Management of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 223

In Croatia, the Ombla project (Figure 116) consisted of damming the
karst circulation feeding the Ombla spring and waterproofi ng the rock with
the help of a 100 m grout curtain supported by fl yschs, in order to form an
underground dam intended to supply water and to feed a 68 MWe electric
plant in the Dubrovnik region (Breznik, 1998).
However, underground dams can modify the stability of the limestone
units in which they are built, or cause the reopening of unused karst
conduits. Following a fl ood, a collapse occurred over the underground
reservoir in Jedres (Herzegovina) (Milanovic, 2001).
Figure 116 Block diagram of the Ombla project (from Breznik, 1998).
4.2 The case of the Lez spring
The vauclusian emergence feeding the Lez river drains the northern
Montpellier limestone plateaus, of which it is the low point. The basin
extends over approximately 400 km 2 . The natural discharge varies between
300 and 10,000 L·s -1 .
The spring has supplied the town of Montpellier since the 19th century.
The demand for water increased regularly, and the contributions from the
Lez went from 25 L·s -1 in 1859 to 400 L·s -1 in 1931. Gravity-driven extraction
then reached the limit that the spring could provide. In order to increase
the available discharge, pumping tests were done in the spring's pool in
1965. They enabled a discharge increase to 1,000 L·s -1 with a drawdown
of 8 m. At this period, advances in sub-aquatic speleological exploration
enabled the exploration of the karst passage upspring of the pool and the
observation that this passage continued deep into the heart of the limestone