ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

The Piave River is an alpine stream 222 km long and with
a drainage basin of 3,899 km. In this study a reach about
100 km long has been examined and in this reach the Piave
River is a braided gravel bed stream.
Channel river changes in the last century have been studied
using historical maps and aerial photos. The oldest
IGM maps at 1:25,000 scale were drawn between 1890
and 1910, and so they give a picture of the river when it
was in natural conditions. Because of the width of the river'
generally some hundred meters, and of the map scales;
1:25,000 and 1:10,000, not only a qualitative analysis of
changes but also some measurements and a quantitative
analysis were possible. In the last century narrowing and
incision of the river channel have been the main processes.
By measuring on the maps and on the aerial photos the river
width at about 100 sites along the studied reach some
average rates of narrowing of the stream during different
periods of time can be evaluated. In the last century the river
has reduced to a half its width. The average of the
channel widths measured at the different sites on the oldest
maps (1890-1910) is about 600 m, whereas it is about
300 m on the more recent maps (1980-1983). In absence of
cross sections, maps and aerial photos can give some qualitative
informations about the degradation of the bed. These
informations allows to compare the average rate of incision
of the river during the Holocene (natural evolution)
and during the last decades (man-induced evolution). The
average incision during the Holocene is few millimeters
per years (0.001-0.004 m/year, considering terrace heights
in the Vallone Bellunese), whereas the average degradation
of the river bed in the last decades is some centimeters per
year (for example 0.06 m/year at Ponte nelle Alpi). In the
last case the highest values of bed degradation are in the
reaches below the dams.
Another issue developed in this study is the identification
of the river corridor. The river corridor is defined by those
areas next to the active channel which, from the hydraulic,
geomorphological, and ecological point of view, are influenced
by river dynamics. The historical analysis, together
with the study of the river terraces, has allowed to
achieve a geomorphological definition of the river corridor
of the study reach and the mapping of the corridor.
From this historical analysis it can be argued that the channel
changes of the Piave River in the last decades are
mainly man-induced. The channel narrowing and the bed
degradation are the river response to the flow regulation
and to the reduction of sediment supply. Anyway, though
a general understandig of processes has been achieved, the
mechanisms of changes of the river need to be explained in
more detailed. For example, it is still not clear the relation
between flows, and particularly of channel forming discharges,
and the observed channel changes.
This study confirms that there are some specific contributions
that fluvial geomorphology can provide for river management.
The value of the geomorphological approach in
river studies is that it considers time and spatial scales that
are generally neglected in engineering approaches. In the
case of the Piave River, the study of the river in the historical
times and during the Late glacial-Holocene allows to
determine the rate and the magnitude of channel changes
and a better understanding of man effects on the fluvial
system.
GRACIELA M. SUVIRES, JORGE A. SISTERNA
& ARMANDO L. IMHOF
Geomorphological models of an arid region of a
southeastern sector of San Juan Province, Argentina
Istituto Seismologico Volponi, Pac. Decs Exactas, Pcas y Nat.
Unsj. Av. Ignacio de la Roza y Meglioli. 5400, San Juan, Argentina
This work shows in a three dimensional shape the relief variations
related with neotectonic activity of this arid region.
The area analysed correspond to the N23 line of Instituto
Geografico Militar, of the Argentine Republic. This line
cross differents geomorphological environments with
many structurals styles such as: intermountain tectonic depression
of Tulum Valley, Pic de Palo Range (included into
the Western Pampean Range System), the Rio Bermejo
Valley Depression and the Valle Fertil-La Huerta Range.
The 1977 earthquake (M=7,3 Richter) had as epicenter the
Pic de Palo Range producing cortical superficial deformations,
registered in the temporal models computer generated.
The relief increased about 100 ern in the post seismc
period (1978-1980). After that the relief decreased slowly
until the present.
HIROSHI SUWA 1 & TAKAO YAMAKOSHI 2
Sediment discharge bystorm runoff at
volcanic torrents affected by eruption
1 Disaster Prevention Research Institute, Kyoto University, Gokasho,
Uji, Kyoto 611, Japan
2 Graduate School of Science, Kyoto University, Kitashirakawa,
Sakyoku, Kyoto 606, Japan
Mount Unzen began to erupt in November 1990 and continued
erupting until March 1995. After several phreatic
eruption, debris flow began to occur at the mid of May
1991 when the first rainy season started. Frequency of debris
flows has been kept very high due to frequent occurrence
of pyroclastic flow and ash fall. Erosion rate of the
drainage area of Mizunashi river reached 175 mm/year.
We estimated that one major cause for this high erosion rate
is brought about from a significant increase in runoff
coefficient of hillslope which is the result of the decrease in
infiltration capacity due to new tephra and pyroclastic deposits
by eruption.
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