Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 301

less rapidly disconnect the hillside aquifers from the lake. This situation can
generate high hydraulic gradients, especially in poorly permeable terrain,
and can lead to movements in the surface cover, or even at depth. In fact,
Antoine (1973) warns of the risk run by light constructions, which are more
and more being built “feet in the water” at the edge of artifi cial lakes.
In France, on the shores of the Sainte-Croix reservoir (Alpes de
Haute-Provence), a slow slide has been active for decades. It is being
carefully monitored, even though the fi lling of the lake slowed its velocity
(Anonymous, 2003).
Behind the Génissiat dam, built on the Rhône, the water level was
brought down to an elevation of 330 m instead of 337 (initial project),
in order to limit the extent of the waves created by the daily oscillations
(which can reach 5 to 6 m). This decision rests on the necessity of protecting
the upstream slopes, which have been highly degraded by active slides
developing in the morainic cover, both on the French right bank (rail line)
and on the Swiss left bank (village of Chancy) (Gignoux & Barbier, 1955).
4.3 Example of the Vajont landslide (Italy)
The Vajont dam was built on a tributary of the Piave, in the Venetian Alps,
for the purpose of hydroelectric exploitation. It is a 262 m high arch dam,
which allowed, by fi lling the reservoir to the elevation of 722.5 m, the storage
of 17 million m 3 of water.
On October 9th, 1963, an exceptional landslide occurred on the left
bank of the reservoir, which it fi lled partially, and displaced the lake water
over the dam, in a wave estimated to be 200 m high, causing 1 700 deaths
in Longarone and in the surrounding areas, while the dam itself remained
unharmed (Figure 141).
The moving mass detached itself from the Mont Toc, on a 1.9 km wide
front and over a thickness of 150 to 330 m, leading to a total volume on
the order of 280 million m 3 . Its estimated travel velocity would have been
between 60 and 90 km·h -1 .
The slide occurred in the Jurassic formations, along a structural surface
between the Dogger and the Malm, where the bedding was locally tilted
to 40°, while it was subhorizontal at the foot of the slope (cross-section in
Figure 141).
It is clear today that ancient landslides had shaped the gentle
morphology of this slope (which can be compared to the abrupt right
bank). Indicators of movement were, in fact, observed during the fi rst
fi lling of the reservoir, as early as June, 1960, which led to the slope being
equipped for topographic and piezometric monitoring, and to a gallery
being excavated under the right bank in order to deviate water should the
valley be obstructed (Letourneur & Michel, 1971).