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The Acquabona debris flow
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Debris flow management and risk assessment in the
alpine region
Research project funded by the European Community
(1997-1998)
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 Contact us
for a FREE copy of : |
- The
12 June 1997 debris flow (video tape recorded in the
initiation area after an intense storm...)
- Acquabona
'98 (CD-ROM
providing a tour on the monitoring activities in
Acquabona by means of interactive maps, pictures,
and videos !)
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General setting
The Acquabona catchment is located
in the eastern italian Alps and is characterised by massive
dolomite cliffs underlain by red marls (schematic
map). Slope below massive
dolomite cliffs consist of thick talus deposits of poorly sorted
material.
Slope angles range from 30°-40°
at the toe of the sub-vertical dolomite cliffs to 10°-15°
approaching the valley bottom. Elevations range from 1000 m to
3200 m a.s.l.
Basin characteristics
The Acquabona drainage basin is
characterised by an upper rock basin consisting of dolomitic
rocks (headwater basin) and of a deep channel cut in poorly
sorted talus deposit (flow channel).
The steep rock basin has an
effective drainage area of 0.3 km2, an average slope
of 38° and an maximum length of only 1300 m. Such
characteristics determine a quick hydrological response to
rainfall: collected water rapidly reaches the outlet of the
headwater basin and flows through a narrow rocky gully straight
to the onset of the flow channel (initiation area), where loose
talus debris fills the channel bottom.
The loose debris is usually dry and constitutes a potential
perched acquifer (1-2 m thick) characterised by very high
hydraulic conductivity and sustained by the less conductive slope
deposits. Following intense rainfalls the water inflow from the
headwater basin causes the temporary saturation of the bed debris
in the upper reach of the channel. If the rainfall magnitude is
high enough, surficial water flow occurs and the channel bed
debris mobilise. The whole process begins 30-35 min after the
rainfall intensity peak and exhausts in the following 20-40 min.
Material characteristics
The Acquabona debris flow
transports gravelly-sandy poorly sorted
material, ranging in size
from silt and clay to big boulders (up to 1-2 m in diameter).
The fine enrichment from the
source to the deposition area is mainly due to the passage of the
flow on the red marls outcropping in the middle reach of the channel.
Fraction coarser than 20 mm is approx. the 25% by weight of the
total.
Debris material shows no plasticity. Index properties can be determined only for the fresh
material deposited downstream the red marls; plasticity is
however very low (WL=28% - PI=4%).
Shear strength has been evaluated through direct shear tests
and isotropically-consolidated triaxial tests on reconstituted
samples under different conditions of relative density, confining
stress and stress path leading to failure. Peak failure envelopes
of saturated samples give effective friction angles ranging from
40° to 42° and effective cohesion close to zero. Dry samples
show similar friction angle but a high effective cohesion (c'=60
kPa) resulting from a slight cementation at particle contacts
which develops during the drying process.
The comparison of the field
conditions (void ratio and confining stress) with the critical
state line (CSL) obtained from TX-ICU tests clearly indicates a contractive
behaviour of the in-situ
material. Field conditions plot well above the CSL indicating the
strong susceptibility of the channel bed material to contract,
and possibly liquefy, at failure.
The monitoring system
(in collaboration with
USGS-Cascades Volcano Observatory)
An observation system, fully
automatic and remotely controlled, was installed at Acquabona. It
consists of 3 on-site monitoring stations and an off-site master
collection station located 1.3 km far from Acquabona. The
monitoring system has been developed and installed in
co-operation with USGS (Cascade Volcano Observatory, Vancouver).
Each on-site station is provided
of a small data logger and of a two-way radio. Data are
radio-transmitted every 5 min if a certain threshold value,
controlled by geophones and raingage, is not exceeded (pre-alarm
mode).
If the threshold is exceeded (alarm mode) data are acquired at 5 Hz and stored in a
memory card placed on-site.
Radio-transmitted data are stored in
the off-site station and can be downloaded via modem from a
remote position (data retrival). Modem connection also allows to fully
control the monitoring system (check the functioning, set the
threshold, etc.).
In summer 1998 the monitoring
system was equipped as follow:
- Station 1 (initiation area): 2 video
systems, 1 geophone, 5 pore pressure transducers, 1
raingage
- Station 2 (middle reach of the channel): 3
geophones, 1 anemometer
- Station 3 (lower reach of the channel): 1
video system, 3 geophones, 1 pore pressure transducer, 1
pressure cell, 1 ultrasonic sensor
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