Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 297

133). On the right bank, a grout curtain was put in place without diffi culty,
extending to the underlying Helvetian sandy clay. On the left bank, the
particular structure of the site (faulted syncline) imposed the digging of a
gallery slightly above the water surface in the reservoir, serving as a work
chamber from which the grout curtain could be built. Its construction
required 5,824 meters of injected boreholes and 2,631 metric tonnes of
injected material. The construction enabled a 90% reduction in leakage
discharge.
The leaks affecting a reservoir in a karst site generally feed preexisting
springs and use previously empty or fi lled-in passageways. The clearing
out of cavities can be very sudden, and resurgences often appear less than
2 km away from the reservoir. Thérond (1973) showed that the natural
catchment basin of the emergences experiencing increased discharge due to
leakage collection extends to the reservoir. He estimates that the presence of
a karst passage less than 150 m away from a spring constitutes a signifi cant
risk. The extent of a spring's drainage basin can be rapidly evaluated, if
the specifi c infi ltration modulus of the concerned area is known (a 10 L·s -1
spring corresponds to a 1 km 2 basin for a 10 L·s -1 ·km -2 modulus).
In the case of the most recent projects, examples of success are happily
more common than failures, due in particular to the undertaking of long
hydrogeologic studies, enabling an understanding of the major outlines
of karst circulation, and of paleo-geographic studies, evaluating the depth
of karstifi cation and therefore the depth to which grout curtains must
extend.
In France, the Génissiat dam on the Rhône (104 m), the Vouglans dam
on the Ain (130 m), and the Castillon (100 m) and Sainte-Croix (90 m) dams
on the Verdon were built on karst sites and were in some cases faced with
sizeable underground cavities (Caborne de Ménouille, Fontaine l'Evêque).
The undertaking of long studies and the construction of large grout curtains
(44,400 m 2 in Castillon, 50,000 m 2 in Sainte-Croix) enabled the success of
these projects.
3.2.3 Special case: storage in poljes
The use of poljes to create reservoirs is particularly common in Dinaric
karst. These are endorheic basins, drained by absorbent karst cavities
called ponors. When the discharge collected by the basin is greater than
the infi ltration capacity of the ponor, the polje fi lls with water and a lake
forms. Lake formation can also be provoked by upwelling from the karst
aquifer, the ponors (in this case termed estavelles) then acting in the reverse
direction. Successive inundations shape the relief, creating vast, multi-