DAM | TYPES OF DAMS | PURPOSES FOR DAM CONSTRUCTION | IMPACTS

 

DAM 

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates, levees and dikes are used to manage or prevent water flow into specific land regions.

DAMS ARE CREATED FOR THE FOLLOWING  OBJECTIVES:

 

HISTORY OF DAMS

The initial dam construction took place in Mesopotamia and the Middle East. Dams were used to control water levels to divert the Tigris and Euphrates rivers to the weather of Mesopotamia, and they are unpredictable. The first known dam was located in Java, Jordan, 100 km northeast of the capital Amman. The ancient Egyptian Saad al-Kafara in Wadi al-Karawi, 25 km south of Cairo, is 102 m long and 87 m wide at its base. The system was built as a diversion dam for fl ood control in 2800 or 2600 BC, but was destroyed by heavy rainfall during or after construction. The Romans were also excellent dam builders with many examples, such as the three dams at Subiaco on the Anio River in Italy. Many large dams live in Merida, Spain.

The world's oldest and tallest dam is believed to be the Quatina Barrage in modern Syria. The dam dates back to the time of the Egyptian pharaoh Seti (1319-1304 BC) and was expanded during the Roman period between 1934 and 1938. This provides more water to the city of Holmes.

Kallanai is a large uncut dam, 300 m long, 4.5 m high and 20 m wide across the mainstream of the Cauvery River in India. The basic structure dates back to the second century AD. The purpose of the dam is to divert the water of the Kaveri for irrigation through canals throughout the fertile delta area.

In Netherlands, dams were often built to regulate water levels and prevent rivers from entering seawater. Such dams often mark the beginning of a town or city because it is easy to cross the river at such a place, and often formed the names of the respective place in Dutch. For example, the Dutch capital Amsterdam began with a dam across the Amstel River in the late twelfth century, with Rotterdam starting with a dam along the Rotte River, a small tributary of the Niue Mass. Amsterdam's Central Square, still believed to be the site of the dam's 800 years old, is still known as Dam Square or simply Dam.

STRUCTURE OF DAMS

 

Heel: contact with the ground on the upstream side

Toe: contact 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 the 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 the 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.

TYPES OF DAMS

Dams can be formed by human agency, natural causes or even by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their size or height and intended purpose or structure.

 

By size

International standards define large dams as higher than 15–20 m and major dams as over 150–250 m in height. The tallest dam in the world is the 300 m high Nurek Dam in Tajikistan.

 

By purpose

Intended purposes include providing water for irrigation to a town or city's water supply, improving navigation, creating reservoirs of water to supply for industrial use, generating hydroelectric power, creating recreation areas or habitats for fish and wildlife, and maintaining monsoon flows. Contain effluent from industrial sites such as mines or factories and minimize downstream flow risks. Few dams serve all of these purposes, but some multipurpose dams offer more than one.

Saddle dams are secondary dams configured to confine the reservoir created by the primary dam to allow for higher water levels and storage, or to limit the extent of the reservoir for increased efficiency. Auxiliary dams are built at low points or saddles from which reservoirs can escape. Sometimes reservoirs are incorporated into similar structures called embankments to prevent flooding of nearby land. Embankments are commonly used for arable land reclamation in shallow lakes. It is similar to an embankment, which is a wall or embankment built along a river or stream, protecting adjacent land from runoff.

The overflow dam is designed to be overtopped. Weirs are a type of small overflow dam often used within river channels to create reservoir lakes for water abstraction purposes and can also be used to measure flow.

Check dams are small dams designed to reduce flow rates and control soil erosion. In contrast, wing dams are structures that only partially confine the waterways, creating faster channels that resist sediment build-up.

Dry dams are dams designed to control flow. It usually allows water channels to flow freely without blocking them.

A diversion dam is a structure designed to divert all or part of a river's flow from its natural path.

By structure

Based on the structure and the material used, dams are classified as timber dams, arch-gravity dams, embankment dams or masonry dams with several subtypes.

 

 

Masonry dams

Masonry dams can be classified into arch dams, gravity dams, embankment dams, rock-fill dams, earth-fill dams, concrete dams, etc.

