HYDROPOWER ENGINEERING

1. Chapter 7
Spillways and Energy dissipaters
Prepared by:
Binu karki
Msc iWRM
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2. Overflow or Ogee Spillway
2

3. Spillway
3

4. Requirements/Purpose of Spillway
• A spillway should have sufficient capacity to
serve as moderation of floods.
• Spillway should be hydrologically and
structurally safe.
i. Location of the spillway should provide safe
disposal of water without toe erosion.
ii. Spillway should provide safe and regulated
release of the surplus water in excess of
reservoir capacity.
• Spillway usually has energy dissipation work on
its downstream side.
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5. 5

6. Ogee spillway
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7. Chute Spillway
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8. Side channel Spillway
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9. Shaft spillway/Glory Hole
Spillway
9

10. Shaft spillway/Glory Hole
Spillway
10

11. 11

12. Straight drop/
Over-fall spillway
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13. Tunnel spillway
13

14. Labyrinth Spillway: Lake Brazos
Labyrinth weir
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15. Labyrinth Spillway
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16. Stepped spillway
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17. Service spillway/Auxiliary
spillway
:Warrangamba Dam, Australia
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18. According to the function
Main Spillway
• Primary spillway
• Operates in general operation condition
• Designed to pass entire spillway design flood
• In most of dams, it is only spillway
Auxiliary Spillway
• Mostly provided in conjunction with smaller main
spillway
• Total capacity=capacity of (main spillway +
auxiliary spillway)
Emergency Spillway
• Comes in operation only during emergency which
may arise at any time and the same might not
have been considered in normal design of main
spillway.
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19. Emergency Spillway:
Oroville Dam, California
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20. Ungated Spillway
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21. Gated Spillway
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22. Orifice Spillway
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23. Energy Dissipation: By
formation of Hydraulic Jump
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24. Overflow or Ogee Spillway
• Constructed by using a portion of dam as overflow
section .
• Provided in valleys with sufficient width to
accommodate the required crest length.
• Might be either controlled or uncontrolled.
• Controlled spillway
i. Permanent gate (lifted automatically or
operationally) OR
ii. Provision of temporary planks(wooden) in small and
less important spillways
• Permanent crest gates increase total cost of dam BUT
they increase reservoir capacity 24

25. Overflow or Ogee Spillway
• The rate of discharge over entire length of the
spillway
Q=CLH15 OR q= Q/L=CH15
Where,
C=coefficient of discharge=2.15-2.2
L=effective length of weir(m)
H=measured head above crest(m)
q=discharge per meter length (m3/s/m)
Q=discharge(m3/s)
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26. Overflow or Ogee Spillway Design
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27. Overflow or Ogee Spillway Design
• Spillway crest needs careful design to stand
maximum flood.
• At design head (h=Hd),water flowing down the
spillway remains in contact with the surface and no
negative pressure gets developed on spillway
surface.
• If h>Hd ,negative pressure gets developed i.e
cavitation on spillway surface BUT no such problem
for h<= Hd
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28. Cavitation Prevention measures
1. Additional quantity of concrete (Ramp) may be
put on the downstream face of the dam.
2. Corbel may be constructed on the upstream face
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29. Overflow spillway profile
• Waterways Experiment Station,
USA (WES) prescribed following
spillway shape equation:
Xn=KHn-1y
Where,
X and y are the co-ordinates of the
crest profile
Origin is at highest point of crest
H is design head without approach
velocity head
K & n depend on slope of upstream
face
For vertical upstream face, K=2 and
n=1.85,the figure shows standard
WES spillway shape.
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30. Overflow spillway profile
• Similarly, US Army Corps of Engineers have
recommended two equations each for
downstream and upstream profile of spillway
apex as:
Where, hD =design head over spillway
x and y are co-ordinates which are right angles to
each other taking apex as origin.
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31. Energy Dissipaters
• The water flowing down from the spillways possess a large
amount of kinetic energy that is generated by virtue of its
losing the potential head from the reservoir level to the
level of the river on the downstream of the spillway.
• If this energy is not reduced, there are danger of scour to
the riverbed which may threaten the stability of the dam or
the neighboring river valley slopes.
• The various arrangements for suppressing or killing of the
high energy water at the downstream toe of the spillways
are called Energy Dissipaters.
• In general, energy dissipation can be achieved in two
ways:
By developing a hydraulic jump
By directing the jet of water using a deflector bucket.
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32. 1.Hydraulic Jump
• For hydraulic jump to
occur, the u/s flow
should be supercritical
i.e. Fr>1
• Type of jump depends
upon value of Fr
number.
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33. Hydraulic Jump
33

