This document discusses dams and their environmental and social impacts. It begins by defining dams and explaining their purposes, which include irrigation, flood control, hydropower, and navigation. It then describes the main types of dams and discusses their environmental performance, noting impacts on terrestrial and aquatic ecosystems from habitat loss and changes to natural flow regimes. Socially, dams can displace large populations and disrupt livelihoods when people are resettled or downstream access to resources is reduced. Planning for dams often undercounts affected people and resettlement efforts are frequently inadequate.

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

2. 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


3. 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.


4. 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 .

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

6. IRRIGATION
RECREATION
FLOOD CONTROL
NAVIGATION
WATER QUALITY
HYDROPOWER

7. Types of Dams

8. 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

9. 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.

10. 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.

11. Environmental Performance

12. 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

13. 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.

14. 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.

15. 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.

16. 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.

17. 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 .

18. Social Performance

19. 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

20. 
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.

21. 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.

22. 
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.

23. 
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.

24. 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


25. THANK YU