Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 298

kilometric closed depressions, with a fl at bottom at an altitude dictated by
the location of the ponors.
The sediments accumulated at the bottom of poljes are relatively
impermeable, often made up of irregularly alternating clay and gravel. It is
therefore tempting to take advantage of this natural coating to store water
outside the high-water period, by blocking the ponors. The success of such
adventure depends on the nature and thickness of the deposits accumulated
in the depression, the possibility of successfully blocking the ponors, and
the absence of risk of other areas becoming unblocked. Examples of failed
attempts are numerous, as water frequently fl ows around any construction
and opens up old passageways.
The Slivlje ponor, draining the Niksicko polje in Montenegro (former
Yugoslavia) was encircled in this manner in the 1950s by a chimney 50
m in diameter and 20 m high, in order to enable the creation of a water
reservoir. The construction was based directly on the limestone bedrock,
through approximately ten meters of clayey silt. As soon as the basin was
fi lled, new absorption points opened around the edges of the blocked
ponor, and old cavities were cleared out, enabling the leakage of sizeable
volumes of water.
In Thorenc, a small tourist resort in the Alpes-Maritimes (France), the
creation of a pleasure lake was attempted around 1,900, with a similar
arrangement. The site was a small polje, at the contact between Cretaceous
marl and Jurassic limestone, fed by a permanent stream and drained by two
ponors located at the lowest point in the depression. The polje is located at
an elevation of 1,165 m and tracer tests revealed a rapid connection between
the infi ltrated water and the Mouna spring, at an elevation of 1,000 m. The
ponors were encircled by chimneys anchored to the bedrock. These were
built out of concrete, and equipped with sluice gates in order to regulate
the water height in the reservoir and to allow the evacuation of overfl ow
into the natural ponors (Figure 139). The lake fi lled on the fi rst attempt,
but hydraulic short circuits soon appeared in the immediate surrounding
of the chimneys, absorbing all of the accumulated water. The project was
abandoned and the two chimneys still standing in the middle of a grassy
plain now serve as a reminder of this abortive project.
In order to create an adequate degree of impermeability in the fl oor of
poljes, the emplacement of an impermeable membrane is often necessary,
but its stability can be compromised by upwelling water in estavelles, when
the karst aquifer rises. The Popovo polje reservoir (Bosnia and Herzegovina)
was waterproofed in this way by a membrane, and equipped with discharge
wells enabling the evacuation of air pushed out by temporary rises in the
water table in the passageways under the polje.