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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
573
DOI: 10.4408/IJEGE.2011-03.B-063
LONG-TERM MONITORING OF BEDLOAD AND DEBRIS FLOWS IN TWO
SMALL ALPINE BASINS OF DIFFERENT MORPHOLOGICAL SETTINGS
m.a. l
enzi
(*)
, l. m
ao
(*)
& m. C
avalli
(**)
(*)
Department of Land and Agro-Forest Environments, University of Padova, Italy
(**)
CNR IRPI, Padova, Italy
load transport) and non-Newtonian (debris and mud
flows) flows, with an intermediate hyperconcentrated
flow phase. The differentiation of those different ty-
pologies is rarely quantifiable, mainly due to a scar-
city of direct measurements. In general, debris flows
are masses of sediments saturated with water, where
the forces exerted by both components contribute to
produce the physics of the flow. Along with thresholds
of precipitation and local slope (G
ReGoRetti
& d
alla
f
ontana
, 2008), the occurrence of debris flows is
controlled by the availability of erodible sediment in
the basin and channel system (b
ovis
& J
akob
, 1999)
and by the linkage between sediment source areas and
channel network. For this reason, debris flow-prone
basins are usually unstable, rich in sediment sources
and the channels are provided with large amounts of
poorly sorted debris. By contrast, in small mountain
basins where sediment availability is limited, debris
flows occurrence requires a long time interval for
sediment recharge. Debris flows in these basins are
thus rare, and even major floods are typically charac-
terized by bedload transport with volumetric sediment
concentrations usually lower than 10%. The quanti-
fication of frequency and sediment volumes (magni-
tude) transported by debris flows and extreme bedload
floods is of crucial importance for hazard assessment,
land use planning and design of torrent control inter-
ventions (J
oHnson
et alii., 1990; z
immeRmann
et alii.,
1997; R
iCkenmann
, 1999; R
iCkenmann
& k
osCHni
,
2010). Several authors have derived magnitude–fre-
ABSTRACT
Sediment transport in steep mountain streams can
occur as bedload or debris flows, depending on ba-
sin geomorphology and sediment supply conditions.
This paper compares two small catchments located in
the Eastern Italian Alps (Rio Cordon and Moscardo
Torrent) where the dominant sediment transport proc-
esses differ substantially. The former hosts a meas-
uring station for water and sediment transport rates
operating since 1986, whereas the latter was set up
in 1989 to monitor debris-flow events. Differences in
sediment dynamics between the two basins are quanti-
tatively investigated by using a magnitude-frequency
analysis that highlights the relatively low sediment
supply of the Rio Cordon and the unlimited sediment
availability in the Moscardo Torrent. This contrasting
sediment transfer activity can be attributed to different
basin and channel morphologies, which are analyzed
in terms of sediment supply conditions and longitudi-
nal profiles curves.
K
ey
worDS
: Bedload, debris-flow, sediment availability,
experimental basins.
INTRODUCTION
Steep streams draining mountain regions are of-
ten characterized by relatively sudden and flashy flood
events which often represents a major threat. Sediment
transport in small headwater streams (<10 km
2
) occurs
both as Newtonian (floods with suspended and bed-
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M.A. LENZI, L. MAO & M. CAVALLI
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
whose catchment drains an area of 4.1 km
2
. The bed-
rock geology of the basin is composed of Carbonifer-
ous flysch, with highly fractured and weathered shale,
slate, siltstone, sandstone and breccias. Quaternary
deposits, mostly consisting of scree and landslide ac-
cumulations, are common in the basin. The presence
of a deep-seated gravitational deformation at the val-
ley head, the loosened rock mass quality and its highly
shattered state make the steep slopes of the basin prone
to rockfalls and shallow landslides which supply large
amounts of debris into the channel (m
aRCHi
et alii,
2002). Sixty-four percent and 18% of the basin area
are covered by conifer forests and shrubs, respectively.
Unvegetated areas, which occupy about 18% of the
basin, provide most of the debris supplied to the chan-
nel network, both in the upper part of the basin and
along the main channel. This causes quasi-unlimited
amounts of sediment availability, resulting in frequent
debris flow events, triggered also by relatively mod-
erate rainstorms. Typical debris flow deposits (lateral
levees) are present along the channel, and the frequent
occurrence of debris flows prevents the formation of
stable bedforms (Fig. 1). The mean gradient of the
main channel is about 37%. Concrete check dams were
built in the main channel in order to limit bed erosion
and to stabilize channel banks in the middle and lower
reaches. The debris-flow deposits are poorly sorted and
show a wide grain size distribution. Lateral levees and
debris-flow lobes mostly consist of pebbles and me-
dium to fine boulders supported in a muddy matrix;
larger boulders with an intermediate diameter of 2–3
m are also common. The particle size distribution of
debris-flow deposits shows D
50
ranging approximately
from 10 to 20 mm and D
84
from 500 to 700 mm.
