Spillways and Flood Control Works

Spillways and Flood Control Works
Unit-V
BTCI09007

Syllabus
• Factors affecting design of spillway, types of
spillways, design principles of ogee spillway,
trough spillway, siphon spillway and shaft
spillway. Design of bucket type energy
dissipater and stilling basin, flood mitigation
reservoirs. Crest gates, types, advantages,
choice, design of radial gate. Outlet works
through dams, intake structures

Spillway
• The various discharging facilities provided in a
storage dams are spillway, intake and outlet works.
Spillway is the escape provided in conjunction with
a dam to dispose of surplus flood water from the
reservoir. The surplus water is drawn from the
reservoir and conveyed back to the river
downstream or some natural drainage.
• The spillway thus acts as the safety valve in the
reservoir against failure of dam due to overtopping.

Spillway
• Spillway are important auxiliary works of dams,
provided to dispose of surplus floodwater safely which
cannot be stored in the reservoir. It is necessary to
provide a spillway of sufficient capacity so as to avoid
water from overtopping the dam. Overtopping of dam
may lead to failure of dam resulting in serious damage
to the property.

Spillway
Requirements of a spillway
• A Spillway should fulfill the following requirements
• The spillway should have sufficient capacity
• The location of spillway should provide safe
disposal of water without toe erosion.
• Spillway should be hydraulically and structure
sufficient
• Usually spillway should be accomplished by an
energy dissipation work on its downstream side.

Essentials of A Spillway
The essentials of a spillway are
• adequate capacity to serve as moderation of
floods.
• While conveying the excess flows downstream
the tail water maintained is such that the
purpose and protection of dam and its
appurtenant work are fully ensured.
• Providing safe & regulated release of the
surplus water in excess of the reservoir capacity.

• Hydraulically and structurally the spillway’s
bounding surfaces are adequate and are erosion
resistant to withstand high scouring velocities
created by drop of flow from reservoir surface
to tail water,
• Device for the dissipation of energy at the
bottom of the dam is provided.

• The spillway size and type depends on the best
combination of storage and spillway capacity to
accommodate the selected inflow design flood
which in turn depends on
• (i) The character of the flood hydrography.
• (ii) Effect of various dams and spillways
combination on probable increase or decrease or
damage above or below the dam
• (iii) Relative cost of increasing spillway capacity
• (iv) Use of combined outlet facilities to serve as
control of release and control or passage of floods.

Spillway Components
• Spillway may be constructed as an integral part of the dam or
as independent structure but essentially comprises
components discussed as under:
Approach Channel
• It is an entrance structure to draw water from the reservoir
and convey it to the control structure.
• The approach channel may be a straight or curved in plan.
Its banks may be parallel, convergent, divergent or
combination of these and may be vertical or sloping.
• It may insure minimum head loss through the channel and to
obtain uniformly of flow over the control structure, the
entrance velocities are limited and channel curvatures and
transitions, if any are gradual.

Components Parts of a Spillway
Control Structure
• Control Structure consists of a weir which may be
sharp. It is a major component of a spillway. It
regulates and controls the surplus water from the
reservoir. It does not allow the discharge of water
below from reservoir level and allows the discharge
of water below from reservoir level and allows only
when water surface in the reservoir rises above that
level.

Spillway Components
Control Structure
• It Consists of any overflow crest provided with a
bridge and gates to regulate and control the
overflow from the reservoir. The control limits the
overflows below fixed reservoir levels and also
regulated when the reservoir rises above that level.
The length of the crest is finalized from economic
considerations and hydraulic model studies as for a
given level of the crest, the height of dam increases
if the crest is made of short length.

Spillway Components
Discharge Carrier
• It is the waterway provided to convey the flows released from
the control structure to the river below the dam, except in the
case of arch dam wherein the discharge falls free from the
crest or where the flow is released directly along the
abutment hill side to cascade down the abutment face.
• The structure of the discharge carrier may be the
downstream face if a concrete dam, an open excavated
channel, a closed cut and cover conduit placed through or
under a dam or a tunnel excavated through an abutment.
• The profile may be variable flat or steep, cross section may
be variable rectangular, trapezoidal, circular or of other shape
and the waterway may be wide or narrow, long or short.

Discharge Channel
• Discharge Channel It is provided to convey the
surplus water released through control
structures to the stream bed below the dam
safely. The discharge channel may be the
downstream face of spillway itself, or open
channel excavated along the ground surface or a
closed conduit placed through or under a dam.

