geographia - Studia

O. POP, J. - C. THOURET, V. SURDEANU
3. METHODS
Our study is based on data obtained from the analysis of the aerial photographs
(the mission 1999 and 2004 of the Institut Géographique National), DEM (digital elevation
model), field investigation, dendrochronology, archives and text references.
The 10 m spatial resolution of the DEM was obtained by digitizing the contour
line on the topographical maps at scale 1:25 000 of the Sancy massif.
Fieldwork allowed localizing and recognizing of the morphological characteristics
for the three zones of each landslide (detachment, track and accumulation zones). Lithological
and geomorphological maps of the landslide areas were finally obtained using the ArcGIS
9.0 software.
All the morphometric measurements (distances and areas) have been obtained by
the GIS analysis. The landslide volumes were calculated by the following formula (Surdeanu,
1998):
V= [0.785 * H * L * (L – A)] * N
Where:
H = the height of deluvial deposits;
L = the width of deluvial deposits;
A = the length of deluvial deposits;
N = number of landslide steps (see fig. 8).
In the case of Egravats landslide, the archives mention events in the past. In this area,
the local press confirms some reactivations of the landslides and debris flows in 1900, 1944,
1951 and 2004. But there aren’t any texts mentioning the time when the landslide occurred.
Because of the presence of the forests, we have used dendrochronology in order to
determine the tree colonization period of this area. On this purpose, 13 increment cores and
3 stem discs were obtained from fir trees (Abies alba) growing on the landslide body that
we have visually estimated to be the oldest trees. Sampled trees were chosen so as to avoid
those affected by the anthropic activities (e.g. the firs lining the forest road). After drying
and sanding of the samples, we have counted their rings and obtained their ages. Because
not all the increment cores reached the pith, we have approximated the missing rings using
a pith locator.
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4. RESULTS
The lithological and geomorphological map of the three landslide areas are
represented in the fig. 3, 5 and 7.
Table 1 presents the values of the main morphometrical parameters of the three
landslides calculated on the DEM. By comparing these data we can observe that the largest
landslide is the one affecting the slope of the glacial valley situated below the Montagne de
la Plate scoria cone and lava flow.
In the case of Egravats landslide, the minimum age colonization of the trees sampled
and determined by dendrochronology is between 1840 and 1940.