Geology Reference
In-Depth Information
establishment of a karst-type drainage system and its associated train of
morphologic indicators: vertical wells, horizontal galleries and chambers in
the heart of the aquifer, dry valleys, lapiaz and sinkhole-covered plateaus,
sometimes-stepped springs in the sides of valleys.
In carbonate rocks, karst evolution is relatively slow on the human
scale and its surface repercussions remain rare in the absence of sizeable
aggravations induced by human activity. Surface manifestations, when
they occur, generally concern the most active sectors of the aquifer system:
appearance of sudden losses in perennial or temporary hanging valleys;
localized collapses in the bottom of closed depression, sometimes near
existing ponors; opening of sinkholes in dry valleys or closed depressions;
temporary modifi cation of emergence conditions at the level of springs.
In the same vein, the Tournaisis collapses in Belgium were a result of
natural active dissolution of the Carboniferous limestone below the Escaut
plain (Derycke, 1979).
Saline and gypsic rocks, on the other hand, are characterized by a high
solubility, which can lead, when they are subject to permanent groundwater
circulation, to the export of large volumes of dissolved material.
Nicod (1991) estimates the annual volume of dissolved substances
evacuated by the Foux de Draguignan (Var), which emerges from the Triassic
units on the left bank of the Nartuby with an average annual discharge of
900 L·s -1 , to be 28.4 million m 3 , 85% of which are sulfates and chlorides.
Reynaud (2000) estimates the following annual quantities, from the Folle
and Mescla springs (Alpes-Maritimes), which respectively drain a Triassic
diapir and a carbonate mass on the right bank of the Var:
• Folle spring (average discharge of 20 L·s -1 ): 4 850 m 3 of halite and 1 180
m 3 of gypsum,
• Mescla spring (average discharge of 120 L·s -1 ): 5 610 m 3 of halite and
1 885 m 3 of gypsum.
To their high solubility can be added the weak mechanical resistance of
these rocks, their alteration in the presence of water frequently generating
an easily movable, pulverulent material. Karst evolution is therefore very
rapid, generally visible on a human time scale, and can be very dangerous
for people and property, particularly in urban and peri-urban areas.
The collapses in the Paris region, a result of the dissolution of Lutetian
gypsic masses, are a perfect illustration (Toulemont, 1984 and 1987).
Similar risks are generated by Triassic evaporites in the Var department,
particularly in the upper basin of the Foux de Draguignan, which has a
sodium chloride and calcium sulfate mineralization on the order of 1.78 g·L -1
(Nicod, 1990 and 1991). The effects of underground dissolution reach both
into the old city of Draguignan and into some its younger neighborhoods
(frequent collapses), as well as the banks of the Nartuby, which are regularly
Search WWH ::




Custom Search