ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

YOSHINORI KODAMA & KEIGO NAKAMURA
An effect of large boulders on forming a longitudinal
profile of bedrock channel
Faculty of Education, Tottori-Univ., Tottori, 680 Japan
We often observe that large boulder reaches in a gorge
show steeper gradients than reaches without large boulders.
The purposes of this study are 1) to illustrate a good
relation between bedrock channel gradients and large
boulder distribution pattern in a gorge, and 2) to consider
an effect of large boulders on gradient forming processes
in a bedrock channel through simple flume experiments
on sediment transport.
The Oshika River, 17 km long, drains 45 km 2 in Tottori
Prefecture, southwest Japan. The drainage area mainly
consists of three kinds of geology; andesite (Pliocene), tuff
breccia (Miocene), and granite (late Mesozoic). The 3-kmlong
Oshika Gorge occurs in the granite area at the center
of the drainage basin, and has mean gradient of 0.08
(1/12), and channel width of 10-25 meters.
Along the Oshika Gorge, a number of large granite boulders
(more than 2 m diameter) suddenly come into view
in many places, but they usually disappear within one
hundred meters downstream. A tributary or a large pool,
which might be source of large boulders, usually exists in
the upstream ends of the large boulder scatter zones.
Even a flood period, the Oshika does not seem to transport
these large boulders. They rest on bedrock for a
long time.
In addition to large boulder scatter zones, step-pool sequence
zones and a waterfall-pool sequence zone are observed
in the Ojika Gorge. According to topographic maps
(1:2,500 scale) and field survey, gradients of each zone
show regularity: step-pool zone, 0.04-0.05; large boulder
zone, 0.08; waterfall-poolzone, 0.13. Abrupt channel slope
changes are observed in four places between adjacent steppool
zone and large boulder zone. Why are bedrock channel
gradients of large boulder zones steeper than those of
step-pool zones?
An experiment was performed in order to investigate the
effect of large boulders on sediment transport rate. In 10
em wide, 5 em deep, 3.6 m long flume, sand was spread to
make the initial channel bed. At a constant water discharge
(60 cc /sec), an adequate quantity of sand was fed by hand
at the upstream end to maintain the dynamic equilibrium
state. In three different channel slopes and three different
densities of rocks on bed, sand transport rates were examined.
The result shows that the higher the density of rocks,
the less the sediment transport rate.
Sediment transport rate along the Oshika Gorge must be
constant, because we could not observe any alluvial reaches
in the gorge. This means that the longitudinal profile
of the Oshika Gorge could be created on the way of bedrock
erosion so as to make the sediment transport rate
constant between adjacent reaches of different channel
morphologies.
YOSUKE KOMATSU 1 & YUICHI ONDA 2
The hydrological characteristics and valley morphology
among serpentinite and other lithologies
in Oe-yama region, Japan
1 Graduate Student, Department of Geography, Tokyo Metropolitan
Univ., Minami-osawa, Hachioji, 192-03, Tokyo, Japan
2 School of Agricultural Sciences, Nagoya Univ.,
Chikusa, 466, Nagoya, Japan
The runoff response to rainfall is known to vary between
underlying geologies (e.g. Freeze, 1972; Onda, 1994), and
the drainage density and valley form also differ between
geologies (e.g. Abrahams & Flint, 1983). The landform underlain
by serpentinite in humid regions, for example Japan,
is characterized by low valley density and convex slopes
profiles, which is significantly different from surrounding
mountains underlain by other geologies. To study the
cause of the landform difference in serpentinite area, spatial
variations of specific discharge of baseflow among geologies
were investigated, and runoff response from small
basins and precipitation were measured in serpentinite and
surrounding geology.
The study area is Mt. Oe-yama regions, Kyoto Prefecture,
western Japan. The geology of this area is serpentinite, granite,
and Paleozoic argillite. In serpentinite area, few deeply
dissected valleys, many shallow valleys and some welldefined
earthslide landforms are found. In contrast,
landform in granite area is characterized by high drainage
frequency, and landform in Paleozoic argillite shows lower
drainage frequency with straight longitudinal profiles. Specific
discharge of baseflow were measured at 15 to 27 tributaries
in three large basins (about 1 km) in snow melt
season (April) and summer dry season (August). A measurement
of baseflow discharge was carried on in each tributary
outlet in the large basin. Three drainage basin areas of
runoff observation site (all about 0.05 km') was located
and discharge were measured with 6-inch parshall flume.
In the serpentinite area, the spatial variations of base flow
is large, and many zero flux tributaries arefound..In contrast'
the spatial variations in granite and Paleozoic argillite
areas are much smaller than serpentinite areas. Specific discharge
in serpentinite areas decreased in summer dry season,
when the relationships between specific discharge and
altitude of measuring site had strong negative correlation
(fig. 1). This suggest that the groundwater surface in serpentinite
mountain body lowered in summer dry season
(Komatsu & Onda, 1996).
The runoff response in serpentinite basin shows that the
lag time from rainfall event to runoff peak is long (more
than 5 hours), followed by small and quick initial peak. In
Paleozoic argillite basin, runoff peak is tenuated and slow
(more than 4 hours), but in strong spike of rainfall after
dry period, runoff peak is evident and respond quickly to
the rain. Runoff peak to rainfall in granite basin are higher
and quicker (less than 1 hour). These results show that
deep groundwater runoff in bedrock would contribute to
runoff in serpentinite and Paleozoic argillite basin, and not
in granite basin. The large spatial variation of springs in
233