ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

The vegetation of the two study areas has been analized by
phytosociological releues: in La Foppa has been carried out
131 phytosociological releues, of which 108 on the active
rock glacier La Foppa I; in La Vallaccia has been carried
out 171 phytosociological releues, of which the first fourty
in the upper part of the valley, on late Holocenic forms.
Both mosses and lichenic flora have been analyzed.
For both the two study areas' flora has been analyzed
Raunkiaer's life forms by which results in La Vallaccia a
great dominance of Hemicryptophyta, particulary Hemicryptophyta
scaposa, on Chamaephyta with a Hemicryptophyta/Chamaephyta
ratio of 2,67. In La Foppa area, the
dominance of Hemicryptophyta is confirmed, but the H/C
ratio is greater: 4,98 considering all the study area; 4,33 referred
to the rock glacier La Foppa I area and 5,71 for the
other glacial and periglacial forms.
The corological spectra of the species has been analized:
there is a dominance of Orophyte species on Circumboreal,
Alpine-Endemic and Artie-Alpine species, but with
different ratios in the considered areas.
Has been carried out the calculation of the ecological indices
for all the species present, according to Landolt, concerning
some soil characteristics. For all the study area has
been carried out the calculation of floristic richness, of biodiversity
and has been made the Principal Components
Analysis and the phytosociological classification of the releues.
From the analysis carried out, the influence on vegetation
of present periglacial phenomena results significant
for both the study areas; it is possible to observe some differences
of the characteristic of vegetation in the two areas,
in particular from the analysis of vegetation of the upper
part of La Vallaccia.
MAURO CARDINALI ', ALBERTO CARRARA2,
FAUSTO GUZZETTI 1 & PAOLA REICHENBACH 1
A predictive model of landslide occurrence,
Upper Tiber Basin, Central Italy
1 Cnr-Irpi, via Madonna Alta 126, Perugia, Italy
2 Cnr-Csite, viale Risorgimento 2, Bologna, Italy
The diffusion of new technological tools, such as hyperspectral
sensors, high-speed workstations, digital photogrammetric
systems and Gis, has made it possible the application
of quantitative techniques and models in assessing
and forecasting natural hazards, among which landsliding
appears to playa central role.
Today, many instability causal factors, mostly morphological
and geological in nature, can be cost-effectively acquired,
stored and analysed in digital form. In particular, by
processing elevation data and its derivatives new morphometric
parameters can be readily generated over wide regions,
and used as predictors of landslide occurrence. Despite
the potential of such technological advancements,
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landslide hazard mapping remains a complex, ill-formalised
operation. The identification of past and present landslide
depositional/erosional areas, which constitutes the
first step for forecasting future slope-failures, is still a highly
subjective task. Likewise, many basic instability determinants
cannot be acquired and mapped with adequate accuracy.
Many of the methods for handling instability factors
and evaluating hazard levels are questionable. The type
of terrain-unit selected to partition the region under investigation
ii.e.: unique-condition unit, slope-unit, topographic
unit, ctc.), exerts a relevant influence on the reliability
and feasibility of the hazard model developed. Models,
based on different types of terrain-units or statistical
approaches, yield responses that may be statistically comparable
but dissimilar in terms of applicability in lad-use
planning. In addition, when different landslide types occur
over a region, each type requires the development of a specific
model.
In spite of such conceptual and operational drawbacks, the
experience gained from the application of predictive multivariate
models in small pilot drainage basins led to initiating
a long-term, major project which attempts to produce
a predictive model of landslide occurrence in the upper
section of the Tiber river (central Italy), an area over 4100
km 2 in size.
The task involved the generation of a high-fidelity (25x25
m) grid Dtm, totalling 6.5 million heights; starting from
this elevation model, the basin was then partitioned into
20,000 slope-units, each one characterised by 24, automatically
derived, morphometric attributes featuring slopes
and channels. Through an extensive interpretation of
1:33,000 scale aerial photographs and field checks, over
8500 landslide deposits were identified, classified and
mapped at a 1:25,000 scale. Existing lithological, structural,
hydrological, and land-use data were acquired, revised,
reinterpreted and integrated with new data collected in the
field or through aerial photo-interpretation. In particular,
bedding attitude was mapped identifying areas of constant
attitude with respect to the local slope. A catalogue of bibliographical
information on slope failures was completed
for the period 1918-1990 through the systematic review of
newspapers, interview of expert witnesses and inspection
of technical reports. All the above data are currently digitised
and stored into a vector Gis which will eventually constitute
one of world largest databases of high-resolution
spatial data for assessing and controlling slope-instability.
At present, a weighted stepwise discriminant function was
applied in order to discriminate landslide-free and landslide-bearing
slope-units pertaining to the uppermost section
of the Tiber basin (over 1100 km 2 in size). Of the 40 variables
entered the function, those reflecting slope morphometry
and attitude of bedding proved to be the most
powerful in successfully classifying stable and unstable terrain-units.
Such results indicate that the Gis-based, statistical approach,
although no lacking conceptual and operational limit ations,
is the most suitable and cost-effective method for
evaluating landslide occurrence and hazard on a regional
scale.