 

Arch dams

In arch dams, stability is ensured by the combination of the arch and the action of gravity. If the upstream side is vertical, the entire weight of the dam must be transferred to the foundation by gravity, and the normal hydrostatic pressure distribution between the vertical cantilever and the arch action depends on the dam strength in the vertical and horizontal directions. The distribution is more complex when the upstream face is sloped. The normal component of the weight of the arch ring can be taken by the action of the arch, and the normal hydrostatic pressure is distributed as described above. For this type of dam, firm and reliable support in the abutment is more important. The most desirable site for an arch dam is a narrow canyon with steep sidewalls made of sound rock. The safety of an arch dam depends on the strength of the sidewall abutments, so not only must the arch sit well against the sidewall, but the rock's properties must also be carefully examined.

Two types of single-arch dams are used: constant angle and constant radius dams. The constant radius type uses the same face radius at all heights of the dam, so the narrower the channel goes toward the bottom of the dam, the smaller the central angle occupied by the dam face. In constant angle dams, also known as variable radius dams, this angle of confrontation remains constant and the radius is changed to handle changes in the distance between the abutments at various levels. Constant radius dams are much less common than constant angle dams.

A similar type is a double curvature or thin-shell dam. This method minimizes the amount of concrete required for construction but transfers large loads to the foundation and abutments. Its appearance resembles a single arched dam, but has a distinct vertical curvature and, when viewed downstream, provides the ambiguous appearance of a concave lens. Multi-arch dams consist of several single-arch dams with supporting buttresses primarily concrete. Multi-arc dams do not require as many buttresses as the hollow gravity type but do require a good rock foundation as the buttress loads are heavy.

 

Gravity dams

In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided that the moment around the turning point caused by the water pressure is smaller than the moment caused by the weight of the dam. This is the case if the resultant force of water pressure and weight falls within the base of the dam. However, in order to prevent tensile stress at the upstream face and excessive compressive stress at the downstream face, the dam cross-section is usually designed so that the result falls within the middle at all elevations of the cross-section. For this type of dam, impervious foundations with high bearing strength are essential.

When situated on a suitable site, gravity dams can prove to be a better alternative to other types of dams. When built on a carefully studied foundation, the gravity dam probably represents the best developed example of dam building. Since the fear of flood is a strong motivator in many regions, gravity dams are being built in some instances where an arch dam would have been more economical.

Gravity dams are classified as solid or hollow. The solid form is the more widely used of the two, although the hollow dam is frequently more economical to construct. Gravity dams can also be classified as overflow and non-overflow. A gravity dam can be combined with an arch dam, an arch-gravity dam, for areas with massive amounts of water flow but less material available for a purely gravity dam.

 

Embankment dams

Embankment dams are made from compacted earth and are of two main types, rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like the gravity dams made from concrete.

 

Rock-fill dams

The Rock-field Dam is an embankment of the compacted free-draining upright district with an impermeable zone. The earth used often contains a large proportion of large particles, so the term rock-filling is used. The impermeable zone may be on the upstream side and is made of the stone, concrete, plastic membrane, sheet pile, wood or other materials. The impermeable area may also be within the embankment, in which case it is referred to as the core. When clay is used as an impermeable material, the dam is called a composite dam. A filter is used to separate the cores to prevent internal erosion of the clay into the rock due to penetrating forces. A filter is a specific grade of soil designed to prevent the movement of fine grain soil particles. When the right materials are prepared, transportation is minimized and costs are reduced during construction. Rock dams are resistant to earthquake damage. However, inadequate quality control during construction can lead to compaction and sanding of the embankment, which can lead to liquefaction of rocks during earthquakes. The likelihood of similarity can be reduced by preventing the brittle material from saturating and providing adequate compression during construction.

 

Earth-fill dams

Earth-fill dams are constructed as a simple embankment of well-compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a locally plentiful shell with a watertight clay core. Most modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. An interesting type of temporary earth dam occasionally used in high latitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it. Earthen dams can be constructed from materials found on-site or nearby and, hence, they can be very cost effective.

 

Asphalt-concrete core

Another type of embankment dam is built with an asphalt concrete core. Such dams are built with rock and/or gravel as the main fill material. Almost 100 dams of this design have now been built worldwide since the first such dam was completed in 1962. All asphalt-concrete core dams built so far have an excellent performance record. The type of asphalt used is a viscoelastic plastic material that can adjust to the movements and deformations imposed on the embankment as a whole, and to settlements in the foundation. The flexible properties of the asphalt make such dams especially suited in earthquake regions.

