Environmental Impacts of Dams-2

1. DAMS & ENVIRONMENT
By:-
RAJ KUMAR BAIRWA
Enrollment no.13512026
M.Tech 1st year (AHES)
IIT ROORKEEE

2. Why Build Dams?
To improve the quality of human life
by providing….

3. RECREATION
IRRIGATION
FLOOD CONTROL
HYDROPOWER
NAVIGATION
WATER QUALITY

4. Types of Dams

5. 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

6. 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.

7. 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.

8. Environmental Performance

9. 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

10. 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.

11. 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.

12. Downstream Aquatic Ecosystems and
Biodiversity:-
➢ 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.
➢ Water temperature and chemistry are altered as a consequence of water storage and the
altered timing of downstream flows.

13. 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.
➢ TheWCD 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.

14. 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 .

15. Social Performance

16. 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

17.  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.

18. 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.

19.  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.

20.  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.

21. References:-
 Dams And Development- A New Framework
For Decision-making The Report OfThe
World Commission On Dams
 www.wikipedia.com
 http://www.internationalrivers.org/
 International Congress On River Basin
Management

22. THANKYU