This chapter is based on the book Hydraulics of Spillways and Energy Dissipators By Rajnikant M. Khatsuria ,concerned with the general procedure of an overall design. An evaluation of the basic data should be the first step in the preparation of the design. This includes the topography and geology as well as flood hydrography, storage, and release requirements.

1. Addis Ababa Science and
Technology
College of Architecture and Civil
Engineering
ADVANCED HYDRALUC STRUCURES
CENG 6605
Fitsum Tesfaye (PhD)

2. ADVANCED HYDRALUC STRUCURES
CENG 6605
Spillway Design: An Overview

3. INTRODUCTION
• The object of spillway design, which involves two steps, is
– to provide a safe and adequate spillway structure for the lowest
combined cost of the spillway and the dam.
• The first step in the design involves determining the type and
overall size of the spillway structure to suit the anticipated
requirements and conditions of the site.
• A detailed hydraulic and structural design of the spillway
structure is the next step.

4. INTRODUCTION
• An evaluation of the basic data should be the first step in
the preparation of the design. This includes
– the topography and geology as well as
– flood hydrography,
– storage, and
– release requirements.
• The type, size, and elevation of the crest and whether it will be
controlled can also be tentatively decided.
• Several alternative arrangements might be possible and a
final layout could be created on the basis of economic
analysis.

5. ANALYSIS OF EXISTING STRUCTURES
• In the projects with concrete dams,
the spillways were mainly located
within the channel portion of the
river valley with any combinations of
L/Hand N.
• About 35% of spillways of concrete
dams are used in combination with
sluices and bottom outlets and the
use of chute spillways and tunnel
spillways makes up about 10% and
5% respectively.

6. ANALYSIS OF EXISTING STRUCTURES
• In the projects with earth-fill dams, gravity spillways
were used with L/H from 8 to 1700.
• In the narrow valleys, gravity spillways were used
when the power of the flow discharged was small.
• In the case of great flows, chute spillways were
preferred.
• On wide valleys, spillway tunnels gave way to
chute spillways, as the power of the flow
discharged was more than 5000 MW.
• Tunnel spillways make up about a quarter of all
flow-release structures of earth-fill dams.

7. VARIOUS ASPECTS INVOLVED IN A SPILLWAY
DESIGN
• The following aspects are involved in the design of
spillways:
– Hydrology
– Topography and geology
– Utility and operational aspects
– Constructional and structural aspects

8. Hydrology
• The hydrological aspects relevant to the spillway design are:
– Estimation of inflow design flood (IDF)
– Selection of spillway design flood
– Determination of spillway outflow discharge
– Determination of frequency of spillway use
• Estimation of inflow design flood for a given dam site is an exercise in
hydrology, therefore, procedures are fairly standardized.
regional
flood
formulae
envelope
curves
refined methods
involving statistical
analyses (flood frequency
series) and numerical
modeling of rainfall–runoff
relationships
Historica
l
Methods

9. Hydrology-Previous Vs Current Practice –Design Flood
Selection
• On the other hand, selection of the spillway design flood is a
function of social, moral, and economic, as well as
technological considerations.
• While the previous design flood–selection criteria considered
factors such as
– dam height,
– storage volume, and
– downstream development,
• Current practice is to select a design inflow flood on the
basis of the consequences of dam failure.

10. Hydrology
• The spillway outflow discharge corresponding to an inflow flood is
determined from the flood-routing analysis.
• At least
– a tentative design of the spillway
– Full Reservoir Level (FRL),
– crest level,
– number of spans, and
– an approximate discharge rating curve for the spillway
• The design of various elements of the spillway such as crest profile,
sidewalls, energy dissapator and downstream protection are all
based on spillway outflow discharge.

11. Hydrology-Frequency of Usage
• Ordinary river flows are usually stored in a reservoir, diverted
through head works, or released through outlets;
therefore, the spillway may not be required to function.
• At diversion dams, where storage space is limited and
diversion flows are relatively small compared to flood flows, a
spillway is used more frequently.
• When the flood flows are generally restricted to a small
duration and are flashy in nature, the spillways are
expected to operate more frequently.

12. Topography and Geology
• These considerations determine the type and location of a spillway as
follows:
1. Ogee spillway: Most commonly used as the integral overflow section of a concrete or a masonry
dam.
2. Chute spillway: Adopted in a site where a suitable foundation with moderate depth of
excavation is available, where topography of the site permits the use of a relatively short channel,
or where spillway
excavation can be used economically in the dam.
3. Side channel spillway: Suitable for earth or rock-fill dams in narrow canyons and for other
situations where direct overflow is not permissible.
4. Shaft spillway/Tunnel spillway: Used advantageously at dam sites in narrow canyons where
abutments rise steeply or where a diversion tunnel or conduit is available for use as the
downstream leg.
5. Siphon spillway: Used when there is a desire for an automatic operation without mechanical
parts and the discharge to be passed is small.
6. Free over-fall spillway: Suitable for arch dams
7. Duck bill spillway: Used when the waterway and foundation for the spillway are limited and a
curved crest-projection into the reservoir is possible.

