Published on February 14, 2014
DAMS & ENVIRONMENT By:RAJ KUMAR BAIRWA Enrollment no.13512026 M.Tech 1st year (AHES) IIT ROORKEEE
CONTENTS:What is Dam?? What is function of Dam?? Why Build Dams?? Types of Dams Environmental Performance of Dams Social Performance of Dams Dam Safety
Dam A dam is an artificial barrier usually constructed across a stream channel to impound water. Timber, rock, concrete, e arth, steel or a combination of these materials may be used to build the dam. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates are used to manage water flow into specific land regions.
What is the function of a Dam? Dams provide a means of regulating downstream surface water flows over space and time by altering the natural spatial and temporal distributions of stream flows .
Why Build Dams? To improve the quality of human life by providing….
IRRIGATION RECREATION FLOOD CONTROL NAVIGATION WATER QUALITY HYDROPOWER
Types of Dams
Gravity dam Gravity dam is an engineering structure that by its own weight resists the forces imposed with a desired factor of safety. Classification of gravity dams based upon the materials comprising the structure is: Concrete dams Embankment dams Earthfill dams Rockfill dams Composite dams
Arch-gravity Dam An arch-gravity dam or arched dam is a dam with the characteristics of both an arch dam and a gravity dam. It is a dam that curves upstream in a narrowing curve that directs most of the water against the canyon rock walls, providing the force to compress the dam. It combines the strengths of two common dam forms and is considered a compromise between the two.
Cofferdam A cofferdam is a temporary enclosure built within a body of water and constructed to allow the enclosed area to be pumped out, creating a dry work environment for the major work to proceed. These cofferdams are usually welded steel structures, with components consisting of sheet piles. Such structures are typically dismantled after the ultimate work is completed. For dam construction, two cofferdams are usually built, one upstream and one downstream of the proposed dam, after an alternative diversion tunnel or channel has been provided for the river flow to bypass the dam foundation area.
Ecosystems and Large Dams: Environmental Performance Terrestrial Ecosystems and Biodiversity Greenhouse Gas Emissions Downstream Aquatic Ecosystems and Biodiversity Impacts of changes in flow regimes Impacts of trapping sediments and nutrients behind a dam Blocking migration of aquatic organisms
Terrestrial Ecosystems and Biodiversity The construction of a storage dam and subsequent inundation of the reservoir area effectively kills terrestrial plants and forests and displaces animals. As many species prefer valley bottoms, large-scale impoundment may eliminate unique wildlife habitats and affect populations of endangered species . Efforts to mitigate the impacts on fauna have met with little success. An alternative to mitigation is a compensatory project approach. For example, in India there is a legal requirement that forests flooded by reservoirs must be replanted elsewhere. However, it is found that only half of the required forest has typically been planted and even this is poorly managed.
Greenhouse Gas Emissions The emission of greenhouse gases (GHG) from reservoirs due to rotting vegetation and carbon inflows from the catchment is a recently identified ecosystem impact of storage dams. A first estimate suggests that the gross emissions from reservoirs may account for between 1% and 28% of the global warming potential of GHG emissions. This challenges the conventional wisdom that hydropower produces only positive atmospheric effects, such as a reduction in emissions of carbon dioxide, nitrous oxides, sulphuric oxides and particulates when compared with power generation sources that burn fossil fuels. It also implies that all reservoirs – not only hydropower reservoirs – emit GHGs. Consequently, reservoir and catchment characteristics must be investigated to find out the likely level of GHG emissions.
Downstream Aquatic Biodiversity:- Ecosystems and Storage dams are intended to alter the natural distribution and timing of streamflow. They compromise the dynamic aspects of rivers that are fundamental to maintaining the character of aquatic ecosystems. Natural rivers and their habitats and species are a function of the flow, the quantity and character of the sediment in motion through the channel, and the character or composition of the materials that make up the bed and banks of the channel. The defining river discharge includes both high- and low-flow elements. Impacts of changes in flow regimes Flow regimes are the key driving variable for downstream aquatic ecosystems. Flood timing, duration and frequency are all critical for the survival of communities of plants and animals living downstream . Small flood events may act as biological triggers for fish and invertebrate migration: major events create and maintain habitats by scouring or transporting sediments. The natural variability of most river systems sustains complex biological communities that may be very different from those adapted to the stable flows and conditions of a regulated river.
