Mekelle University

Ethiopian Institute of Technology-Mekelle

Chapter Four: Spillways & Energy

Dissipator:
Principles of Design and Construction
Course Name: Dam
Engineering
Course Code: CENG
6032

Bizuneh Asfaw Abebe

1 (Ph.D)
What does it mean by a spillway?
• The spillway is among the most important
structures of a dam project.
• It provides the project with the ability to release
excess or flood water in a controlled or
uncontrolled manner to ensure the safety of the
project

2
Introduction
A spillway is a structure constructed at or near the dam site to

dispose of surplus water from the reservoir to the channel

downstream.

Spillways are provided for all dams as a safety measure against

Overtopping and the consequent damages and failure.

A spillway acts as a safety valve for the dam. because as soon as

the water level in the reservoir rises above a predetermined level,

3 excess water is discharged safely to the downstream channel,

Cont’d…
 The spillway must have adequate discharge capacity to pass the

maximum flood d/s without causing any damage to the dam or its

appurtenant structures.

 A spillway may be located either in the middle of the dam [Fig 1.1(a)]

or at the end of the dam near abutment.

 the spillway must be hydrodynamically and structurally safe

 The design of a spillway requires utmost attention. Many failures of

dams occurred in the past because of improperly designed spillways or

4 by spillways of inadequate capacity.

Essential Requirements of a Spillway
The essential requirements of a spillway, as discussed above,
may he summarized as follows:
 It must have adequate discharge capacity

 It must he hydraulically and structurally stable

 The surface of the spillway must be erosion resistant.

 The spillway must be so located that the spillway discharge does not

erode or undermine the downstream toe of the dam.

 It should be provided with some device for the dissipation of excess

energy.
 The spillway discharge should not exceed the safe discharge capacity

5 of the downstream channel to avoid its flooding.

Required Spillway Capacity
The required spillway capacity is usually determined by flood

routing. The following data are required for the flood routing:
Inflow flood hydrograph, indicating the rate of inflow with

respect to time. It is the same as the design flood hydrograph of

the spillway.
Reservoir-capacity curve, indicating the reservoir storage at

different reservoir elevations.

Outflow discharge curve, indicating the rate of outflow through

6
spillways at different reservoir elevations.
Factors affect the spillway capacity

 Inflow flood hydrograph

Available storage capacity

Capacity of outlets

 Gates of spillway

 Possible damage, if the capacity is exceeded

7
Best combination of the Storage capacity, and the
spillway capacity
 For determining the best combination of the storage capacity and the

spillway capacity to accommodate the selected inflow design flood,

 It is necessary to consider all pertinent factors of hydrology, hydraulics

of spillway, design cost and possible damages.

 After a spillway of a particular type and dimensions has been selected,

the maximum water level and the maximum spillway discharge can be
determined by flood routing.
 Various combinations of the spillway capacity and the dam height (or

storage capacity), for the assumed spillway types are selected and
flood routing is done.
8
Spillway Capacity-Surcharge Relationship.
Component Parts of a Spillway
A spillway generally has the following component parts

Entrance channel

Control structure

Discharge channel (or waterway)

Terminal structure (energy dissipator)

Exit channel

However, entrance and exit channels may not be required

for some spillways.

10
A tail channel

Energy dissipater

Flip bucket
Earth dam
Chute

Approach channel
Classification of Spillways

The spillways can be classified into different types based on

the various criteria,

Classification based on purpose

Main (or service) spillway

Auxiliary spillway

Emergency spillway

 Classification based on control

Controlled (or gated) spillway

12 Uncontrolled (or ungated) spillway

Cont’d…
Classification based on prominent feature
Free overfall (or straight drop) spillway
Overflow or Ogee spillway
Chute (or open channel or trough) spillway
Side-channel spillway
Shaft (or morning glory) spillway
Siphon spillway
Conduit (or tunnel) spillway
Cascade spillway
13
1. Free Overfall Spillway
A free overfall spillway (or a straight drop

spillway) is a type of spillway in which the

control structure consists of
low-height, narrow-crested weir and the
downstream face is vertical or nearly vertical
so that the water falls freely more or less vertical

14
1. Free Overfall Spillway
The overflowing water may discharge as a free

nappe, as in the case of a sharp-crested weir,

The water flowing over the crest drops as a free

jet clear of the downstream face of the

spillway.

