Data acquisition and processing in a small debris flow prone ...

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
Abstract
Data acquisition and processing in a small debris flow prone
catchment
M. Arattano 1 , L. Marchi 2 , A.M. Deganutti 2
The Moscardo Torrent is a small creek, with an area of about 4 km 2 , located in the
Eastern Italian Alps (Fig. 1). The bedrock of the Moscardo basin is made of Carboniferous
flysch, consisting of shale, slates, siltstone, sandstone and breccia that
continuously outcrop in the upper portion of the basin and locally along the torrent.
Fig. 1 - The Moscardo Torrent basin and its alluvial fan. 1: debris flow
initiation site; 2: raingauges; 3 and 4: instrumented channel stretches.
The poor mechanical properties of the rocks, the instability caused by the presence
of a deep seated gravitational slope deformation and the steepness of the upper basin
slopes facilitate frequent widespread falls and toppling of rocks. The very large quantity
of debris that reaches the drainage network by gravitational and erosion processes and
by snow avalanches explains the strong tendency of the Moscardo Torrent to generate
debris flows and hyperconcentrated flows. The debris flows of the Moscardo Torrent
led to the construction of a huge fan that progressively invaded the receiving valley
bottom forcing the receiving stream to flow on the opposite side of the valley floor. The
1 1CNR-IRPI, Strada delle Cacce 73, 10135 Torino, Italy E-mail: m.arattano@irpi.to.cnr.it.
2 CNR IRPI, Corso Stati Uniti 4, 35127 Padova, Italy E-mail: lorenzo.marchi@irpi.pd.cnr.it

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
main morphological parameters of the basin closed at the fan apex are listed in Table 1;
the length of the channel reach on the fan is about 1000 m (Arattano et al., 1997).
The Moscardo debris flows are composed by material of a wide range of dimensions.
Lateral levees and debris flow lobes are mostly formed by small and medium boulders;
larger boulders of over two meters are also common. Grain size analyses have been
carried out on several matrix samples (Arattano et al., 1997, Moscariello and Deganutti,
2000) from old and fresh deposits. The high percentage of fine particles in the matrix
(from 13 to 35% less than 40 µm) underlines the muddy character of the Moscardo
debris flows.
Table 1. Main morphometric parameters of the Moscardo basin.
Basin area
[km 2 ]
Maximum
elevation
[m]
Minimum
elevation
[m]
Average
basin slope
[%]
Channel
length [km]
Average
channel slope
[%]
4.1 2043 890 63 2.76 37
The Moscardo debris flow material has been studied also from the rheological point
of view, using a large amount of material sampled from a fresh deposit left by a debris
flow occurred on July, 5 1995. Several suspensions, obtained mixing the material with
water, were analysed for a range of different solid concentrations using two parallel
plates rheometers and a tilting plane. The material showed a shear thinning viscous
behaviour which can be well represented by a Herschel-Bulkley model (Coussot et al.,
1998).
This basin was chosen in 1989 by the Research Institute for Hydrogeological Protection
of the Italian National Research Council (CNR IRPI) for the installation of a debris flow
monitoring system. This latter consisted of two ultrasonic sensors, placed at a distance
of about 300 m on the fan. A raingauge was also installed in the upper portion of the
basin. In 1995, thanks to the fundings of a European Project, the ultrasonic sensors
installed in 1989 were replaced by new ones and a third ultrasonic sensor was added
150 m upstream of these latter. A fixed video camera was positioned close to the
intermediate of the three ultrasonic sensors and a network of four seismic detectors was
set up about 1 km upstream from the ultrasonic gauging stations. Finally, in 1997,
a second raingauge was installed in the centre of the basin and two new seismic sensors
were set up close to the intermediate ultrasonic gauge on the fan.
From 1990 to 1998, 15 debris flows occurred, 14 of which were recorded by the
installed gauges. Several types of measurements have been carried out on the recorded
debris flows during the years, including mean front velocity, surface velocity, volume,
flow stage, hydrograph deformation, triggering rainfalls etc..
The ultrasonic sensors measure the torrent stage, making it possible to record the debris
flow hydrographs. A time lag of 60 seconds was initially set between two consecutive
recordings of the ultrasonic sensors. This time lag was then reduced to 10 seconds in
1990, to grant a better accuracy of the data, and further reduced to 1 second in 1995,
thanks to the updating and improvement of the recording system mentioned earlier. The
mean propagation velocity of the front can be calculated in the monitored reach as the
ratio of the distance between the sensors to the time interval between the appearance
of the peak of the debris flow surge in the two recorded hydrographs. The analysis