RIO CORDON
The Rio Cordon is a boulder-bed, step pool stream
draining an area of 5 km
2
. Due to its high elevation
and past use for cattle grazing, forests cover only
the lower part of the catchment (7% of the area). Al-
pine grasslands dominate (61%), followed by shrubs
(18%) and bare land (14%). The bedrock mainly con-
sists of dolomites, volcaniclastic conglomerates and
tuff sandstones. Quaternary deposits are widespread.
The Rio Cordon mean channel slope is 13.6% and the
longitudinal channel profile displays an alternation of
high-gradient and low-gradient stretches. The aver-
age bed surface grain size distribution is characterized
quency relationships for debris flows (e.g. m
aRCHi
&
d’a
Gostino
, 2004; H
unGR
et alii., 2008). Differences
among the various magnitude-frequency relationships
have been related to differences on triggering condi-
tions and debris availability (v
an
s
teiJn
, 1996). Most
of the evaluations of debris flow magnitude and fre-
quency have been conducted by using indirect meth-
ods, such as stratigraphic techniques (b
laiR
, 1999),
lichenometric methods (H
elsen
et alii, 2002), tree-
ring records (s
toffel
et alii, 2006) or aerial photog-
raphy interpretation (J
akob
et alii, 2005). Despite a
significant diversity as to their transport mechanics,
the monitoring activity of such impulsive, high-ener-
gy processes in remote areas poses problems that are
similarly complex. Their short duration and relatively
low frequency of occurrence require the implemen-
tation of robust and reliable systems for performing
direct field observation in remote areas. Monitoring
activities carried out through permanently installed
devices are very costly but are of extreme value when
long-term series of data are eventually produced.
In the present paper, the long-term (>10 years) data
on sediment volumes in two instrumented channels in
the Eastern Italian Alps, will be comparatively ana-
lysed. The two study sites are the Moscardo Torrent and
the Rio Cordon, both instrumented for the continuous
monitoring of sediment transport. The two basins are
characterized by comparable size and climatic condi-
tions, but differ as to the typology of the dominant sedi-
ment-transporting flows, because the Moscardo Torrent
commonly features debris flows in contrast to the Rio
Cordon, which is characterized by bedload events.
STUDY BASINS
The Moscardo Torrent and the Rio Cordon are
two small headwater basins (around 5 km
2
) of the
Eastern Italian Alps. Their mean hillslopes gradient is
63% and 52%. Their climatic conditions are typical of
Alpine environments, with precipitation (annual aver-
age 1660 mm in the Moscardo torrent, and 1100 mm
in the Rio Cordon) occurring mostly as snowfall from
November to April and snowmelt-dominated runoff
in May and June. Short-duration summer floods and
floods occurring in early autumn represent important
contribution to the flow regime.
MOSCARDO TORRENT
The Moscardo Torrent is a debris-flow channel,
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LONG-TERM MONITORING OF BEDLOAD AND DEBRIS FLOWS IN TWO SMALL ALPINE BASINS OF DIFFERENT MORPHOLOGICAL
SETTINGS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
575
installed instrumentation includes rain gauges, ultra-
sonic sensors for the measurement of flow depth, seis-
mic detectors for recording the vibrations caused by
the passage of debris flows (a
Rattano
, 1999), and a
video camera (Fig. 2). The mean propagation velocity
of the front is calculated as the ratio between the sen-
sors (ultrasonic and/or seismic) distance and the time
interval between the debris-flow peaks. The methods
used for estimating peak discharge and flowing vol-
ume from ultrasonic records are discussed in m
aRCHi
et alii (2002).
