ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

to the theoretical maximum predicted by intact rock
strengths because pervasive discontinuities such as faults,
joints, and bedding planes weaken rock masses. Thus, determining
representative strength properties of a mountain
is difficult because such properties integrate both material
and structural discontinuities and may also be time dependent.
Here we investigate bedrock slope stability and
quantify material strength by 1) comparing hillslope profiles
at the landscape scale in areas exhibiting widespread
bedrock landsliding with the use of a model for the maximum
size of stable hillslopes and 2) by estimating rock
mass strength (Rms) values for individual discontinuities at
the outcrop scale.
We hypothesize that in a threshold landscape integrated
rock strength properties limit local relief development and
effectively bound the size of stable hillslopes for mountain
drainage basins in a given lithologic, climatic, and tectonic
regime. In the absence of external forces, such as extreme
pore-water pressures or earthquakes, bedrock landslides
occur where relief development produces topographically
induced stresses that exceed rock strength. A model for
bedrock landsliding provides a framework for predicting
the maximum size of stable hillslopes or mountain fronts
and thereby for the evaluation of the influence of material
properties on relief development. Integrative rock strength
properties back-calculated from the upper limit to hillslope
relief and gradient (limit to topographic development,
Ltd) in the Chuckanut Formation, an alternating sequence
of coarse and fine-grained alluvial strata, of the northern
Cascade Range, Washington, U.S.A are a friction angle of
21 0
and cohesion of 150 kPa. Back-calculated values for
the suite of sedimentary rocks in the Santa Cruz Mountains,
California, U.S.A. have a friction angle of 20° and
cohesion of 60 kPa. Laboratory experiments on fractured
shale recovered from drill cores through landslide slip-surfaces
in the Santa Cruz Mountains (friction angle = 20±6°
and cohesion = 69±32 kPa) corroborate material strengths
determined from the Ltd. The close agreement between
strength parameters back-calculated from the Ltd method
and those obtained through field and conventionallaboratory
measurements on the weakest members of each rock
formation supports the idea that large-scale rock strength
controls the limit to relief.
While material properties back-calculated from observed
hillslope profiles provide an estimate of rock strength at
the landscape scale, rock mass strength (Rms) values determined
for joint surfaces and bedding planes provide quantitative
measures of outcrop-scale strength encompassing
the spatial variability, character, and frequency of discontinuities
in a rock mass, features impossible to evaluate via
conventional laboratory analysis. Strength quantification of
the Chuckanut Formation using bedding planes from 61
hillslopes, including 17 rockslides, reveals distinct populations
of data for stable hillslopes and rockslides, with the
latter exhibiting Rms values below 69 (possible total of
100). In contrast, Rms values obtained from joint surfaces
in the study area do not correlate with deep-seated landslide
susceptibility in the Chuckanut Formation. Analysis of
the seven weighted variables in the Rms scheme indicates
that bedding plane orientation relative to hillslope orientation,
together with intact rock strength, serve as the primary
controls on deep-seated rocksliding within the
Chuckanut Formation. The association of low Rms values
with sites of bedrock landsliding highlights the influence
of spatial variations in material strength on landscape evolution.
Heterogeneities in mountain-scale rock strength,
however, should impart significant spatial variability to the
maximum stable relief, and hence to the rate and depth of
channel network incision. As large rivers carve into bedrock,
rock mass strength regulates the magnitude of local
ridge relief through the mass transfer provided by bedrock
landsliding. Moreover, as exhumation of a landscape proceeds,
the spatial locus of bedrock landsliding may concentrate
where rock uplift exposes units with relatively low
mountain-scale material properties. Thus, if mountain-scale
material strength limits the stable relief in a given geologic
unit, the Ltd approach estimates material properties for
the entire geologic unit while the Rms approach identifies
those localized portions of the landscape most prone to bedrock
landsliding.
SUSANNE SCHNABEL & ANTONIO CEBALLOS
Gully erosion and temporal variability of discharge in a
small watershed in semi-arid Spain
Departamento de Geografia, Universidad de Extremadura,
Avda. de la Universidad sin, 10.071 Caceres, Spain
Gully erosion is studied as part of a research project about
the erosional and hydrological processes operating in areas
of open evergreen woodland under silvo-pastorallanduse
in Spain. Investigations are carried out in a catchment of
35.6 ha since 1990. Gullying is observed in the valley bottoms
filled with sediments of 1.5 metre depth, which contrast
with the shallow soils on the hillslopes. The amount
of gully erosion is estimated by repeated monitoring of
transverse sections. Discharge production and rainfall is
measured with a time resolution of five minutes. Furthermore,
soil erosion and runoff on hillslopes is determined
on an event basis. Mean gully erosion for the six year period
amounted to 12.3 m' a-I.
Temporal variability of gullying is high and can be attributed
to rainfall variations. However, the results indicate that
high sediment losses are either caused by intensive rainstorms
of short duration and moderate 'frequency, or by
continuous rainfall of long duration. In the first case overland
flow in the basin is exclusivley of the Hortonian type
and discharge in the channel is rapidly produced with peak
flows occurring 5 to 10 minutes after the rainfall peak. In
the second case large amounts of precipitation produce
water saturation of the whole catchment giving rise to saturation
overland flow on footslopes and in the valley bottoms.
This situation was observed during winter of 1995-
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