Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 48
8.2.3.6 River Management Measures
The release of a residual flow serves the purpose to keep intact, to the extent possible,
the functioning of the affected 4 km of river which would otherwise be completely dry
during a considerable period of the year. The main objective of a residual flow is to
maintain a certain amount of water flowing in the river bed at all times in order to
preserve is function as an aquatic habitat for fish and other aquatic organisms, and in
this way also maintain the possibilities for using this river by the local population
(mainly for fishing and washing, since there is no major use of the river for drinking
water, and there is no irrigation).
Still, it is clear that the river will not remain unchanged. The Project will inevitably
have effects on it, the main being the fact that river flow would be limited to the residual
flow during the dry season, but can still be quite substantial during periods with spilling.
However, as explained above, 8 m³/s would be sufficient for maintaining a constant
flow during the dry season, preventing it from drying up completely.
In the present situation, it is impossible to say how this amount of 8 m³/s would flow in
the river bed (distribution on several channels, or all in one channel; visible river over
the entire stretch, or running hidden below boulders and gravel in some places; joining
all the important parts, i.e. the deep ponds, or leaving out some of them, which could
lead to the death of fish trapped in them; etc.). It is therefore recommended to carry out
a study, as part of the monitoring of fish populations proposed), to identify measures
that might be required for improving the situation, i.e. for getting the most benefit
(environmentally and economically) from this measure.
The following is recommended:
� To make an inventory of relevant habitats (spawning areas, deep ponds, etc.) in
the affected part of the river.
� To observe the flow pattern, especially of the residual flow during the dry season
(channels with and without water, characteristics of the residual flow in the river
bed).
� To identify (potentially) important structures (ponds, gravel beds suitable for
spawning, feeding places) and the degree to which they are affected by the
changed river flow pattern.
These observations will have to be carried out during the first years of operation of the
power plant. Monitoring results obtained during the construction phase will serve as a
baseline.
The results of this study will then show if the situation is satisfactory, or if there is a
need for improving it, and possibilities to do so. Some measures that might be useful for
improving the situation could be the following:
� Modifications of the river bed immediately below the spillway in a way which
would allow to distribute the released water to separate into two or more
channels (or for keeping it in one channel, if this should be considered as more
beneficial).
� Digging a longitudinal channel into the river bed in order to concentrate the
limited flow there and to prevent it from spreading too thinly in the existing river
bed. This is a measure applied mainly in rivers with low gradients and a wide