15
Cont’d…
A free overfall spillway is commonly used for a low arch

dam whose downstream face is almost vertical.

It used as a separate structure for low earth dams. The

design of a free overfall spillway is similar to that of a

vertical drop weir

16
Cont’d…
A free overfall spillway is not suitable when the

foundation is weak and yielding, because the apron at the

stream bed is subjected to large impact forces at the point
of impingement of the falling jet.

17
2. Ogee - Shaped (or Overflow) Spillway
 An ogee-shaped (or overflow) spillway is the most commonly used
spillway.
 It is widely used with gravity dams, arch dams and buttress dams.
 Several earth and rockfill dams are also provided with this type of
spillway as a separate structure.
 When the head is greater than the design head, the overflowing
water tends to break contact with the spillway surface and a zone of
separation is formed, in which a negative or suction pressure occurs.

18
Shape of the crest of the overflow spillway
The shape of the ogee-shaped spillway depends upon a number

of factors such as

(1) head over the crest,

(2) height of the spillway above the stream bed or the bed of the
entrance channel and

(3) the inclination of the upstream face of the spillway

 The U.S. Army Crops of Engineers developed several standard shapes of the

crests of overflow spillways on the basis of U.S.B.R. data.

 Because the shapes were developed at U.S. Waterways Experiment Station

at Vicksberg (U.S.W E.S.), the shapes are known as the W.E.S, standard
19
spillway shapes.
Cont’d…
1. Downstream profile The d/s profile of the
spillway can be represented by the following
general equation:

Where:
 x and y are the coordinates of the point on the

spillway surface, with the origin at the highest

point 0 of the crest.
 Hd is the design head, excluding the head due to

the velocity of approach, and

 K and n are constants, which depend upon the
20
inclination of the upstream face of the spillway
2. Upstream profile of the crest
(a) Vertical upstream face: The upstream profile of the

crest should be tangential to the vertical face and should

have zero slope at the crest axis to ensure that there is no
discontinuity along the surface of flow. The upstream
profile should conform to the following equation

21
Cont’d…
 (b) Sloping upstream face:The coordinates of the upstream profile in the case

of sloping upstream face can be determined from Table l.2 for slopes of 1:3,
2:3 and 3:3. For intermediate slopes. the values may be interpolated.

22
3. Offsets and risers on upstream face
 If structural requirements permit, offset and risers can be provided on the

upstream face by removing some portion of concrete, and thus economy can
be effected
 The maximum permitted projection from the crest line is 0.315 Hd and the

vertical depth of the maximum bulging is 0.25 Hd [Fig. 1.8 (a)]

23
Cont’d…
4. Pressure over spillway surface
 positive (i.e. above atmospheric pressure) i.e the actual head is less
than the design head, the pressure on the crest will be positive
 negative (i.e. less than the atmospheric pressure) for heads greater
than the design head, it may lead to cavitation.
5. Orifice Flow
 In a gated spillway, orifice flow occurs at partial gate openings,
U.S.B.R. permits a negative pressure of 4.3m of water (about 42.0
kN / m2). But
As far as possible, the negative pressure should be avoided, because it has
the following ill effects.
It increases the overturning moment on the crest.
It increases the force required for lifting the gates.
It causes a decreases in capability for automatic control.
It causes vibrations which eventually extend. all over the structure.
The vibrations also cause cracks in the mortar of stone lining of the
24 masonry crest, for which
Cont’d…
6. Corbel
 it is found that it extends beyond the downstream face of the non-

overflow section [Fig. 1.9 (a)]. In other words, the spil1way

section is thicker than the non-overflow section of the gravity
dam.

25
 Discharge Computation for an Ogee Spillway
The discharge over an ogee spillway is computed from the basic

equation of flow over weirs, given below:

Where Q is discharge (cumecs), Cd is the coefficient of

discharge, Le is the effective length and He is the actual effective

head including the head due to the velocity of approach. i.e.

1. Coefficient of discharge (Cd) An ogee spillway has a relatively high

value of the coefficient of discharge (Cd) because of its shape.
26
Cont’d…
The maximum value of Cd is about 2.20, if no negative pressure

occurs on the crest. However, the value of Cd is not constant.