Spillway Components
Terminal Structure
• At the downstream end of the discharge carrier
when flow from reservoir to downstream river
level manifests itself in the form of high
velocities enough to cause scour, erosion and
subsequent damage to adjacent structures, a
terminal structure usually in the form of stilling
basin is provided for dissipation of residual
energy.

Energy Dissipaters
• They are usually provided on the downstream
side of the spillway. High Voltage water coming
through Spillway may cause serious damage to
the toe of dam and to the adjacent structures.
This high energy of flow must be dissipated
before it flows back to river. For this, energy
dissipaters are provided

Spillway Components
Types & Design of Spillways
• The site, shape and type of spillway and the arrangement
of its components depend upon the various factors as
stated below:
Topographical
• Site conditions such as steepness of terrain traversed by the
spillway control and discharge carrier, type and amount of
excavated material, possibility of utilization of the
excavated material for the dam embankment, chances of
scour of the bounding surfaces and the need for lining,
permeability and safe bearing capacity of the foundation,
stability of the excavated slopes, and geological site
conditions.

Spillway Components
Hydrological
• Inflow and reservoir storage conditions such as
inflow discharges, its frequency and shape of
hydrography, reservoir capacity at various
levels, and length and height of crest..
Purpose of Storage
• Purpose of storage determines whether the
spillway is required to be gated and the type of
gates.

Spillway Components
Types of Storage
• Types of dam governs the spillway design flood.
Spillway of certain types are eminently suitable
for earth and rock-fill dams, while the other are
adapted for concrete dams.
Outlet facilities
• Possibility of combined outlet facilities to serve
more than one function, such as control of
release and control or passage of floods.

Spillway Components
Downstream flow conditions
• Certain types of spillways greatly alter the shape
of flood hydrograph downstream of the
spillway. Siphon spillway, for example, gives rise
to a wave travelling downstream in the river
which is detrimental to navigation & fishing.

Spillway Components
Safety Considerations
• Spillway of inadequate capacity or improperly
designed spillway may cause failure of the dam.
Economic
• Several study of alternative types of spillway are
undertaken to finalize the most economical and
hydraulically efficient spillway suited to the
discharging requirement and site conditions

Entrance and Outlet Channels
• They are not required in case of an
overflow spillway. However entrance
channels are provided to draw water from
reservoir and convey it to control structure.
Similarly Outlet channels are provided to
carry the spillway flow in river channel
below the dam. Similarly, outlet channels are
provided to carry the spillway flow to river
channels below the dam.

Types of Spillways
• According to the prominent features related to various
components of spillway viz. control Structures, discharge
channel etc. The spillways may be classified in the
following types.
• Free Overfall
• Ogee or overflow spillway
• Trough Spillway
• Shaft Spillway
• Side Channel Spillway
• Tunnel Spillway
• Siphon Spillway.

Types of Spillways
Free Over-fall
• This is the simplest type of Spillway which consists of a
low height narrow crusted Weir having downstream
face either vertical or nearly vertical. This type of
Spillway is suitable for low earth dam, low concrete
masonry gravity dam or low thin arch dam. It is not
suitable for high Dam.

Types of Spillways
Ogee or Overflow Spillway
• It is the modified form of drop spillway
suitable for high gravity dam, arch dam and
buttress dam. The overflow water is guided
smoothly over the crest and profile of
spillway. This type of spillway is more
preferable on valleys where width of river
is more to provide sufficient crest length and
river bed can be protected from scouring at
reasonable cost.

Types of Spillways
Trough Spillway
• Trough spillway Is provided when it is not possible to
provide an overflow spillway such as in case of
embankment dam or due to erodible nature of stream
bed in case of concrete masonry dams. It discharges
the surplus flood through a steep sloped open
channel. The crest of spillway is kept normal to its
centre line. It consists of a discharge channel to the
river.

Types of Spillways
Side Channel Spillway
• It is the Spillway in which, the flow after passing
over a weir or ogee crest, is carried away by a
side channel. It is best suited for non rigid dam
like earthen dam.

Types of Spillways
Siphon Spillway: When available space is limited
and surplus discharge is not large siphon
spillway is often preferred. It is based on siphon
action in the shape of an inverted pipe. Usually
siphon spillway is provided in concrete gravity
dam through its body.

Energy Dissipation Below Spillway
• Water flowing over a spillway has high potential energy
as it glides along spillway. This large kinetic energy rises
to high velocity of flow which may cause large scale
scour/erosion at the downstream toe, if proper
arrangements are not made to dissipate this high energy.
The arrangements provided to dissipate this energy are
known as energy dissipaters.
• In general the dissipation can be achieved in two ways
• By developing a Hydraulic jump
• By directing the jet of water using a deflector bucket.

