Energy dissipator below spillway

• An energy dissipator in a spillway is a structure or device installed at the

downstream end of a spillway to reduce the high energy and velocity of water
flowing from a dam or reservoir. Without energy dissipation, the fast-moving water can
cause severe erosion, scour, and structural damage to the riverbed, banks, and nearby
infrastructure.
• Types of Energy dissipators
• i. Bucket type energy dissipators
• ii. Stilling basin type dissipators
• iii. Baffle type dissipators
• Bucket type dissipators
• a. Roller bucket type
• b. Ski jump bucket type
• The high kinetic energy of water is reduced by
providing a hydraulic jump at the end of
spillway.
• The hydraulic jump can be achieved by
providing bucket type dissipators.
• By hydraulic jump of water some part of
energy is dissipated by aeration.
• Stilling Basins Type
• Stilling basins are usually provided after the buckets.
• Due to the hydraulic jump of water, the water falling on the ground may cause
cavitations on the ground.
• These cavitations can be avoided by providing the stilling basin.
• The stilling basin consists of water which reduces some part of energy of water.

Horizontal Apron Type Sloping Apron Type

• Baffle type dissipators
• After passing the stilling basin water has
still some energy.
• If any amount of energy exists, it can be
fully dissipated by providing baffle
dissipators.
• In this, baffle type structures are provided
in a number of series depending on the
amount of energy.
 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.
• The depth before the jump is always less than the depth after the jump is called the
initial depth (y1) and the depth after the jump is called the sequent depth (y2)
• For hydraulic jump to be developed in a horizontal rectangular channel, the
following equation must be satisfied b/w pre-jump depth (𝑦1 ) and post-jump depth
(𝑦2 ).
𝑦 𝑦12 2𝑞 2
• 𝑦2 = − 1 + +
2 4 𝑔𝑦1

• Hydraulic jump formation depends considerably upon the Froude number of the
incoming flow ( F1)
𝑦2 1 𝑣1
• = 1 + 8𝐹12 − 1 , where 𝐹1 =
𝑦1 2 𝑔𝑦1
• For a given intensity and given height of
spillway, 𝑦1 and 𝑦2 are fixed.
• Availability of a depth equal to 𝑦2 in the channel
on the downstream depends on the water level,
which depends on the hydraulic dimensions and
slope of the river channel below.
• If a graph is plotted b/w q and tail water depth,
the curve obtained is known as tail water curve
(T.W.C).
• If a curve is plotted on the same graph, between
q and 𝑦2 , the curve is known as the jump height
curve (J.H.C) or 𝑦2 curve.
• When TWC and JHC plot in the same graph
paper then there are five possibilities
a) T.W.C coinciding with 𝑦2 curve at
all discharges.
b) T.W.C lying above 𝑦2 curve at all
discharges
c) T.W.C lying below 𝑦2 curve at all
discharges
d) T.W.C lying above 𝑦2 curve at
smaller discharges and lying below
𝑦2 curve at larger discharges
e) T.W.C lying below 𝑦2 curve at
smaller discharges and lying above
𝑦2 curve at larger discharges.
• a) T.W.C. Coinciding with 𝑦2 curve at all discharges:
• This is most ideal Condition for jump formation
• The hydraulic jump will form at the toe of the spillway at all
discharges
• In such cases, a simple concrete apron of length 5 (y2- y1) is generally
sufficient to provide protection in the region of hydraulic jump.
• b) T.W.C. Lying above the 𝑦2 curve at all discharges
• In this case when 𝑦2 is always below the tail water, the jump
forming at toe will be drowned out by tail water , and little
energy will dissipated as follow
• i) By constructing a sloping apron above the river bed level.
• Jump will form on the sloping apron where the depth equal
to 𝑦2 .
• Slope of apron is made in such a way that proper conditions
for jump will occur somewhere on the apron at all
discharges.
• ii)By providing a roller bucket type dissipators
• Roller which is formed d/s of the bucket, tends to move the
scoured bed material towards the dam, thus preventing
serious scour at toe of the dam
• Sometimes, the scoured material may enter the bucket under
the action of u/s roller, and may cause severe abrasion.
• C) T.W.C. lying below the Y2 curve at all discharges
• i) By providing ski jump bucket type dissipater
• If the tail water is very low, the water may shoot up out the bucket, and fall
harmlessly into the river at some distance downstream of the bucket
• In such cases ski jump bucket energy dissipater is used
• ii) By providing a sloping apron below the river bed
• D) T.W.C. Lying above the 𝑦2 curve at smaller discharges and lying below the 𝑦2
curve at large discharges:
• In this case, at low discharges the jump will be drowned and at high discharges tail
water level is insufficient in such cases sloping apron partly above and partly
below the river bed is provided.
• At low discharges, the jump will form on the apron above the river bed, where the
available depth is equal to the required depth and Jess than the T.W. depth.
Similarly, at high discharges, the jump will form on the apron below the river bed,
where the available depth is more than the T.W. depth and equal to the depth
required for jump formation.
• E) T.W.C. Lying below the 𝑦2 curve at smaller discharges and lying
above the 𝑦2 curve at large discharges
• This is the reverse of case (d)
• Same arrangement which was made in cade (d) will serve the purpose
• Only difference will be that at low discharges, the ump will form on
the apron below the bed; and at high discharges, jump will form on the
apron above the bed
Types of Indian Standard Stilling Basins (IS: 4997–1968)
• Type I Basin – Low Energy, Low Type II Basin – Medium Tailwater,
Tailwater Medium Flow
• Used for: Small weirs, canal falls Used for: Medium discharge spillways
• Flow condition: Froude Number < 4.5 Flow condition: Froude Number ~
4.5–7
• Design: Simple floor and end sill
Design:
• No baffle blocks Horizontal apron
• Hydraulic jump forms naturally Chute blocks
Baffle blocks
End sill
• Type III Basin – High Velocity, Moderate Tailwater
• Used for: High-velocity spillways
• Flow condition: Froude Number > 7
• Design:
• Longer apron
• Strong baffle blocks
• Dentated end sill
• Heavier reinforcement
• Good for larger dams, where flows are turbulent and more energy needs to be
dissipated
• Type IV Basin – Deep Tailwater Basin
• Used when: Tailwater is much deeper than the conjugate depth of jump
• Design:
• Deeper basin to hold jump
• High vertical sidewalls
• Stronger blocks and sill
• Used when hydraulic jump won’t form naturally due to deep tailwater
• Type V Basin – Roller Bucket (Special Case)
• Used when: Tailwater is extremely high, or terrain doesn’t allow a horizontal
basin
• Design:
• Circular or bucket-shaped flip at the end
• Flow lifts into air and falls in downstream pool (forms a surface roller)
• Common in mountain dams (e.g., Bhakra Dam)
 Hydraulic
Type Flow Energy Tailwater Components
Jump Form
I Low Low Natural Floor, sill
Chute+ Baffle
II Medium Medium Forced
Block, sill
Strong block,
III High Medium Forced
dented sill
Deep floor,
IV High High Contained
vertical walls
V Very High Very High External (air) Roller Buckets