Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 223
Greenhouse gas emissions: large reservoirs submerging high amounts of vegetation
can become substantial sources of greenhouse gasses (GHG). This is especially the case
in for deeper reservoirs, which will develop a stable thermal stratification. In such a
case, the deeper parts of the reservoir will quickly become anoxic, since all the available
oxygen will have been consumed by the process of vegetation decomposition. The
ongoing breakdown of organic matter (mainly leaves, herbs and finer twigs, the socalled
“soft biomass” will lead to the generation of methane, which is a very powerful
GHG. While this will not occur in the case of MC HPP (small and shallow reservoir,
large amount of water flowing through and therefore constant replenishment of oxygen
also in its deeper parts, see analysis in Section 8.3.5), there is certainly a high
probability for such a situation to occur in the case of the very large Via reservoir,
which is also deep enough for most probably developing a stratification. Other negative
effects on water quality can occur in such a situation, most noticeably the development
of hydrogen sulphide (H2S), which is very toxic for fish and very corrosive for metal
and concrete structures.
Impact on forests and wildlife: as mentioned, the two uppermost reservoirs are located
in a forested area, and at least partially in a forest reserve. These forests are probably
also an important habitat for wildlife. A part of the reserve will be destroyed
permanently by becoming submerged. In addition, the construction of access roads and
the presence of a large work force during the construction period will likely generate
additional pressure on these habitats and their wildlife.
Impacts on riparian habitats: all along the river, there are habitats which depend on or
are adapted to the high seasonal fluctuations in river discharge. A large storage reservoir
in the upper parts of the river basin will considerably reduce these seasonal fluctuations
(lowering peak flow in the rainy season and increasing low flows in the dry season, see
above). This can have positive effects (reduction of the risk of floods, higher availability
of water during the dry season), but it can negatively affect riparian habitats like
floodplains, gallery forests etc., and the wildlife as well as human uses of such areas.
Effects on the estuary: the effect just described will also be observed in the estuary of
the river. This can have mainly two indirect effects. The first one is on salt and the risk
of salt intrusion. Higher flows in the dry season will result in a reduction of the risk of
salt intrusion and therefore be beneficial. On the other hand the reduction in seasonal
high flows might reduce the self-regulating capacity of the system by reducing its
ability to wash out salt. Without in-depths analyses, it is difficult to recognise which of
these contrasting effects will prevail. A second effect, closely linked to the first one,
could be on the mangroves in this area. They depend on the dynamic of the system, and
especially on a balance between salt and fresh water, and a change in this balance could
put an additional stress on them.
Impacts on water users d/s of dams: this could be a subject especially in the 25 or so
river km below SP-4 dam. If no measures are taken, all the water from the St. Paul
River will be diverted to the Via reservoir, leaving nothing (except the runoff from the
small intermediate catchment) in this part of the river. This would have a detrimental
impact on any aquatic biota in this area, and could severely affect human populations
which use the water there in any way (for fishing, drinking, washing, transport etc.). A
residua flow will have to be defined for this part of the river.