ADDIS ABABA UNIVERSITY

School of Civil and Environmental Engineering

5th CED

CHAPTER THREE
DAM OUTLET WORKS

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Hydraulic Structures I
 3.1. DESIGN AND HYDRAULIC CALCULATION OF SPILLWAYS

 The spillway is a hydraulic structure intended to discharge the excess water from the
reservoir, during times of flood, in such a manner so as to insure the safety of the dam and
appurtenant works at all times.

 It is of paramount importance for the spillway facilities to be designed with sufficient

capacity to avoid overtopping of the dam, especially when an earth-fill or rock-fill type of dam
is selected for the project.

 The spillway must be hydraulically and structurally adequate and must be located so that
spillway discharges do not erode or undermine the downstream toe of the dam.

 A spillway may be located either in the middle of the dam or at the end of the dam near
abutments. In some case, it is located away from the dam as an independent structure if
there is suitable saddle (natural depression).

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Bottom outlet

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 The essential requirements of spillway may be summarized as below;
 It must have adequate discharge capacity;
 It must be hydraulically and structurally safe;
 The surface of spillway must be erosion resistant;
The spillway must be located that the spillway discharges 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 of the downstream
channel to avoid flooding.
3.12 REQUIRED CAPACITY OF THE SPILLWAY
The required spillway capacity is usually determined by flood routing (the spillway capacity
should be equal to the maximum outflow rate determined by flood routing).
The following data are required for flood routing:
 Inflow flood hydrograph, indicating the rate of inflow with respect to time.
Reservoir-capacity curve, indicating the reservoir storage at different elevations
Outflow discharge curve, indicating the rate of outflow through spillways at different reservoir
elevations.

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3.13 COMPONENT OF A SPILLWAY
Spillway generally has the following components:
Entrance channel:
 are required in those types of spillways in which the control structure is away from the
reservoir.
 the entrance channel is used to draw water from the reservoir and carries to control
structure.
 it may not be required for spillway types which draw water directly from the reservoir.
 Control structure: which regulates the outflows from the reservoir.
Control devices limit or prevent outflows below fixed reservoir.
 Discharge channel (or waterway)
 Terminal structure (energy dissipater):
 When spillway flows drop from reservoir pool level to downstream river level, the static
head is converted to kinetic energy.
 This energy manifests itself in the form of high velocities that, if impeded, result in high
pressures.
 Means of returning the water to the river without serious scour or erosion of the toe of the
dam and without damage to adjacent structures must usually be provided.
 Exit channel: - in some types of spillway, the exit channels are provided to convey the
discharge from the terminal structure to the river downstream.
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Energy
Dissipator
Exit
channel

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3.14 CLASSIFICATION OF SPILLWAYS
Spillways can be classified in to different types based on the various criteria as below:
Classification based on Purpose
i. Main (or service) spillway:
It is designed to bypass a prefixed of the design flood. This spillway is necessary for all dams
and in most case it is the only.
ii. Auxiliary spillways
The main spillway is designed to pass floods which are likely to occur more frequently and
when the flood greater than this, auxiliary spillway comes in operation.
iii. Emergency spillways
Emergency may arise when such conditions occur that have not been anticipated or
considered in the design of the main spillway.(i.e flood exceeds the design flood, when the
gates or other parts of spillway are not working)

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Classification based on Control
i. Controlled (gated) spillway ii. Uncontrolled spillway
Classification based on prominent features
i. Free overfall (straight drop) spillway
ii. Shaft (morning glory) spillway
iii. Overflow or ogee spillway
vi. Siphon spillway
iii. Chute (open channel or tough) spillway
vii. Conduit (tunnel) spillway
iv. Side-channel spillway
ix. Cascade spillway

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 Free overfall (straight drop) spillway
In this type of spillway, the control structure consists low-height, narrow-crested weir and the
downstream face is vertical or nearly vertical so that the water falls freely.
The overflowing water may discharge as a free nappe, as in the case of sharp crested weir.
It is commonly used for low arch dam for the downstream face is almost vertical. It is also used
for as a separate structure for low earth dams. It is suitable for sound rock foundation.

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Ogee (overflow) spillway
 The ogee spillway has a control weir which is ogee or S-shaped in profile.
 Flow over the crest adheres to the face of the profile by preventing access of air to the
underside of the sheet.
The shape of crest profile depends upon the head, the inclination of the upstream face of the
overflow section, and the height of the overflow section above the floor of the entrance
channel (which influences the velocity of approach to the crest).
For most conditions the data can be summarized according to the form shown on
below, where the profile is defined as it relates to axes at the apex of the crest.
 That portion upstream from the origin is defined as either a single curve or a tangent or as a
compound circular curve.
The portion downstream is defined by the equation:
n 1
x n  k .H d .y
Where: ‘K’ and ‘n’ are constants whose value depend upon the upstream inclination and
approach velocity. ‘X’ is taken as positive towards the downstream and ‘y’ is taken as positive in
the downstream direction and Hd is the design head including the velocity head.

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The u/s curves are given by different slopes and the equation used to compute as given:

0 . 724 ( x  0 . 27 H d )^1 . 85
y  0 . 126 H d  0 . 4315 H 0 . 375
d * ( x  0 . 27 H d ) 0 . 625

H d0 .85

The u/s profile extends up to X= - 0.27Hd

The slope of d/s face of the over flow dam usually varies in the range of 0.7:1 to 0.8:1. At the
end of slopping surface terminal structures for energy dissipation will be provided to prevent
scour.