• A. Roller Basin
B. Deflector Bucket
C. Flip Bucket
D. Non-radial spillway and sluice buckets
E. Schoklitsch Dissipater

Spillway Crust Gates
• Gates provided over the crest of a controlled
spillway are termed as spillway crest gates. An
additional storage can be obtained by providing
gates on the spillway crest. However, during
floods, these gates are lifted to make full use of
spillway capacity. Great care is needed to be
taken while operating crest gates of earth dams
so as to avoid the overtopping of earth dam.

• The following are some of the common types of crest
gates.
(i) Dropping shutters or flash boards
• They are some sort of temporary gates used only for
smaller spillway of minor importance. They consists of
wooden panels usually 1.0 to 1.25 m high. They are
hinge at bottom and supported by struts to resist water
Pressure. These wooden panels can be raised or
lowered from an overhead cableway or a bridge.

Dropping shutters or flash boards

Radial Gates or Tainter Gates
• Radial gate has a curve water supporting face which
is made of steel. Its shape is just as sector of a
circle properly braced and hinged at the pivot.
• The gate is thus made to rotate above a horizontal
axis. The load of the gate, water etc. is carried on
bearing which are mounted on piers. An operating
plateform is provided to lift the the gate by means
of ropes and chain

Drum Gates
• This gate is developed by United States Bureau of Reclamation
(USBR) this gates is suitable for longer span of the order of 40
or 50 m. The drum gate consists of circular sector in cross
section formed by skin plates attached to internal bearing. The
entire section may be raised or lowered such that upper surface
coincides with the crest of spillway. The buoyant force due to
head water pressure underneath the drum assists in its lifting.
The drum Gate needs large recess and hence is not suitable for
smaller spillway. Some other types of spillway gates are also
used namely Vertical lift gates, rolling Gates, Tilting Gate or
Flap Gate, Bear Trap gate.

Intake and Outlet Works
• The stored water in a reservoir is essentially drawn
to meet the intended purpose of irrigation, hydro
power generation, public water supply, etc. through
the discharge facilities termed as intake structure.
An intake structure may take on many forms
depending on the functions it is to serve, the range
in fluctuations of reservoir water levels, quality of
water drawn, discharge to be released, frequency
and amount of reservoir drawdown, trash removal
requirement, wave action in the reservoir and other
such considerations such as sedimentation control
devices, etc.

• The general requirement of an Intake Structure are:
Structural Stability
• The Intake structure is stable to resist water and wave
thrust besides wind pressure when reservoir is empty as
also against the shock of earthquakes.
Hydraulic efficiency
• There is smooth entry into the water conductor system
to ensure gradual transformation of static head to
conductor system to ensure gradual transformation of
static head to conduct velocity so as to involve hydraulic
losses.

Velocity Limitation
• The velocity through trash rack gates and ports
is within economic and safe limits.
Operational efficiency
• The intake and the equipments are such as to
prevent/ minimize ice, floating trash and coarse
sediment entering the water conductor system
to ensure good operational efficiency.

• The main components of an irrigation intake
structure are
(i) Trash rack and supporting structure
(ii) Anti-Vortex devices,
(iii) Bell mouth entrance with transition and
rectangular circular opening, and
(iv) Gate slot closing devices with air vents.

Gate slot closing devices with air vents.

Function of Intakes
• Intake structure serve to permit withdrawal of water in the reservoir over a
predetermined range of reservoir levels to the outlet.
• The outlet may release water at a retarded rate than the incoming flow from
the intake or release the impounded water at controlled rate constituent with
the requirement of downstream such as feeding the power turbines, feeding
the irrigation channels, discharging heavily silt laden inflow, depleting the
reservoir to facilitate inspection channels, discharging heavily silt laden
inflow, depleting the reservoir to facilitate inspection, as flood control
regulator to release water temporarily impounded in flood control storage
space. The other functions served by an intake are to support necessary
auxiliary appurtenances such as trashrack, fish screens and bypass devices,
etc,.

Location of Intakes
• The various factors influencing the choice of
location of intake structure are
• (i) Type of storage reservoir
• (ii) Location & type of dam/weir.
• (iii) Type of water conductor system that is
canal or tunnel,
• (iv) Topographical features of the river.

Quality Water
• Location of intake is required to be such as to
draw the best quality of water from the
reservoir. Depth of water at intake is important.
Quality of water varies at different levels in the
reservoir and it is necessary to draw water from
different elevations of the reservoir at different
seasons of the year for which multi-level intakes
are frequently provided.