It depends upon the shape of the ogee profile, and also upon the

following factors.
(i) Height of spillway crest above the stream bed
(ii) Ratio of actual total head to the design total head.
(iii) Slope of the upstream face of spillway
(iv) Extent of the downstream submergence of crest
(v) Downstream apron

27
 i) Height of spillway above stream bed
 The height P of spillway above the stream bed affects the discharge coefficient because

the velocity of approach depends upon this height.

 With an increase in the height P, the velocity of approach deceases but the coefficient of

discharge Cd increases. Fig. 1.10 shows the variation of Cd with the ratio (P/HD)
P/HD where
HD is the design total head, including the head due to the velocity of approach. Thus

28
(ii) Ratio of actual total head (He) to the design total head
(HD)

Fig 1.11 shows the variation of actual discharge coefficient Cd’. It is

plotted between (Cd’/ Cd), as ordinate, and (He /HD), as abscissa.

The plot is applicable to high overflow spillways; with P>=1.33 Hd.
Similar curves are available for low overflow spillways.

29
(iii) Slope of the upstream face of spillway
 Fig. 1.12 shows the variation of (Cd’/ Cd) with the ratio (P/HD) for three

different slopes of the upstream face. For small ratios of (P/HD). the actual
coefficient Cd’ is slightly more than the coefficient Cd for the vertical face.
However, as the ratio (P/HD) increases, the ratio (Cd’ / Cd) decreases.

30
(iv) Extent of downstream submergence
 The actual coefficient of discharge Cd’ is decreased due to downstream submergence,

 Fig. 1.13 shows the variation of Cd’/ Cd, with the degree of submergence h/Hd, where

h is the depth of water over the crest on the downstream and Hd that on the upstream.
 It may be noted that the effect of submergence is negligible for smaller degree of

submergence.
 It is about 5% for the degree of submergence of 60%.

31
(v) Downstream apron
 Fig. 1.14 shows the effect of downstream apron on the coefficient of

discharge. When the value of (hd + d)/ HD exceeds about 1.70, the d/s floor
apron has little effect on the coefficient of discharge, but for lower value,
the coefficient of discharge Cd is lower. In this expression d is the tail water
depth, and hd is the depth of d/s water level below u/s TEL. Thus

32
Cont’d…
2. Effective length of crest The effective length of crest of an overflow spillway is
given by

Where:
 Le is the effective length of crest;
 L’ is the net (clear) length of crest, which is equal to the sum of the clear spans
of the gate bays between piers;
 He is the actual total head of flow on crest, including the head due to velocity of
approach;
 N is the number of piers,
 K is the pier contraction coefficient and

33
Kp is the abutment contraction coefficient.
(a) Pier contraction Coefficient
The value of the pier contraction coefficient Kp depends
upon several factors, such as
 Shape and location of the pier nose,

 Thickness of pier,

 The velocity of approach,

 The ratio of actual total head on crest He to the design head HD.

34
(b) Abutment contraction coefficient
The value of the abutment contraction coefficient Ka depends upon a

number of factors, such as (i) shape of abutment, (ii) the angle

between the upstream approach wall and the axis of flow,
(iii)approach velocity and (iv) ratio of the actual head to design head.

• Higher value of Ka should be used for spillways involving extreme

angularity of approach flow.

35
Cont’d…
Discharge formula at partial gate opening
Eq 1.8 is the discharge formula for an ungated overflow
spillway or for a gated overflow spillway at full gate
opening.
The discharge for a gated spillway at partial gate opening
is given by the low-head orifice formula (or large
orifice formula),

36
 Design of Side Walls
The profile of flow on spillway surface determines the height of

sidewalls required to retain the flow on the spillway.

The profile determined by rigid calculation is not the true

profile of flow, since air entrainment occurs in the flow giving

the phenomenon of white water.
To the solid stream profile could thus be added the effect of air

entrainment which would increase the water depth.

The pressure on the training wall is taken as the component of

37
weight of water normal to the surface of flow.
3. Chute Spillway
A chute spillway (or trough spillway or open channel
spillway) consists of a steep-sloped open channel called a
chute or trough, which carries the water passing over the
crest of spillway to the river downstream
For earth dams and rockfill dams, a separate spillway is
generally constructed in a flank or a saddle away from
the dam if a suitable site exists.

38
4. Side Channel Spillway
In the side channel spillway, the crest of the control weir
is placed along the side of the discharge channel.

39
Cont’d…
 The crest is approximately parallel to the side channel at the entrance.

Thus the flow after passing over the crest is carried in a discharge channel
running parallel to the crest.
 Water flows over the crest into the narrow trough of the discharge channel

opposite the weir, it turns approximately at right angle and then

continues in the discharge channel
The side channel spillway is usually constructed in a narrow canyon

where sufficient space is not available for an overflow spillway.

A side channel spillway is also usually required in a narrow valley

where there is neither a suitable saddle, nor wide side-flanks to

40accommodate a chute spillway.
5. Shaft Spillway (or morning glory)
A shaft (or morning glory) spillway consists of a large vertical

funnel, with its top surface at the crest level of the spillway and its
lower end connected to a vertical (or nearly vertical) shaft. The
other end of the vertical shaft is connected to a horizontal (or
nearly horizontal) conduit or tunnel,
A shaft spillway is used at the sites where the conditions are not

favourable for an overflow spillway or a chute spillway.

Ideal site A shaft spillway is ideally suited for a site where a rock

spur projects into the reservoir a little distance upstream of the

41 dam.
42
6. Siphon Spillways
A siphon spillways operates on the principle of siphonic
action. There are basically two types of siphon spillways.
1. Saddle siphon spillway 2. Volute siphon spillway
1.Saddle siphon spillway A saddle siphon spillway (also called
saddle siphon) is a closed conduit of the shape of an inverted
U-tube with unequal legs.
 Saddle siphon spillway is commonly used in practice. Saddle
siphon spillways are usually of two types:
 (a) Hood type and
 (b) Tilted outlet type,

43
 (a) Hood type (b) Tilted outlet type,

2. Volute Siphon Spillway

The volute siphon spillway (or volute siphon) is a special type of
siphon spillway which makes use of volutes (curved vanes) for
priming.
The volute siphon spillway consists of a vertical shaft (or barrel),
44
which has a funnel shape at its top.
Cont’d…
At the bottom end, it is connected to a horizontal or nearly

horizontal outlet conduit through a right-angled bend, which

leads the water to the downstream channel (Fig. 1.24).

45
7. Conduit (or Tunnel) Spillway
A conduit (or tunnel) spillway consists of a closed conduit to

carry the flood discharge to the downstream channel. Fig 1.26

It is constructed in the abutment or under the dam.

46
Cont’d…
The closed conduit may take the form of a vertical or

inclined shaft, a horizontal tunnel, or a conduit constructed

in an open cut and then covered. Such a spillway is suitable
for dam sites in narrow canyons with steep abutments.
The conduit should be designed to flow partly full, because

if it runs full, the negative pressure may develop due to

siphonic action. The area of the flow is usually limited to
75% of the total cross-sectional area of the conduit.

47
8. Cascade Spillway
A cascade spillway consists of a cascade of falls, with a
stilling basin at each fall (Fig. 1.27).
It is ideally suited for very high dams in which the energy
cannot be dissipated by a hydraulic jump or a bucket.
In the case of a high rockfill dams, already excavated quarry
benches on d/s may be utilized for the formation of cascades.

48
Spillways
 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 discharged directly
into the channel downstream, serious scour of the channel bed may
occur.

49
Cont’d…
 Dissipation of the kinetic energy generated at the base of a spillway is essential
for bringing the flow into the downstream river to the normal almost pre-dam
condition in as short of a distance as possible.
 This is necessary, not only to protect the riverbed and banks from erosion, but
also to ensure that the dam itself and adjoining structures like powerhouse, canal,
etc. are not undermined by the high velocity turbulent flow.

50
Cont’d…
 The energy-dissipating devices can be broadly classified into two types.

1. Devices using a hydraulic jump for the dissipation of energy.

2. Devices using a bucket for the dissipation of energy.

The choice of the energy-dissipating device at a particular

spillway is governed by the tail water depth and the

characteristics of the hydraulic jump, if formed, at the toe.
If the tail water depth at the site is not approximately equal to

that required for a perfect hydraulic jump, a bucket-type

energy dissipating device is usually provided.
51
Characteristics of a Hydraulic Jump
 Hydraulic jump is a sudden and

turbulent rise of water which

occurs in an open channel when the
flow changes from the supercritical
flow state to the subcritical state.
 It is accompanied by the formation

of extremely turbulent rollers and

considerable dissipation of energy.
 Thus a hydraulic jump is a very
effective means of dissipation of
energy below spillways.
52
Location of a Hydraulic Jump
For a given discharge intensity (q), the sequent depth y2 and

the tail water depth y2' are fixed.

The location of hydraulic jump will depend upon the relative

magnitudes of y2 and y2', and hence on the JHC and TWRC.

There are five cases, depending upon the relative positions of

JHC and TWRC, as discussed below

53
Case-1 JHC and TWRC coincide throughout
 In this case, the JHC and TWRC curves coincide for all discharges [Fig. 2.3 (a)].

 As the tail water depth y2' is exactly equal to the sequent depth y2 required for the formation of

hydraulic jump, a perfect jump is formed just at the toe of the spillway as shown in Fig. 2.1.
 However, this case indicates a highly idealised condition, which rarely occurs in practice.

54
Case-2 TWRC always lower than JHC
In this case, the tail water rating curve (TWRC) is below the jump height

curve JHC for all discharges [FIG. 2.3 (b)]. Such a condition occurs when the
tail water is carried away quickly due to a rapid or a fall somewhere on
the downstream of the spillway.
In this case, the jump will be located at a point on the downstream of the toe

of spillway. The high velocity jet would sweep down the toe and scour the
river bed. Therefore, severe erosion may occur in the portion of the river
between the spillway and the section where the hydraulic jump is formed.

55
Case-3 TWRC always higher than JHC
 In this case, the tail water rating curve is above the jump height curve for all

discharges [Fig. 2.3 (c)].

 This condition usually occurs when the river cross-section on the downstream of

the spillway is narrow and therefore the tail water backs up. The hydraulic jump in
this case is located upstream of the toe on the spillway face.
 The hydraulic jump is drowned or submerged, and the high velocity jet dives

under the tail water.

 The energy dissipation in a drowned hydraulic jump is not good.

56
Case-4 TWRC lower than JHC at low discharges, but higher at
high discharges
 In this case, the tail water rating curve is lower than the jump height curve
at low discharges, but it becomes higher at a particular discharge and then
remains higher than the jump height curve [Fig. 2.3 (d)].
 It is a combination of cases 2 and 3. The hydraulic jump is formed further
 downstream of the toe at low discharge, as in the case 2; but at higher
discharges, it is drowned, as in the case 3.

57
Case-5 TWRC higher than JHC at low discharges, but lower at
high discharges

It is also combination of cases 3 and 2.

However, in this case, at low discharges, the jump is drowned;

whereas at high discharges, it is formed further downstream of

the toe [Fig. 2.3 (e)].

58
Measure Adopted For Dissipation of Energy
 Various measures are adopted at or near the toe of the spillway so that a perfect jump is
formed for the dissipation of energy.
 The measures adopted will depend upon the relative positions of the tail water rating curve
(TWRC) and the jump height curve (JHC).
 Measures are discussed separately for all the five cases discussed in the preceding section.

59
Case-1
 There is no need of any special measure for the formation of hydraulic jump,

as a perfect jump will always form at the toe.

 A horizontal apron is however provided on the downstream of the toe for the

protection of the river bed (Fig. 2.4).

 The length of a horizontal apron is taken equal to the maximum length of the

hydraulic jump. Sometimes, baffle blocks are also constructed on the

horizontal apron for dissipation of energy. However, if the baffle blocks are
placed too near the toe, they may be subjected to cavitation and abrasion.

60
Case –2

The hydraulic jump forms at a certain section downstream of the toe. The following
measures are adopted.
 A depressed horizontal apron is formed by excavating the river bed on the

downstream of the toe of the spillway to increase the tail water depth [Fig. 2.5 (a)].
 A low secondary weir (or dam) is constructed downstream of toe to raise the tail

water [Fig. 2.5(b)].

61
Case 3
The following measures are adopted.
 A sloping apron is constructed above the river bed level extending from

the spillway surface to the toe [Fig. 2.6 (a)].

 The river bed may be excavated to provide a drop in the river bed to

lower the tail water [Fig. 2.6(b)].

 A roller bucket is provided near the toe, which forms rollers for the

dissipation energy (Sect 2.6).

