Technique: UNDERGROUND CATCHMENT TUNNELS - Foggara
Local name: Foggara
Site: Wadi Saoura, ALGERIA
LOCATION
 |
Continent: Africa
Country: Algeria
Site: Wadi Saoura
Coordinates:
|
DESCRIPTION OF THE LOCAL VARIANT OF THE TECHNIQUE
In the Gourara and Tuat regions of the Algerian Sahara, these antique methods of water production and the complex procedures tied to their management are still used. There are about a thousand foggara, half of which are still working, with underground development from 3,000 to 6,000 kilometers. On the surface, a series of wells are recognizable by a raised edge, the result of debris from excavation, which identify the tunnel. The wells, at a distance of about 8 to 10 meters apart, guarantee the aeration during works in the subsoil and are useful for maintenance works but do not have the ability to draw water. The excavation of the foggara, contrary to what is done in the Iranian qanat, begins from the place of establishment returning then to the edge of the alluvial cones of oued fossils.
The foggara, unlike an adductive channel, are not conveyors of water from springs or other sources such as underground wells at the point of use, but across their linear development they pick up the micro flows infiltrated in the rocks and create open water; they work as manufacturing devices or water mines. The tunnel excavated parallel to the ground does not sink into the groundwater, but drains the upper part of an aquifer without causing the lowering of it and only absorbing an amount of water compatible with the renewable capacity. The area of the underground water supply resembles a large sponge rock rather than an underground basin. This is fed by micro flows headed towards the sebkha, the surfacing of deep aquifers, consisting of non-renewable geological persistence and atmospheric supplies differentiable into three types.
The first type of atmospheric input comes from flows that run under the sand of the erg and are due to the rains that have fallen to the north on the high plains and the Saharian Atlas Mountains. These mountains are located thousands of kilometers away, the distance that it has taken the micro flows 5,000 years to fill up under the sands of the erg in order to reach the oasis where it is collected, and this water supply is therefore made up of rain precipitated in prehistoric times.
The second atmospheric input is made up of normal precipitations that in this area don’t exceed 5 to 10 millimeters annually. Given the enormous dimensions of the water collection basin however, even these minimal components can, if not wasted, provide a significant contribution of water. In fact, if the rains of the Gourara seem miniscule in respect to the temperate zones where the precipitation reaches 3000 millimeters and to the arid zones which are characterized by an annual rainfall of less than 300 millimeters, it must be considered that even 5 millimeters of precipitation on a surface 10 millimeters in height has significance, if 5000 liters of water could be collected.
The third is due to phenomena even more unthinkable. It has to due with quantities of water produced by means of condensation on surfaces. It is the cosiddette phenomenon of precipitation fundamentally concealed in the ecology of the desert. This permits the gazelles to quench their thirst by licking the nocturnal dew from stones and beetles and lizards to absorb water from the atmospheric humidity; these water supplies are indispensible to them. Due to the difference in temperature between day and night, often exceeding 60 degrees, a nocturnal condensation forms in the soil that wets the sands and, dried from the rays of sun, provokes the typical hard crust from the cracking that occurs when it is stepped on. Precisely managed, the precipitations succeed in creating an important water reserve. Appropriate hydraulic devices allow the collection of water vapor from the atmosphere and for the conservation of it in the subsoil before it disappears with the new day. In certain conditions, it is possible to obtain four liters of water on a surface of only one meter squared during the night in a desert. In this way some networks of foggara typical of the Tuat are nourished because they are not dug in deep soil and therefore are called superficial foggara.
PROSPETTIVE DI SOPRAVVIVENZA
Technique at risk
IMAGES
Outlet to a foggara (source: IPOGEA)
