Ethiopia, a country which relies principally on hydro power for its electricity, has been embarking on an enthusiastic programme to refurbish existing and construct new hydro power projects.
The country’s current power consumption is 1670GWh/year and this is supplied by an estimated installed capacity of 410MW, generated by six hydro power plants.
Two projects which will supply an additional 190MW are under construction and two further projects with a total installed capacity of 453MW are ready to go out to tender. An increase in electricity generation is essential if economic growth is to be sustained.
A joint venture comprising Brown & Root (Howard Humpreys), PB Kennedy & Donkin from the UK and coyne-et-bellier of France have engineered three projects in Ethiopia: the Tis Abbay II (73MW); the Tekeze (300MW); and the Gojeb project (153MW).
Tis Abbay II
The Tis Abbay II project in northwest Ethiopia is currently under construction and due for completion in early 2001. Located on the Blue Nile on the outlet of Lake Tana, the project diverts the water flow around the Tis Issat falls, which is the second largest waterfall in Africa and also provides the head needed for power generation. The discharge from Lake Tana is regulated by a 7m high weir with radial gates. The project also has a 3km
long headrace canal which leads to steel penstocks and supplies a surface power station with an installed capacity of 73MW.
Environmental considerations
Before construction could go ahead the environmental issues surrounding the scheme required careful consideration, particularly as the spectacular Tis Issat falls are an important tourist attraction.
The falls attract around 7000 inter-national and 9000 domestic visitors each year and a survey performed at the airport showed that 40% of international arrivals were anticipating visiting the falls.
An interesting approach was taken to justify the economic viability of the hydro power plant by comparing the costs and benefits of the project with the income gained from tourism and the environment. It was calculated that if the hydro project was built and the falls were removed, Ethiopia would experience a loss of US$14M per year and the region would lose US$11M per year in comparison to the US$23M gained from the potential power generated annually. However, the evaluation is only in terms of expenditure and does not take into account the cultural aspect of the falls, and its value to local people.
It was therefore important to keep the falls and not to cause an adverse impact on tourism by reducing the flow so dramatically that it no longer attracted enough tourists.
The project developers also proposed to avoid disruption to a settlement on a nearby basalt plain, and avoid interfering with the visual appeal of the 17th century Fasil Bridge which is regarded as an important cultural feature.
Benefiting tourism
After consideration was given to the above factors, the consultant recom-mended keeping the falls by regulating the flows from Lake Tana while maintaining its visual appeal. It was discovered that the highest rainfall resulting in the most spectacular falls occurred two months before the tourist season. It was possible that the project could benefit tourism by making high flows coincide with the tourist season. Photographs of the falls were taken at different times of the year. This allowed the team to compare the appearance of the falls when altered by varying amounts of rainfall throughout the year.
The team was then able to decide on how much it should regulate the waterflow to maintain the attractiveness of the falls, and at the same time generate sufficient electricity to make the project economically viable.
Successful balance
Once all the issues and measurements had been taken into consideration a flow of 60m3/sec, which produced a respectable falls but did not extend the whole width of the horseshoe created by the heaviest flow, was chosen. In this way, a successful balance could be achieved, providing ample power production and maintaining the spectacular Tis Issat falls for future generations to enjoy. The project is however reversible, and the flow may be regulated on a diurnal basis with reduced flows at night.
A Water Resources Commission will be established in the area and will meet at six-month intervals to discuss what the water allocation should be and possibly re-allocate it as needed.