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
of the hydrographs recorded by the ultrasonic sensors may show the aggradation or
degradation of the channel bed at the recording sites. Flow stage measurements and
topographic survey of the monitored sections make it possible to estimate peak
discharges and total volume of debris flows (Arattano et al., 1997, Marchi et al., 2002).
Debris flow volumes Vol have been estimated as:
Vol
t
f
∫ vA ( t ) dt = v ∫ A ( t ) dt
(1)
f
=
t 0
t 0
t
where v is the mean velocity of the flow, which was assumed constant for the entire
debris flow wave and equal to the mean front velocity, A(t) is the cross section area
occupied by the flow at the time t, known from topographic surveys and the ultrasonic
data, t 0 is the time of arrival of the surge at the gauging site and t f is the time at the end
of the debris flow wave. Mean velocity of the main front was used for volume
computation because it is the only velocity datum available for all recorded events:
additional information on velocity variations during the debris flow wave have been
obtained only for two events. This approach to the computation of the flowing volume
assumes that the material flows through the considered section at a constant velocity
during the surge. Thus computed debris flow volumes should be regarded as
approximate estimates.
The seismic detectors (seismometers and geophones) record ground vibrations induced
by the passage of a debris flow. The purpose of the seismic sensors installation, in the
initial phase of the research, was essentially to verify which information could be
obtained through this type of devices on the occasion of a debris flow occurrence.
However, the first results that have been obtained showed the possibility of using these
detectors also as tools for velocity measurements (Arattano and Moia, 1999). The debris
flow passage generates ground vibrations whose amplitude graph corresponds to the
stage hydrograph. The ground vibrations peak is detectable by a seismic sensor placed
at a safe distance of some tens of meters from the channel bed. The mean front velocity
can be then measured placing a couple of these detectors at a known distance from each
other along the torrent adopting the same procedure previously described for velocity
measurements with ultrasonic sensors.
A fixed video camera like that installed in 1995 on the alluvial fan of the Moscardo
Torrent allows a visual interpretation of the debris flow features. The video camera
shoots slantwise a straight channel reach about 80 meters long and is triggered by the
upstream ultrasonic sensor by means of a triggering software that identifies abrupt
increases of the stage in the torrent and starts the video recordings. The possibility was
also investigated of using the video recordings for estimating debris flow surface
velocity. A simple method to process the recorded images was developed that maps 2D
image points on the screen and points in the 3D space (Arattano and Marchi, 2000).
Average velocity of the features floating on the surface was then computed as the ratio
of their traveled distance to the time elapsed between the shooting of the video frames
that contained them. Average debris flow velocities estimated through image processing
were consistent with measurements based on the recordings of the ultrasonic gauges;
velocity variations in debris flow waves are discussed in Arattano and Marchi (2000).
The instrumentation installed in the Moscardo Torrent basin (raingauges and sensors
which detect debris flow passage) gives the possibility of analyzing the relations
between rainstorm characteristics and debris flow occurrence more precisely than

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
usually possible in alpine basins. Rainfalls recorded in the Moscardo Torrent from 1990
to 1998 were analyzed and storm characteristics of two classes, i.e. debris flow causing
storms (15 cases) and storms which did not cause debris flows (58 cases) were
compared (Deganutti et al., 2000). Several storm variables were taken into account,
including total storm rainfall, average intensity, maximum 60 minutes intensity,
antecedent precipitation: a statistical analysis showed that only total storm rainfall and
maximum 60 minutes intensity were significantly different between debris flow storms
and storms that did not trigger debris flows. The analysis of rainfall has shown that
rainstorm characteristics and antecedent precipitation play an important role but are not
sufficient to define debris flow initiation conditions. A critical combination of sediment
availability and hydrologic conditions is necessary to cause debris flow formation. This
is particularly true when sediment moisture is influenced by a complex groundwater
flow regime and sediment availability depends on bank and slope failures, as well as on
the previous occurrence of debris flows, like it occurs in the Moscardo basin.
Even though debris flow monitoring in the Moscardo Torrent was intended for research
purposes, the results that have been obtained provide suitable indications for the
possible use of different gauges in debris flow alarm systems. In particular, the use
of seismic devices as warning system, as it has been tested also for snow avalanches
appears to be encouraging. It is part of the plans of CNR IRPI to continue the
monitoring activities in the Moscardo Torrent through the next years.
References
Arattano, M., Deganutti, A.M., Marchi, L., 1997. Debris flow monitoring activities in
an instrumented watershed of the Italian Alps. In: Chen, C. (Ed.), Proceedings, First
International Conference on Debris-flow Hazard Mitigation: Mechanics, Prediction,
and Assessment. Water Resources Engineering Division / ASCE, New York, pp.
506-515.
Arattano, M., Moia, F., 1999. Monitoring the propagation of a debris flow along a
torrent. Hydrological Sciences Journal 44(5), 811-823.
Arattano, M., Marchi, L., 2000. Video-derived velocity distribution along a debris flow
surge. Physics and Chemistry of the Earth Part B 25(8), 781-784.
Coussot, Ph., Laigle, D., Arattano, M., Deganutti, A.M., Marchi, L., 1998. Direct
determination of rheological characteristics of debris flow. Journal of Hydraulic
Engineering ASCE 124(8), 865-868.
Deganutti, A.M., Marchi, L., Arattano, M., 2000. Rainfall and debris flow occurrence in
the Moscardo basin (Italian Alps). In: Wieczorek, G., Naeser, N. (Eds.), Proceedings,
Second International Conference on Debris-flow Hazard Mitigation: Mechanics,
Prediction, and Assessment. A.A. Balkema, Rotterdam, pp. 67-72.
Marchi, L., Arattano, M., Deganutti, A.M., 2002. Ten years of debris-flow monitoring
in the Moscardo Torrents (Italian Alps). Geomorphology, in press.
Moscariello, A., Deganutti, A.M., 2000. Sedimentary and hydrologic processes of a
debris-flow dominated alluvial fan - Moscardo Fan, Italy. In: Wieczorek, G., Naeser,
N. (Eds.), Proceedings, Second International Conference on Debris-flow Hazard
Mitigation: Mechanics, Prediction, and Assessment. A.A. Balkema, Rotterdam, pp.
301-310.