Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 302

Vajont
Figure 141 Vajont landslide (Italy). Map and representative cross-section of the site (from
Letourneur & Michel, 1971).
During the second fi lling, in 1962, the slope reactivated when the
water level reached an elevation of 700 m, but its stability returned with
a lowered level.
During the third fi lling, in 1963, it was also beginning at an elevation of
700 m that noticeable accelerations in the movement of the slope appeared. It
was decided that to keep the water level at an elevation of 700 m in order to
minimize the risk, but starting in early October, the slope movement velocity
increased, reaching 20 cm·day -1 on October 9th (Anonymous, 2003).
The disaster was, on the one hand, a result of a poor choice of location
on a site that had previously been shaped by ancient landslides of great
magnitude, and on the other hand a result of the successive fi lling and
emptying of the reservoir, and, particularly, of the last rapid emptying to
return to the elevation of 700 m, which favored the saturation of already-
reworked terrain, then initiated the failure mechanism by suddenly
increasing the hydraulic gradient.