Spillways2000 101110063804-phpapp02 - copy

Types of Types of spillwaysspillways Presented by: Bakht Presented by: Bakht

ZaminZamin

CU-PC1-2013CU-PC1-2013

SUBJECT: DAM ENGINEERINGSUBJECT: DAM ENGINEERING

Dated: 21-12-2013

IntroductionIntroduction

A spillway is an integral part of a Dam (large wall built to

hold back water usually across rivers). A spillway is usually

used to remove excessive water from a reservoir to

prevent overflow and to release pressure on a dam from

increasing quantities of water. It is generally composed of

four major components:

Control Structure

Discharge Channel

Terminal Structure

Entrance and outlet channels

Different types of spill waysDifferent types of spill ways Spill ways are generally classified as controlled

and uncontrolled spill ways.

Controlled Spillway:

A controlled spillway has mechanical structures or

gates to regulate the rate of flow of water from the

reservoir. This design allows nearly the full height

of the dam to be used for water storage year-

round, and flood waters can be released as

required by opening one or more gates.

Continued……..Continued…….. Uncontrolled Spillway:

An uncontrolled spillway, in contrast, does not have

gates; when the water rises above the lip or crest of

the spillway it begins to be released from the

reservoir. The rate of discharge is controlled only by

the depth of water within the reservoir. All of the

storage volume in the reservoir above the spillway

crest can be used only for the temporary storage of

flood water, and cannot be used as water supply

storage because it is normally empty.

Sub classificationSub classification Each of controlled and uncontrolled spillways are

further classified on the basis of the their shapes,

structures and main working procedure.

Straight drop spillway (free over fall).

Ogee spillway.

Side channel spillway.

Labyrinth spillway.

Chute (open channel) spillway.

Continued……..Continued……..

Conduit and Tunnel spill way.

Baffled chute spill way.

Culvert spill way.

Straight drop spill wayStraight drop spill way

Flow drops freely from the crest of dam.

It may be provided with gates.

Flows may be freely discharging or may be supported

by narrow channel section.

This type of spillways may be economical for low heads

as compared to other types.

Floor blocks and an end sill are provided.

Widely used on Gravity dams, Arch dams, and Buttress

dams.

Not recommended for very high dams.

free fall spill way

Side channel spillwaySide channel spillway

A side channel spillway is one whose control weir is placed

alongside and approximately parallel to the upper portion

of the spillway discharge channel.

Flow may be directed into an open channel, or a closed

conduit or in an inclined tunnel.

It may be used as principal spillway or emergency spillway.

Lining of the side channel spillway with concrete is

required.

If a sufficient crest length is not available for an over flow

spillways in narrow valleys, excess water is removed from

the reservoir through a side channel spillway.

Ogee Spillway Ogee Spillway

The ogee spillway has a control weir that is ogee

shaped (s-shaped)

It is also an overflow type spillway.

Reduce the impact of water at downstream.

To drop off the water at downstream from the

foundation of dam.

Reduce scouring etc.

Used for small concrete dams. Provide recreational impact at downstream side.

Labyrinth spillwayLabyrinth spillway The concept behind labyrinth spillway is to provide added crest for a

given spillway width, so that less head is required to pass a given

discharge.

The additional crest length is obtained by a series of trapezoidal or

rectangular walls.

Labyrinth design is particularly beneficial when spillway width is fixed,

u/s water surface elevation is restricted and large discharge must be

passed.

An excellent alternative to traditional method of adding another spillway.

Labyrinth spillways are more economical than gated structures.

Flow pattern are very complicated.

Ratio of Length of spillway width to the spillway height is an important

parameter responsible for flow pattern.

Different shapes of labyrinth spillways

Chute spillwayChute spillway

A spillway whose discharge is conveyed from the reservoir to

the d/s through an open channel or through a saddle (chute).

Used for earth fill dam.

Factors influencing the selection of chute spillways are

simplicity of their design, and construction, their adoptability

to any foundation condition and overall economy.

Profile of chute spillway is greatly influenced by site

topography.

These types of spillways may be provided with gated orifice,

a side channel crest or other control device etc.

Abrupt slope changes should be avoided.

Rectangular depression Triangular depression

Conduit and tunnel spillwayConduit and tunnel spillway It is a spillway in the form of a closed channel used to convey the

discharge around or under the dam.

The closed channel may be in the form of vertical or inclined shaft,

a horizontal tunnel through earth or rock filled dam.

For free flow in the tunnel, the ratio of the flow area to the total

tunnel area is often limited to about 75%.

Designed for partially full flow throughout the length of tunnel.

Air slots are provided for prevention of cavitations.

Suitable in rockslide and snow slide areas.

Expensive type of spillways.

Inclined tunnel

Baffled chute spillwayBaffled chute spillway

The spillway where water is to be lowered from one level

to another level without a stilling basin.

Similar to chute type spillway with exception of baffled

structure.

The baffle pier obstruct the flow partially, dissipating

energy so that the flow velocity entering the d/s is

lowered.

Slope is normally 2:1.

Economical , smooth operation, and no effect on d/s

condition are the advantages of such types of spillways.