Impacts of trapping sediments and nutrients behind a dam The reduction in sediment and nutrient transport in rivers downstream of dams has impacts on channel, floodplain and coastal delta morphology and causes the loss of aquatic habitat for fish and other species. Changes in river water turbidity may affect biota directly. Reduction in sediment moving downstream from the dam leads to degradation of the river channel. Blocking migration of aquatic organisms As a physical barrier the dam disrupts the movement of species leading to changes in upstream and downstream species composition and even species loss. River-dwelling species have several migratory patterns. The WCD Survey found that impeding the passage of migratory fish species was the most significant ecosystem impact, recorded at over 60% of the projects for which responses on environmental issues were given. In 36% of these cases, the impact of the large dam on migratory fish was not anticipated during project planning. Migratory fish require different environments for the main phases of their life cycle: reproduction, production of juveniles, growth, and sexual maturation. Many anadromous fish populations have died out as a result of dams blocking their migratory routes.
Dams and Water Quality: The chemical, thermal and physical changes which flowing water undergoes when it is stilled can seriously contaminate a reservoir and the river downstream. The extent of deterioration in water quality is in general related to the retention time of the reservoir — its storage capacity in relation to the amount of water flowing into it. Water in a small headpond behind a run-of-river dam will undergo very little or no deterioration; that stored for many months or even years behind a major dam may be lethal to most life in the reservoir and in the river for tens of kilometres or more below the dam. Water released from deep in a reservoir behind a high dam is usually cooler in summer and warmer in winter than river water, while water from outlets near the top of a reservoir will tend to be warmer than river water all year round. Warming or cooling the natural river affects the amount of dissolved oxygen and suspended solids it contains and influences the chemical reactions which take place in it. . Altering natural seasonal changes in temperature can also disrupt the lifecycles of aquatic creatures .
People and Large Dams – Social Performance Socio-economic Impacts through the Planning and Project Cycle Irrigation, Drainage & Flood control Hydropower Displacement of People and Livelihoods Under-counting of the displaced Physically displaced populations enumerated but not resettled Downstream Livelihoods Cultural Heritage
Socio-economic Impacts through the Planning and Project Cycle Planning and design: At the planning and design stage an important social impact is the delay between the decision to build a dam and the onset of construction. Dams are often discussed years before project development is seriously considered and once a site is identified a form of planning blight can take place, making governments, businesses, farmers and others reluctant to undertake further productive investments in areas that subsequently might be flooded. A related problem is the fear felt by many people living in a possible reservoir area. Such psychological stress cannot be effectively quantified in economic terms, but it is a real issue. Construction: During the construction period, dam projects require a large number of unskilled workers and smaller but significant amounts of skilled labour. New jobs are therefore created both for skilled and unskilled workers during the construction phase.
Irrigation , Drainage & Flood control A major portion of water stored behind dams in the world is withdrawn for irrigation which mostly comprises consumptive use, that is, evapotranspiration needs of irrigated crops and plantations. The total quantum of flow and size and frequency of peak floods in the flood season reduce in the down stream due to a dam, reducing flood hazard . It also reduces congestion of runoff in plains and coastal lands. Dams, reservoirs, flood levees, embankments, and river training works constitute structural measures for better flood management. Hydropower Energy provided by dams have benefited urban populations and others connected to power distribution systems.
Displacement of People and Livelihoods An estimated 40 to 80 million people have been displaced by dams worldwide. Yet mitigation, compensation, and resettlement attempts are often inadequate. Under-counting of the displaced At the planning stage, the numbers of both directly and indirectly affected people have frequently been under-estimated and there has been inadequate understanding of the nature and extent of the negative impacts. For example For the Sardar Sarovar project, the 1979 Narmada Water Disputes Tribunal gave the number of displaced as 6 147 families, or about 39 700 people. The World Bank’s 1987 mission placed the total at 12 000 families (60 000 people). In 1991, the project authorities provided an estimate of 27 000 families. According to three state governments, the current estimate of displaced families stands at 41 000 (205 000 people) . The current estimate does not include at least 157 000 people displaced by canals. Nor does it include those moved to make space for the creation of a wildlife sanctuary and for the resettlement of people displaced by the dam, or the 900 families displaced in the early 1960s to make room for construction site infrastructure.
Physically displaced populations enumerated but not resettled Among physically displaced people officially recognised as project affected not all are given assistance to resettle in new locations. In India, those actually resettled range from less than 10% of the physically displaced in the case of the Bargi dam to around 90% for the Dhom dam. Cash compensation is a principal vehicle for delivering resettlement benefits, but it has often been delayed and, even when paid on time, has usually failed to replace lost livelihoods. Cultural Heritage Large dams have significant adverse effects on this heritage through the loss of local cultural resources and the submergence and degradation of archaeological resources. Dams can also cause loss or damage of cultural heritage through land reclamation and irrigation projects and the construction of power lines, roads, railways and workers towns.
References:Dams And Development- A New Framework For Decision-making The Report Of The World Commission On Dams www.wikipedia.com http://www.internationalrivers.org/ International Congress On River Basin Management
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