Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 213
All the structures were regularly monitored and well maintained. In the 1980s, these
controls revealed that the condition of the foundations of the dam indicated the
approach of the end of the operating life of this structure. It was therefore necessary to
either remove or replace dam and plant. Extensive studies were then conducted to
carefully assess the various options (technical studies, EIA: EWI 1987). These studies
were designed to identify the best solution, which would then be implemented from
2000 to 2006.
In addition to technical considerations, both economic and financial (cost of
decommissioning work with or without replacement of structures, power generation),
the most important points to consider were environmental issues:
� Landscape: In over 100 years of existence of the reservoir, the surrounding
landscape, including the developed and inhabited zones, had adapted to a "lake"
situation; lowering the water level by about ten metres (and more in the vicinity of
the dam) would cause a significant change in this situation.
� Sediment: thanks to the great lake of Constance and the other reservoirs upstream
of the dam, the amount of sediment that had accumulated in the reservoir was
relatively small and did not in any way affect the continuous operation of the plant.
Nevertheless, the dismantling of the dam without replacement would have meant
mobilizing these sediments. Now, there was a risk that these sediments could
contain unknown amounts of harmful substances, the result of industrial activities
for a period of almost 50 years upstream of the dam. As long as the sediments were
still in place, these substances did not constitute a risk, but mobilising the sediments
would have necessitated preventive measures (excavation and disposal on land of
these substances), which would have been very costly.
The solution adopted in this case, taking into account the considerations described
briefly, involved the construction of a new dam and power plant a hundred metres
downstream of the existing dam, which was then removed. With this solution, energy
production could be maintained and even increased significantly (installed capacity of
the new plant: 100 MW), and the situation in the reservoir area (water level and thus the
landscape, sediment) remained unchanged. The construction started in 2006, and the
new plant was commissioned in 2010.
In the case of Mount Coffee power plant, when the time comes, a similar procedure will
have to be followed, with a detailed analysis of the options. It will certainly be
necessary to carry out technical feasibility studies, and as with the construction, to
prepare an ESIA for the dismantling or replacement of the dam. Just like in the case of
Rheinfelden, the issue of sediments will be something to consider, among others.
According to the available technical studies, sedimentation is not of major concern in
the Mount Coffee reservoir. Depending on the development of the cascade (with the
large Via storage reservoir and additional dams and power plants between it and Mt.
Coffee) sediment input to the latter could still be further reduced. Nevertheless, it is
obvious that over an operation period of 50 to 100 years still a large amount of sediment
will be accumulated in the reservoir, which would be transported downstream should
the dam be removed. This could cause serious problems downstream of the site, e.g. for
the fauna (fish), water use (like drinking water intakes) and the estuary (mangroves).
This will have to be evaluated carefully, taking into account the situation at that
moment.