34. Stilling Basin
• Use stilling basin to initiate jump.
• Allows dissipation of energy within a structure that
will minimize damage.
• Baffle blocks are used to make jump position more
stable.
• Chute Blocks and End sill are also used for control
of jump
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35. Chute Blocks, Baffle Blocks and End Sill
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36. Chute Blocks
• These are triangular blocks with their top
surface horizontal.
• They are installed at the toe of the spillway just
at upstream end of the stilling basin.
• These blocks stabilize the jump, improve jumps
performance and decrease the length of
hydraulic jump.
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37. Baffle Blocks or Piers/Friction
Blocks
• They are installed on stilling basin floor between
chute blocks and end sill.
• They stabilize the formation of jump.
• They assist in dissipation of energy.
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38. End Sill/Dentated Sills
• Provided at the end of stilling basin
• They diffuse residual portion of high velocity jet
reaching end of basin
• They help to reduce length of jump or basin
38

39. 2.Roller Bucket /Flip Bucket type
energy dissipaters
• Used when tail water condition is
not favorable for adopting
hydraulic jump.
• Roller bucket is a spoon type
structure at the toe of spillway.
• This requires relatively short
structure in comparison to
hydraulic jump type stilling basin.
• The high velocity of water slides
down and get arrested by tail
water.
• For successful roller action, the
tail water depth has to be higher
than that required by hydraulic
jump type basin.
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40. 2.Roller Bucket /Flip Bucket type
energy dissipaters
• Main variables of design:
Radius of bucket & lip angle
• Radius=15-25 m
• Lip angle=20 to 40 degrees
• The optimum dimensions are
decided with model studies.
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41. By directing jet of water through
deflector bucket: Flip bucket
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42. By directing jet of water through
deflector bucket: Flip bucket
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43. By directing jet of water through
deflector bucket: ski jump bucket
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44. 3.Ski-Jump Bucket type Energy
Dissipater
44
• Similar to roller bucket type in construction
• The water jet flows over the bucket and springs up
clearly in air and after a trajectory hits the river bed at
some distance away from the toe of the dam.
• Suitable when foundation rock is of good quality and
can withstand erosive action of plunging jet.
• Tailwater has to be low so that clear ski jump formation
can take place.

45. 3.Ski-Jump Bucket type Energy
Dissipater
45
• It acts as ski-jump type bucket at certain discharges
and as a roller bucket at lower discharges.
• For example:
In Rihand Dam in India, bucket acts as ski at Q of
33m3/s/m and above BUT as roller below this value.

46. By directing jet of water through
deflector bucket: ski jump bucket
46

47. Design of Stilling Basin
47

48. Design of Stilling Basin
• Equation of flow over crest,𝑄 𝑑 =
2
3
𝐶 𝑑 2𝑔 ∗ 𝐿𝐻
3
2
Where,
L-Length of Crest
Qd-Design discharge
Cd-Coefficient of Discharge(assume 0.22)
H-Height of water above crest
• discharge per unit width, q=Q/B
• Since, Upstream specific energy=Specific energy
at 1 i.e. Eu=E1):H+h =y1+
𝑣1
2
2𝑔
• Fr1=
𝑣1
𝑔𝑦1
where, v1=
𝑄 𝑑
By1
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49. Design of Stilling Basin
• Sequent depth
y2=
𝑦1
2
(−1 + 1 + 8𝐹𝑟1
2 )
• Length of Basin
Length=5(y2-y1)
• Tail water depth(yt or yn)
𝑄 𝑑 =
1
𝑛
𝐴𝑅
2
3 So
1
2
n=Manning’s rough coefficient
A-Area of flow(yn*B)
R-(
𝑦𝑛∗𝐵
2𝑦𝑛+𝐵
)
2
3
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50. Design of Stilling Basin
• Find yc=(q2/g) Τ
1
3
If yn>yc-Hydraulic jump is formed
If yn<y2-ski-jump type energy dissipator
recommended/repelled jump is formed/further
excavation for stilling basin required
If yn>y2-submerged jump is formed
• Energy loss in the jump, ∆E
∆E=
𝑦2−𝑦1 3
4𝑦1𝑦2
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