Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 111
This downstream site showed no discernible flow velocity or direction. Still, all the
values recorded here did not suggest an intrusion of salt water due to tidal movements.
16.2 Saltwater Intrusion
16.2.1 Prevailing Situation
Mt. Coffee dam is located at the point where the river enters the coastal plain. This plain
is very flat and only very little above sea level. In fact, the bed of the tailrace channel, or
at least its deeper parts, is below sea level (see Stanley 2012, Inspection Report p. 2-28),
and this is presumably true also for the entire St. Paul river d/s of the power house. This
means that during high tides sea water is likely to flow into this part of the river. This
could lead to an increase in salt concentrations in the ground water table in the coastal
plain, but mainly it could lead to higher salt content in the water taken into White Plain
water treatment plant, potentially making this water unsuitable as drinking water.
The difference between high and low tide in Monrovia is about 1.4 m (see Figure
below).
m
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
Tides in Monrovia, April 2012
1 2 3 4 5
Figure 16-1: Daily tidal fluctuation in Monrovia
Source: http://tides.mobilegeographics.com/calendar/month/3875.html
During periods of high river flow, the pressure of fresh water from the river will be
sufficient for preventing salt water intrusion beyond the immediate area of the river
mouth. However, during periods of low river flow, i.e. in the dry season, mainly
February and March, the backflow from the sea during high tide could actually reach
the area of the White Plain water intake.