Sardar Sarovar Dam (SSD), Gujarat, India


Sardar Sarovar Dam (SSD), on the Indian Narmada river, is located in the village of Kevadia in the state of Gujarat. It is one of the largest and most controversial interstate, multipurpose river valley infrastructure development projects in the country. The Sardar Sarovar Project (SSP) also consists of auxiliary works and a 1,450MW power complex.

SSP was estimated to have cost INR400bn ($8bn) in 2010-2011, revised from the initial estimate of INR64bn ($1.25bn) in 1988. It is part of the Narmada Valley Development Project, a major plan to generate power and supply water for drinking and irrigation to states of Gujarat, Madhya Pradesh and Maharashtra.

The scheme was conceived by the late Sardar Vallabhbhai Patel in 1946-1947. It envisages the construction of 30 major dams, 135 medium and 3,000 smaller dams along the river, with SSD being the largest of them all. They are expected to generate about 4,000MW of power in total.

 

Height of the Sardar Sarovar Dam in Gujarat was increased to 121.9m in 2006.

 

Sardar Sarovar Dam (SSD) specifications and capacities:

The SSD is a 1,210m long concrete gravity dam with a proposed final height of 163m above the deep foundation. Its present height is 121.9m. Its construction required pouring of about seven million cubic metres of chilled concrete. The Sardar Sarovar reservoir, built for the main dam, has 0.95 million hectare metre (M.Ha.m) of gross storage capacity and 0.586M.Ha.m of live storage capacity.

It occupies an area of 37,000ha with an average length of 214km and width of 1.7km. The river catchment area above the dam site is 88,000 square kilometres. It has a spillway discharging capacity of 87,000 cubic metres a second.

The dam and the reservoir periphery have been installed with advanced seismological instruments for calculating the stresses. Seven chute spillway radial gates and 23 service spillway gates are installed for flood control. The full reservoir level of the SSD is 138.6m, the maximum water level is 140.2m and minimum draw down level is 110.6m. The tail water level is about 25.9m.

 

The Narmada Main Canal is the largest irrigation lined canal in the world, supplying water at a capacity of 1,133 cubic metres a second. Image courtesy of Nvvchar.

The Sardar Sarovar Dam and associated water infrastructure is expected to supply water for irrigation of 1.84 million hectares in Gujarat. Image courtesy of Nvvchar.

 

Purpose of the SSD as part of the Narmada Valley Development Project:

Narmada is the fifth largest river in India, traversing 1,312km from the Amarkantak range in Madhya Pradesh to drain into the Arabian Sea at the Gulf of Cambay. The river has a basin area of 97,410 square kilometres.

The SSP is expected to supply water for irrigation of 1.84 million ha of land across 15 districts and 73 suburbs including drought prone regions in Gujarat, as well as two districts of Rajasthan. It is also expected to supply drinking water to 29 million inhabitants across 131 towns and 9,633 villages in the state.

It will supply water to wildlife sanctuaries and industries, as well as secure the needs of Gujarat’s expected population of 40 million, projected by 2021.

Significance of the SSP for Gujarat and Narmada Main Canal (NMC) details:

A state-wide drinking water grid is expected to supply for about 75% of the Gujarat population. Gujarat Water Infrastructure and the Gujarat Water Supply and Sewage Board (GWSSB) are executing this Narmada Master Plan.

The 66,000km network of conveyance and distribution system includes the Narmada Main Canal (NMC), about 2,500km of branch canals, 5,500km of distributaries and other associated channels. The 458.3km long, 1,133 cubic metres a second capacity NMC in Gujarat is the largest irrigation lined canal in the world. It further extends by 74km in Rajasthan. It also has 38 off-taking branch canals being built in phases and is scheduled for completion by 2025.

Construction history and opposition protests against the Narmada dam:

Plans for the dam projects in the Narmada basin were initiated in 1946. The SSP project has been under constant criticism since its early stages. Its construction was halted several times due to court stay orders and oppositions.

Works could not start due to water sharing disputes among the concerned Maharashtra, Rajasthan, Gujarat and Madhya Pradesh states. The Narmada Water Dispute Tribunal (NWDT) was formed in October 1969 to resolve the issues and allocate the water shares. NWDT reached an agreement in 1979 and construction of the dam was started in April 1987. The construction was backed by funds from the World Bank.

A group of local people opposed the project in 1989 under the Narmada Bachao Andolan (NBA, or Save the Narmada Movement). The World Bank withdrew the funding in 1993 following immense protests, resulting in delays to the project.

