DAMS AND BARRAGES

Irrigation projects

Requirement :

Source of water – River (Surface water), Underground water

Source of water – River (Surface water), Underground water

Cultivable area – Area suitable for irrigation

Good Agriculture practice

Cultivable area – Area suitable for cultivation

Area sown – Area where seeds are placed

Command area – Area suitable for cultivation and receive water

Irrigable area – Area irrigated receiving water

Water is a natural resource and precipitation flow along natural contours

Water is available through surface flows – Rivers and Tanks

Through ground water resources – wells, springs

Civilization started first near water source – that is man settled near water source

The Mohenjodaro is the city built 5000 years ago.

The city was destroyed and rebuilt not less than seven times.

Indus Valley Civilization is first known one where man constructed cities

Indus Valley Civilization is similar to Nile Valley Civilization, Tigris Civilization.

Recent excavations show the city is fairly big

Dwelling houses are many in number

The construction is with brick

Big houses have two or more storyies

There are big structures containing large pillered halls

Streets of the city are wide and straight

Nagarjuna Konda where civilization survived near Krishna river

Civilization extended to distant places far away from rivers

Man through his ingenuity able to bring water by gravity to even parched deserts by constructing diversion structures or storage reservoirs

Now it is also possible to lift waters to a greater heights by lift schemes and by canal system to the higher fields

Harnessing of surface water for irrigation / water supplyRivers are of two categoriesi. Perennial Riversii. Non Perennial RiversPerennial river – water flows throughout the year – Ganga, Brahmaputra (North Indian Rivers)Non Perennial river – water flows only for few months – Godavari, Krishna (South Indian Rivers)Irrigation works are classified asi. Diversion works – Weir, Barrageii. Storage work – Dams

In diversion work, on obstruction called Weir or an anicut is constructed across the river to raise the bed.

Before Obstruction

After Obstruction

This rise in bed results raise in water level.

With increased water level, waters are diverted

There will be no storage and function only when there is water in level

Sometimes shutters will be provided to store the water

Shutters in raised position Shutters in fallen position

This rise in bed results raise in water level.

With increased water level, waters are diverted

There will be no storage and function only when there is water in level

Sometimes shutters will be provided to store the water

The elements of diversion work are

i. Body wall

ii. Head sluice

iii. Scour sluice

iv. Canals and Distributory System

A Diversion work

Scour

Sluice

Anicut

Dowlaiswaram Anicut Constructed by Sir Arthur Cotton (1852)

Sir Arthur Cotton Barrage (1983)(Dowlaiswaram Anicut is replaced by Barrage)

Storage work – Solid Barrier of considerable height across the river at suitable location – Dam

A reservoir (big tank) is formed in the river valley

Components of storage work

A non over flow section

A spill way to dispose off flood water

Head sluices on either side to tap water from reservoir

River sluices to dispose off early floods canal system

Components of Storage Work

Types of DamsRigid dams and Non Rigid dams

Rigid dams are rigid like rock – constructed with masonry, concrete, steel etc.

Example : Concrete Dam, Masonry Dam, Timber Dam and Steel Dam

Non Rigid Dams – constructed with soft materials

Earth Dam, Rock Fill dam

Classified as per material used -

Example : Concrete Dam constructed with concrete

Masonry Dam constructed with masonry

Likewise Timber Dam and Steel Dam

Earthen dam with soil

Rockfill dam with rocks

Anicut

Power House

Concrete dam

Concrete Dams constructed with concrete

Masonry Dams constructed with masonry

Likewise Timber and Steel Dam

Timber DamSteel Dam

Rubber Dam

Earthen dam

Rockfill dam

Dams are classified as how loads are resisted

1. Gravity Dam

2. Arch Dam

3. Butress Dam

Classification of Dams Gravity Dam – External forces are resisted by weight of the dam

N.S. Dam (1957-74)(Reservoir of 408.24 TMC)

Arch Dam – External forces are resisted by the arch action

Arch Dam

Buttress Dam – External forces are resisted by multiple arches built in between the piers

Buttress Dam – Himayat Sagar, Hyderabad

Forces on Dam

Water thrust

Weight of the Dam

Wave Action

Up lift forces

Buoyancy

Earthquake forces

Masonry and concrete dams constructed

Forces are resisted by weight

Earth Dam – is an Engineering structure

• Designed and built with available soils

• Earth Dam with one type of soil – Homogenous section

Earth Dam with different types of material – Zonal section or composite section

Casing soils – Gravelly soils

Gravelly clays (G.C), Sandy clays (C.S) and Silty sand (S.M)

