Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 249

de-Tinée and a slice of water equivalent to 600 to 900 mm induced by the
fusion of mountain mass's snow cover. One can, indeed, observe a very
clear correlation between phases of acceleration and local precipitation,
and especially with snowmelt (Follacci, 1987).
The most recent hydrogeologic research, based on a detailed inventory
and quantitative monitoring of the outlets, as well as on isotopic and
chemical analyses of the extracted water, detail the role of water in the
disequilibrium of the slope (Compagnon et al. , 1997; Guglielmi et al. , 2000
and 2002):
• infi ltration occurs preferentially over a recharge area above the slide,
where a perched superfi cial aquifer is developed over an area of 4 to 5
km 2 . There exists a lateral perennial drainage of this spring, but most
water is injected through vertical exchange into the moving mass,
where it rejoins a deeper aquifer;
• a secondary infl ow of water appears to originate from the tectonically
deformed Triassic syncline on the southeastern edge of the slide
and clearly marks certain deep aquifer outlets with a strong sulfate-
magnesium signature, with, in particular, sulfate concentrations
reaching 800 mg·L -1 ;
• a rough budget can account for only a small portion of the quantities
infi ltrated discharged at various outlets (13% in lateral springs and
1% in basal springs), which indicates signifi cant internal circulation
and hidden infl uxes directly connected to the basal alluvial aquifer
(86% of infi ltration);
• it is essentially during the melting of the snow mantle at higher
elevation that the infl ow quantities are large enough to result in a
signifi cant recharge of the rock's fracture network and to maintain the
mechanism under way.
Given the size of the phenomenon, the topography of the area, the
apparent depth of the ruptures (100 to 200 meters) and the impossibility of
considering an economically feasible reinforcement system, the responsible
parties opted for careful and continuous monitoring of the phenomenon
and for measures directly oriented towards the protection of people and
property:
• topometric monitoring of the slope's surface has been operational
since 1982 and enables the following, with a good level of precision
and total regularity, the kinetics of the slide;
• an alert and rescue plan was created in order to coordinate the response
in the event of a sudden rupture;
• prescriptions were included in the PPR beginning in 1983 in order to
defi ne at-risk zones requiring evacuations or construction bans;