The Exploitation of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 181

cleared of any unconsolidated covering and of its weathering mantle, in
order to limit the risk of contamination by lower quality surface or shallow
water (Figure 95a).
If the water is emerging from a heterogeneous material, for example
within a paleovalley fi lled with rock debris and scree (porous reservoir), the
catchment system will be completely closed and equipped with drainage
channels at the base of its uphill wall. It is dug down to the impermeable
substratum, in order to avoid leakage below the fl oor. (Figure 95b).
In all situations, the catchment system also includes a stabilization
chamber for the water, revealing its natural piezometric level, and a header
chamber, fed by the stabilization chamber.
The stabilization chamber also plays the role of decanter, and should
be equipped with a basal drain.
The header chamber is equipped with a lateral fl ood drain, and a
submerged section of perforated tubing at the head of the outgoing pipe.
Figure 95 Typical diagrams of point-source catchment systems directly touching the water-
bearing formation (a) and at the base of a relay reservoir (b).
2.2 Drainage galleries and trenches
When the emergence is diffuse and extends along the contact between the
aquifer and its impermeable substratum over a given length, it becomes
necessary to reconcentrate the discharge by cutting across the maximum
number of small outfl ows.
This objective can be attained only through the emplacement of
drainage trenches or galleries.
￿ Trenches are levelled, with a high longitudinal slope, then fi lled with
clean, draining material, after the emplacement of a gutter and of
perforated tubing in the lower section,