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.
  • 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  • Dead Storage level: Level of permanent storage below which the water will not be withdrawn.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.

Factors Affecting Selection Of Type Of Dam

  • Topography
  • Geology and Foundation Conditions
  • Availability of materials
  • Spillway size and location
  • Earthquake zone
  • Height of the Dam
  • Other factors such as cost of construction and maintenance, life of dam, aesthetics etc.

Selection Of Dam Site

  • Suitable foundation must be available.
  • For economy, the length of the dam should be as small as possible, and for a given height, it should store the maximum volume of water.
  • The general bed level at dam site should preferably be higher than that of the river basin. This will reduce the height of the dam.
  • A suitable site for the spillway should be available in the near vicinity.
  • Materials required for the construction of dam should be easily available, either locally or in the near vicinity.
  • The value of land and property submerged by the proposed dam should be as low as possible.
  • The dam site should be easily accessible, so that it can be economically connected to important towns and cities.
  • Site for establishing labor colonies and a healthy environment should be available near the site.

Advantages of dams

  • Clean, efficient, and reliable form of energy.
  • Does not emit any direct pollutants or greenhouse gases.
  • While the initial cost is high, they are very inexpensive to operate.
  • Electricity generated by hydro-electric power plants is the cheapest electricity generated.
  • Dams prevent floods.
  • Dams store water for irrigation in summer seasons and dry months. Many desert areas can now farm due to dams and canals that supply water.
  • Dams supply water for local drinking needs.
  • Allows for fish farming.

Negative impact of dams

  • In flat basins large dams cause flooding of large tracts of land, destroying local animals and habitats.
  • People have to be displaced causing change in life style and customs, even causing emotional scarring. About 40 to 80 million people have been displaced physically by dams worldwide.
  • Large amounts of plant life are submerged and decay anaerobically (in the absence of oxygen) generating greenhouse gases like methane. It is estimated that a hydroelectric power plant produces 3.5 times the amount of greenhouse gases as a thermal power plant burning fossil fuels.
  • The migratory pattern of river animals like salmon and trout are affected.
  • Dams restrict sediments that are responsible for the fertile lands downstream. Farmers use chemical fertilizers and pesticides to compensate for the loss in productivity.
  • Salt water intrusion into the deltas means that the saline water cannot be used for irrigation.
  • Large dams are breeding grounds for mosquitoes and cause the spread of disease.
  • Farmers downstream who used to wait for the flooding of the fields to plant their seeds are affected.
  • Dams serve as a heat sink, and the water is hotter than the normal river water. This warm water when released into the river downstream can affect animal life.
  • Peak power operations can change the water level thirty to forty feet in one day and can kill the animals staying at the shorelines.
  • Around 400,000 km2 of land worldwide has been submerged due to the construction of dams.

Solution to the problems

Negative effects on flora, fauna, and the local population can be reduced by the following methods:
  • Fish passages should be created to aid in the migration of the fish.
  • New dam sites should be chosen with the environmental impacts in mind.
  • Local people should be led into confidence and must be suitably re-settled.
  • Proper compensation as per the market rate should be given.
  • Religious monuments of historic significance should be shifted.
  • Endangered species can be relocated.
If the political will to change and do a good job is there a dam can be constructed in a way to minimize its effects on people and the environment.

Conclusion

Dams have made an important and significant contribution to human development, and the benefits derived from them have been considerable. Dam building has been one of the most disputed topics affecting the environment today. The push and pull between the pros and cons have created conflicts among different groups. While dams destroy the nature and people surrounding the area in which they are built, they do provide people with water and products from water. The solutions are minimal, but the damages could be decreased depending on the placement of the dam. 


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