Chapter 7 spillway and energy dissipators

CHAPTER 7: SPILLWAY AND ENERGY
DISSIPATORS
1
0401544 - HYDRAULIC STRUCTURES
University of Sharjah
Dept. of Civil and Env. Engg.
DR. MOHSIN SIDDIQUE
ASSISTANT PROFESSOR

LEARNING OUTCOME
After taking this lecture, students should be able to:
(1). Obtain in-depth knowledge on various types of spillways
used in dams and their design guide lines
(2). Apply the design guide lines for the design of selected
Spillway
3
References:
Khatsuria, R. M., Hydraulics of Spillways and Energy Dissipators,
Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 4th Ed. CRC Press
Santosh, K. G., Irrigation Engineering and Hydraulic Structures, Khanna Publishers
BULU, A., Lecture noted of water resources, Istanbul Technical University

SPILLWAY
A spillway is a structure
designed to 'spill' flood waters
under controlled (i.e. safe)
conditions.
The Spillways can be
Uncontrolled (Normally)
Controlled
Note: Concrete dams
normally incorporate an over-fall
or crest spillway, but
embankment dams generally
require a separate side-channel
or shaft spillway structure
located adjacent to the dam.
Sketch of conventional weir/spillway
4

CLASSIFICATION OF SPILLWAYS
I. According to the most
prominent feature
• A. Ogee spillway
• B. Chute spillway
• C. Side channel spillway
• D. Shaft spillway
• E. Siphon spillway
• F. Straight drop or overfall
spillway
• G. Tunnel spillway/Culvert
spillway
• H. Labyrinth spillway
• I. Stepped spillway
II. According to Function
• A. Service spillway
• B. Auxiliary spillway
• C. Fuse plug or emergency
spillway
III. According to Control
Structure
• A. Gated spillway
• B. Ungated spillway
• C. Orifice of sluice spillway
5

CLASSIFICATION
OF SPILLWAY
Classification of Spillway (Vischer et al, San
Francisco,1988). 6

ANALYSIS OF EXISTING STRUCTURES
Semenkov (1979) analyzed more than 400 projects in terms of
parameters L/H and N for the three main types of spillways: gravity
spillways, chute spillways, and tunnel spillways for concrete and
earth-fill dams.
Where, L and H are the length and height of the dam crest respectively, and
N is the power of the flow
Types of spillways for concrete and earth-fill dams. T: Tunnel spillways, C:
Chute spillways, G: Gravity spillways (Semenkov, 1979).
7

VARIOUS ASPECTS INVOLVED IN A
SPILLWAY DESIGN
The following aspects are involved in the design of spillways:
1. Hydrology
• Estimation of inflow design flood
• Selection of spillway design flood
• Determination of spillway outflow discharge
• Determination of frequency of spillway use
2. Topography and geology
• Type and location of spillway
3. Utility and operational aspects
• Serviceability
4. Constructional and structural aspects
• Cost-effectiveness
8

ECONOMIC ANALYSIS
Comparative costs: spillway-dam combinations. A:Minimum cost: gated
spillway, B: Minimum cost: ungated spillway (USBR,1960).
9

SPILLWAY DESIGN FLOOD
Probable Maximum Flood (PMF)
This is the flood that may be expected from the most severe
combination of critical meteorological and hydrological conditions that
are reasonably possible in the region. This is computed by using the
Probable Maximum Storm.
Standard Project Flood (SPF)
This is the flood that may be expected from the most severe
combination of hydrological and meteorological factors that are
considered reasonably characteristic of the region and is computed by
using the Standard Project Storm (SPS).
In US, generally, large dams are designed for PMF, intermediate for
SPF/PMF, and small dams for floods of return period of 100 years to
SPF.
10

ESTIMATION OF SPILLWAY DESIGN FLOOD
The estimation of spillway design flood or the inflow design flood is an
exercise involving diverse disciplines of hydrology, meteorology,
statistics and probability.
There is a great variety of methods used around the world to determine
exceptional floods and their characteristics. ICOLD (1992) groups all
these methods under the two main categories:
1. Methods based mainly on flow data.
2. Methods based mainly on rainfall data.
(discussion on the methods is not scope of this course)
11

SPILLWAY DESIGN
Ogee or Overflow Spillways
12

OGEE OR OVERFLOW SPILLWAYS
The ogee or overflow spillway is the most common type of spillway. It
has a control weir that is Ogee or S-shaped. It is a gravity structure
requiring sound foundation and is preferably located in the main river
channel.
13

