ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

MANDY CAROLISSEN
Investigating bedload movement in the Jonkershoek
catchment, in the Western Cape, South Africa
Department of Geography and Environmental Sciences, University
of the Western Cape, Private Bag X17, 7535 Bellville, South Africa
Bedload movement was monitored on a long-term and
short-term basis in two streams, namely Bosboukloof and
Langrivier, in the Jonkershoek catchment in the south-western
Cape. Bosboukloof is afforested with Pinus radiata,
has a low slope of 29° and area of 200.9 ha, while Langrivier
is a natural catchment, consisting of several fynbos
species, has a steeper slope of 49° and an area of 245,8 ha.
The long-term bed load movement for both catchments highlights
the spatial and temporal variability of bedload
movement. The high sediment peaksfor Bosboukloof were
a result of extrinsic factors, namely clearfelling and bushfires.
Short-term bedload movement with the aid of tracer
particles was monitored during the wet season (June - August)
of 1995. Langrivier yielded greater sediment movement
than Bosboukloof, with a maximum displacement of
11.75 m. The particle sizes of Bosboukloof consisted
mainly of sand sizes, while in Langrivier, particle sizes ranged
from sands to boulders. The banks and river beds were
the main sources of sediment for the catchments.
ALBERTO CARRARA 1, GABRIELE BITELLI 2 & LUCA VITTUARI 2
Derivation of high-quality Dtms from contour lines
and digital photogrammetry
1 Cnr-Csite, viale Risorgimento 2, Bologna, Italy
2 Distart, Universita di Bologna, viale Risorgimento 2, Bologna, Italy
In the recent years, digital terrain models (Dtms) and their
derivatives have found application in many investigations
appertaining to the realm of applied and theoretical geomorphology.
Meanwhile, in several countries governmental
organisations are increasingly involved in the production
of high-resolution digital ground models over large
areas.
Grid (raster) and Tin formats, which are the two basic
structures for electronically storing and handling elevation
data, are derived either from digitised contour lines of existing
maps, or directly from aerial photographs.
The first is still the most common approach followed by
public institutions in order to produce large elevation databases
within Gis projects. By exploiting automatic scanner/vectoring
technology, contours can be cost-effectively
digitised and used as source data for Dtms. The extent to
which these contours reflect the real ground morphology
will greatly vary depending on many factors that cannot be
readily assessed. By generating Dtms from such contours,
all source errors are transferred to the elevation model
which will also incorporate the errors related to the operation
of contour interpolation (grid Dtms) or space tasselation
(Tin Dtrns).
The second approach consists in the direct derivation of
the Dtm from analytical plotting of stereo aerial photographs;
however, the costs involved are unaffordable when
dealing with large regions.
An alternative direct method is recently becoming available
on the market: the automated processing of stereo aerial
data through digital image correlation techniques
(Krzystek & Ackermann, 1995). However, in spite of the
recent technological advancements, the quality and accuracy
of Dtms obtained by this technique do not appear fully
documented as yet.
Within the framework of an investigation on Dtm generation
and application (Bitelli & alii, 1996), an attempt was
made to investigate the quality and accuracy of Dtms
derived from both contour lines and image matching techniques.
For a morphologically complex sample area located in Calabria
(southern Italy), grid-based digital terrain models
were derived from both digitised contour lines and digital
photogrammetry. The first was obtained by interpolating
with a high-quality algorithm scanned and vectored contours
of existing topographic sheets. The second was extracted
from scanned aerial photographs using softcopy
photogrammetric technology. By comparing each Dtrn
with a third terrain model generated by traditional analytical
plotting, advantages and pitfalls of each approach were
highlighted. They can be summarised as follows.
Owing to the current technology, contour-derived Dtms
can be produced for wide areas at effective cost, but they
are affected by unpredictable widespread errors which are
due to both inaccuracy of input contour lines and inefficiency
of the interpolating algorithm. Digital image correlation-derived
Dtms faithfully reflect the actual relief of
most of the ground surface; however, the whole process is
still costly and, most importantly, it may leads to very large
local errors where slopes are steep, ground mantled by
thick forest and aerial photographs masked by shadows.
MAURO CASADEI & ENZO FARABEGOLI
Estimation of the effects of slope map computing
on landslide hazard zonation.
A case history in the Northern Apennines
Dipartimento di Scienze della Terra e Geologico-Ambientali
Universita di Bologna, via Zamboni 67, 40127 Bologna, Italy
The geomorphological setting of the Northern Apennines
features a widespread distribution of landslides, most evident
within badland-type terrains. The evaluation of slope
angle plays a relevant role in the process of landslide ha-
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