Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 299

Figure 139 Thorenc polje (Alpes-Maritimes). Conceptual outline of the development
project.
4 SHORE INSTABILITY OF RESERVOIRS
4.1 Natural instability
Large-scale landslides (rockfalls or rockslides) can sometimes block a valley
and cause the creation of a lake.
This lake is generally quickly resorbed due to the erosive action of water
arriving from upstream, which opens a breach in the natural dam that had
been created. Such was the case in the Alpes-Maritimes department, where
the exceptional rainfall in November, 1926 led to the multiplication of large
landslides in the upper Vesubie basin and the complete obstruction of the
Madone de Fenestre valley and of the Vesubie's course at the place locally
known as “La Muselle” (Perriaux, 1927).
The emptying of these lakes is sometimes artifi cially prompted by
humans, when it is necessary in order to protect nearby infrastructure and
development. On January 30th, 1948, a large landslide affected the slope
on the right bank of the Var valley, immediately downstream of the village
of Puget-Théniers (Alpes-Maritimes). The mobilization of a hundred or
so thousand m 3 of gypsic clay, at the place locally known as “Le Breuil,”
blocked the river across its entire width, creating a reservoir of 440 000 m 3
and cutting off the road and rail line along the left bank. Breaches were
then opened with explosives in the displaced mass in order to accelerate
the return to the initial hydraulic state.
Finally, permanent lakes resulting from such processes do exist: the
Lac de Sillans in the Jura, and the Lac de Parouard, in the Haute-Ubaye
(Goguel, 1967).
Ancient landslides can also lead, over time, to a diversion of the river's
course (characteristic dogleg turn) or can produce a simple local narrowing
of the valley, which creates an ideal dam emplacement. There is therefore