Environmental Engineering Reference
In-Depth Information
Figure 27.8
Contrasting river discharges above and below
the Aswan Dam.
of vast lakes impounded by large dams. The Volta dam in Ghana can store water within
an area the size of Lebanon and Lake Kariba on the Zambezi can cover over half a
million hectares (Table 27.1). These artificial lakes have become a feature of Earth's
surface. There is no disputing that the control of river systems by dams can have a
positive effect on the local area and the national economy. If we take Egypt as an
example, we see that the construction of the Aswan High Dam in 1970 has stabilized the
flow of the Nile (Figure 27.8). It has also allowed the generation of about 20 per cent of
Egypt's electricity, which saves on import costs of fossil fuel. The area of perennially
irrigated cropland has been increased, which is of vital importance in a country with a
high natural increase of population. Less obviously the control of water level on the Nile
has had a beneficial impact on tourism. Tourists can visit the Nile temples more easily
with improved navigation and assist the country's economy.
Unfortunately not all the results of big dam construction have been benefits. There
have been a number of disadvantages, some natural, some unexpected and some induced
through subsequent human activities.
Siltation
. Like all natural lakes, reservoirs are prone to silting. Dams built in dryland
environments are likely to have large areas of partial vegetation cover, especially during
the dry season. When the rains start, or floods occur, there is likely to be major
movement of sediment which would normally be transported in the river. When a river
flows into a new lake, its velocity will fall and so the suspended sediments will be
deposited in the lake. Some of the dams on the Huang Ho in China have the unenviable
global reputation of being filled most rapidly by sediment. The catchment area of the
Huang Ho includes the loess plateau of western China, which is easily eroded during the
rainy season. The river used to be known as the Yellow River because its sediment load
coloured the water. The Sanmenxia dam on the Huang Ho began impounding water in
1960 but within seven and a half years the reservoir had lost 35 per cent of its capacity; it
was estimated that 3391 million m
3
had been deposited in that time! The lake is now
filled with sediment and can generate electricity for only a few months in winter. Silt
continues to pit the turbine blades and shorten their working life. All dams suffer this
problem to varying degrees. The life of the dam is also important in economic terms. If
power is generated only for, say forty years, instead of the predicted 100 years, it will be
much more expensive per kilowatt of electricity generated.
