Module 4 - Construction of a gravity fed system

Reinforced concrete constructions will be preferred because they guarantee an easy
maintenance (catchment box easily accessible). Gabion constructions must be avoided when
possible because they rarely have inspection doors and are buried in the ground.
Three rules must be imperatively respected to build a spring catchment:
Catchments should never be subject to back-pressure: the water level in catchment
box or in the drain must always be lower than the water discharge area level before work.
Catchments must allow draining the aquifer layer by authorizing a drawdown of the
piezometric level, but not to increase the pressure, which could make the spring disappear. A
stake can be used (planted quite far in order not to disturb the excavation works) to keep
the reference of the initial level of the water discharge. This level is used as reference mark
during work construction: the outlet and the overflow pipes have must be always located
below this level. Still to avoid an accidental back-pressure, the installation of an overflow
pipe is obligatory; if you do not have any information on the spring maximal flow, the overflow
must be oversize (two pipes of 2” for example).
The dam must be constructed on an impermeable ground: the excavation should not be
stopped before reaching the substratum (rocky watertight layer). It sometimes represents
large works, but it is essential to prevent water from flowing under the catchments few
weeks after the end of the construction work. The substratum is sometimes difficult to
identify in the field: thus, you must consider it as a layer of soil less permeable on which
water circulates without infiltrate.
Catchment must be protected: protection works should always be included in the
catchments work, on the level of the drain cover (plastic cover) or the catchment box.
Fig.4: “Gabion type” spring catchments
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