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Significance of Dams and their impact on Environment

by Yashwant Singh
Introduction
 
A barrier that is impervious in nature and is built across a river to create a reservoir on its upstream. This is a hydraulic structure made for various purposes. They are used to increase the amount of water available for generating hydroelectric power, to reduce peak discharge of floodwater created by large storms or heavy snowmelt, or to increase the depth of water in a river in order to improve navigation and allow barges and ships to travel more easily. Their construction involves geospatial data for planning and other engineering work.
 
Terminologies
 
a) Crest
 
This is on the top of the dam and is usually used as a roadway or a pathway.
 
b) Parapet walls 
 
This is a protective wall on either side of the crest.
 
c) Heel
 
This part of the dam is in contact with the river bed on upstream side.
 
d) Toe
 
This part of the dam is in contact with the river bed on downstream side.
 
e) Spillway
 
 This is an outlet for the reservoir water when its level raised above the maximum water level mark.
 
 
f) Abutments
 
The valley slopes on either side of the dam wall to which the left & right end of dam are fixed.
 
g) Gallery
 
This is a long tunnel like passage at transverse or longitudinal within the dam. The normal function of a gallery is to provide access for inspection purposes, to monitor the behaviour of the dam, and to carry out remedial work if required.
 
h) Sluiceway
 
Opening in the dam near the base, provided to clear the silt accumulation in the reservoir.
 
i) Freeboard
 
The space between the highest level of water in the reservoir and the top of the dam.
 
j) Dead Storage level
 
 Level of permanent storage below which the water will not be withdrawn.
 
k) Diversion Tunnel
 
Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The dam is built while the river flows through the diversion tunnel.
 
Types of Dams
 
1) Based on the function
 
Storage Dams
 
As the name indicates this type of dam is used for the storage of water during rainy season when there is a large flow in the river. This water can be used later in dry summer. 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.
 
Diversion Dams
 
These type of dams are used to divert the water of the river into an off-taking canal (or a conduit). Such shorter dams are used for irrigation and for diversion from a stream to a distant storage reservoir. A diversion dam is usually of low height and has a small storage reservoir on its upstream. This is a sort of storage dam which diverts the water too.
 
Detention Dams
 
These type of dams are basically used for flood control. Detention dam restore some of the flood water, thus the effect of the sudden floods is reduced to some extent. This water is later released gradually.
 
 
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.
 
Coffer Dams
 
It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A cofferdam is thus a temporary dam constructed for facilitating construction. 
 
2) Based on structure and design
 
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. 
 
Examples of Gravity dam: Grand Coulee Dam (USA), Nagarjuna Sagar Dam (India) and Itaipu Dam ( Between Brazil and Paraguay).
 
 
Earth Dams
 
An earth dam is made of earth (or soil) built up by compacting successive layers of 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.
 
Examples of earth fill 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.
 
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. An arch dam 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.

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.
 
Examples of Buttress Dam: Bartlett dam (USA) and The Daniel-Johnson Dam (Canada)
 
                                Following are the dam types around the world as per ICOLD
 
Steel 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 steel dams, and (ii) Cantilever type steel dams. In a direct strutted steel dam, the water pressure is transmitted directly to the foundation through inclined struts. In a cantilever type steel dam, there is a bend supporting the upper part of the deck, which is formed into a cantilever truss. 
 
 
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.
 
2) Based on the size
 
 
 
2) Based on the Use
 
Saddle dams
 
A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir. This is similar to a levee, which is a wall or embankment built along a river or stream to protect adjacent land from flooding.
 
Weir dams
 
A weir (also sometimes called an overflow dam) is a type of small overflow dam that is often used within a river channel to create an impoundment lake for water abstraction purposes and which can also be used for flow measurement or retardation.
 
Check Dams
 
This is used to control the soil erosion. This is a small dam and it reduce the flow velocity to control soil erosion.
 
Dry dams
 
A dry dam, also known as a flood retarding structure, is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.
 
 
Diversionary dams
 

A diversionary dam is a structure designed to divert all or a portion of the flow of a river from its natural course. The water may be redirected into a canal or tunnel for irrigation and/or hydroelectric power production.

Underground dams
 
Underground dams are used to trap groundwater and store all or most of it below the surface for extended use in a localized area. In some cases they are also built to prevent saltwater from intruding into a freshwater aquifer. Underground dams are typically constructed in areas where water resources are minimal and need to be efficiently stored.
 
Tailings Dams
 
A tailings dam is typically an earth-fill embankment dam used to store tailings, which are produced during mining operations after separating the valuable fraction from the uneconomic fraction of an ore. Conventional water retention dams can serve this purpose, but due to cost, a tailings dam is more viable. Unlike water retention dams, a tailings dam is raised in succession throughout the life of the particular mine.
 
