Dams Definition, Classification, Factors Affecting Selection of Type of Dam Selection & Investigation of Site for a Dam

12-07-2016

Dams Definition, classification, factors affecting selection of type of dam Selection & investigation of site for a dam

Earth dams: Types, foundation requirements, causes of failure, design criteria, suitable preliminary sections, seepage line determination & analysis, stability analysis - side slope, foundation, checking of stability, seepage control measures

Rockfill dams: Foundation requirements, typical sections, design consideration for a rockfill dam

Gravity dam: Definition, forces acting on dams, load combination for design, modes of failure & criteria for structural stability, stability analysis, elementary & practical profile, design of gravity dam, openings in dams 1

Definition :

Dam is a solid barrier constructed at a suitable location across a river valley to store flowing water.

• Storage of water is utilized for following objectives: • Hydropower • Irrigation • Water for domestic consumption • Drought and flood control • For navigational facilities • Other additional utilization is to develop fisheries

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Structure of Dam

Crest Upstream Down stream Spillway(inside dam) MWL Maximum water level

NWL Normal water level

Free board Sluice way

Gallery

Heel Toe

• Heel: contact with the ground on the upstream side

• Toe: contact with the ground on the downstream side

• Abutment: Sides of the valley on which the structure of the dam rest

• Galleries: small rooms like structure left within the dam for checking operations.

• Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.

• Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side

• Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side. 4

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Classification of Dams

1. Based on Use

2. Based on Design

3. Based on Materials

4. Based On Structural Behaviour

5. Based On Size : Hydraulic Head And Gross Storage In The Reservoir

Classification of Dams

1. Based on Use • Storage dam : Gravity dam, Earth dam, Rock fill dam, Arch dam • Detention dam • Diversion dam : Weir, Barrage • Coffer dam • Derbies dam

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1. Storage dam: • Most common type. • Store water during the rainy season when there is a large flow in the river to be utilized later during period when flow in the river is less than the demand.

2. Detention dam: • Constructed for flood control. • Retards the flow in the river on its downstream during floods by storing some flood water. • The water retained in the reservoir is later released gradually at a controlled rate according to the carrying capacity of the channel downstream of the detention dam.

3. Diversion dam: • Diverts water of the river into an off-taking canal (or a conduit). • Usually of low height and has a small storage reservoir on its upstream.

4. Coffer dam: • Constructed around construction site to exclude water so construction can be done in dry. • Usually constructed on upstream of main dam to divert water into a diversion tunnel during construction of the dam.

5. Debris dam: • Constructed to retain debris such as sand, gravel, reservoir buttress dam drift wood • The water after passing over a debris dam is relatively clear. 7

Classification of Dams

2. Based on Design • Overflow dam : Spillway • Non overflow dam : Gravity dam, Earth dam, Rock fill dam

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1. OVERFLOW DAM • Crest to permit overflow of surplus water that cannot be retained in reservoir. • Generally dams are not designed as overflow dams for its entire length.

2. NON-OVERFLOW DAM • Water is not allowed to overflow over its crest.

• In most cases, dams are so designed that part of its length is designed as an overflow dam (spillway) while the rest of its length is designed as a non-overflow dam. • In some cases, these two sections are not combined. 9

Classification of Dams

3. Based on Materials • Rigid dam : Gravity dam, Arch dam, Steel dam, Timber dam • Non rigid dam : Earth dam, Rock fill dam

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1. RIGID DAM • Constructed with rigid material such as stone, masonry, concrete, steel, timber. • Steel dams and timber dams are rarely constructed only for small heights.

2. NON-RIGID DAM (EMBANKMENT DAMS) • Constructed with non-rigid material such as earth, tailings, rock fill etc… • Earthen dams are provided with a stone masonry or concrete overflow (spillway) section. Such dams are called composite dams. • In some cases, part of the length of the dam is constructed as earth dam and the rest (excluding the spillway) as a masonry dam. Such dams are called masonry cum earthen dams.

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Classification of Dams

4. Based On Structural Behaviour • Gravity Dam • Arch Dam • Buttress Dam • Embankment Dam

1. GRAVITY DAM

Masonry/Concrete dam which resists forces acting on it by own weight.

