6 Spillways and Energy Disspators
6.1 Functions of Spillways
 To release the surplus water or flood water that can not be contained in the allotted
storage space
Bottom outlet
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-2013
6.2 Necessity of Spillways
 Many failures of dams have been caused by hydraulic failure (40%), i.e
overtopping
 Improperly designed spillways
 Spillways of insufficient capacity (Underestimation of the design flood)
 Malfunctions of spillways components
 Special precaution for embankment dams since any over topping may lead to the
Dam’s failure
 In concrete dams, the impact of overtopping is less serious,
 It may increase the stress on the dam body
 Undermine the stability of the dam by eroding the foundation at the toe of the
dam mainly by the overflowing impinging jet,
 Spillways are relatively expensive
 Spillways cost account for about 20% for earth and rock fill dams,
 Small portion for concrete dams
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6.3 Component parts of A spillway
A spillway generally has the following 5 component parts
1. Approach channel / Entrance channel
2. Control structure
3. Discharge channel / conveyance features / waterways
4. Terminal Structures / Energy Dissipators
5. Exit channels
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1.
2.
3.
4.
5.
6.
7.
River
Dam
Control Structure
Approach Channel
Discharge Channel
Terminal Structure
Exit channels
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6.3.1 Approach / Entrance Channel
 It draws water from the reservoir and carries it to control structure
 It is required in those types of spillways in which the control structure is away from
the reservoir
 They are not required for spillways which draw water directly from the reservoir
Control structure
Reservoir
Dam
Fig 5 Approach channel
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-2013
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Energy
Dissipator
Exit
channel
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2011-2013
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6.3.2 Control Structure
 It is a control device which regulates the outflow from the reservoir
 It limits or prevents outflows below fixed reservoir levels
 It regulates releases when the reservoir rises above the level
 Definite relationship between discharge and head
 A control structure usually consists of
 Weir (sharp crested, broad crested, and ogee)
 Orifice
Fig 6 Control Structure
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-2013
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6.3.3 Discharge Channel
 Conveys the water released through the control structure to the river downstream
 The conveyance structure may be
 The downstream face of a concrete dam
 Open channel excavated along the ground surface
 Closed conduits place through or under a dam
 Tunnels excavated through an embankment
 The channels may have
 Rectangular, trapezoidal or circular x-sections
 Flat or steep slopes
Discharge
Channel
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6.3.4 Terminal Structures
 When spillways flow drop from the reservoir pool level to downstream river level
The static head is converted to kinetic energy
The energy manifest itself in form of high velocity which may cause
 scour of the toe of the spillway and dam and other appurtenant structures
scour the bed of the receiving river
 Terminal structures are provided at the downstream end to dissipate this excess
energy
Hydraulic jumps with stilling basins
Bucket type
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6.3.5 Exit Channels
 They are provided to convey the spillway discharge from the terminal structure to the
river downstream
An exit channel is not required for the spillway which discharges water directly into the
river downstream
Spillways placed through abutments, saddles, exit channels are usually required
.
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-2013
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6.4 Location of Spillways (link)
Spillways should be located so that spillway discharge will not undermine the toe of
the dam
Spillways may be located either in the middle or at the edges
Place the spillways in the main gorge so that the flood flows will be confined with in
the banks of the river,
Locate and align the spillways so that the main direction of the flow in the
downstream is maintained unchanged
If a suitable saddle can be found, they may be located away from the dam
Although hydraulically more suitable, it would not be always possible to design
heavy structures such as spillways, if a suitable foundation is not available in the main
gorge. Then, a choice has to be made to locate the spillway structure on the flank.
Saddle
Dam
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6.5 Spillway Design Considerations
In designing a spillway, the following factors have to be given due considerations
a) It must have adequate capacity
• It should be sized so that there will not be any overtopping
b) It must be hydraulically and structurally safe.
