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WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD. Mount Coffee HPP ESIA and RAP 2012-09-18 ESIA Report Page 212 27 DECOMMISSIONING PLAN 27.1 Present Situation Mt. Coffee HPP presents, in a way, a rather unique situation in that it is a reconstruction of a plant that existed already. It is not a scheme to be newly developed, but at the same time it is much more than a rehabilitation of an existing project, given the state of the remaining structures. At present, there are mainly three parts which still exist, namely: � The spillway: the gates are not in working order, but the structure is still in place. � The power house: it has been completely striped of all equipment, only the concrete shell of the building remains. � The embankment dam: it was breached close to the power house, but large parts of it still exist. Rehabilitation of existing and/or replacement of missing structures is being defined in the technical study (prepared by Stanley Consultants). According to first discussions with them and their preliminary reports (February 2012), the remaining structures are in a good state and can probably be used for rehabilitating or rebuilding the power plant. Details remain to be clarified. 27.2 Final Decommissioning Normally, the expected operating life for dam projects is about 50 years. However, if the quality of construction and maintenance of the structures is good, the dam and plant can last much longer. Some dams and power plants have been in operation for 100 years. The end of the operating life of a dam is usually signalled by the fact that the structures, particularly the dam, have reached an age where their safety can no longer be guaranteed. Once this stage is reached, the concerned structures should be removed. There are generally two main options, either (i) to remove the structures (dam, plant and ancillary structures) so as to, if possible, restore the original situation, or (ii) to replace the structures, the dam in particular, in order to continue operations with a new plant. The option eventually chosen will depend on the situation at that time (technical considerations, state of the site and the plant, economic situation, energy supply, environmental considerations, etc...), which cannot be predicted now. Currently, experiences with the dismantling of hydro structures are few, and there is nothing that could be considered as "normal procedure" in these cases. For these reasons, opportunities and key points to consider are illustrated by a recent example: the case of the Rheinfelden plant. The Rheinfleden hydroelectric plant was built in the 1890s on the River Rhine, which forms the border between Switzerland and Germany, about a hundred km downstream from Lake Constance. It was a run-of-rive scheme, part of a cascade of 11 hydroelectric plants between Lake Constance and the city of Basel, with a dam 18 m high and an installed capacity of 25.7 MW.
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.
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Temperature - European Investment Bank

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