ADDIS ABABA UNIVERSITY

School of Civil and Environmental Engineering

5th CED

CHAPTER THREE (PART TWO)

DAM OUTLET WORKS

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Hydraulic Structures I
 3.2 ENERGY DISSIPATION BELOW SPILLWAYS

3.21 Introduction
 Water flowing over a spillway has a very high kinetic energy because of the conversion of
the entire potential energy to the kinetic energy.
 If the water flowing with such a high velocity is discharge directly into the downstream
channel, serious scour may occur if not sound rock.
 In order to protect the channel bed from scour, the kinetic energy of the water should be
dissipated before it discharged in to the d/s channel.
 The energy-dissipation devices can be broadly classified in to two types:
– Devices using hydraulic jump for dissipation of energy
– Devices using a bucket for the dissipation of energy
 The choice of the energy dissipation device is governed by the tail water depth and the
characteristics of the hydraulic jump at the toe.

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 3.22 Forms of Energy dissipation on the Spillway
The passage of water from a reservoir into the downstream reach involves a whole number of
hydraulic phenomena such as the transition into supercritical flow, supercritical non-aerated
and aerated flow on the spillway, possibly flow through a free-falling jet, entry into the stilling
basin with a transition from supercritical to sub-critical flow, after turbulence the transition
into the stream beyond the basin or plunge pool.

It is; therefore, best to consider the energy dissipation process in five separate stages, some
of which may be combined or absent.

1. On the spillway surface;

2. In a free-falling jet;
3. At impact into the downstream pool;
4. In the stilling basin; and
5. At the outflow into the river.

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i. Region 2 and 3 (Free falling Jet and Impact on the downstream pool)
 In many modern spillways design, increased energy dissipation is achieved by using free falling
jet, either at the end of ski-jump or d/s of flip bucket.
 The head loss in the jet is only up to 10 – 12% whether solid or disintegrated. But, if jets are
colliding substantial energy will be dissipated.
 The main benefit from jet spillways is in the 3rd phase at the impact in to the down-stream pool.
ii. Energy Dissipation on Stilling Basins /Region – 4/
 The stilling basin is the most common energy dissipater converting the supercritical flow from the
spillway in to sub-critical flow compatible with the down stream river.

Figure : Hydraulic jump at the stilling basin

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3.23 Characteristics of Hydraulic Jump
 A hydraulic jump is a sudden and turbulent rise of water which occurs in an open channel when
the flow changes from the supercritical to sub critical state of flow.
 It is accompanied by considerable dissipation of energy.
Types of jumps:
 the type and characteristics of the jump depend mainly upon the Froude Number of the incoming
flow given by: v1 the mean velocity before the jump and y1 pre-jump height.
 For the formation of hydraulic jump, the initial Froude Number should be greater than unity.
The different types of hydraulic jumps are discussed as below:
 Undular Jump (Fr=1.0 to 1.7):- the water surface shows some undulation and the energy
dissipation is about 5%.
 Weak Jump (Fr= 1.7 to 2.5):- a series of small rollers develops on the surface of the jump, but
the d/s water surface of remains quite smooth.
The energy dissipation is about 20%
 Oscillating Jump (Fr=2.5 to 4.5):- there is oscillating jet entering the jump bottom to surface
and back again without any periodicity. The energy dissipation is between 20 to 40%. 5
Steady Jump (Fr=4.5 to 9.0):-
oThe jump is quite stable and balanced.
o It is not much sensitive to variation in the tail water depth.
o It has very good performance and most of the hydraulic structures utilize this
type of jump for the dissipation of energy and its dissipation is between 45 to 70%.
 Strong Jump (Fr>9.0):-
o the jump is quite rough but effective.
oIt causes rough water surface with strong surface waves downstream.
oThe energy dissipation is between 70 to 85%.

