[Date Prev][Date Next][Thread Prev][Thread Next][Date Index
][Thread Index
]
Salween Dams: a study
TERRA-BURMA BULLETIN
DAM IMPACTS.1
PART I: DAMMING THE SALWEEN RIVER
This report is an introduction to the impacts of large dams in general,
the environmental problems associated with existing large dams in
Asia, and the potential impacts of damming the Salween.
For the purpose of discussion this report deals with damming the
Salween -- regardless of exact location of dam site -- excluding the
proposed dams on Salween tributaries and other Thai-Burma border
rivers.
The idea of damming the Salween has been studied for many years by
dam proponents. The dams are intended to generate hydroelectricity
for export to Thailand and possibly also to divert water from the
Salween into river basins in Thailand.
The dams to date proposed represent only a theoretical estimate of
the hydroelectric power that could be generated by harnessing flows
of the Moei and the Salween.
What actually ever gets built will depend not upon the engineers
proposals but more upon political decision makers and international
financiers who will weigh the political and economic stakes of each
dam project separately.
In any case, it is improbable that all of these large dams would ever be
built because there are some serious problems and risks associated
with damming the Salween or Moei rivers which dam builders would
not be able to solve and which will make the dam projects far less
economically attractive than originally projected.
Regardless of where exactly the Salween would be dammed, any dam
on the Salween is cause for concern for people throughout the
Salween river basin who depend upon the forests, land and rivers for
their livelihoods and local economies.
1.1 Reservoir Sedimentation Shortens Dam Lifespan
All rivers carry sediment and other organic matter which accumulates
in the reservoirs created by dams. Eventually all reservoirs fill with
sediment but long before this happens, the buildup of sediment in the
reservoir can interfere with the operation of the dam and shorten the
dam's operating lifespan.
The Salween is a Himalayan river with a naturally high sediment load
but logging and road building in the upper reaches of the Salween
river basin has made the Salween extremely muddy most of the year.
Therefore a reservoir on the Salween could be expected to slit up
rapidly.
Dam reservoirs on other Himalayan rivers in China and India, for
example, have filled with sediment only a few years after completion
even though the dam builders predicted a 50 year operating life.
To date there are no successful measures to keep reservoirs sediment
free and therefore the economic benefits of the dam are often far less
than the dam builders originally estimated.
1.2 Aging and Decommissioning of Dams
Dams are usually designed to have an economic lifespan of anywhere
from 50 to 100 years but their useful lifespans are inevitably
shortened as the reservoirs become clogged with sediment and as the
concrete and other dam materials age. As with nuclear power plants,
careful monitoring and costly maintenance of older dams must
eventually be followed by a difficult decommissioning process, for
which the dam building industry is unprepared.
Dams designed to hold back the force of the standing water can
require expensive new construction in order to withstand the greater
force of a wall of dense sediment.
Over the course of several decades, the structural integrity of dams
not designed to function as artificial waterfalls can be undermined,
threatening to collapse in a catastrophic flood of mud and debris.
In the fifty years of building large dams, no large dam has yet been
decommissioned although it is now being discussed in the USA
where some of the first megadams were built.
1.3 Dams and Earthquakes
The pressure applied to often fragile geological structures by the vast
mass of water impounded by a large dam can -- and often does -- give
rise to earthquakes.
Although it is difficult to establish the geological conditions under
which induced earthquakes can occur, scientists now conclude that
all large reservoirs can be considered potential sources if induced
seismic activity.
The first hint that dams could cause earthquakes came in the late
1930s, when increased seismic activity was recorded after the Lake
Mead reservoir was impounded by the Boulder Dam in Colorado,
USA.
Since then major earthquakes have occurred at large reservoirs in
China, Africa, Greece and India. The Aswan dam and Akosombo
dams have experienced earthquakes even though the dams are
located in what were classified as low risk areas.
Originally it was thought that earthquakes can only occur when a
reservoir was being filled -- or immediately after it reached its
maximum height. But earthquakes can also occur when a reservoir is
emptied and then refilled, as has occurred at dams in France and
Greece.
