ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

ALASTAIR J. KIRK
A field-based rainfall simulation experiment to examine
the movement of fines into arid soil profiles during
the development of a soil crust
Department of Geography, University of Durham, Durham,
DHI3LE, U.K.
The formation of soil crusts in arid landscapes has important
effects upon the short term sediment mobility and the
hydrological fluxes acting at the soil surface. The research
to be reported examines the processes of soil crust development
in arid environments, where land has recently
been cleared of basalt boulders and is now being used for
agriculture. Soil crust formation in arid regions is endemic
of soil structural instability. The phenomenon of soil crusting
involves the break-up of soil aggregates by a combination
of physical and chemical processes. Once developed,
soil crusts reduce infiltration and gaseous exchange,
increase runoff and are therefore seen as indicators of soil
degradation. They do, however, act as a sedimentary indicator
for the movement of fine material over the soil surface.
Rainfall simulation experiments were carried out in the
Northern Badia area of Jordan during the spring of 1995.
Silt-sized hematite particles, acting as physical tracers, were
deposited over the soil surface to examine the effect of
the washing-in of fines during rainfall events. Fabric analysis
using thin sections of the resultant soil crusts has been
carried out and image analysis has been used to quantify
the amount and distance of sediment movement.
MIKE KIRKBY
A model for downstream changes in grainsize
School of Geography, University of Leeds, UK
Stream grainsize characteristics are related to source materials,
long profile form, and rates of erosion or deposition.
Where there is net channel erosion, abrasion and other
breakdown processes are thought to be dominant, whereas
in depositional environments selective transportation is
much more important.
The model presented here first considers a simplified scheme
describing the processes of sediment breakdown and
selective transportation. Breakdown is considered in terms
of rates of change over time and downstream. Abrasion
weight loss over time is related to grain area, curvature
within the grain, and the relative velocities of other grains
in motion. Weight loss over time is linked to via grain velocity,
which is also dependent on diameter. Abrasion rates
therefore respond to size, shape and, to a less extent, size
distribution. Abrasion also produces fines from the grin-
ding process. Breakdown also occurs rapidly by grain partition
in suitable lithologies. Selective transportation is separated
into detachment and grain travel. The detachment
process is considered to take place under conditions of
equal mobility, in response to flow power in relation to
average surface grain size. Grain travel is treated as highly
size-selective, with weak interactions between the various
grain sizes present. These processes are considered for
both a specified distribution of grains and through changes
in an assumed log-normal distribution, or mixture of distributions.
The integrated finite difference model then budgets,
breaks down and routes sediment from its sources as hillslope
basal sediment delivery. Sediment is routed through
the channel and flood plain system under a variety of regimes
for flow variability, using the flood event as the iteration
time unit. Uniform source material throughout the
catchment, or simple contrasts of lithologies are used at
this stage. The linearised stream systems are placed in stable
or simple tectonic settings which provide base level
boundary conditions of uniform erosion or deposition rate.
The models are being applied to the evolution and change
in semi-arid gully systems in S.E. Spain, as part of the EUfunded
Medalus project.
It is argued that, in «graded» time spans, hillslope sediment
supply and channel gradient strongly influence the
prevailing grainsize by sorting between channel and floodplain,
and also control local rates of erosion or deposition
with the possibility of variations in Sediment Delivery Ratios
(Sdr). Over «cyclic» time spans, however, in which appreciable
net change occurs,
1. grain size is more strongly constrained by lithology,
2. Sdr is generally close to Unity and
3. abrasion has a much more dominant effect on the typical
patterns of downstream fining.
Exceptional conditions are found for rivers passing through
arid areas, where, with decreasing discharge downstream,
grain size and gradient trends may depart from the
normal pattern. Similarly river profiles respond in apparently
anomalous ways when passing across lithological
contrasts.
KAZIMIERZ KLIMEK
Historical phases of human impact in upper Odra
Valley System, Upper Silesia, Poland
Earth Sciences Faculty, University of Silesia, ul.Bedzinska 60,
41-200 Sosnowiec, Poland
The Upper Odra Catchment, Upper Silesia, drains an area
filled primarily by Quaternary deposits overlaying the older
sedimentary rocks. The maximum extent of scandinavian
ice-sheet during the Middle Polish (Saalian) glaciation
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