ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

cially during intense rainfall periods, increasing the probability
of triggering' shallow landslides. In fact, during the
extreme rains (400 mm/24 hours) of last summer (February/96),
when landslides (rock debris avalanche type)
have spread onto these steep slopes, 42.3 % occurred under
the degraded forest cover and 43.2 % in grasslands. An
extensive field survey around the 1996-landslides generation
have shown that root systems have ambivalent hydrological
and erosive functions: when alive, it favours infiltration
and soil formation into the bedrock joint-network and
hold the residual blocks, specially in homogeneous rocklithology;
when dead, it favours turbulent pipe flow generation
and ceases its holding function, leaving unstable the
residual blocks. It seems probable that block detachment
was at least partially responsible for the initiation and propagation
of the extensive rock debris avalanches, which
were triggered in the very uppermost portion of the mountain,
where a highly jointed granitic bedrock dominates.
One cannot say that a well preserved forest cover would
impede such catatrophic events, but certainly the prevailing
degraded forest vegetation have increased the propagation
effects, as indicated by some field evidences. Some
of the 96-landslide events were recurrent from older landslide-scars,
particularly from February 1988. The short time
interval for such recurrence was mainly associated to
forest degradation due to border effects around the previous
scar. Therefore, we have started another investigation
line searching for a methodology to recover the vegetation
within landslide scars of different sizes and soil conditions:
the main goal is to improve the vegetation growth
and quality within. the scar and its surrounds in order to
get back (and faster) those elements which are relevant in
controling hillslope hydrology as' we found in the local,
well-preserved forest ecossystem (litter cover, fauna activity,
macropores in the A-Horizon and deep Toot systems)
and, consequently, to promote the mitigation of hillslope
erosion (with deep root systems anchored within cohesive
materials) on steep slopes. In the last two years we started
to monitore the vegetation succession by spontaneous and
induced processes, within a few landslide scars together
with measurements of both hydrological and erosive responses
to rainfall inputs. Field data will be presented and
discussed during the meeting.
ANTONIO C. COLANGELO & OLGA CRUZ
Spatial magnitude-frequency index ofmassmovement
event deposits in an humid tropical precambrian
plateau, and its connection with MFI of daily ra·infalI:
according to Ahnert's approach
Departamento de Geografia,Universidade de Sao Paulo,
Av. Lineu Prestes 05.508-900 Sao Paulo, Brasil
Rapid mass movement processes are discontinuous events,
of varying spacial magnitude and frequency, which depo-
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sits may be mapped. In this way it's possible to apply the
Ahnert's (1987) approach wich respect to this kind of
events, slump in particular. In humid tropical relief sistems,
mass movements is not only the result of breakdown
in strength parameters, its play too an important
geoecologic role: scars offers an opening to the sun light,
scarce under the forest canopy and is a renewer factor to
the soil minerals; «colluvium» deposits constitute a special
ambient with thick soils rich in organic mather and water.
Cruz (1974), De'Ploey & alii, (1978) and De'Ploey & Cruz
(1979) already presented mass movement as integrated
part of dinamics of slope evolution: thick weather materials,
high temperatures and rainfall. It explains the mapping
of 750 slump deposits (Holocene) in 150 km 2 area
(5/km 2 ) , at Sao Luis do Paraitinga, in the «Planalto Atlantico»,
Sao Paulo State. Nine kinds of other deposits are
identified and mapped (total 1900 unities). This paper focuses
slump unities deposits because it's the more representative
process and for the ease identification and delimitation
in airphotos Colangelo (1995).
The Ahnert's (1987)semilogarithmic approach was applied
directly in the slump unities deposits mapped in the
airphotos and transfered to the topografic maps (1:10.000
scale). In this way, the magnitude-frequency index refers
to the spacial recurrence interval or frequency (deposits
uniries/km') for deposits of varying magnitudes. The results
are: A (m') = 1.667 + 1.667. 10gRI , for migmatites
with very steep slopes, and A(m 2 ) = 3.000 + 5125. 10gRI,
for basement with terciary sediments, with more smooth
relief (moderate gradients): A is the deposit area magnitude
(in square meters) and RI is the spacial recurrence interval
of the unity deposit of determined magnitude. The
daily rainfall analisis of two years data, at Paraibuna station
(source of data: Cesp), in the proximity of the study area
(10 km), present the regression equationrPj, (mrn) = 62+
36,3.logRIy, Mfi (62.0; 36.3), P 24 > 15 mm. These results
suggest, apart from the constraint strutural lito-tectonic
factors, that there is a relationship between magnitude-frequency
index of rainfall and spacial magnitude-frequency
index of specific process deposit unities: slumps, for exernple,
The map exibits the spatial arrangement for this kind
of process, and it's possible to evaluate the total surface represented,
unity for unity in the space, which may be combined
with magnitude-frequency index of rainfall and hidraulic
conductivity (K) for establising the relationship
above in terms of thresholds of magnitude-frequency, and
quantify the prodution of mobilized material.
In these terms it is possible discuss some results with respect
to evaluation and mapping the hazard areas. Both laboratorial
and field tests are very important in this approach.
In the field, app.arent cohesion and apparent angle of
internal friction are recorded by means ,of the soil sheargraph
apparatus. Combining this results with analysis of
magnitude and frequency, it is possible to evaluate the production
of surface of rupture, and correlated deposits, in
the space and time, in a determined range of scales.