Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 44
o Application of a value which would be reached according to Swiss
legislation. Here, residual flow is based on Q347, and determined by means
of a stepwise increase, leaving a higher proportion of water in smaller
rivers. This rule was formulated mainly for highly seasonal rivers in alpine
conditions. Applied to St. Paul river (which is not an alpine, but definitely
a highly seasonal river), this would result in a residual flow of 4.3 m³/s.
However, this law also specifies that this is a minimum value, which might
have to be increased depending on specific site characteristics.
o Using the 7Q10 value; this value represents the lowest 7-day average
flow that occurs once every 10 years. This would result in a residual flow
of 8 m³/s. This corresponds approximately to the lowest flow values
recorded in St. Paul river.
� Determination of a value based on site-specific considerations.
8.2.3.3 Evaluation of Options
The affected stretch of the river, between dam and tailrace channel, is rather short, and
given the fact that the slope is very gentle and some riverbed pools even her might
actually be below seas level, it will not dry out completely. Still, it is certainly a habitat
for fish, and a minimum flow, i.e. a minimum amount of actually flowing water, as
opposed to stagnant water in some pools, must be maintained. It has also to be
considered that the river in this part is divided into several branches. If the residual flow
is too small, it would not have the desired effect. On the other hand, the water left
flowing from the spillway means a loss in energy production, a fact which also has to be
taken into consideration. The following Figure illustrates the effect of different amounts
of residual flows on water availability for electricity generation (only situation during
low flow conditions).
m³/s
160
140
120
100
80
60
40
20
St. Paul river water availability for different residual flows
0
70.00 75.00 80.00 85.00 90.00 95.00 100.00
-20
% exceedance
Figure 8-7: Water availability with different amounts of residual flow
total flow
1 unit 100%
1 unit 40%
residual flow 1
available flow 1
residual flow 2
available flow 2
residual flow 3
available flow 3