel is used to convey the discharge around a
dam through the adjoining hill sides, the spillway is often called a
tunnel or conduit spillway.
The closed channel may take the form of a vertical or inclined
shaft, a horizontal tunnel through earth or rock, or a conduit
constructed in open cut and backfilled with earth materials.
Tunnel spillways are advantageous for dam sites in narrow gorges
with steep abutments or at sites where there is danger to open
channels from rock slides from the hills adjoining the reservoir.

12. Shaft Spillway
If sufficient space is not available for an overflow spillway, a shaft
spillway may be considered.
In the site of shaft spillway
 Seismic action should be small,
 Stiff geologic formation should be available, and
 Possibility of floating debris is relatively small.
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.

13. Siphon Spillway
A siphon spillway may be constructed in the body of a concrete
dam when space is not available for an overflow spillway.
Disadvantage of siphon spillway:
 The flow would result excessive vibrations in the dam body
which may cause expansion problems in the joints.
 There is a possibility of cavitation for negative pressures,
which is affected by the head between upstream and
downstream water levels.
 Repair and maintenance of siphon spillways are difficult.
 It has a limited capacity.

14. Ogee Spillway
(Design Principles)

19. 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.

22. JHC vs TWC
Jump Height Curve (JHC):
 It is a curve representing post jump depth
(d2) with discharge (q)
Tail Water Curve (TWC):
 It is a curve representing tail water depth (D)
with discharge (q)

23. Five conditions that govern the relationship between JHC and TWC

24. JHC lies lower than TWC at all
Discharges
(PROTECTION MEASURES)

28. JHC lies above TWC at all
Discharges
(PROTECTION MEASURES)

33. JHC lies lower than TWC at Small
Discharges
& JHC lies above TWC at large discharge
&
Vice-Versa
(PROTECTION MEASURES)

35. Stilling Basins

36. USBR Stilling Basin
Type – II
(Fr > 4.5)
for Medium and High Dams

37. USBR Stilling Basin
Type – IV
(2.5 < Fr < 4.5)
for Weirs and Barrages

38. Spillway Gates
1.Flash Boards, Stop Logs and Needles
2.Radial Gates
3.Drum Gates
4.Vertical Lift Gates
5.Bear Trap Gates
6.Rolling Gates

46. Reference
Chapter 11
Irrigation and Water Power Engineering
By Dr. B. C. Punmia,
Dr. Pande Brij Basi Lal,
Ashok Kr. Jain,
Arun Kr. Jain