Construction was halted as per the Supreme Court’s stay in May 1995. It resumed in February 1999 and the dam’s height was increased from the planned 80m to 88m. The Court allowed the increase of the dam’s height to 90m in October 2000, but ordered adherence to the rehabilitation and resettlement issues. The dam’s height was increased to 110.6m with the approval from Narmada Control Authority, in June 2004. It was further raised to its current 121.9m height in December 2006.

NBA claims the government has overlooked the environmental impact, resettlement of 320,000 people and exaggerated project benefits. About one million people are estimated to be affected by the river canal and associated works. Several other local groups across the states concerned also opposed the SSP.

Power generation capabilities and distribution of SSD:

The SSP has two hydropower generating units. The 1,200MW underground river bed power house (RBPH) station has six, 200MW units of reversible Francis type turbines, supplied by Sumitomo and BHEL.

The 250MW surface canal head power house (CHPH) consists of five, 50MW Kaplan turbines. The CHPH power units were commissioned by December 2004 and RBPH by November 2006. The power stations are connected to a Gas Insulated Switch Gear and bus bars switchyard complex in RBPH. Electricity is distributed to Gujarat (16%), Madhya Pradesh (57%) and Maharashtra (27%) through a 400kV power transmission line.

Key players involved with the controversial Sardar Sarovar Dam project:

Sardar Sarovar Narmada Nigam (SSNL), a state-owned company, is responsible for implementing and managing the SSP. Jaiprakash Associates was the engineering, procurement and construction contractor for the dam and power house. Gujarat State Electricity Company operates and maintains the power complex.

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Spillway Definition – Types of Spillway – Classification of Spillway


Definition:

Spillways are structures constructed to provide safe release of flood waters from a dam to a downstream are, normally the river on which the dam has been constructed.

Every reservoir has a certain capacity to store water. If the reservoir is full and flood waters enter the same, the reservoir level will go up and may eventually result in over-topping of the dam. To avoid this situation, the flood has to be passed to the downstream and this is done by providing a spillway which draws water from the top of the reservoir. A spillway can be a part of the dam or separate from it.

Spillways can be controlled or uncontrolled. A controlled spillway is provided with gates which can be raised or lowered. Controlled spillways have certain advantages as will be clear from the discussion that follows. When a reservoir is full, its water level will be the same as the crest level of the spillway.

This is the normal reservoir level. If a flood enters the reservoir at this time, the water level will start going up and simultaneously water will start flowing out through the spillway. The rise in water level in the reservoir will continue for some time and so will the discharge over the spillway. After reaching a maximum, the reservoir level will come down and eventually come back to the normal reservoir level.

Spillway Design Spreadsheet

The top of the dam will have to be higher than the maximum reservoir level corresponding to the design flood for the spillway, while the effective storage available is only up to the normal reservoir level. The storage available between the maximum reservoir level and the normal reservoir level is called the surcharge storage and is only a temporary storage in uncontrolled spillways. Thus for a given height of the dam, part of the storage – the surcharge storage is not being utilized. In a controlled spillway, water can be stored even above the spillway crest level by keeping the gates closed. The gates can be opened when a flood has to be passed.

Parameters considered in Designing Spillways

Thus controlled spillways allow more storage for the same height of the dam. Many parameters need consideration in designing a spillway. These include:

  1. The inflow design flood hydro-graph
  2. The type of spillway to be provided and its capacity
  3. The hydraulic and structural design of various components and
  4. The energy dissipation downstream of the spillway.

The topography, hydrology, hydraulics, geology and economic considerations all have a bearing on these decisions. For a given inflow flood hydro graph, the maximum rise in the reservoir level depends on the discharge characteristics of the spillway crest and its size and can be obtained by flood routing. Trial with different sizes can then help in getting the optimum combination.

Types of Spillways – Classification of Spillways

There are different types of spillways that can be provided depending on the suitability of site and other parameters. Generally a spillway consists of a control structure, a conveyance channel and a terminal structure, but the former two may be combined in the same for certain types. The more common types are briefly described below.

Ogee Spillway

The Ogee spillway is generally provided in rigid dams and forms a part of the main dam itself if sufficient length is available. The crest of the spillway is shaped to conform to the lower nappe of a water sheet flowing over an aerated sharp crested weir.

Chute (Trough) Spillway

In this type of spillway, the water, after flowing over a short crest or other kind of control structure, is carried by an open channel (called the “chute” or “trough”) to the downstream side of the river. The control structure is generally normal to the conveyance channel. The channel is constructed in excavation with stable side slopes and invariably lined. The flow through the channel is super-critical. The spillway can be provided close to the dam or at a suitable saddle away from the dam where site conditions permit.

Side Channel Spillway

Side channel spillways are located just upstream and to the side of the dam. The water after flowing over a crest enters a side channel which is nearly parallel to the crest. This is then carried by a chute to the downstream side. Sometimes a tunnel may be used instead of a chute.

