Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 43
2.1
-1.2
-1.2
-1.2
-1.5
8.2.3.2 Options
-1.2
-2.1
-2.4
-2.4
-2.1
-0.9
-1.5
-0.3
Figure 8-6: Tailrace channel
0.3
1.2
1.2
2.1
3.1
The situation is different for the original river bed below the dam and spillway structure.
Once the dam in place, no water will be left flowing there if the spillway gates are
completely closed. During much of the time (47%, see Figure 8-4), there will be some
amount of spilling, but the rest of the time basically all water will be used for producing
energy. This means that unless a minimum flow is defined which will have to be
released at all times from the spillway structure, these approximately 4 km of river
would be dry for about 50% of the time, or at least without flowing water, with just
some pools left in the deeper parts of the river bed. Since this would be detrimental to
aquatic fauna in this part of the river which is an important habitat for the rock fish
(Labeo curriei, see Chapter 11), and would be in contradiction to international best
practice, a riparian flow needs to be defined and maintained.
Liberia does not have a legal basis for defining such a minimum flow, and there is no
generally accepted rule for this. In this case, the following possibilities can be
considered for determining the amount of water to be released as minimum flow:
� Application of a rule used in or defined for other sites. Such rules could be e.g.:
2.7
1.8
3.1
o Defining the residual flow as Q(95%) or Q347, i.e. as the flow exceeded in
95% of the time or on 347 days per year on average; this is a rather
simple approach used in a number of countries, with an easily
determined quantity of water; in the case of Mt. Coffee HPP this would
be 22 m³/s.
-0.6
1.5
1.5
1.5
0.6
0.3 3.1
-2.1
Note: elevation values displayed in
black are ft asl and in read are m asl