13. Utility and Operational Aspects
From the standpoint of serviceability, spillways may be defined in three broad
classes as follows.
1. Service Spillways
Include any spillway that may be utilized without significant damage to the structure or downstream channel. As a
general rule, service spillways have paved
channels and suitable energy dissipators.
2. Limited Service and Additional Spillways
Include any spillway that may be utilized infrequently for operation of the reservoir without incurring excessive
damage. Some extraordinary maintenance at infrequent intervals would be acceptable in order to reduce initial
construction costs, but not to the extent of imposing significant limitations on the optimum utilization of the
reservoir’s controlled storage capacity under normal operating
conditions.
3. Emergency Spillways
Include any spillway, the use of which to be avoided as long as possible, used to prevent major damage to the
spillway structure or to downstream areas. Emergency spillways may involve partial control by so-called ‘‘Fuse Plugs’’
or ‘‘Flash Boards.’’

14. Utility and Operational Aspects
• Uncontrolled Crest
• Uncontrolled crests permit water to discharge whenever the reservoir
surface is higher than the crest.
• The height of the dam is determined from the maximum flood
required to be discharged and the necessary free board.
• Since the longest crest requires the least head, an economic balance
may be found between length and height of spillway if the
topography does not limit the length of the crest.

15. Utility and Operational Aspects
• Controlled Crest
• Gates may be used to control the reservoir water surface.
• The top of the gates is usually at the normal water-surface elevation of the
reservoir; to keep the maximum elevation constant, the gates are opened
sufficiently to pass the floods.
• While an uncontrolled crest requires a dam higher than a controlled crest, an
uncontrolled crest offers the following advantages:
– flood storage is always available, the necessity of gates and their maintenance is
eliminated, and the crest has a greater ability to pass the logs and other debris
without interference.
• An uncontrolled crest requires less discharge capacity than a controlled crest for
a given flood, since part of the flood is stored in order to acquire a head
necessary to pass the discharge.

16. Utility and Operational Aspects
• Spillways for Flood and Sediment Disposal
• While the prime objective of a spillway is disposal of floods, if designed specifically, it may
also serve for the disposal of sediment deposited in the reservoir.
• Mountainous streams with dependable flows and considerable heads favor selection of run-
off river plants.
– However, such streams carry large amount of sediment that could ultimately settle in the reservoir
and reduce its capacity.
• In such projects, spillways can be designed to serve the dual purpose of flood disposal and
sediment disposal to flush material deposited in the reservoir downstream, but these
requirements are often conflicting.
• While flood disposal warrants a larger spillway capacity with a wider waterway, sediment
disposal requires low level spillways or bottom outlets of large capacity placed deep below
the water surface. .

17. Constructional and Structural Aspects
• River valley projects with multiple purposes are usually phased over long periods of
time to suit the requirements of irrigation and power, financial allocations, and
progress of rehabilitation of the project-affected population.
• The construction of major dams and spillways involves large quantities of
excavation, earthwork, and concreting, and may be required to be constructed in
stages.
• The flood flows in the intervening periods, diverted over partly constructed spillways,
may set up undesirable flow conditions, thus resulting in damage to the adjacent
structures already constructed by then.
• Hydraulic model study is the best means to visualize these effects and to evolve
suitable designs.

18. Constructional and Structural Aspects
• The choice of earth- and rock-fill dams is often based on the availability of
material from the excavation for the spillways
• A detailed discussion on this is presented in the chapter ‘‘Unlined Rock
Spillways.’’
• Many future dams are likely to adopt the technology of roller-compacted
concrete. This method of construction is cost-effective, typically faster, and
causes minimum project disruption.
• This technique also facilitates the provision of stepped spillways since the rolling
in lifts of 30 to 60 cm favors the construction of a stepped surface as the height
increases.
• The stepped spillways ensure energy dissipation on the flow surface itself at
almost double that of an unstepped, smooth spillway.

19. ECONOMIC ANALYSIS
• The procedure for economic analysis has been illustrated by the USBR
(1960).
• Factors in EA Optimization may be:
– (1) the characteristics of the flood hydrograph;
– (2) the damages that would result if a flood occurred without the dam;
– (3) the damages that would result if such a flood occurred with the dam in
place;
– (4) the damages that would occur if the dam or spillway were breached;
– (5) the effects of various dam-spillway combinations on the probable increase
or decrease of damages above or below the dam;
– (6) the relative cost of increasing the spillway capacity; and
– (7) the use of outlet facilities to serve more than one function, such as control
of releases and control or passage of floods

20. Comparative costs: spillway-dam combinations. A:Minimum cost:
gated spillway, B: Minimum cost: ungated spillway (shown in
USBR, United States,1960)