Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 292

The creation of a drainage network immediately downstream of the
impermeable curtain remains essential in order to account for the real
“discontinuity” of the impermeable screen. Its goal is to intercept residual
water circulation and to dissipate interstitial pressure below the dam. It is
made up of vertical or inclined outfl ow boreholes, discharging downstream
of the dam or collected in a drainage gallery incorporated into the body
of the dam.
Similarly, it is desirable to create a representative piezometer network,
in order to enable the long-term monitoring of the aquifer's behavior, to be
integrated into security checks.
3 IMPERMEABILITY OF RESERVOIRS
When a reservoir is created over permeable terrain, risk of leaks is greater
given the greater hydraulic load.
The least favorable conditions are when a reservoir is built on fl uvial
or glacial fi ll in valleys slowly migrating laterally (epigenetic phenomenon)
and by intensely karstifi ed rock masses, if the geometry of the formations
truly allows the bypassing of the dam.
Dam losses can have a signifi cant impact on the use of the water resource
and can, in extreme cases, lead to the impossibility of exploiting certain
works or to an abandonment of other projects. This is particularly the case
when the revealed or estimated leakage discharge is greater than or equal
to the natural discharge fl owing into the reservoir.
3.1 Epigenetic phenomena
The example of the hydroelectric infrastructure in the Drac valley in Isère
(France) is totally representative of this type of diffi culty.
Carved out during the Riss-Würm interglacial period, the Drac valley
was then fi lled with rocky alluvium, then by glacial clay containing large
blocks, deposited during the Würmian. After the retreat of the glacier,
the Drac re-excavated its course following a different path than the initial
channel, which it cuts back across in places.
It is within this particular context that the Drac valley storage
infrastructure was built, with the establishment of dams in the new sections
of the river channel, termed epigenetic, which cut deep gorges into the