• It has to be safe against
• Overturning,
• Sliding,
• Failure by crushing and tension cracks for various load combinations
c) Its surface should be erosion resistant.
d) It must be located so that its discharge will not erode or undermine the downstream toe
of the dam
e) Its capacity should not exceed that of the receiving river
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-2013
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6.6. Types of Spillways
6.6.1 Classification scheme
Based on various criteria, spillways can be classified
A) Classification based on purpose
A) Main / service spillways
B) Auxiliary spillways
C) Emergency spillways
B) Based on control
A) Controlled spillway
B) Uncontrolled spillway
C) Classification based on prominent features
A) Free over-fall or straight drop
B) Overflow Ogee spillways
C) Chute spillway
D) Side channel spillways
E) Siphon spillways
F) Shaft spillways
G) Others
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2011-2013
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6.6.2 Classification Base on Purpose
A. Main / Service spillways
 Designed to pass the Inflow Design Flood (IDF)
 Necessary for all dams
B. Auxiliary Spillways
 Where site conditions permit, service spillways are supplemented by auxiliary spillways to
gain an overall economy
 The main / service spillway are designed to handle the most frequent floods
 Floods exceeding the capacity of the service spillway are handled by Auxiliary spillways
 Auxiliary spillways are designed in such a way that their capacity is less than that
required for the IDF

Total spillway capacity = service spillway + auxiliary spillway capacity
 Auxiliary spillway crest is kept higher than the service spillway
 beyond the dam to avoid the possibility of damage to the dam or other structures.
C. Emergency spillways
 It is used during emergencies and acts as an additional safety valve of the dam
 Emergencies:
 Floods exceeding design flood
 Malfunctioning of spillway gates
 It is usually provided in a saddle or a depression along the reservoir rim,
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 Its crest level is kept above the maximum water level
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6.6.3 Classification based on Mode of Control
A)



Controlled (Gated) spillways
Gates are provided over the crest to control the outflow from the reservoir.
In such spillways, the full reservoir level is usually kept at the top level of the gates.
The outflow from the reservoir can be varied by lifting the gates to different elevations.
B) Uncontrolled (free) spillways
 Gates are not provided over the crest to control the outflow from the reservoir. Water
flows over the crest uncontrollably
 The FRL is at the crest level of the spillway. The water escapes automatically when the
water level rises above the crest level.
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6.6.4. Classification based on Pertinent Hydraulic Features
6.6.4.1 Free Overfall Spillways
 The control structure is a narrow crested weir with vertical or near vertical
downstream face and the water falls freely more or less vertical
Arch dam
Scour hole
 It is suited for thin arch or buttress overflow dams or to a crest which has a nearly
vertical downstream face.
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El Atazar Dam (Spain)
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Gebidem2011-2013
Dam (Switzerland).
Fig 2.8 Some pictures of free overfall spillways
 Two challenges
1. The underside of the jet / nappe should be sufficiently ventilated to prevent a
pulsating, fluctuating jet.
2.
If the stream bed does not consist of sound rock, scouring of the stream bed
may occur.
d – max scour hole depth
H – the height of the drop
q - specific discharge of the free jet
 Where erosion can not be tolerated, artificial pool or stilling basins are
provided.
 It is not suitable where the foundation is weak the apron is subjected to large
impact forces which may cause vibration
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6.6.4.2 Ogee (Overflow) Spillway
Description
It is an improvement up on the free overfall spillway.
 Main Feature
 The control structure isan ogee-shaped (inverted S) weir.
The shape is made to conform the profile of the lower surface of napee of a
ventilated jet issuing from a sharp crested weir when the head over the weir is
the design head. The nappe shaped profile is ideal: Atmospheric pressure
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2011/2012
Hydraulics of Overflow (Ogee) Spillway:
It is divided in to 3 zones:
The crest : control structure
The face: discharge channel
The toe: energy dissipator
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2011/2012
A. The Spillway Crest
The crest is shaped in the form of an ogee.
 It is made to conform closely to the profile of the lower surface of the nappe from
a sharp crested weir
A.1 Profile of the Crest
 The profile of the crest is described by first separating the crest it into two quadrants
upstream and downstream from the high point (apex) of the lower nappe. The apex is
normally defined as the crest axis (origin of coordinates).
Appex, Origin
x
Downstream
Upstream
y
Dr.-Ing. Asie Kemal Jabir, AAiT, 2011/2012
 Extensive experiments by USBR resulted in the development by USACE (WES) of
curves which can be described by simple equations
Downstream Profile
where
o x and y are the coordinates of the point on the spillway surface, with the origin at
the apex (see Fig).
o He is the design head, excluding the head due to the velocity of approach, &
o K and n are constants, which depend upon the inclination of the upstream face of
the spillway.
