Saturday, December 06, 2003

A dam perspective

A dam perspective

By F.H. Mughal

Dams have had a far more adverse effect on the environment than previously thought. Can we reverse this adversity?

According to a report of the World Commission on Dams (WCD), Dams and Development: A New Framework for Decision-Making, 2000, less than 2.5 per cent of water is fresh, less than 33 per cent of fresh water is fluid and, less than 1.7 per cent of fluid water runs in stream. However, mankind have dammed half of world's rivers at a rate of one per hour and, at unprecedented scales of over 45,000 dams (as of 2000), more than four stories high.

The aggregate storage capacity of large dams, based on dam design, is about 6,000 cubic kilometres (cukm). Assuming that half the design storage is achieved in reality, the aggregate real storage capacity of large dams globally is similar to total freshwater withdrawals estimated at around 3,800 cu km.

While, dams have made significant contribution to the development, they have altered river regimes; their ecosystem and have deprived local riparian settlers of benefits, which they have been receiving. Environmental impacts and, impacts on people clearly outweigh the benefits provided by dams. Global estimates suggest that, the magnitude of impacts include some 40 to 80 million people displaced by dams, while 60 per cent of the world's rivers have been affected by dams and diversions.

There has been limited success in countering the impacts of the ecosystem. This is due to a number of reasons, including lack of attention to anticipating and avoiding impacts and poor quality and uncertainty of predictions. Failure to assess the range of potential negative impacts and inadequate implementation of mitigation, resettlement and development programmes for the displaced and, the failure to account for impacts on downstream livelihoods, have lead to impoverishment and suffering of millions.

According to the International Rivers Network (IRN), more than 400,000 square kilometres (sq km), have been inundated by reservoirs worldwide.

A 1990 internal survey of the World Bank hydroelectric dam projects, showed that 58 per cent were planned and built without any consideration of downstream impacts, even when these impacts could be predicted to cause massive coastal erosion, pollution and other problems.

In view of large-scale problems and risks, associated with large dams, the current trend is towards the decommissioning of large dams.

China, United States, India, Spain and Japan are the top five dam building countries that account for nearly 80 per cent of all large dams worldwide.

ENVIRONMENTAL IMPACTS: An important impact of large dams is the fragmentation of rivers. According to the World Resource Institute, one large dam modified 46 per cent of the world's 106 primary watersheds.

Among the factors that impact the riverine ecosystems, dams are by far the main threats to the riverine ecosystems, fragmenting and transforming aquatic and terrestrial ecosystems, with a range of effects that vary in duration, scale and degree of reversibility.

In case of Sindh, construction of storage reservoirs and increased water withdrawal upstream, have adversely affected the ecology of riverine areas downstream of Kotri barrage. Sea intrusion has occurred to a large extent; mangroves population has been badly affected; there is increase in average groundwater salinity; soil erosion has occurred; lands have been rendered unfertile; fishermen have been rendered jobless and; people's livelihood has been affected due to significant economic losses.

Dams retain sediments on their upstream side, especially the heavy gravel and cobbles. The river, which looses its sediment load, seeks to recapture it by eroding the downstream banks, undermining bridges and other river structures. Within nine years of closing, the riverbed below Hoover Dam in the US had lowered by more than four meters. Riverbed deepening will also lower the groundwater table along a river, threatening vegetation and local wells in the floodplain and requiring crop irrigation in places where there was previously no need. The depletion of riverbed gravels reduces habitat for many fish that spawn in the gravelly bottom and, for the invertebrates such as insects, mollusks and crustaceans.Dams and river diversions have wiped out vital habitat, decimating fish populations and have pushed numerous aquatic species to the brink of extinction.

In the Colorado River Delta, flow diversion caused severe environmental consequences. Today, the delta is a desiccated place of mud-cracked earth, salt flats and murky pools.

In the Nile River basin, the Aswan High Dam has greatly altered the river system. Out of 47 commercial fish species in the Nile prior to the dam's construction, only 17 were still harvested a decade after its completion. The annual sardine harvest in the eastern Mediterranean dropped by 83 per cent, likely a side-effect of the reduction in nutrient-rich silt entering that part of the sea.