The basic shape of the overfall (ogee) spillway is derived from the
lower envelope of the overall nappe flowing over a high vertical
rectangular notch with an approach velocity, Vo,=0 and a fully aerated
space beneath the nappe (p=po)
14

DISCHARGE CHARACTERISTICS
Similar to the crest profile, the discharge characteristics of the standard
spillway can also be derived from the characteristics of the sharp
crested weir. The weir equation in the form:
If the discharge, Q, is used as the design discharge in above Eq, then the term
He will be the corresponding design head (Hd) plus the velocity head (Ha). i.e.,
He= Hd +Ha
For high ogee spillways, the velocity head is very small, and He≅ Hd.
2/3
2 eLHgCQ =
He
15

Overflow spillways are named as high-overflow, and low-overflow
depending upon to the relative upstream depth P/HD.
In high-overflow spillways, this ratio is (P/HD>1.33) and the approach
velocity is generally negligible.
Low spillways have appreciable approach velocity, which affects both
the shape of the crest and the discharge coefficients.

Definition sketch of overflow spillway cross-section

Figure gives variation of CD, the value of C when H equals the design
head HD, with the relative upstream depth P/HD. Here P is the height of
the spillway crest with respect to the channel bed.

Overflow spillways
frequently use undershot
radial gates for releases
over the dam. The
governing equation for
gated flows:
Where C is a coefficient of
discharge, and H1 and H2
are total heads to the
bottom and top of the gate
opening. The coefficient C
is a function of geometry
and the ratio d/H1, where d
is the gate aperture.

THE SPILLWAY CREST PROFILE
On the crest shape based on a design head, HD, when the actual head
is less than HD, the trajectory of the nappe falls below the crest profile,
creating positive pressures on the crest, thereby reducing the
discharge. On the other hand, with a higher than design head, the
nappe-trajectory is higher than crest, which creates negative pressure
pockets and results in increased discharge.
H=HD
H>HD
H<HD

Accordingly, it is considered desirable to under design the crest shape
of a high overflow spillway for a design head, HD, less than the head on
the crest corresponding to the maximum reservoir level, He (~Hmax).
However, with too much negative pressure, cavitation may occur. The
U.S. Bureau of Reclamation (1988) recommendation has been that
He/HD should not exceed 1.33.
The Corps of Engineers (COE) has accordingly recommended that a
spillway crest be designed so that the maximum expected head will
result in an average pressure on the crest no lower than (-4.50m) of
water head (U.S. Department of Army, 1986). Pressures of (-4.50m)
can be approximated by the following equations (Reese and Maynord,
1987).

He/HD <=1.33

Crest shapes have been studied extensively in the USBR hydraulic
laboratories with various approach depths. On the basis of the USBR
data, the US Army Corps of Engineers, WES (1952)** has developed
several standard shapes, designated as WES standard spillway
shapes, represented on the downstream of the crest axis by the
equation:
**WES Spillway for Genegantslet dam,. New York, Tech Memo 2–351, 1952.

THE SPILLWAY CREST PROFILE (typical values)

In the revised procedure developed by Murphy (1973), using the same
basic data of USBR, the upstream quadrant was shaped as an ellipse
with the equation
and the downstream profile conformed to the equation
Where K is a parameter depending on the ratio approach depth and
design head
For vertical u/s face
origin at the base of apex

Figure. Coordinate coefficients for spillway crest (USACE, 1986)

Typical WES crest profiles.

In a high-overflow section, the crest profile merges with the straight
downstream section of slope α, as shown in Fig. (i.e., dy/dx = α).
Differentiation and expressing that in terms of x
yield the distance to the position of downstream tangent as follows:
where
xDT = Horizontal distance from
the apex to the downstream
tangent point
α = Slope of the downstream
face.

With respect to origin at the apex, the equation of the elliptical shape
for upstream quadrant is expressed as,
where
x = Horizontal coordinate, positive to the right
y = Vertical coordinate, positive downward
A, B = One-half of the ellipse axes, as given in Fig. above for various
values of approach depth and design head.

For a inclined upstream face of slope
FS, the point of tangency with elliptical
shape can be determined by the
following equation.

The coefficient of discharge (or say discharge) is influenced by a
number of factors such as
(1) the relation of the actual crest shape to the ideal nappe shape,
(2) the depth of approach,
(3) the inclination of the upstream face,
(4) the contraction caused by the crest piers and abutment,
(5) the interference due to downstream apron, and
(6) the submergence of the crest due to downstream water level.

