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Large dams promise large, often deliver small or made to look delivering large. Actually they are large scale planned destructions, as far as environment is concerned. As the dams cause lot of problems, onus is on the civil engineers and environmental scientists to identify them and find solutions.
Pore over the following snippets and know the cascading impact of large dams on environment and ecology of affected areas.
- Destruction begins before construction of dam. The laying of communication routes, digging of diversion tunnels or channels etc. start the destruction of environment in vicinity but their effects may be minimal.
- As the dam work progresses, the quarries expand, destroying the forests and its dwellers’ habitats.
- As the dam work picks up or after its completion, the doors open up for outsiders to come and settle and in many cases, for cultural clashes with indigenous people or tribals. Outsiders, generally from affluent alien cultures tend to plunder natural resources of the dam area. Accelerated large scale destruction of pristine forests beyond the direct consequence of dam building and smuggling out of banned timber like sandalwood and red sanders become common place.
- Fishing activities will immediately be affected in the area, due to restrictions on movement of fisher folk, migration of fish to distant placid spots.
- As the dam begins to impound water, vast areas of forests and biological hotspots may get flooded. Loss of habitats of localized fauna and flora in those forests lead to their eventual extinction. Even other exotic species would also be threatened.
- Migration of anadromous fish like salmon, hilsa, steel snouted trout etc., will permanently be stopped. These fish migrate upstream by instinct in winter to spawn and die. New born fish return later to seas after spring. Fish ladders, the slow cascading step like structures laid across dams and other measures not met with success. Fisheries in the upstream side will be worst hit. Lack of natural spawning grounds causes the fish downstream to dwindle, affecting fisheries there as well.
- As the dam stops silt load of river, supply of water borne micro-nutrients severely affects crops in down stream flood plains, aquatic life in the river downstream, estuarine ecosystems, mangrove forests and finally marine ecosystems. Falling fish catches already confirmed this cascading effect of large dams on major river deltas and adjoining seas.
- In many cases, the downstream river side areas face drought like situation. With depleted water supply the ground water table too plummets in many areas. Notably, in estuaries and deltas the depleting water table recedes inland giving way to sea water intrusion and salination of ground water.
For example, after construction of High Aswan Dam across river Nile in Egypt, which carried 100 m tonnes of silt downstream annually, the Nile delta itself receded by some two kilometers at some places. Fisheries have been wiped out. Further, silting rates in tropics and sub-tropics are much more than those in temperate zones. Hence, western design based dams silted faster rendering them useless. Akosombo reservoir in Lake Volta in Ghana is a classic example.
- As it happened often, deforestation in catchments led to accelerated silting in reservoirs reducing their life and killing them before their avowed goals are met.
- Compensatory afforestations taken up in place of lost virgin forests are never a match. They simply can not recreate the biodiversity.
- Dam induced changes in catchment areas range from water logging, increased salinity, increased erosion, change in water quality etc.
- The large reservoirs of shallow dams pose a threat to public health, by spreading vector borne diseases.
- The complete or near complete diversion of flows in the rivers flowing into small seas, dried them. Aral Sea in Central Asia and Dead Sea in the Middle East are classic cases. Particularly, the former presents a ghastly picture of destruction.
All these problems have some role for civil engineering in their mitigation. Achieving the power and irrigation potential through other means, protecting catchment area from erosion, doing most possible to maintain silt load in river and so on may go a long way. Last but not least, there are rehabilitation and other problems to deal with. Let us discuss it in another part.
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