Hearting soils – Clayey soils

Clay of high plasticity (C.H), Clay of low plasticity (C.L), Clay of intermediate plasticity (C.I)

Earth Dam is economical – as materials are available at site

Foundations are simple – can be constructed almost on the ground

Only cut off trench have to be keyed to impervious zone, if impervious zone is deeper, a sheet pile or a slurry wall be taken and sometimes curtain grouting can be taken up

Earth Dam consists of

Main section

Cut off trench

Up stream rivetment and down stream turfing

Filters – Rock Toe – Toe drain

Spill way have to be with Rigid material

Earth Dam settlement – 10% due to shrinkage

Rock Fill Dam

Rock Fill Dam consists of boulders

Sluicing by sand to fill up the voids

The impervious membrane of the upstream face

Rock fill dam settles by 20 – 25%

Normally the construction of dams will be in the river with flowing water

Area has to be cleared i.e. water has to be diverted so that, excavation can be taken up and bed is exposed for work

Water will be diverted and area of work has to be dry

The temporary structure to divert the water is called Cofferdam

Sometimes, wells will be sunk, upstream and downstream to divert the water

Sometimes, a tunnel will be constructed to divert the water. It is called a diversion tunnel.

Construction of Rigid Dams

Area is cleaned and over burden removed

Area is divided into blocks

Blasting will be taken till sheet rock or approved strata is reached

Grouting the area – Consolidated Grouting – Pressure Grouting

Concrete is laid in layers and blocks

Block 1 Block 2 Block 3

Width of foundation

Construction of Earth Dams

Area is cleaned by removing trees and shrubs

The top soil to be scarified to 15 cm depth

Cut off trench excavated

Impervious soils are placed with optimum moisture content and consolidated in the cut off trench

Layers should not be more than 30 cm

Barrow Area

Barrow area to be identified so that specified soils are available

Close to be dam site

Top surface to be scrapped

Barrow pits not more than 25 m x 10 m x 11/2m

Should be away from dam by 10 times height of the upstream and 2 time height from down stream

Consolidated by roller till Proctors Density of 100 is achieved

After filling cut off trench main earth dam section will be taken

Filters are provided as and when the dam is raised

Dam have to be constructed with extra height of 10% or specified to take care of shrinkage

Construction aspects of Dams

i. Whenever a dam has to be constructed, the approach roads connecting to the nearest town have to be formed so that the site is approachable for materials and men comfortably for the work.

ii. On the down stream side of the proposed site, a bridge will also be constructed to have the connectivity between the banks.

iii. Frequent inspections are required. A guest house and quarters for the staff have to be constructed.

iv. The area where the dam will be constructed have to be marked.

v. Vegetation has to be removed

vi. The line indicating the width of the dam at the foundation level has to be marked

vii. For work convenience, the area will be divided into blocks. Since the excavation has to be carried out to the required depths where good rock is available. This requires blasting operations.

viii. For good progress, earth work excavation, blasting and concreting have to be in a phased manner. Initially, three blocks will be taken and excavation in the first block will be started and after completing the excavation, the machinery will be subjected to next block.

ix. Now the blasting operations which require to reach the foundation level will be taken. The blasting will be started in the first and second block and the debris will be deposited far away without creating any hindrance for the future works

x. Then the blasting operations will be continued to the next blocks

xi. In the first block, grouting operations will be started both consolidation and curtain grouting and the area will be made ready for laying the concrete

xii. It has to be ensured that before laying the concrete, there should not be any blasting operations at the adjacent two blocks that is when concreting has to be started in the first block, the blasting operations in the second and third blocks have to be completed. Similarly, when concrete operations have to be started in second block, the blasting operations for the third and fourth blocks have to be completed. The blasting in no way should disturb the already laid young concrete.

xiii. In this sequence, earthen excavation, blasting of the rock, concreting of the dam will be proceeded.

xiv. If the river is very big, the operation will be started from both sides.

xv. Once the concreting operation is started, it is only the concrete

which can go uninterruptedly.

xvi. Curing arrangements has to be made for the concrete and all precautions for concreting have to be taken up

xvii.During floods, the work will be put to stop and all precautions has to be taken that there should not be any damage to the concrete already laid and the concrete that is already laid is not disturbed by

the flood water.

xviii.All machinery have to be shifted to safe place well before the floods

xix. Any protection works required for the main work of dam has to be planned and completed before the on set of floods.