(1). The relation of the
actual crest shape to the
ideal nappe shape,
R. M. Khatsuria, Hydraulics of Spillways and Energy Dissipators,

(2) the depth of approach

(3) the inclination of the upstream face

(4) The effective length (L’) of Ogee spillway
Crest piers and abutments cause contraction of the flow, reduction in
the effective length of the crest, and cause reduction in the discharge
as compared to that of an otherwise uncontrolled crest. The following
relationship applies:
The values of KP and Ka depend mainly upon the shape of the piers
and that of the abutments.

(5 & 6): Submerged Discharge on Overflow Spillways
The coefficient of discharge decreases under the condition of
submergence. Submergence can result from either excessive tailwater
depth or changed crest profile.
The effect of tailwater submergence on the coefficient of discharge
depends upon the degree of submergence defined by hd/He and the
downstream apron position, (hd+d)/He shown in Fig. (7.5).
For a value of (hd+d)/He up to approximately 2, the reduction in the
coefficient depends on the factor (hd+d)/He and is independent of hd/He
as shown in Fig. (7.5.a), i.e., it is subject to apron effects only.

(5 & 6): Submerged Discharge on Overflow Spillways
Atıl BULU, Lecture noted of water resources, Istanbul Technical University

When (hd+d)/He is above 5,
the reduction depends only
on hd/He as shown in Fig.
(7.4.b), i.e., tailwater effects
control.
For (hd+d)/He between 2 and
5, the reduction of the
coefficient depends on both
factors, given in Fig. (7.5.c).

SPILLWAY TOE
The spillway toe is the junction between the discharge channel and the
energy dissipator. Its function is to guide the flow passing down the
spillway and smoothly in the energy dissipator
A minimum radius of 3 times the depth of flow entering the toe is
recommended.

EXAMPLE 7.1: Design an overflow spillway section for a design
discharge of 1500 m3/sec. The upstream water surface level is at
elevation 240m and the upstream channel floor is at 200 m. The
spillway, having a vertical face, is 50 m long.

Solution:
1. Assuming a high overflow spillway section, for P/HD ≥ 3, discharge
coefficient CD =0.49 from Fig.
2. From the discharge equation

5. Calculate height of the crest,
P = 40.00 − 5.73 = 34.27m
6. Calculate design head
Since He=5.76 m<10m
Design head=HD=0.7He=0.7*5.76=4.03m
7. Calculate P/HD
P/HD=34.27/4.03=8.5 >1.33 high overflow

8. Shape of downstream quadrant
for P/HD=8.5 K= 2 (from Fig)
Therefore,

Coordinates of the downstream shape computed by the equation
are as follows:
9. Calculate point of tangency: Assume a downstream slope of (2/1).
From Eq.

10. Shape of upstream quadrant:
Eq.
Therefore ,

Coordinates of the downstream shape computed by
the equation are as follows:

sketch of overflow spillway cross-section

EXAMPLE 7.2: A spillway has been designed for a head of 2.80 m with
a length 200 m. The discharge coefficient is C = 0.49. Calculate the
discharge for this head.
What will the discharge be for heads of 0.20 m and 1.50 m?
What is the maximum discharge that can be passed over this spillway
without cavitation?

Solution:
At the design head,

Similarly,

Maximum head:

EXAMPLE 7.3: Determine the length of an overflow spillway to pass 60
m3/s with a depth of flow upstream not to exceed 1.50 m above the
crest. The spillway is 2.50 m high. The upstream face is sloped 1/1. For
60 m3/s, the tailwater rises 1.00 m above the crest. The spillway is
designed for the maximum head.

1. Since the spillway is designed for maximum head,
HD= He = 1.50 (without the approach velocity head)
2. From the given figure,
>2 but <5

Problem 1:
Design a suitable section for the overflow portion of a concrete gravity
dam having the downstream face sloping at a slope of 0.7H: 1V. The
design discharge for the spillway is 8,000 m3/s. The height of the
spillway crest is kept at RL 204.0 m. The average river bed level at the
site is 100.0 m. Thickness of each pier may be taken to be 2.5 m.
(Take He=HD)

Problem 2:
Design a suitable section for the overflow portion of a concrete gravity
dam having the downstream face sloping at a slope of 0.7H: 1V. The
design discharge for the spillway is 8,000 m3/s. The height of the
spillway crest is kept at RL 204.0 m. The average river bed level at the
site is 100.0 m. The spillway length consists of 6 spans having a clear
width of 10 m each. Thickness of each pier may be taken to be 2.5 m.
(Take He=HD)

THANK YOU
Slides are prepared from various sources(References). It may have
discrepancies/ inconsistency. If you find any, kindly rechecked with
sources list in “references” .
64

ENERGY DISSIPATERS
(STILLING BASIN)

LEARNING OUTCOME
After taking this lecture, students should be able to:
(1). Obtain knowledge on energy dissipators (stilling basin)
used in hydraulic structures and their design guide lines
(2). Apply the design guide lines for the design of selected
energy dissipators
66
References:
Khatsuria , R. M., Hydraulics of Spillways and Energy Dissipators,
Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 4th Ed. CRC Press
Santosh, K. G., Irrigation Engineering and Hydraulic Structures, Khanna Publishers
Mays, L. W., Hydraulic design handbook (CHAPTER 18), Mcgraw hills

ENERGY DISSIPATION
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.
Low velocity
Very high velocity
V1=(2gH1)0.5
y1=q/V1
67

ENERGY DISSIPATION
CLASSIFICATION
1. Based on hydraulic action: Turbulence and internal friction as in
hydraulic jump stilling basins, roller buckets, and impact and pool
diffusion as with ski jump buckets and plunge pools.
2. Based on the mode of dissipation: Horizontal as in the hydraulic
jump, vertical as with ski jump buckets/free jets, and oblique as with
spatial and cross flows. The vertical dissipation may be in the downward
direction as with free jets and plunge pools and in upward direction as
with roller buckets.
3. Based on geometry or form of the main flow: Situations involving
sudden expansion, contraction, counter acting flows, impact, etc.
4. Based on the geometry or form of the structure: Stilling basin
employs hydraulic jump with or without appurtenances like chute blocks,
baffle piers, etc. Buckets (ski jump or flip buckets) include special
shapes like serrated, dentated buckets, and roller buckets that are either
solid roller bucket or slotted buckets.
68

ENERGY DISSIPATION
PRINICIPAL TYPES OF ENERGY DISSIPATORS
The energy dissipators for spillways can be grouped under the following
five categories:
1. Hydraulic jump stilling basins
2. Free jets and trajectory buckets
3. Roller buckets
4. Dissipation by spatial hydraulic jump
5. Impact type energy dissipators
69

ENERGY DISSIPATION
ANALYSIS OF PARAMETERS
E
g
V
y
g
V
y
g
V
y o
o ∆++=+=+
222
2
2
2
2
2
1
2
∆E= Energy dissipation between
u/s and d/s
Energy equation:
Mass conservation:
Q1=Q2=Q3
70

ENERGY DISSIPATION
In case of hydraulic jump at the d/s
V1=(2gH1)0.5
y1=q/V1
Thus, q/y1=(2gH1])0.5








+−








+==∆
g
V
y
g
V
yE
22
2
2
1
2
2
2
71
Energy dissipation
Assumption of Horizontal bed !!!

ENERGY DISSIPATION
Hence, for a given discharge intensity and given height of spillway, y1 is
fixed and thus y2 (required for the formation of hydraulic jump) is also
fixed.
But the availability of a depth equal to y2 in the channel on the d/s cannot
be guaranteed as it depends upon the tail water level, which depends
upon the hydraulic dimensions and slope of the river channel at d/s.
The problem should, therefore, be analyzed before any solution can be
found by plotting the following curves:
Tail Water Curve (TW Curve): A graph plotted between q and tail water
depth,
Jump Height Curve (JH Curve) also called y2 curve: A curve plotted on
the same graph, between q and y2,
72

ENERGY DISSIPATION
(1)
IdeaI condition
73

ENERGY DISSIPATION
(1). When TW curve coincides with y2 curve
This is the most ideaI condition for jump formation. The hydraulic
jump will form at the toe of the spillway at all discharges. In such a case,
a simple concrete apron of length equivalent to length of jump (e.g.,5 [y2
- y1]) is generally sufficient to provide protection
75

ENERGY DISSIPATION
(A). When TW curve is above the y2 curve
When y2 is always below the tail water, the jump forming at toe will be
drowned out by the· tail water, and little energy will be dissipated.
The problem can be solved by:
(i). constructing a sloping apron above the river bed level
(ii). providing a roller bucket type of energy dissipator
76

ENERGY DISSIPATION
iii. Providing a higher apron level followed by a drop

ENERGY DISSIPATION
(B). When TW curve is below the y2 curve
When the tail water depth is insufficient or low at all discharges, the
following solution can be applied:
(i). Ski jump bucket type: This type of energy dissipator requires
sound and rocky river bed, because a part of the energy dissipation
takes place by impact, although some of the energy is dissipated in air
by diffusion and aeration
78