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The following crest profile has been found to give good agreement with the prototype
measurement by U.S. Waterways Experimental Station (WES). Such shapes are known as
WES Standard Spillway shapes as shown in figure below

Figure ,Standard Spillway crest (after US Army Waterways Experimental Station, 1959)

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Table, Values of a, b, R1, R2, K and n for different U/S slope

U/S slope b/Hd a/Hd R2/Hd R1/Hd K n

0H:1V 0.175 0.282 0.50 0.20 2 1.85

1H:3V 0.139 0.237 0.68 0.21 1.936 1.836

2H:3V 0.115 0.214 0.48 0.22 1.936 1.81

Note: For U/S face vertical as shown on figure above, a=0.282*Hd, b=0.175*Hd, R1=0.2*Hd and
R2=0.5*Hd

 Discharge over ogee Spillway

The spillway discharge is given by: Q  C d Le H e 3 / 2
Where: Q- discharge, Cd – Coefficient of discharge, Le- effective crest length, He- the actual
effective head including the head due to the velocity of approach. I.e. He = Hd + Ha.

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Coefficient of discharge (Cd):
 An ogee has relatively high value of coefficient of discharge because of its shape.
The maximum value of Cd is about 2.20 if no negative pressure occurs on the crest. However,
the value of value of Cd is not constant and depends on the shape of the ogee profile,
and also on the following factors:
a) Height of spillway crest above the stream bed; Model tests of spillways shown that the
effect of velocity of approach on coefficient of discharge is negligible when the hight is
equal to or greater than 1.33Hd and such spillways are known as high overflow spillways,(
in high overflow spillways, the velocity of approach is sometimes neglected).

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b. Slope of the u/s face of spillway

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c. Extent of downstream submergence: the effect of submergence is negligible for
smaller degree of submergence.

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d. Downstream apron
If (hd + d) / HD exceeds 1.7 the effect of downstream apron on the coefficient of
discharge will be negligible. But there may be decrease in coefficient due to tail water
submergence.

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Effective length of crest;
 Where crest priers and abutments are shaped to cause side contractions of the overflow, the
effective length, Le, will be less than the net length of the crest. The effect of the end
contraction may be taken into account by reducing the crest length as follows:
L e  L '  2 NK p  K a H e

Where: L’- net length of the crest, N- Number of piers, Kp- piers contraction coefficient and
Ka- abutment contraction coefficient.
The pier contraction coefficient, Kp, is affected by the shape and location of the pier nose, the
thickness of the pier, the head in relation to the design head, and the approach velocity.
The average pier contraction coefficient may be assumed as follows:
Value of pier contraction coefficient

Pier condition Kp
Square nosed pier with corners rounded on a radius equal to about 0.1 of the pier thickness 0.02
Rounded nosed piers 0.01
Pointed nose piers 0

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 The abutment contraction coefficient is affected by the shape of the abutment, the angle
between the upstream approach wall and the axis of flow, the head in relation to the design
head, and the approach velocity.
 The average abutment contraction coefficient may be assumed as follows:

Value of abutment contraction coefficient:

Abutment condition Ka
Square abutments with head wall at 90o to direction of flow
0.20

Rounded abutments with head wall at 90o to the direction flow

0.10

Rounded abutments with head wall placed at not more than 45o to the direction of flow
0

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 Chute (open channel or tough) spillway
 A chute spillway is a steep channel conveying the discharge from a low overfall, side
channel, or special shape spillway over the valley side into the river downstream.
A chute spillway essentially consists of a steeply sloping open channel, placed along a dam
abutment or through a flank or a saddle.
 It leads the water from the reservoir to the downstream channel.

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 Side-channel spillway
 Side channel spillways (Figure below) are mainly used when it is not possible or advisable
to use a direct overfall spillway as, e.g., at earth and rock fill dams.

(a) (b)
figure :- Side channel spillway: (a) plan (b) section A-A, side view
They are placed on the side of the dam and have a small ogee spillway, the flume (channel)
downstream of the spillway, followed by the chute or tunnel.
 Spillway crest is usually designed as a normal overfall spillway.

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The depth, width, and bed slope of the flume must be designed in such a way that even
the maximum flood discharge passes with a free overfall over the entire horizontal
spillway crest, so that the reservoir level is not influenced by the flow in the channel.
The width of the flume may therefore increases in the direction of the flow.

Side Channel Trough

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 Shaft (morning glory) spillway
 It is the spillways designed like an inverted bell so that water can enter all around
the perimeter.
These uncontrolled spillway devices are also called morning glory spillways.
The structure is considered to comprise three elements: an overflow control weir, a
vertical transition, and a closed discharge channel.

 Discharge during crest control is given byQ  Cd (2Rs ) H 3 / 2

Where: Cd is discharge coefficient, Rs is the radius of the funnel at crest and H is the head
over the crest. The above equation holds true for the limit up to H/Rs = 1.0.
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When the head over the crest is increased beyond the limit (H/Rs ≤ 1) the orifice flow
occurs and the crest is completely submerged and the water surface has a slightly depression.
 The discharge is given by: Q  C d (R 2 ) 2 gH a

 Where: Cd is discharge coefficient, R is the radius of the vertical shaft and Ha is the depth of
the orifice formed.

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 Siphon spillway
 Siphon spillways (figure below) are closed conduits in the form of an inverted U with an
inlet, short upper leg, throat (control section), lower leg, and outlet.

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