ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

future landsliding. If reforested areas were included as an
independent variable the tendency to map landslides
rather than susceptibility to landsliding would be increased.
Therefore, when building the statistical model, it is
important to select and define variables in such a way that
they reflect conditions before rather then after landslide
occurrence. Then, the model is applied to present conditions
since we are interested in future landslide susceptibility.
Particularly in the case of dormant landslides, properties
surveyed at the present time are more likely to reflect
conditions post-failure and are, therefore, redundant. Too
often, the indiscriminate inclusion of such variables in the
analysis has resulted in simple indirect landslide mapping
(i.e., recognition of geologic and geomorphologic features
of area already failed) rather than real landslide susceptibility
mapping.
Generalised linear modelling was used to model the relation
between landslides and several independent variables
for a small area of the central Apennines in Italy. Raster
maps of landslides and independent variables were produced
from air photographs, topographic and geological maps,
and field survey. Logistic regression relations were obtained
between landslides and the independent variables
surveyed, chosen to reflect conditions prior to landsliding
(Atkinson & Massari, 1996). Nine variables were used in
the case of all landslides and dormant only, and 17 in the
case of active landslides. Geology and slope angle were
found to be always highly significant factors in the models.
Slope aspect and strike were also significant, particularly
for dormant landslides. In the case of active landslides, vegetation
cover and concavity/convexity of the slope were
more significant than geology and slope angle.
However, the extreme variability of causing factors, and
the diverse influence of each factor in each type of landslides
made the single model, while useful in the understanding
of the overall processes occurring in the area, imprecise.
Moreover, since more than 500/0 of landslides in the
area are of the Slump & Flow type, the model was heavily
weighted by this group.
In this paper, different models were constructed for different
types of landslides, again subdivided into dormant
and active. Landslides were classified following Varnes
(1978) and Wieczorek (1984). Several types of landslides
were detected (slump & flows, rock-falls, debris-flows,
rock-slides, earth-flows, etc.). For the present analysis,
landslides were separated into five groups and each group
into active and dormant. Within each landslide body, rupture
and deposit areas were identified, since only the features
of the former are relevant when modelling landslide susceptibility.
The resulting statistical models are very interesting. Each
independent factor has a different influence in each group
of landslides. Some factors, such as the density of lineaments,
dip and strike of the strata, are influential only in a
few types of landslide. Others, such as slope angle, have a
different behaviour in each group. From each model it is
clearly possible to produce a map of the relative landslide
susceptibility for each landslide type. These form the output
that is generally needed by the end user, even if asses-
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sing the accuracy of the predictions is difficult since one
would require data on future landslide occurrence.
The present study highlighted the high potential of the
multivariate statistical approach in a geographical information
system for understanding the geomorphic processes
connected to the presence of landslides.
TAMARA M. MASSONG & DAVID R. MONTGOMERY
Regional controls on bedrock and alluvial channels
Department of Geological Sciences, University of Washington,
Seattle, WA 98195, USA
Field data from five drainage basins in Washington and
Oregon, USA, (Willapa River Basin, Finney Creek Basin,
Boulder Creek Basin, Satsop River Basin, WA, and Deton
Creek, OR) show that bedrock channels generally have a
higher slope given a similar drainage area, but that the threshold
values are lithology dependent. Data were collected
during the summers of 1992, 1995 and 1996. The most
comprehensive data set (Willapa River Basin, WA) contains
a total of eighty-three sample sites with reach average
slopes of 0.002 to 0.300 and drainage areas of 10 4 to 10 8
m'. Channels were classified as either alluvial, bedrock or
mixed; the latter are reaches with alternating sub-reaches
of bedrock and alluvium-floored channel that extend over
at least one channel width in length. The study basins host
a variety of land management practices including agricultural,
timber harvest, and recreation (national parks).
Lithology in the study areas range from basalt and siltstone
in the Willapa River Basin to more competent low-grade
metamorphic rocks in Boulder Creek and Finney Creek
Basins. For each lithology, data from bedrock and alluvial
reaches define distinct fields on area-slope graphs. Data
from the Willapa River Basin show that the threshold slope
for bedrock channels in a basalt lithology is higher than
the threshold slope in the sedimentary lithology, indicating
that a larger drainage area for a given channel slope is required
to form a bedrock channel in a marine basalt rather
than in a more friable siltstone. Bedrock channels in Boulder
Creek Basin require larger drainage areas for a given
channel slope than bedrock channels in the Willapa Basin.
Deton Creek Basin (underlain by sandstone) has a similar
threshold to the sedimentary portion of the Willapa River
Basin. Several reaches surveyed in the Willapa River Basin,
however, did not conform to a simple area-slope threshold.
These exceptions that lie outside the general field for their
channel type appear to be controlled by differences in sediment
supply, scour, or in-stream stuctures. A local low
sediment supply, due to upstream sediment impoundments
(i.e., local low-gradient valley floors, beaver dams,
and log jams), appeared to characterize the few mixed or
bedrock channels that plotted within the field defined by
data from alluvial reaches. Also, several steep channels