ENERGY DISSIPATION
(ii). Providing of a sloping apron as below the river bed
79

ENERGY DISSIPATION
(iii). Constructing a subsidiary dam below the main dam
80

ENERGY DISSIPATION
(iv) Providing upward slope

ENERGY DISSIPATION
(D). When TW curve is above the y2 curve at low discharges and
below the y2 curve at high discharges: In this case, at low
discharges, the jump will be drowned and at high discharges, tail water
depth is insufficient. The following solutions can be applied by:
(i). Providing a sloping apron partly above and partly below the river bed
(ii). A combination of energy dissipator performing as a hydraulic jump
apron for low discharges and flip bucket for high discharges
At low discharges, the jump
will form on the apron above
the river bed.
Similarly, at high discharges,
the jump will form on the
apron below the river bed
82

ENERGY DISSIPATION
(C). When TW curve is below the y2 curve at low discharges and
above the y2 curve at high discharges (inverse of case D)
83
The following solutions can be applied:
(i). Sloping-cum-horizontal apron such that the
jump forms on the horizontal portion for low
discharges and on the sloping portion for high
discharges

ENERGY DISSIPATION IN HYDRAULIC JUMP
Hydraulic jump can be used as Energy Dissipator








+−








+=∆
g
V
y
g
V
yE
22
2
2
1
2
2
2
yqV /=
However, the real problem in the design of stilling basins, is not the absolute
dissipation of energy, but is the dissipation of this energy in as short a length
as possible.







 −
=∆
21
12
4 yy
yy
E








=
gy
V
F
84
V1=(2gH1)0.5
y1=q/V1
Thus, q/y1=(2gH1])0.5

STILLING BASIN
• In general, a stilling basin may be defined, as a structure in which the
energy dissipating action is confined.
• If the phenomenon of hydraulic jump is basically used for dissipating
this energy; it may be called a hydraulic jump type of stilling basin.
• The auxiliary devices may be used as additional measures for
controlling the jump, etc.
• Stilling basins are placed at the ends of dam spillways and at the
ends of steep-sloped canal sections where elevation change has
generated high kinetic energy.
• Stilling basin come in a variety of types and can either contain a
straight drop to a lower elevation or an inclined chute
• Inclined chutes are the most common design for stilling basins
and the most used inclined chutes are: USBR Stilling Basins
Type II-IV, SAF Stilling Basins
85

STILLING BASIN
In practice, the following types are highly recommended:
• USBR Type II basin for large structures and Fr > 4.5;
• USBR Type III basin and the SAF basin for small structures;
• USBR Type IV basin for oscillating jump flow conditions
The designs are selected based on the Froude Number of the flow and
the flow velocity:
1
1
1
1
1
y
q
V
gy
V
Fr
=








=
86

STANDARD STILLING BASINS
Elements of Stilling Basin
Chute blocks
Baffle blocks
End sill or Dentated Sill
87

• Chute blocks -concrete blocks built into the inclined sections of the
spillway. These features are commonly placed at the head of the
stilling basin to create turbulence prior to the hydraulic jump
• Baffle blocks -freestanding concrete blocks built in the main basin.
These blocks are only used for flows <20m/s due to the high force
they are subjected to and the potential for cavitation
• End sills -a built-up lip at the tail of the basin, with or without blocks.
The sill height has the most significant impact on energy dissipation
and taller sills are used to reduce the overall length of the stilling
basin
88

USBR Stilling Basin Type II
Fr1 > 4.5
89

USBR Stilling Basin Type III
Fr1 > 4.5 & V<18m/s
D1=y1
90

USBR Stilling Basin Type IV
Fr1=2.5-4.5
91

Saint Anthony Falls
Effective for Fr1= 1.7 and 17
92

d=y1
dconj=y2
Summary
93

ENERGY DISSIPATION
DEFLECTOR BUCKETS
Sometimes it is convenient to direct spillway into the river without
passing through a stilling basin. This is accomplished with a deflector
bucket designed so that the jet strikes the riverbed a safe distance from
the spillway and dam. This type of spillway is often called a flip bucket
or ski jump spillway.
94

ENERGY DISSIPATION
The trajectory of the jump
Where,
hv = Velocity head
d = Thickness of the jump
When the free jet discharging from the deflection bucket falls into an
erodible riverbed, a plunge pool is eroded to a depth, D, given by:
95

THANK YOU
Slides are prepared from various sources(References). It may have
discrepancies/ inconsistency. If you find any, kindly rechecked with
sources list in “references” .
96

Chapter 7 spillway and energy dissipators

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