Shaft (Morning Glory or Glory hole) Spillway

This type of spillway utilizes a crest circular in plan, the flow over which is carried by a vertical or sloping tunnel on to a horizontal tunnel nearly at the stream bed level and eventually to the downstream side. The diversion tunnels constructed during the dam construction can be used as the horizontal conduit in many cases.

Siphon Spillway

As the name indicates, this spillway works on the principle of a siphon. A hood provided over a conventional spillway forms a conduit. With the rise in reservoir level water starts flowing over the crest as in an “ogee” spillway. The flowing water however, entrains air and once all the air in the crest area is removed, siphon action starts. Under this condition, the discharge takes place at a much larger head. The spillway thus has a larger discharging capacity. The inlet end of the hood is generally kept below the reservoir level to prevent floating debris from entering the conduit. This may cause the reservoir to be drawn down below the normal level before the siphon action breaks and therefore arrangement for de-priming the siphon at the normal reservoir level is provided.

Types of dams: Introduction and classification


 A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir on its upstream side for impounding water for various purposes. These purposes may be Irrigation, Hydro-power, Water-supply, Flood Control, Navigation, Fishing and Recreation. Dams may be built to meet the one of the above purposes or they may be constructed fulfilling more than one. As such, it can be classified as: Single-purpose and Multipurpose Dam.

Different parts & terminologies of Dams:

  • Crest: The top of the dam structure. These may in some cases be used for providing a roadway or walkway over the dam.
  • Parapet walls: Low Protective walls on either side of the roadway or walkway on the crest.
  • Heel: Portion of structure in contact with ground or river-bed at upstream side.
  • Toe: Portion of structure in contact with ground or river-bed at downstream side.
  • Spillway: It is the arrangement made (kind of passage) near the top of structure for the passage of surplus/ excessive water from the reservoir.
  • Abutments: The valley slopes on either side of the dam wall to which the left & right end of dam are fixed to.
  • Gallery: Level or gently sloping tunnel like passage (small room like space) at transverse or longitudinal within the dam with drain on floor for seepage water. These are generally provided for having space for drilling grout holes and drainage holes. These may also be used to accommodate the instrumentation for studying the performance of dam.
  • Sluice way: Opening in the structure near the base, provided to clear the silt accumulation in the reservoir.

Illustration of dam-parts in a typical cross section

  • Free board: The space between the highest level of water in the reservoir and the top of the structure.
  • Dead Storage level: Level of permanent storage below which the water will not be withdrawn.
  • Diversion Tunnel: Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The hydraulic structures are built while the river flows through the diversion tunnel.

CLASSIFICATION OF DAMS

Dams can be classified in number of ways. But most usual ways of classification i.e. types of dams are mentioned below:
Based on the functions of dams, it can be classified as follows:
  1. Storage dams: They are constructed to store water during the rainy season when there is a large flow in the river. Many small dams impound the spring runoff for later use in dry summers. Storage dams may also provide a water supply, or improved habitat for fish and wildlife. They may store water for hydroelectric power generation, irrigation or for a flood control project. Storage dams are the most common type of dams and in general the dam means a storage dam unless qualified otherwise.
  1. Diversion dams: A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit). They provide sufficient pressure for pushing water into ditches, canals, or other conveyance systems. Such shorter dams are used for irrigation, and for diversion from a stream to a distant storage reservoir. It is usually of low height and has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir which also diverts water and has a small storage. Sometimes, the terms weirs and diversion dams are used synonymously.
  2. Detention dams: Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water. Thus the effect of sudden floods is reduced to some extent. 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. Thus the area downstream of the dam is protected against flood.
  3. Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water. The water after passing over a debris dam is relatively clear.
  4. Coffer dams: It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A coffer dam is thus a temporary dam constructed for facilitating construction. These structure are usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam. When the flow in the river during construction of hydraulic structures is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.
Based on structure and design, dams can be classified as follows:
  1. Gravity Dams: A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.

Gravity dams are well suited forblocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation ofbedrock.
Examples of Gravity dam: Grand Coulee Dam (USA), Nagarjuna Sagar (India) and Itaipu  Dam (It lies Between Brazil and Paraguay and is the largest in the world).

