(May 23, 1994) 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.
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.
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.
Categories: Mekong Utility Watch