Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 300

signifi cant danger in building such a dam, due to the risk of failure or of very
onerous stabilization work. The fate of the Cheurfas dam (Algeria) should be
kept in mind, where the right bank support, founded on an old scree slope,
was swept away in 1885 (see chap. D4-2.1 and Figure 133). Its reconstruction
had to be relocated immediately upstream, out of the reworked zone.
This risk also concerns reservoirs, when they fl ood the base of old
landslides. Remobilization is always a potential danger, even if the reservoir
creates a stabilizing counter-pressure. The most delicate phases consist of
large variations in the water level (fi lling and emptying of the reservoir),
which modify the pre-existing hydrodynamic conditions and can generate
landslides of varying magnitude.
Gignoux & Barbier (1955) cite the abandonment of a dam project on
the Romanche, near the village of Villar-d'Arène (Hautes-Alpes), after
the preliminary investigations revealed large masses of schisty Lias slide
material making up the right bank (Figure 140). Indeed, below these
reworked materials, a borehole cut across ancient alluvium from the
Romanche, revealing the dam created by the ancient landslide over a
height of 40 m and the displacement of the river's course approximately
70 m towards its left bank.
Figure 140 Villar-d'Arène dam project site (Hautes-Alpes) (from Gignoux & Barbier, 1955).
4.2 Role of emptying
When the reservoir level is relatively stable, hillside aquifers are sustained
by the water surface, with which a stable hydrostatic level, and therefore
a new state of equilibrium, is established.
The variations in water level resulting from the exploitation of the dam
(periodic oscillations of hydroelectric reservoirs) or from monitoring or
maintenance operations (partial or total emptying), will, however, more or