Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 294

3.2 Karst substratum
In limestone and dolomite formations, the narrowness of the canyons and
the good mechanical characteristics of the rock are factors favorable to
dam construction. Nevertheless, the presence of more or less evolved karst
networks and of natural cavities of various sizes can be a daunting obstacle
to the fi lling of reservoirs.
3.2.1 Extreme risk of empty reservoirs
The most representative example is that of the Montejaque arch dam in
Andalucia (Spain), 73.5 m high. It was built on the Rio Gaduares, which
fl ows over a polje before disappearing below ground in the del Gato cave.
After a 3.8 km underground course, the water returns to the surface to the
south, on the edge of the Rio Guadiaro, 160 m lower in elevation.
The dam, located 500 m upstream of the cave, was to have enabled the
creation of a reservoir with a capacity of 5 million m 3 over impermeable
fl ysch. As soon as it was fi lled, leaks appeared and increased the discharge
flowing in the del Gato cave and emerging at the Benaojan springs
(Figure 138).
The losses reached 4 m 3 ·s -1 and tracer tests showed groundwater
circulation velocities reaching 160 m·h -1 . The average discharge into the
reservoir being of 0.8 m 3 ·s -1 , these leaks prevented the creation of a reservoir.
They proved uncheckable, despite ten years of studies and construction,
and led to the abandonment of the project in 1925. The dam today serves
only to dampen fl ood peaks during the winter.
The same fate befell the Maria-Christina dam, near Castillon-de-la-
Plana (Spain) and the small Saint-Guilhem-le-Désert dam in the Hérault
(France), both built on highly karstifi ed limestone, without a real capacity
to store water (Lugeon, 1933).
Such failures, often ancient, are clearly a result of inexistent or very
inadequate geologic and hydrogeologic studies.
3.2.2 Effects of waterproofi ng projects
Relatively limited construction can sometimes solve the problem, when the
leaks are very much localized. In this manner, the high losses generated
by the activation, in 1925, of the Bouvante dam in the Vercors (France)
were reduced in 1937, by covering the absorbent zone at the bottom of the
reservoir with a layer of concrete, reinforced with a metal trellis (Gignoux
& Barbier, 1955).