Optimum water Utilization
• Intake is located in the deepest part of the
impounding reservoir to enable full utilization of
the capacity of the reservoir and to protect intake
from sediments in the reservoir. In the reservoir
with wide variations in the water level. The intake
is better located at the lowest stage so that one inlet
is always submerged and operative to draw supply
and minimum operating head is always available.

Minimum Silt Entry
• In order to prevent silt from the reservoir being
carried into the outlet system, location of intake
at low points or pockets in the reservoir is
avoided. Bottom sediments are kept out of the
intake if the lowest entrance ports are
sufficiently above the reservoir floor.

Wave action
• The Intake structure is placed sufficiently
removed from the currents that might threaten
the safety of the structure. Waves exert pressure
on the superstructure of the intake. Waves may
also stir up the bottom sediments if the intake is
not so located deep in the water below the
water surface. Locations where winds may drift
debris into the intake are avoided.

Ice Pressure
• Ice may push down and clog ports. Ice troubles are
reduced in case entrance velocities are kept
between 7 to 10 cm/sec. low velocities do not
transport ice and hold the entry of leaves and
debris into the intake as also fish can escape from
being drawn into by intake currents. Leaves and
debris into the intake as also fish can escape from
being drawn into by intake currents.

Types of Dams
• Usual location of intake in an earthen dam is in deep water
near the upstream toe of the dam while in concrete or
masonry dam, the intake may be in the structure itself.
Sewage Pollution
• The intake is located to avoid sewage pollution. Locations
where stagnant water or algae can accumulate are unsuitable
for intake structure.
Shore location
• Usual location of the intake for public water supply is some
distance away from the shore to avoid turbid water from
surface wash along the banks.

Types of Intakes
• Depending on the function served and the range in
reservoir head under which it is to operate, the
discharging capacity and the frequency of the
reservoir drawdown, intakes for hydroelectric
projects may be relatively simple submerged
intakes or more elaborate structure raised as a
tower above maximum reservoir level.
• Broadly the Intakes are classified as
• (i) Run-of-river type
• (ii) Reservoir type.

Run-of-River Intakes
• In a run-of-River plants, intake is apparent to
power house and draws water from the river
without any appreciable storage upstream of the
diversion structure. Characteristics of river
flows., the intake is designed to withstand high
peaks and short duration flood flows and high
sediment loads. The bell mouth entrance is
essentially provided with trash racks.

Canal Intake
• It is also a variant of the run-of-river intake, that is
provided adjacent to the diversion weir/ barrages to
admit water into the canal. It is designed to
function under low heads and the topography and
geology permits straight reach suitable for it.
Sediment excluder is an essential component of
the intake. The crest of the intake is generally
raised to prevent entry of coarse fraction of bed
load into the canal.

Reservoir Type Intakes
• Intake tower classified as Submerged, dry and wet intakes fall in
this category.
(i) Submerged Intake
• An Intake Structure which remains entirely under water during
its operation is termed as submerged intake. It is provided
where the structure serves only as an entrance to the outlet
conduct and where ordinarily cleaning of the trash is not
required. The conduct intake may be inclined, vertical or
horizontal in accordingly with the intake requirements. . An
Inclined Intake may be provided with gates and operated on
the upstream slopes of a low dam.

• The submerged Intakes remain entirely under
water and possess the advantages
• (i) Simple Structure, less costly to Construct,
• (ii) No Obstruction to navigation and little
obstruction to river flow
• (iii) Little danger from floating material and a
minimum of trouble from ice.
• (iv) Trash rack cleaning is not generally required,
• (v) Particularly suitable as public water supply
intake from a river.

Intake tower
• An Intake tower is used to draw water from the
reservoir in which there are huge fluctuations in
water level or quality water is to be drawn at the
desirable depth or both. It Consist of an elaborate
exposed or tower like structure rising above
maximum reservoir level and closely located to the
dam body or the bank of the stream so as to be
approached by a connecting bridge of minimum
span.

• The Intake tower consist of circular concrete
structure provided with openings or ports for water
entry fitted with trash racks to prevent the entry of
debris and ice large enough to injure the
equipment and gates that control the flow through
intakes into the feeding conduct outlet.
• It has a merit that best quality of water available at
different depths at different seasons of the year can
be drawn through port openings at different
elevations.

• There are two types of intakes as under
• (i) Dry Intake Tower
• In dry intake tower the entry ports are directly
connected with the withdrawal conduit and water
inside the tower when gates are in a closed position.
Dry Intake tower has a merit that the intake tower
being dry is made accessible for inspection and
operation besides that the water can be withdrawn
from any level by opening the port at that level.
• However, dry intake tower is massive in structure,
than wet intake to withstand additional buoyant forces
to which it is subjected when the port gates are closed.

