Environmental Engineering Reference
In-Depth Information
Evaporation
. When water is stored at the surface it will evaporate. In dryland areas
this is a particular problem, as rates of evaporation are high because of the dry
atmosphere, high inputs of solar energy and temperature levels. In Lake Nasser, behind
the Aswan High Dam, it is estimated, about 10 billion m
3
of water are lost each year
through evaporation. Although much of this evaporated water may have previously run
off direct to the sea, it is nevertheless an important loss of water resources. Unfortunately
there is little that can be done to reduce it. The water surface can be covered with
chemicals or artificial skins but that would be uneconomic for large lakes.
Environmental and ecological changes.
The removal of sediment from the river into
the lake means that the water released from the dam is relatively sediment-free. As we
saw in Chapter 14, if a river loses sediment it has more energy available for erosion.
Construction of the Danjiangkou dam on a tributary of the Yangtze led to degradation of
the river bed and banks up to 500 km below the dam. Reduction of silt load in the Hwang
Ho through damming has reduced siltation and increased scouring in its lower course and
reduced the risk of flooding, though the effect is only short-term. Similar effects have
been noticed on the Nile. The lack of annual sediment accumulation has caused rapid
retreat of the delta coastline, with consequences for the coastal fishing industry. The loss
of silt reduces the natural fertilizing effect of flooding, so, to sustain yields, artificial
fertilizers have to be used.
Water tables are affected by dam construction, leading to waterlogging of the
immediate surrounds and the potential for salinization if there is insufficient downward
movement of water. Along the coast, reduced freshwater flow can lead to an incursion of
salt water into the water table.
The new lake will cover and destroy all existing vegetation. Trees may be left to
decay, habitats will be lost. Such environmental impact is increasingly causing concern
over major dam construction such as the Three Gorges project in China. Thorough
surveys of the impact of a dam are not always made. The Rasi Salai Dam in eastern
Thailand was built on a huge salt dome, so the water soon became too saline for irrigation
use.
An interesting biological consequence of the increase of freshwater surface has been
the encroachment of water weeds. The water hyacinth is a major problem. Within two
years of construction, 50 per cent of the surface area of a lake in Surinam (South
America) was covered by this plant, and Lake Kariba has experienced water-fern
encroachment.
Pests and diseases
. Bodies of still water provide an attractive environment for many
pests and diseases in dryland areas. Malaria can be an increased problem, though in
smaller dams changing water levels may strand larvae. Bilharzia is another water-related
disease which has increased near major dam projects. People contract the disease through
bathing, fishing, washing clothes or collecting water from areas infected by the parasitic
larvae. After the Volta dam in Ghana was constructed the incidence of bilharzia in
children under ten had risen to 90 per cent. Positive effects can occur. River blindness is
caused by a fly which breeds in fast-flowing sections of rivers. As some of these habitats
have disappeared under the reservoirs, so the incidence of the disease has declined.
Management
. Many dam schemes have been less successful than expected because of
poor management and maintenance following construction. Frequently too much
attention is paid to the design and construction of the project, little training being given to