62
 Case 4
The following measures are adopted.
 A sloping apron is provided which lies partly above and partly below the river

bed level so that a perfect jump will form in the lower portion of the apron at low
discharges and in the higher portion of the apron at high discharges (Fig. 2.7).
 A low secondary dam (or a sill)

 If the velocity is not greater than 15 m/s, baffle blocks or dentated sills may be

constructed to break up the jet

63
Case 5
The case is similar to case 4 but the range of discharge is

different. The following measures are usually adopted.

A sloping apron

A low secondary dam (or a sill)

64
Stilling Basins
A stilling basin is a basin-like structure in which all or a part

of the energy is dissipated.

In a stilling basin, the kinetic energy causes turbulence and it

is ultimately lost as heat energy.

The stilling basins commonly used for spillways are of the

hydraulic jump type, in which dissipation of energy is

accomplished by a hydraulic jump.

65
Cont’d…
A hydraulic jump can be stabilised in stilling basin by using

appurtenances (or accessories such as chute blocks, basin blocks and

end sill.
Chute blocks These are triangular blocks with their top surfaces

horizontal.
These are installed at the toe of the spillway just at upstream end of

the stilling basin.

Basin blocks (or baffle blocks or baffle piers) These are installed

on the stilling basin floor between chute blocks and the end sill.
These blocks also stabilise the formation of the jump.
66
Cont’d…
End sill It is constructed at the downstream end of the stilling basin.

It may be solid or dentated. Its function is to reduce the length of the

hydraulic jump and to control scour.

Types of stilling basins There are various types of stilling basins.

The type of stilling basin most suitable at a particular location

mainly depends upon the initial Froude number (F1) and the
velocity V1 of the incoming flow.
The length of the basin, measured in the direction of flow, depends

upon the sequent depth y2 and the initial Froude No. F1. It is
different for different type of basins.
67
Cont’d…
U.S.B.R Stilling basins
1. Type I basin

2. Type II basin

3. Type III basin

Indian Standards basins

1. Horizontal floor-Type I

2. Horizontal floor – Type II

3. Sloping apron – Type III

4. Sloping apron – Type IV

68
U.S.B.R Stilling basins
No special stilling basin is required to still flow if F1 is less than

1.70.
However, the channel length beyond the point from where the water

depth starts increasing, should not be less than 4.00 y2, where y2 is
the sequent depth.
For F1 between 1.7 to 2.5, there is not much turbulence, only a

horizontal apron is provided. However, the apron should be

sufficiently long to contain the jump.
A length of 5.0 y2 is usually provided. No accessories such as

69
baffles or sills are provided.
U.S.B.R. Type I basin for Froude number F1 between 2.5
to 4.5

 For this range of F1, type I basin has proved to be quite effective

for dissipating most of the energy (Fig. 2.9).

 However, it is not able to dampen the oscillating flow entirely. The

water depth in the basin should be about 1.10 y2 to check the

tendency of the jump to sweep out and to suppress wave action.
 The basin is provided with chute blocks of the size, spacing and

location as shown in the figure. All the dimensions are in terms of

the initial depth y1.

70
Cont’d…
The length L of the stilling basin varies from 4.3 y2 to 6 y2,

depending upon the value of F1, as given in Table 2.1

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USBR Type II basin for Froude number F1 greater than 4.5 and
V1 less than 15 m/s.
 For F1 greater than 4.5 and V1 less than 15 m/s, type II basin
shown in Fig. 2.10 is provided.
 The basin is provided with chute blocks, baffle blocks (baffle
piers) and end sill, as shown. The size, spacing and location of the
chute and baffle blocks are as shown in Fig.2.10. The length L of
the stilling basin, the height h3 of the baffle block and the height h4
of the end sill are obtained from Table 2.2

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U.S.B.R. Type III basin for Froude number F1 greater than 4.5 and V1 greater
than 15m/s.

 When F1 is greater than 4.5 and V1 is greater than 15 m/s. Type III basin is

provided. In this case, baffle blocks are not provided, because of the following
reasons:
 The block would be subjected to very high impact forces due to high velocity V1 of incoming

flow.
 There is a possibility of cavitations on the downstream faces 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 basin

is greater than that in Type II basin.

 The size, spacing and location of the chute blocks are the same as in Type II basin.

 The length of the basin is obtained from Table

73
Indian standards stilling basins
 IS : 4997 – 1968 recommends 4 types of stilling basin so, two types are with horizontal

apron, and two types are with sloping apron.