Wet Intake Tower
• A wet intake tower has entry ports at various levels
and the vertical shaft is filled with water up to
reservoir level. It differs from the dry intake tower is
that the water enters from the ports into the tower and
then into the withdrawal conduict through separate
gated openings. As such it consists of a circular shell
made of concrete filled with water up to reservoir
level, housing another inside shaft directly connected
to the withdrawal conduit. It is less costly to construct
and is usually not subjected to flotation and certain
other stress may not be the consideration.

Low Pressure Intake
• It is the usual form of intake used for relatively smaller
drawdowns as in the case of hydel power plants of
smaller units catering for daily and weekly water
surface variations.
• Low Pressure Intakes usually performs the function of
a dam and is designed accordingly based on the theory
of stability of a dam.
• The maximum velocity through the gross cross
sectional area of the track is about 0.75 m/sec which
may be of the order of 1.5 m/sec in case of large units
and mechanical racks of wider rack bars spacing.

High Pressure Intake
High Pressure Intake
• High Pressure Intake, in general, is used in a reservoir
which serves as a storage reservoir and head water for
hydel power generation involving considerable
drawdown under normal operation conditions. The
Intake is constructed in a variety of forms depending
on the type of dams whether of concrete or earthen
dam. In concrete dam, high pressure intake varies
little in detail from low pressure intake except in the
type of gates installed. In earthen or rock-fill dam, the
intake is generally a tower built near the foot of the
upstream slope of the dam, the structural details of
which vary greatly from the low pressure intake.

Trash Racks
• Trash rack is defined as a screen or grating
provided at the entrance of intake to prevent
entry of debris. Trash racks usually consists
of trash sections 1.5 to 2 m wide and not too
long for handling, made up of mild steel flats
on edge 5 to 15 cm. Coarse trash racks are
provided near the ports to prevent large drift,
such as cakes of ice, roots, trees and timber
from being drawn into the intake.

Trash Racks
• In some part of the intake fine trash racks are
provided to protect the machine & machine parts
through which water flows. In cold region, trash
racks is often clogged with fragile ice. Electrical
heating for small trash racks are provided to
prevent ice formation on the racks.
• The floating debris accumulated, as are denied
entry into the intake, are removed with the help
of power driven rack-rakes.

Outlet Works
• Outlet works comprise discharging facilities for
conveyance of water from a dam for power generation
and irrigation . Outlet work for power generation is
termed as penstock and that as a means for releasing
water to meet the demand of irrigation water
downstream is called irrigation outlet.
• The outlets are placed sufficiently below minimum
reservoir level to provide necessary head for effective
flows. Occasionally the outlet may be placed at a
higher level to deliver water to a canal. In a
multipurpose reservoirs, the irrigation outlets are
operated rarely, the release being usually made
through penstock to serve for power generation as
well as to meet irrigation demands.

Classification of Outlet Works
• Outlet works are classified according to their purpose,
their physical and structural arrangement, or their
hydraulic operation, as follows
• Classification according to their purpose
(i) River Outlet
• An outlet work which implies directly into a river
(ii) Canal outlet
• An outlet which discharges into a canal
(iii) Pressure Pipe Outlet
• An outlet which delivers water into closed pipe system

• Classification according to their physical and
structural arrangements.
Open Channel
• It consists of a conventional open flume or
rectangular channel with a gate similar to
that of a spillway. Open Channel outlet work
is suitable for a low dam from release of
water to a canal or to the water.

Cut & Cover conduict
• Cut and cover conduit outlet is suitable where
the water way is too small to justify a minimum
size tunnel or where the foundation conditions
are not suitable for a tunnel.
Tunnel
• It is closed conduit waterway. It is suitable for
high earth fill dams, where open channel outlets
cannot be provided. It may be carried through,
under or around the dam as a cut-and –cover
conduit or as a tunnel through dam abutment.

• Classification according to their hydraulic Operation
Gated Outlet
• It is outlet provided with operating gates and
regulating gates to control and regulate flow. The
gates may be located at the upstream end of the
conduit, at an intermediate point along its length or at
the downstream end of the structure.
Ungated Outlet
• Ungated Outlet flow in the conduit, as in the case of
detention dams, is similar to that in a culvert spillway.

References
• Modi P.N. (2011), “Irrigation water resources and water power
engineering”, Standard Book House
• Garg S.K. (2010), “Irrigation Engineering and Hydraulic
Structures”, Khanna Publishers
• Internet Websites

Spillways and Flood Control Works

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