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Jump height curve (JHC) and Tail water rating curve (TWRC)
Jump Height curve (JHC): - It will occur in a rectangular channel if the following
equation between the initial depth (y1) and the sequent depth (y2) is satisfied.

y
y2  1
2
 1  8F  1 , where
1
2
F1 
v1
gy1

q
gy13
A 1
H
H1
Hydraulic Jump
P

y v2
y v1

Figure: Hydraulic Jump formation

The mean velocity (V1) of the incoming flow for an ogee-shaped spillway can be
determined by applying the Bernoulli equation at points A and 1, neglecting the loss and
velocity of approach:
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 2
v
P  H  y1  1
2g

and the mean velocity of (v1) at the toe of the spillway v1  q / y1

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 q  1
Therefore: P  H  y 1    Hence the value of y1 can be easily determined for
 y1  2g

the given discharge intensity (q) over the spillway.

 A plot between q as abscissa and the corresponding y2 as ordinate can be drawn and is
called jump height curve (JHC) or jump rating curve (JRC).
Tail Water Rating Curve (TWRC):- It gives the relation b/n the tail water depth (y2’) (the
actual water depth in the river on the d/s) as ordinate and q as abscissa.

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for case a for case b

for case c

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for case d
Case a represents the ideal condition in which the two rating curves always coincide.
• This means the jump forms at the desired place on the apron at all discharges.
Case b: In this case the jump forms at a certain place far downstream.
•An effective method of ensuring that the jump will occur on the protected apron is to use
sills to create a stilling basin.
Case c: The jump may be controlled at the desired location by providing a drop in the channel
bottom or by letting the jump form on a sloping apron
Case d: The tailwater curve is below the jump curve at low discharges and above it for higher
discharges.
• The stilling basin may be designed so that the jump is formed in the basin at low rates
of discharges and the jump moves on to a sloping apron at higher discharges.
Case e: This is opposite to case (d) in the sense that the tailwater curve is above the jump curve
at low discharges and below the jump curve at high discharges.
• An effective method to ensure a jump is to increase the tailwater depth sufficiently high by
providing a stilling pool, thus forming a jump at high discharges. 12
3.24 Stilling Basin
 A stilling basin is a channel structure of mild slope, placed at the outlet of a spillway, chute or
other high velocity flow channel, whose purpose is to confine all or part of the hydraulic
jump and dissipate some of the high kinetic energy of the flow.
 It is a structure which is necessary to prevent bed scour and undermining of the structure in
situation where high velocity flow is discharged into the downstream channel.
 Usually flow entering a stilling basin is at super critical velocity.
The stilling basin on the mild slope supports only sub critical flow.
The transition from super critical to sub critical flow takes place in the form of a hydraulic jump.
The design of a particular stilling basin will depend on the magnitude and other characteristics
of the flow to be handled, and particularly the Froude number of the approaching flow;

v1 q
F1  
gy1 gy13

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3.241 Stilling Basin Types
i. Basins for Froude Numbers Less Than 1.7:- No special stilling basin is needed , except
that the channel lengths beyond the point where the depth starts to change should be not
less than about 4y2.
ii. Basins for Froude Numbers between 1.7 and 2.5:- Flow phenomena for these basins
will be in the form designated as the pre-ump stage. Because such flows are not attended
by active turbulence, baffles or sills are not required.
iii. Basins for Froude Numbers Between 2.5 and 4.5 (Type-IV):-
o Flows for these basins are considered to be in the transition flow stage because a true
hydraulic jump does not fully develop.
o stilling device must be provided to dissipate flows for this range of Froude number, the
basin shown on figure below, which is designated a Type-I V basin, has proved relatively
effective for dissipating the bulk of the energy of flow.
o The basin is provided with chute blocks of size, spacing and location as shown on the
figure. The length of the stilling basin can be determined on the table below: 14
Table 3.2-1: Stilling Basin length for Froude Number from 2.5 to 4.5

Fr1 2 3 4 5

L/y2 4.3 5.3 5.8 6.0

Figure : Stilling basin characteristics for Fr between 2.5 and 4.5 (Type-IV)