• Straight gravity dam – A gravity dam that is straight in plan. • Curved gravity dam – Resists forces acting on it by combined gravity (own weight) and arch action. • Solid gravity dam – consists of a solid mass of masonry/concrete • Hollow gravity dam – Has hollow spaces within.

 Most gravity dams are straight solid gravity dams.

Bhakra Dam : Highest Concrete Gravity dam in Asia and Second Highest in the world.

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2. ARCH DAM • Curved masonry/concrete dam, convex upstream, • Resists the forces by arch action. • Arch shape gives strength, Less material (cheaper), Narrow sites • Shape helps to transmit the major part of water load to abutments • Need strong abutments • Built across narrow, deep river gorges • Recent years considered even for wider valleys. • Good for rocky locations. • Only arch dam in India – Idukki dam

3. BUTTRESS DAM • Buttresses : equally spaced triangular masonry or RCC walls that retains water. • Consists sloping membrane (deck) on u/s supported by series of buttresses. • Structural behaviour is similar to that of a gravity dam. • Can be considered even if the foundation rocks are little weaker.

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4. EMBANKMENT DAM • Non-rigid dam which resists the forces acting on it by its shear strength and to some extent also by its own weight (gravity). • Structural behaviour is in many ways different from that of a gravity dam, earth dam.

5. Earth Dams •They are trapezoidal in shape. •Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth. •Earthen dams are relatively smaller in height and broad at the base. •They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam.

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Classification of Dams

5. Based On Size : Hydraulic Head And Gross Storage In The Reservoir

Sr. No. Classification Gross Storage Hydraulic Head (MCM) (m)

1 Small 0.50 – 10.00 7.5 – 12.0

2 Intermediate 10.00 – 60.00 12.0 – 30.0

3 Large > 60.00 > 30

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Classification of Dams 1. Based on Use • Storage dam : Gravity dam, Earth dam, Rock fill dam, Arch dam • Diversion dam : Weir, Barrage • Coffer dam • Derbies dam 2. Based on Design • Overflow dam : Spillway • Non overflow dam : Gravity dam, Earth dam, Rock fill dam 3. Based on Materials • Rigid dam : Gravity dam, earth dam, Rock fill dam, arch dam, steel dam, timber dam • Non rigid dam : Earth dam, Rock fill dam

4. Based On Structural Behaviour • Gravity Dam • Arch Dam • Buttress Dam • Embankment Dam

5. Based On Size : Hydraulic Head And Gross Storage In The Reservoir 21

SITE SELECTION FOR A DAM ƒ

• Dam is a huge structure requiring a lot of funds.

• ƒExtreme care shall be taken while selecting the site of a dam.

• ƒA wrong decision may lead to excessive cost

• Difficulties in construction and maintenance. ƒ

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Good Site for dam • Large storage capacity • Shape of reservoir basin • Water tightness of the reservoir • Good hydrological conditions • Deep reservoir • Small submerged area • Low silt inflow • No objectionable minerals • Spillway site • Availability of materials • Accessibility • Healthy surroundings • Minimum overall cost • Other considerations • Topography 23 • Suitable Foundation

Factors affecting selection of type of dam • Requires a lot of judgment and experience. • ƒTopography and valley shape • ƒGeology and foundation conditions • ƒAvailability of construction materials • ƒOverall cost ƒSpillway size and location • ƒEarthquake hazards • ƒClimatic conditions • ƒDiversion problems ƒ • Environmental considerations ƒ • Roadway ƒ • Length and height of dam ƒ • Life of dam ƒ • Miscellaneous considerations • Preliminary designs and estimates are usually required for several types of dams before making the final selection on economic basis.24

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Dams Definition, classification, factors affecting selection of type of dam Selection & investigation of site for a dam

Rockfill dams: Foundation requirements, typical sections, design consideration for a rockfill dam

Sources /Further reading : • 1.Modi, P.N., Irrigation Water Resources and Water Power Engineering, Standard Book House, New Delhi.

• 2. Garg, S.K., Irrigation Engineering and Hydraulic Structures, Khanna Publishers, New Delhi.

• 3. Sharma, R.K., Text book of Irrigation Engineering and Hydraulic Structures, Oxford and IBK Publishing House, New Delhi. • http://www.nptel.ac.in http://theconstructor.org/geotechnical/stability-of-slopes/2632/ http://ecoursesonline.iasri.res.in/mod/page/view.php?id=2194 26