U/s Slope
K
n
Vertical
2.000
1.850
1:3 (H:V)
1.936
1.836
2:3 (H:V)
1.936
1.810
3:3 (H:V)
1.873
1.776
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
The curved profile of the crest section is continued till it meets tangentially the
straight sloping surface of the downstream face of the overflow dam (Figure). The
location of this point of tangency depends upon the slope of the downstream face of
the overflow dam, which is determined from the stability requirements of the overflow
section.
Upstream profile
 The upstream profile is defined as a compound
circular arc. It should be tangent to the upstream face
and should have a zero slope at the crest
U/s Slope
a/Hd
b/Hd
R1/Hd
R2/Hd
Vertical
0.175
0.282
0.5
0.20
1:3 (H:V)
0.139
0.237
0.68
0.21
2:3 (H:V)
0.115
0.214
0.48
0.27
3:3 (H:V)
0
0.199
0.45
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2011/2012
Discharge over the crest
The discharge over the spillway crest is given by the weir equation
Where
Co is coefficient of discharge (maximum value of 2.2)
Le is effective crest length
Hd is the total head on the crest
Hd = H + Ha
H is the water depth at crest
Ha is the head due to the velocity of approach
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Effective Length of Crest Le
Piers and abutments cause side contractions of the overflow. Under such
conditions, the effective length, Le, is less than the net length of the crest.
The effect of the end contraction may be taken into account by reducing the
net crest length as follows:
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,
Hd is the actual total head of flow on crest,
N is the number of piers,
Kp is the pier contraction coefficient, and
Ka is the abutment contraction coefficient.
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2011/2012
Pier Contraction
The pier contraction coefficient, Kp, is affected by the shape of the pier nose, the
thickness of the pier, the design head, and the approach velocity. For conditions of
design head, Ho, average pier contraction coefficients may be assumed as
follows:
Pier condition
Square nosed pier
Rounded nosed piers
Pointed nose piers
Kp
0.02
0.01
0
Abutment Contraction
The abutment contraction coefficient, Ka, is affected by the shape of the abutment, the
angle between the u/s approach wall, the axis of flow, and the head in relation to the
design head, and the approach velocity. The average abutment coefficients may be
assumed as follows:
Abutment condition
Square abutments at 90o
Rounded abutments at 90o
Rounded abutments at 45o
Ka
0.2
0.1
0
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6.6.4.3 Side Channel Spillway
Side Channel Trough
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2011-2013
 The control structure, i.e., the control weir, is placed approximately parallel to the
upper portion of the discharge channel.
Discharge Channel
Control
 It has advantages that make it desirable for certain spillway layouts.
 where a long overflow crest is needed to limit the surcharge head and the
abutments are steep and precipitous,
 where the control must be connected to a narrow discharge channel or tunnel, the
side channel spillway is often the best choice,
 where a dam spillway is not feasible, such as in the case of an earth dam,
 The factor limiting its adoption is that this type of spillway is hydraulically less efficient.
 Side channels are definitely not considered for dams with a large design flood due to
the limited overflow head of say 3 m.
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2011-2013
6.4.4.4 Chute Spillway
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2011-2013
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Energy
Dissipator
Exit
channel
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2011-2013
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Description
 A spillway whose discharge is conveyed from the reservoir to the downstream river level
through a steep open channel, placed either along a dam abutment or through a saddle.
It is used
 often in conjunction with embankment dams.
 some times, even for a gravity dam, a separate spillway is required when the
valley is narrow and an overflow spillway cannot be provided at the dam site. It can
be conveniently provided independently in a saddle at a low cost.
Factors influencing the selection of chute spillways are
 the simplicity of their design and construction,
 their adaptability to almost any foundation condition, and
 the overall economy often obtained by the use of large amounts of spillway
excavation in the dam embankment.
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6.4.4.5. Shaft Spillway
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A shaft spillway is an uncontrolled spillway in which the water enters over a weir and
drops through a vertical or sloping shaft into a conduit which discharges into the
downstream channel.
The structure comprise of three elements:
an overflow control weir,
a vertical transition, and
a closed discharge channel.
Shaft spillways are typically used for dams with small to medium design discharges, with
a maximum of approximately 1000m3/s. The overfall height of the structure may be almost
100m although 50m is more relevant.
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2011-2013
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