Significance of a dam
 
           

Following is the distribution chart of the purpose of the dams as per ICOLD.

(Single Purpose Dams)

 
 
 
                                                                                   Following is the distribution chart of the purpose of the dams as per ICOLD. (Multipurpose Dams)
 
  • Power generation
  • Water supply
  • Stabilize water flow/irrigation
  • Flood prevention
  • Land reclamation
  • Water diversion
  • Navigation
 
Effect of dam construction on Environment
 
Several impacts are there on biological, physical and chemical properties of a river.
 

1) The dam wall itself blocks fish migrations, which in some cases and with some species completely separate spawning habitats from rearing habitats.

2) Dam also traps sediments, which are critical for maintaining physical processes     and habitats downstream of the dam (include the maintenance of productive deltas, barrier islands, fertile floodplains and        coastal wetlands).

3) Transformation upstream of the dam from a free-flowing river ecosystem to an artificial slack-water reservoir habitat. Changes in temperature, chemical composition, dissolved oxygen levels and the physical      properties of a reservoir are often not suitable to the aquatic plants and animals that evolved with a given river system.

 
 

4) Riverbeds downstream of dams are typically eroded by several meters within the decade of first closing a dam; the damage can extend for tens or even hundreds of kilometres below a dam.

5) Riverbed deepening (or "incising") will also lower groundwater tables along a river, lowering the water table accessible to plant roots (and to human communities drawing water from wells).

6) During the construction of dams in the past, the geology of the area was rarely or never put into consideration. The sudden filling of reservoirs with millions of gallons of water has caused seismic instability     and causes tremors and minor earthquakes.

7) Reservoirs have been identified as breeding grounds for various disease-carrying pests, including mosquitoes that transmit malaria disease and snails that transmit schistosomiasis. Some of these reservoirs have become habitats for crocodiles which bring an imbalance in the ecosystem and are also dangerous to the surrounding human settlement.

8) Archaeological and historical places that are rare with their exceptional beauties, disappear after lying under the reservoir.

9) Dams affect the social, cultural and economical structure of the region   considerably. Especially forcing people, whose settlement areas and lands remain under water to migrate, affect their psychology           negatively.

Geospatial services and Dams
 
GIS and remote sensing techniques in dam site selection
 
Application of remote sensing and GIS techniques in hydrology is today one of the most effective approaches. Recently, remote sensing has provided valuable datasets to examine hydrological variables and morphological changes over large regions at different spatial and temporal scales.
 
Data from different geospatial sources as an input
 
Data is collected from different GIS, Photogrammetry, LiDAR and other geospatial sources. A proper geological mapping, Land cover mapping, Digital Terrain Model (DTM), Orthorectified imagery or high resolution satellite imagery serve as input for the planning and construction work. A DGPS survey or a UAV survey of the site is needed to be done for the planning and engineering inputs. Slope, settlement and rainfall are the three main criterion on the basis of which the data is collected.
 
Input data, software and output data
 

Input data for the study area covers satellite imagery, Digital Elevation Model (DEM) and Runoff data

 

Software related to geospatial science like ERDAS, ArcGIS and global mapper along with other advance CAD and algorithm based software can be used for data processing and evaluation.

 

Output data include Land Use and Land Cover (LULC) map, DEM map, Slope and aspect maps, Fill Sinks and Flow, Contour and Triangular Irregular network (TIN) maps, 3D surface map, Drainage pattern map over the DEM. These output contents are helpful in construction of the GIS database. This further leads to the selection of the suitable dam site and it is followed by engineering design of the DAM cross section.

UAVs (Drones) for Dam security and vigilance
 
UAV(Drones) are playing a vital role in providing security to dams and keeping a close eye on them. This is a time saving and a robust way of doing a routine surveillance of the dams.
 
 
Countries with more than 1000 dams
 
 
 
Summary
 
Dam is as an obstruction constructed across a stream or river. Behind this barrier water is collected forming a pool. Dams are significant for Power generation, Water supply, Stabilize water flow / irrigation, Flood prevention, Land reclamation, Water diversion, Navigation and many more. Dams have a negative impact on environment too like blocking fish migration, trapping sediments, changes in temperature, chemical composition, dissolved oxygen levels and the physical properties of a reservoir are often not suitable to the aquatic plants and animals. Geospatial science is playing a vital role in dam construction and their security.

SATPALDA is a privately owned company and a leading provider of satellite imagery and GeoSpatial services to the user community. Established in 2002, SATPALDA has successfully completed wide range of photogrammetric and Remote Sensing Projects.

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