Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 207
Figure 26-1: Overflow and erosion of an embankment dam
Source: Powledge and al, 1989b
Subsurface erosion is caused by weathering of the dam material by permeating water.
This takes place in many ways, for example: suffusion, piping. It begins downstream
and proceeds towards upstream. The increase in the percolation rate and of the velocity
of flow continuously increases erosion of the dam materials or the foundation. When
finally a part of the dam gives way, this leads to the formation of a breach.
The final width of the breach that is formed can be in the order of magnitude of 5 to 10
times the height of the dyke. In the case of the MC dam, the height of the embankment
is 18 m, therefore a breach of a width of 90 to 180 m could be assumed (note: in the
case of MC dam, this hypothesis can be checked, since it was breached already 20 years
ago).
Still, the rupture of an embankment dam is a rather slow process in comparison to the
breaking of a concrete structure, and therefore less dangerous. It gives more time for
taking measures, like lowering the reservoir level by opening the spillways, and alerting
the population in the endangered downstream area. Likewise, the increase in river
discharge is, while still being rapid, gradual and not a massive surge wave.
The linear shape of the MC dam and its shallow depth (in fact, it is a long dyke) makes
complete rupture of the dam impossible. The common type of risk of rupture envisaged
on this type of dams is the gaps in the impoundment. The rate of outflow will therefore
be equal to a partial or complete opening of the spillway gates.
The upstream villages around the reservoir would not be affected by dam rupture; on the
other hand, all the settlements downstream of the dam just bordering on the river will
suffer a more or less rapid rise of the water level depending on the distance from the
dam. This excessive rise in water level can cause partial or complete flooding of
villages, and accidents to persons close to or on the river, as well as loss of material and
property.
26.1.3 Prevention and Protection Strategies
26.1.3.1 Dimensioning of the Dam
The most important prevention measure is the dimensioning of the dam, which will
have to be done in a way as to minimise the risk of a dam break.
Evaluations of the dam break risk situation were made in Stanley Consultant's Dam
Safety Report (Draft, February 2012).