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iv. Basins for Froude Numbers greater than 4.5 and velocity less than 15m/s
(Type-III):-
o For this condition, type-III basin should be provided.
o The basin is provided with chute blocks, baffle blocks and end sills.
o The length of the stilling basin, the height h3 of the baffle block and the height h4 of end
sill are obtained from Table below:
Table 3.2-2: Length of basin and baffle block parameters

Fr1 5 6 8 10 12 14 16

L/y2 2.3 2.5 2.6 2.7 2.8 2.8 2.8

h3/y1 1.5 1.7 2 2.3 2.7 3 3.3

h4/y1 1.2 1.3 1.5 1.6 1.7 1.8 1.9

Figure : Stilling basin for Fr above 4.5 where incoming velocity, V1 ≤ 15/s (Type-III)
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v. Basins for Froude Numbers greater than 4.5 and velocity greater than 15m/s
(Type-II):
 In this case the baffle blocks are not provided because of the following reasons:
a.The blocks would be subjected to very high impact forces due to high velocity V1 of
incoming flow; and
b.There is a possibility of cavitations on the d/s face of the blocks.
 The stilling basin therefore consists of only chute blocks and a dentated sill.
 As the dissipation of energy occurs mainly by hydraulic jump, the length of the basin is
greater than Type-II basin.
Table 3.2-3: Stilling Basin length for Fr greater than 4.5 and velocity greater than 15m/s

12 14
Fr1 5 6 8 10
4.3 4.3
L/y2 3.85 4.0 4.2 4.3

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Figure : Stilling basin for Fr above 4.5 where incoming velocity, V1>15/s (Type-II)

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 3.3 OUTLET AND INTAKE STRUCTURES

 Outlet works: Outlet works consist of a combination of structures designed to control the
release of water from the reservoir as required for project purposes or operation.

The components of outlet works: starting from the upstream end generally

 an approach channel

 an intake structure

 a conduit or tunnel (Water passage through or around the dam is provided through
tunnels in rock abutments or through cut-and-cover conduits through the base of an
embankment type dam).

 a control gate chamber (located in the intake structure, within the conduit, or at the
downstream end of the conduit),

 an exit chute

 and a discharge channel

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 Intake structures: The intake structure may serve several different
functions in the outlet works system.
Besides forming the entrance, it may include
• a trashrack to block debris,
• fish entrances
• multilevel ports or weirs for water temperature control
• water supply and irrigation intakes,
• control gates and devices.
Intake Structure Types:
Common categories of intake structures
A. Tower Intakes
B. Dam Intake
C. Submerged Intakes
Selection of the appropriate type depends on a number of considerations including site
conditions, economics, and effectiveness in meeting project requirements.
 An intake structure may be submerged or may extend above the maximum reservoir
water surface, depending on its function

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A. Tower Intakes
Huge fluctuation in water level or quality and to withdraw water at right depth
 Usually used with embankment dams

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II) Dam Intake
The intake structure is usually provided in the body of the dam,
It is rectangular in shape, and often flared in all four direction,

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C .Submerged Intake
 Intake structure entirely under water

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Intake Tower and Outlet Works Design Considerations:
The outlet works is an integral part of a project that includes a dam and spillway.
Its layout and configuration therefore should be associated with the planning and
development of the complete project .
In all cases, selection of the best overall plan for the outlet works should be made after careful
comparative studies of alternative plans and consideration of the site conditions.
 Functional and service requirements, interrelationships and compatibility, economy, safety,
reliability, and repair and maintenance requirements should all be considered in the studies.
 Site conditions include topography, climate, foundation geology, and seismicity .
 Hydrology and minimum flow requirements are important for determining the range of design
releases for the outlet and diversion conditions.
 All operation and maintenance requirements must be identified in order that a safe, reliable,
and economical outlet works will be designed.

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 After all the purposes of the project are established and the functions and criteria for the
outlet works have been clearly defined, the geometry and layout of the intake tower can
proceed.
 Multiple alternatives should be developed and evaluated to determine the optimum plan.

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