  • Earth Dams: An earth dam is made of earth (or soil) built up by compacting successive layersof earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides. A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam. Earth dam resists the forces exerted upon it mainly due to shear strength of the soil. Although the weight of the this structure also helps in resisting the forces, the structural behavior of an earth dam is entirely different from that of a gravity dam. The earth dams are usually built in wide valleys having flat slopes at flanks (abutments).The foundation requirements are less stringent than those of gravity dams, and hence they can be built at the sites where the foundations are less strong. They can be built on all types of foundations. However, the height of the dam will depend upon the strength of the foundation material.
    Examples of earthfill dam: Rongunsky dam (Russia) and New Cornelia Dam (USA).
  • Rockfill Dams: A rockfill dam is built of rock fragments and boulders of large size. An impervious membrane is placed on the rockfill on the upstream side to reduce the seepage through the dam. The membrane is usually made of cement concrete or asphaltic concrete.

    In early rockfill dams, steel and timber membrane were also used, but now they are obsolete. A dry rubble cushion is placed between the rockfill and the membrane for the distribution of water load and for providing a support to the membrane. Sometimes, the rockfill dams have an impervious earth core in the middle to check the seepage instead of an impervious upstream membrane. The earth core is placed against a dumped rockfill. It is necessary to provide adequate filters between the earth core and the rockfill on the upstream and downstream sides of the core so that the soil particles are not carried by water and piping does not occur. The side slopes of rockfill are usually kept equal to the angle of repose of rock, which is usually taken as 1.4:1 (or 1.3:1). Rockfill dams require foundation stronger than those for earth dams.
    Examples of rockfill dam: Mica Dam (Canada) and Chicoasen Dam (Mexico).

  • Arch Dams: An arch dam is curved in plan, with its convexity towards the upstream side. They transfers the water pressure and other forces mainly to the abutments by arch action.An arch dam is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action.The section of an arch dam is approximately triangular like a gravity dam but the section is comparatively thinner. The arch dam may have a single curvature or double curvature in the vertical plane. Generally, the arch dams of double curvature are more economical and are used in practice.
    Examples of Arch dam: Hoover Dam (USA) and Idukki Dam (India).
  • Buttress Dams: Buttress dams are of three types : (i) Deck type, (ii) Multiple-arch type, and (iii) Massive-head type. A deck type buttress dam consists of a sloping deck supported by buttresses. Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members. Buttresses are typically spaced across the dam site every 6 to 30 metre, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced.In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete. In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses. The buttress dams require less concrete than gravity dams. But they are not necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement and more skilled labor. The foundation requirements of a buttress are usually less stringent than those in a gravity dam.
    Examples of Buttress type: Bartlett dam (USA) and The Daniel-Johnson Dam (Canada).
  • Steel Dams: Dams: A steel dam consists of a steel framework, with a steel skin plate on its upstream face. Steel dams are generally of two types: (i) Direct-strutted, and (ii) Cantilever type . In direct strutted steel dams, the water pressure is transmitted directly to the foundation through inclined struts. In a cantilever type steel dam, there is a bent supporting the upper part of the deck, which is formed into a cantilever truss. This arrangement introduces a tensile force in the deck girder which can be taken care of by anchoring it into the foundation at the upstream toe. Hovey suggested that tension at the upstream toe may be reduced by flattening the slopes of the lower struts in the bent. However, it would require heavier sections for struts. Another alternative to reduce tension is to frame together the entire bent rigidly so that the moment due to the weight of the water on the lower part of the deck is utilised to offset the moment induced in the cantilever. This arrangement would, however, require bracing and this will increase the cost. These are quite costly and are subjected to corrosion. These dams are almost obsolete. Steel dams are sometimes used as temporary coffer dams during the construction of the permanent one. Steel coffer dams are supplemented with timber or earthfill on the inner side to make them water tight. The area between the coffer dams is dewatered so that the construction may be done in dry for the permanent dam.
    Examples of Steel type: Redridge Steel Dam (USA) and Ashfork-Bainbridge Steel Dam (USA).
  • Timber Dams: Main load-carrying structural elements of timber dam are made of wood, primarily coniferous varieties such as pine and fir. Timber dams are made for small heads (2-4 m or, rarely, 4-8 m) and usually have sluices; according to the design of the apron they are divided into pile, crib, pile-crib, and buttressed dams.The openings of timber dams are restricted by abutments; where the sluice is very long it is divided into several openings by intermediate supports: piers, buttresses, and posts. The openings are covered by wooden shields, usually several in a row one above the other. Simple hoists—permanent or mobile winches—are used to raise and lower the shields.
  • Rubber Dams: A symbol of sophistication and simple and efficient design, this most recent type of dam uses huge cylindrical shells made of special synthetic rubber and inflated by either compressed air or pressurized water. Rubber dams offer ease of construction, operation and decommissioning in tight schedules.
  • These can be deflated when pressure is released and hence, even the crest level can be controlled to some extent. Surplus waters would simply overflow the inflated shell. They need extreme care in design and erection and are limited to small projects.
    Example of Rubber type: Janjhavathi Rubber Dam (India).