 

Cofferdams

Cofferdam is a temporary barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, concrete or steel sheet piling, cofferdams are used to allow construction on the foundation of permanent dams, bridges and similar structures. When the project is completed, the cofferdam may be demolished or removed. Common uses for cofferdams include construction and repair of offshore oil platforms. In such cases, the cofferdam is fabricated from sheet steel and welded into place underwater. Air is pumped into space, displacing the water, allowing a dry work environment below the surface. Upon completion, the cofferdam is usually deconstructed unless the area requires continuous maintenance.

 

Timber dams

Timber dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times by humans due to the relatively short lifespan and limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly or difficult to transport and either a low-head the diversion dam is required or longevity is not an issue. Timber crib dams were erected of heavy timbers or dressed logs in the manner of a log house and the interior filled with earth or rubble. The heavy crib structure supported the dam’s face and the weight of the water. Timber plank dams were more elegant structures that employed a variety of construction methods utilizing heavy timbers to support a water retaining arrangement of planks. Very few timber dams are still in use.

 

Steel dams

A steel dam is a type of dam that uses steel plating and load-bearing beams as the structure. Intended as permanent structures, steel dams were an experiment to determine if a construction technique could be devised that was cheaper than masonry, concrete or earthworks, but sturdier than timber crib dams. The spillway can be gradually eroded by water flow, including cavitations or turbulence of the water flowing over the spillway, leading to its failure. Erosion rates are often monitored, and the risk is ordinarily minimized, by shaping the downstream face of the spillway into a curve that minimizes turbulent flow, such as an ogee curve.

 

PURPOSES FOR CONSTRUCTION OF DAMS

The common purposes for the construction of dams are as follows:

Power generation: Hydroelectric power is a major source of electricity in the world. Many countries have rivers with adequate water flow that can be dammed for power generation purposes.

Water supply: Many urban areas of the world are supplied with water abstracted from rivers pent up behind low dams or weirs. Other major sources include deep upland reservoirs contained by high dams across deep valleys.

Stabilize water flow/irrigation: Dams are often used to control and stabilize water flow, often for agricultural purposes and irrigation.

Flood prevention: Dams that are created for flood control.

Land reclamation: Dams are used to prevent ingress of water to an area that would otherwise be submerged, allowing its reclamation for human use.

Water diversion: Dams that are constructed for diverting water for various purposes.

Recreation: Dams built for any of the above purposes may find themselves displaced by the time of their original use. Nevertheless, the local community may have come to enjoy the reservoir for recreational and aesthetic reasons.

 

 

HOW TO IDENTIFYING A LOCATION FOR THE CONSTRUCTION OF A DAM

One of the best places for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam’s structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable. Significant other engineering and engineering geology considerations when building a dam include:

·       Permeability of the surrounding rock or soil

·       Earthquake faults

·       Landslides and slope stability

·       Water table

·       Peak flood flows

·       Reservoir silting

·       Environmental impacts on river fisheries, forests and wildlife

Also read: Functions of a weir

·       Impacts on human habitations

·       Compensation for land being flooded as well as population resettlement

·       Removal of toxic materials and buildings from the proposed reservoir area

IMPACT ASSESSMENT

The impact is assessed in several ways:

·       The benefits to human society arising from dams, such as agriculture, water, floods and hydroelectric power.

·       Harm to nature and wildlife (especially rare species) and impact on the geology of an area.

·       Whether the change to water flow and levels will increase or decrease stability, and the disruption to human lives.

·       A the large dam can cause the loss of entire ecospheres, including endangered and undiscovered species in the area, and the replacement of the original environment by a new inland lake.

HUMAN SOCIAL IMPACT

The impact on human society is also significant. For example, the Three Gorges Dam on the Yangtze River in China will create a reservoir 600 km long, to be used for hydro-power generation. Its construction required the loss of over a million people’s homes and their mass relocation, the loss of many valuable archaeological and cultural sites, as well as significant ecological change. It is estimated that to date 40–80 million people worldwide have been physically displaced from their homes as a result of dam construction.

 

 

ECONOMICS

Construction of a hydroelectric plant requires a long lead time for site studies, hydrological studies and environmental impact assessment, and are large-scale projects by comparison to traditional power generation based upon fossil fuels. The number of sites that can be economically developed for hydroelectric production is limited; new sites tend to be far from population centres and usually require extensive power transmission lines. Hydroelectric generation can be vulnerable to major changes in the climate, including a variation of rainfall, ground and surface water levels and glacial melt, causing additional expenditure for the extra capacity to ensure that sufficient power is available in low water years.

 

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