Despite the risks associated with large dams and reservoirs many
dams are still being built -- or planned -- in areas of seismic activity.
In the last two decades, the science of seismology has advanced
rapidly, with new research and recognition of reservoir-induced
earthquakes. But many large dams have been designed using earlier
optimistic estimates of fault displacement and ground accelerations.
Dam safety experts now fear there are tens of thousands of existing
dams in the USA, India and former Soviet Union that are unsafe.
Independent researchers in China estimate there might be as many as
85,000 small and medium-sized dams built since the 1950s that are
unsafe.
In August 1993, the Gouhou dam in Qinghai province, northwest
China, collapsed killing hundreds of people and unleashing torrents
of water on town and villages downstream. Dam experts suspect that
the 1990 earthquake weakened the dam leading to its collapse under
this year's floodwaters.
If the Salween dams are built, millions of people living downstream of
the dams would be forced to bear the burden of risk that someday an
earthquake could cause the dam to crack or burst unleashing a terrible
flood downstream.
1.4 Forced Eviction
The people who are now living near the dam sites and reservoir areas
would be forced to leave their homes and land.
It is unlikely that the dam proponents would make any provision for
these people given that the SLORC [Burma's ruling State Law and
Order Restoration Council] is waging war against the people living in
the Salween river basin, and that Thai authorities would claim no
responsibility for people affected beyond their border.
Exact location of the dam sites and details of the areas that would be
flooded (towns, army bases, villages, natural/historical points of
interest etc.) by the dams are as yet unrecorded and will depend upon
exact dam location, height and local topography.
1.5 Effects of Dam Construction
Tonnes of explosives would be used to blast through the river gorges
and rocky rapids to prepare the foundation for construction and the
channel downstream of the dam and power station.
There would be a large-scale extraction of building materials -- such
as stones, gravel, sand and clay for dam construction.
Forests would be cleared for access roads to the site on either side of
the Thai-Burma border.
Temporary housing for a large labour force of thousands of workers
would have to be constructed for the years that it will take to build
the dam. Wastewater and sewage will be dumped directly into the
river.
This labour force would rely heavily on the forests in the vicinity of
the construction site for their daily fuelwood.
The people evicted to make way for the construction site and worker
camps would then be forced to clear new farm plots in the forest.
Construction and blasting of the river to transform it into straight
intake and outflow channels will scare away animals and aquatic life
as well as destroy their habitat.
1.6 From River to Reservoir
The physical form of a natural river and the ecosystem that depends
on it have evolved together over thousands of years, created by the
natural flows and sediment moved from the river's watershed.
The construction of a large dam destroys this balance and within
several decades the changes in the river system would be far more
costly than the economic benefits of the dam.
When a river is dammed a body of water is impounded behind the
dam and is known as a reservoir. This impoundment has an immediate
impact on the physical and biological systems within the reservoir.
Physical Changes
Transformation of a free-flowing river into a regulated reservoir
systems would initiate the following changes in the river systems:
The free flowing river would be transformed into a deep, slow moving
or still water system (depending on actual operation), up to several
hundred kilometres or more in length (depending on dam height and
topography).
Migration of fish either upstream or downstream would be blocked by
the dam.
Forests and fertile, low-lying land along the river and in the tributary
valleys would be permanently submerged by the reservoir. These
areas are now used for seasonal cultivation of crops which serve the
needs of local families and communities.
The flooding of the river will disrupt aquatic and land-based animal
habitat along the river banks. In the case of the Lower Salween dam,
the western edge of the Salawin Wildlife Sanctuary in Thailand would
be drowned.
As water velocity is reduced, fine particles of sand and clay, known
as sediment, would settle to the bottom of the reservoir.
Initially, as the water level in the reservoir rose, land would be
inundated and nutrients (and pollutants) would be drawn out from the
flooded soils and decomposing vegetation.
The flooded vegetation would provide new habitat for young fish and
there might be a population boom over the short term for some fish
species.