A significant impact of large dams is the displacement of large populations, who thrived on downstream riverine resources. The loss of access to natural resources of riverine communities and, uprooting of their heritage has profound economic, social and cultural impacts on them.

The construction of large dams has led to the displacement of some 40 to 80 million people worldwide. Many of them have not been resettled or received adequate compensation. Between 1986 and 1993, an estimated four million people were displaced annually by an average of 300 large dams starting construction each year.

Dams are notorious in increasing the population of vector-borne diseases. In case of Aswan Dam, dramatic increases in snail's population lead to the propagation of schistosomiasis.

Water resources development in the Senegal River Basin has resulted in epidemics of bilharzias (schistosomiasis) and rift valley fever in areas that had previously been unaffected by the depilating diseases. Malaria cases have proliferated as mosquito vectors found many new breeding sites.

In China, a high degree of primary liver cancer has been linked to the presence of cyanobacterial toxins in drinking-water. Bacteria feeding on the rotting biomass in reservoirs transforms the naturally-present, harmless mercury to methyl-mercury, which is harmful for the central nervous system.

Although, it has now become very difficult to build destructive river projects in the US and many other dammed countries, the dam proponents and financial institutions continue to export this obsolete technology, much in the same way the chemical industry continued to export pesticides long after they had been banned in the county of origin. At dam conferences, the talk these days always centers around finding fresh markets to exploit and new ways to sell dams to skeptical public.

REAL WORLD EXAMPLES: Prior to the construction of Aswan Dam, the Nile River carried about 124 million tons of sediment to the sea each year, depositing nearly 10 million tons on the floodplain and delta. Today, 98 per cent of that sediment remains behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt's floodplain agriculture. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river.

Along the mouth of the Volta River in Ghana, Akosombo Dam has cut off the supply of sediment to the Volta Estuary, affecting also neighbouring Togo and Benin. Their coasts are now being eaten away at a rate of 10-15 meters per year. A project to strengthen the Togo coast has cost US$3.5 million for each kilometre protected.

Since the 1950s, people in the area of the Aral Sea have been trying hard to increase the irrigated agricultural production. This drive has been the root-cause of turning the Aral Sea region into the largest ecological disaster.

In 1957, the Aral Sea had 1,075 cu km, with surface area of 66,085 sq km and, a mean sea level of 53 meters. An annual inflow of water of 56 cu km from the Amy-Darya and, another five cu km of atmospheric precipitation, maintained the water balance of the Aral Sea.

Unhindered development of the irrigated agriculture took place, in the last over three decades that followed. This resulted in a rapid decline in the annual flow of waters from the Amu-Darya and Syr-Darya to the sea. In some years, the water of Amu-Darya did not even reach the seashore.

In 1990, a total of seven million hectares in the Aral area were irrigated. Annual water withdrawals were 60 cu km and 45 cu km from Amu-Darya and Syr-Darya, respectively. As a result, the Aral Sea began to shrink. The water level of the sea fell by 14 meters. Its area shrunk by more than 40 per cent and its volume decreased by more than 60 per cent in 30 years. In 1989, the sea receded into two separate portions.

The level of the southern Greater Sea stood at 38.6 meters. Together, the two water bodies covered an area of 36,500 sq km, with 330 cu km volume of water. The average salinity of the water increased from nine per cent in 1957 to 30 per cent in 1989.

All these actions lead to a trail of adverse environmental impacts. Most of the 20 species of fish, which the sea originally had, died due to declining food, increase in water salinity and, drying-up of shallow spawning areas.

The exposed seabed of the Aral Sea has become a source of large dust storms, blowing up to 75,000 tons of dust each year from the saline solonchak soils. Deposition of this salty material on the irrigated cropland of the delta, has badly affected the crop and soil yields. Massive use of toxic chemical pesticides has affected the drinking-water supplies. The incidence of diseases, like typhoid and hepatitis, has increased, due to unsound water management practices.

The case history of Aral Sea clearly shows, how just one mistake in the implementation of irrational water resource development project, can lead to a chain of unmitigated ecological and environmental disasters.

Copyright C. 2002 - 2004
Pakistan Science and Engineering Forum (R)
"Kindling the Flame of Science in Pakistan (TM)"
PakSEF (TM) Daily Science News Update

No comments: