Energy dissipators

Need for Energy Dissipaters
 When water is released over the spillway, the potential
energy is converted into kinetic energy at the base of
spillway.
 This energy must be dissipated in order to prevent the
possibility of severe scouring of downstream .
 For this purpose energy dissipators must be used
which perform the energy reduction by converting the
kinetic energy into turbulence and finally into heat.

Hydraulic Energy Dissipation Devices
 Water Cushion
 Baffle Walls
 Biff Walls
 Deflectors
 Staggered blocks
 Ribbed Pitching
 Hydraulic Jumps

Baffle Walls
 It is an obstruction constructed across the canal
downstream of the fall.
 It is in the form of a wall of low height.
 It heads up the water just upstream of it. Thus it
tries to create a water cushion on the upstream.

Biff Walls
 It is an end wall of the cistern. It is a vertical wall with a
horizontal projection extending in the cistern.
 Due to the projection, the flow of water returns back
in the cistern. It creates an obstruction to the fast
moving water down the fall. As a result the energy of
flow is dissipated.

Deflectors
 It is a short wall constructed at the end of a
downstream side.
 This end wall deflects the high velocity flow of water.
Due to deflection the velocity of flow in the direction
of motion is reduced.

Staggered Blocks
 They are nothing but rectangular blocks or cubes
generally made of concrete.
 They are arranged in a staggered manner on the
downstream horizontal apron.
 They deflect the high velocity flow in a lateral
direction. It provides an obstruction to high velocity
downstream flow and the energy of flow is dissipated
effectively.

 For a given value of specific energy, the critical
depth gives the greatest discharge, or conversely, for a
given discharge, the specific energy is a minimum for
the critical depth.
 For rectangular channels, the critical depth, dc ft
(m), is given by
dc =[Q2/b2g]1/3
Where
dc =critical depth, (m)
Q= quantity of flow or discharge, (m3/s)
B= width of channel, (m)

 Theoretical depth after jump – alternate depth
 Actual depth after jump – sequent depth
 Length of jump – about 5-7 times the subcritical
depth

Uses of Hydraulic Jumps
 Hydraulic jump is used to dissipate or destroy the energy
of water where it is not needed otherwise it may cause
damage to hydraulic structures.
 It may be used for mixing of certain chemicals like in
case of water treatment plants.
 Hydraulic jump usually maintains the high water level on
the down stream side. This high water level can be used for
irrigation purposes.

Case 1
 This is the ideal case in
which the horizontal
apron provided on the
riverbed downstream
from the toe of the
spillway would suffice.
 The length of the apron
should be equal to the
length of the jump
corresponding to the
maximum discharge
over the spillway.

Class 1
 Jump height rating curve is always above tailwater rating
curve. In this class, the depth of in the river is insufficient
for all discharges for the formation of a jump at the toe of
the structure.
 The jump will form at certain place for downstream (Case
2). The energy dissipation can be achieved in any of the
following ways:
1. Lowering the floor level downstream of the dam in
order to make the tailwater depth in the stilling basin equal
to the jump height for all discharges.
2. Stilling basin with baffles or sills at bed level.
3. Stilling basin with a low secondary dam downstream.
4. Bucket type energy dissipators (ski-jump).

Note – Length is same for
all. You cannot break any
of matchsticks.

Class 2
 The jump height curve is always below tailwater rating
curve. This means that Case 3 occurs at all times and
the jump will move upstream consequently, little
energy will be dissipated.
 A method of energy dissipation can be achieved by:
1. Sloping apron.
2. Roller bucket type energy dissipator.

Roller Bucket Energy Dissipation
 A roller bucket energy dissipator consists of a circular
arc bucket tangent to the spillway face terminating
with an upward slope.
 This geometry when located at an appropriate depth
below tailwater will produce hydraulic conditions
consisting of a back roller having a horizontal axis
above the bucket and a surge immediately downstream
from the bucket.
 Solid and slotted buckets have been used successfully.

Class 3
 Jump height curve is above tailwater rating curve at
low discharges and below at higher discharges. An
effective method of dissipating energy is by:
1. Stilling basin for forming a jump at low discharges
and to combine with the basin a sloping apron for
developing a satisfactory jump at high discharge.
2. Stilling basin with baffle piers or dentated sill.

Apron partly above & below G.L.

Class 4
 Jump height curve is below tailwater curve at low
discharges and above at high discharges.
 An effective method to insure a jump is to increase the
tailwater depth sufficiently high by providing stilling
pool (basin), this forming a jump at high discharges.

Siphon Spillway
 Contains siphon pipe
 On end at upstream end and other end at downstream
end

Baffled Walls
 The baffled chute spillway relies upon multiple rows of
baffles to aid in dissemination of energy flowing down
a spillway chute.
 The USBR has developed a set of design guidance
which can be used in preliminary design of such a
structure.
 Model studies are recommended for design
verification when the design discharge exceeds 50
ft3/sec and/or the slope is steeper than 1V on 2H.

Presented By
RAHUL GUPTA
(D4 CE A 120091)

Energy dissipators

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