ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

KENICHI TAKAHASHI l , YUKINORI MATSUKURA 2
& MAMORU KOBAYASHI 2
Thermo-infrared images of sandstone blocks used
for a masonry bridge piers in the coastal spray zone
1Faculty of Literature, Chuo University, Hachioji-shi,
192-03 Tokyo, Japan
2 Institute of Geoscience, University of Tsukuba, Tsukuba-shi,
305 Ibaraki, Japan
Yayoi Bridge, connecting Aoshima island with Kyusyu
main island in Japan, is supported by four piers whose surface
is composed of sandstone blocks. The four side walls
of the piers, have a height of 3 m and a slope of 70°, and
face approximately east, south, west and north. The bases
of the piers are situated at Mean Tide Level (mean tidal
range: 1.6 m), Each sandstone block has developed a dishor
bowl-like depression due to weathering and erosion.
The greatest depth of each depression in all blocks was
measured in 1971 and 1989 corresponding to 20 and 38
years, respectively, since the construction of the bridge.
The erosion data indicate that the rate of erosion is not linear
function of time but an exponential function such as
D = A (1 - exp(-btl), where D is the erosion depth, t is the
time, and A and b are constants. The erosion features are
summarized: (1) the erosion depth is the largest on the
south-facing wall (maximum value is about 15 ern during
38 years), and gradually becomes smaller on the west- and
east-facing wall, and is smallest (about 5 em) on the northfacing
wall, i.e. the erosion depth and the amount of insolation
accepted are positively related; (2) altitude of maximum
erosion is situated at just High Tide Level on the
south-facing wall, and at 3 m above Mtl on the north-facing
wall where the sea spray zone, i.e. wetting zone, is located
higher due to prevailing waves. These findings suggest
that the difference in the depth of depressions according
to aspect and altitude is due to the combined effect of
insolation and sea water spray.
The joints between the sandstone blocks are filled with
mortar. The surface of the mortar joints protrudes from
the surface of the sandstone blocks except in the lower
part of each pier. The resistant sandstone (64-100 MPa in
compressive strength) is eroded to form the depressions,
whereas the less-resistant mortar (50 MPa) is nowhere eroded,
irrespective of aspect and altitude. This indicates that
(1) these depression never form on fresh (unweathered)
sandstone, and (2) they form only after the surface layers
of sandstone blocks have suffered some loss of strength
due to physical weathering. The deepening of the depression
is, therefore, weathering-controlled erosion.
The field evidence shows that (1) the depth of the depressions
is controlled by the frequency and intensity of both
wetting by the supply of sea water spray and drying (evaporation)
by insolation and (2) the shape of some depressions
is similar to «tafoni» and the sand grains produced
by disaggregation are found on the surface of depressions.
Tafoni have recently been suggested to be formed by salt
weathering. These findings indicate that the salt weathering
in superficial part of the sandstone blocks plays the
most important role in the strength reduction and resulting
formation of the depressions.
The formation and deepening of these depressions is caused
by the sequential process of granular disintegra- tion
and removal of the detached sand grains and weakened
sub-surface layer of sandstone through the action of such
external agents as wind and waves.
In order to elucidate the weathering processes mentioned
above, the measurement of the temporal changes in water
content of the surface layer of sandstone blocks was tried.
There is, however, no instruments for non-destructive in
situ rock moisture monitoring. Then, the rock surface temperature,
which reflects water content, was measured with
a thermo-infrared image tracer. The images of the piers
taken with two hour interval indicates that the blocks with
high changes in rock surface temperature, i.e., high changes
in rock moisture, have a large depth of the depression.
PETER TALLING & MATTHEW SOWTER
Erosion, deposition, and stream power
in large alluvial basins
Department of Geology, Bristol University, Queens Road,
Bristol BS8 1RJ, Great Britain
The boundary between the relative flat depositional parts
of alluvial basins, and the adjacent rugged areas from whichsediment
is eroded is often very abrupt. This observation
is particularly striking in areas of active tectonic deformation.
Alluvial sediment underlying these depositional
areas may be up to several kilometers in thickness. A key
question is what controls the position of this boundary,
and by what processes can such large thicknesses of sediment
accumulate over geological (> 10 ka) time scales. Previously,
such large thicknesses of sediment have been attributed
to the filling of a subsiding «hole» initially caused by
tectonic deformation of the lithosphere. However, such
an explanation does not provide a link to the physical processes
which act on actual sediment particles in alluvial
systems.
This presentation aims to illustrate a striking coincidence
between large-scale patterns of erosion and deposition in
the present-day Po Basin of northern Italy, and downstream
changes in stream power. Stream power is measured
in this study as the product of channel gradient and
upstream drainage area (typically proportional to mean annual
discharge). Relatively rapid downstream decreases in
stream power are found to occur only along the reaches of
rivers which have deposited sediment during the last 5-25
ka. (Patterns of man-induced erosion or deposition occurring
during the last two centuries have been excluded from
this study). Such rapid decreases in stream power are pro-
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