(a) Stilling basins with a horizontal apron

 Stilling basins with a horizontal floor may be provided when the jump height curve

(JHC) and the tail water rating cure (TWRC) are not much different from each other
i.e one curve may be slightly above or below the other.
 In this case, the required condition for the development of the hydraulic jump are

 obtained on a horizontal apron or near the bed level, and hence there is no necessity of

a sloping apron.
 Depending upon the froude number F1, there are two types of basins with a horizontal

apron.
 (i) Type I for F1 < 4.5

74 (ii) Type II for F1 >,. 4.5


1. I.S Type I basin for F1 < 4.5.
 This type of the basin is provided when F1 is less than 4.5. Such a
case usually occurs on weirs, barrages and low dams.
 The basin is provided with chute blocks, basin blocks (baffle
blocks) and a dentated sill (Fig. 2.11).However, the basin blocks
should not be used if the velocity of flow exceeds 15m/s. The
tailwater depth y2' should normally not exceed 1.10 y2 where y2 is
the conjugate depth.

75
IS Type II basin for F1 >= 4.5
 This type of basin is used when F1 is equal to or greater than 4.5.
This basin is usually required in the case of spillways of the
medium and high dams.
 The stilling basin is provided with chute blocks, basin blocks (or
baffle blocks) and a dentated sill (Fig. 2.12)

76
Stilling basins with a sloping apron floor (IS Type III basin
and type IV basins
 A sloping apron is provided when the tail water depth y2' is very large as

compared to the sequent depth y2. In that case, a drowned jump would develop if
no sloping apron is provided.
 With a sloping apron, an efficient hydraulic jump is formed at a suitable level

on the sloping apron. There are two types of basins:

 (i) IS Type III basin [Fig. 2.13 (a)] (ii) IS Type IV basin [Fig. 2.13 (b)]

77
 Cont’d…
IS basin Type III is recommended where the tail water rating curve

(TWRC) is higher than the jump height curve (JHC) at all discharges.
IS Basin Type IV is suitable where the tail water depth y2' at the

maximum discharge exceeds y2 considerably but is equal to or

slightly greater than y2 at lower discharges.
The design criteria for sloping aprons have not been standarised to

the same extent as in the case of the horizontal apron.

The slope and overall shape are determined from economic

considerations.
The length of the basin is fixed, depending upon the type and

78 soundness of the river bed.

Bucket Type Energy Dissipators
Bucket type energy dissipators are commonly used for the dissipation of

energy below the overflow (ogee-shaped) spillways.

The dissipator consists of an upturned bucket ( or curved apron)

provided at the toe of the spillway in continuation of its downstream face.

Bucket type energy dissipators are usually of small size and are more

economical than conventional hydraulic jump stilling basins.

These are especially useful when the Froude number F1 exceeds 10,

because in that case, the difference between the initial depth and the
sequent depth is quite large and a very long and deep stilling basin is
required.
79
 Cont’d…
The bucket type energy dissipators are

basically of the following three types.

Solid roller bucket

Slotted roller bucket

Ski-jump (or flip or trajectory ) bucket.

The solid and slotted roller buckets are used

where the tail water rating curve (TWRC) is

above the jump height curve (JHC).
Both these types of buckets remain

submerged in the tail water, and hence these

are also called submerged buckets.

80
Solid roller bucket:
This type of bucket shown in fig. 2.20 performs well when

deeply submerged.
It consists of a circular bucket type apron with a concave

profile of considerable radius and a lip which deflects the

high velocity flow away from the stream bed.
The dissipation of surplus energy in the water is

accomplished by the diffusion of the water jet in a large mass

of water and by overcoming the boundary resistance of the
scooped bed and material.
81
Cont’d…
 ii) Slotted or dentated bucket: - High position of the river bed

downstream of roller bucket and unsymmetrical gate operation

reduces to a great extent the usefulness of the solid roller. This type
of bucket can also be tried for flows where tail water depth is less
than y2 curve.
 Various designs are available for slotted roller bucket. The

U.S.B.R. design

82
(iii) Ski Jump Bucket:
This type of energy dissipator is suitable where the stream bed

is composed of firm rock and the tail water depth is less than
required for the formation of hydraulic Jump. Fig. 2.24 shown
the definition sketch of ski jump bucket.

83