In the Moscardo Torrent, 15 debris flow events oc-
curred from 1990 to 1998, 13 of which were recorded
by the installed devices (m
aRCHi
et alii, 2002). Record-
ed debris flows range from small events (around 700
m
3
), which would had probably remained undetected if
a monitoring system had not been installed, to intense
phenomena with volumes up to about 60,000 m
3
. The
hydrographs recorded by the ultrasonic sensors show
differences from event to event. In particular, surge ve-
locities and hydrograph shapes differ considerably. In
some events, debris flows show a single, well-defined
wave with a steep front followed by a continuous de-
crease in flow depth; a few smaller waves may follow
the main surge. In other cases, the recession limb is very
irregular with abrupt stage fluctuations (m
aRCHi
et alii,
2002). There are features common to all recorded debris
flows and these are the short duration of the event and
the presence of a sharp rising limb in the hydrograph,
corresponding to the passage of the debris flow front at
the monitoring station. The largest debris flow was re-
corded on July 8, 1996 (Fig. 3). The volume of the flow-
ing mass (water and solid particles), calculated through
flow stage measurements and topographical survey,
was estimated around 65,800 m
3
(m
aRCHi
et alii, 2002),
by D
16
= 37 mm, D
50
= 119 mm and D
84
= 357 mm
(l
enzi
et alii, 2004). Some reaches of the Rio Cor-
don channel feature step-pool morphology (Figure
1). Through detailed field surveys of the longitudinal
profile carried out before and after floods of different
magnitude, l
enzi
(2001) demonstrated that the step-
pool sequences are bed structures that fail only dur-
ing low-frequency, intense flood events. In the Rio
Cordon, active sediment sources, represented by bare
slopes, shallow landslides, eroded stream banks and
minor debris flow channels, cover about the 5% of the
basin area. However, about 50% of the total sediment
source area is located upstream of a low-gradient belt
where sediment deposition takes place, thus making
sediment supply from the upper part of the basin to be
of minor relevance (d
alla
f
ontana
& m
aRCHi
, 2003;
l
enzi
et alii, 2004). The generally limited sediment
availability within the main channel can occasion-
ally be increased either during low-frequency events
able to remove the bed armour layer (as during the
1994 flood) or by minor mud flows and debris flows
entering the main channel from the steeper tributaries
(l
enzi
et alii, 2004).
MONITORING DEVICES AND REGISTE-
RED EVENTS
MOSCARDO TORRENT
A debris flow monitoring system, designed and
maintained by the Research Institute for the Hydro-
geological Protection of the Italian National Research
Council (CNR IRPI), has been operating since 1989.
The Moscardo Torrent appeared suited for the instal-
lation of such a system because it is characterized
by frequent debris flows, easy accessibility and by a
stable channel on the fan (m
aRCHi
et alii, 2002). The
Fig. 1 - Pictures of the Moscardo Torrent (on the left) and the Rio Cordon (on the right) main channels.
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M.A. LENZI, L. MAO & M. CAVALLI
576
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
stream (Fig. 2). The volume of bedload is measured
at 5 min intervals by 24 ultrasonic sensors fitted on a
fixed frame over the storage area (l
enzi
et alii, 1999,
2004). Suspended sediment is measured by two tur-
bidimeters: a Partech SDM-10 light absorption and a
Hach SS6 light-scatter instrument. Flow samples are
gathered automatically using a Sigma pumping sam-
pler installed at a fixed position in the inlet channel.
Overall, 24 bedload events characterized by bed
load transport (grain size greater than 20 mm) were
recorded by the Rio Cordon station from 1986 to 2006
(l
enzi
et alii, 2004; m
ao
et alii, 2008). On 14 Septem-
ber, 1994, an intense flood featuring a peak water dis-
charge of 10.4 m
3
s
-1
and a peak bedload transport rate
of about 157 kg s
-1
(25 kg s
-1
m
-1
) was recorded (l
enzi
et alii, 2004). Such a high-magnitude event features
the typical flash-flood pattern, i.e., a very high peak
flow rate, a very short duration (4 h), and 900 m
3
as
total bedload volume (Fig. 3). The coarsest boulders
(around 1 m) of the bed surface were entrained and
transported to the station. Most sediment was supplied
by the channel-bed – the bed armour layer was re-
moved – and channel banks, plus some point sources
which is approximately 30% of the total volume record-
ed in the monitored period. Since 1999, control works
have been implemented on the Moscardo Torrent. More
grade-control dams have been constructed in the mid-
dle and lower parts of the main stream within the basin.
On the alluvial fan, the channel was straightened and
widened through sediment displacement, and sills were
constructed to reduce channel slope. Torrent control
works have affected the evolution of the debris flows,
so that data collected in the most recent years are not
homogeneous with those recorded from 1990 to 1998
and will be not analyzed in this paper.
RIO CORDON
A station for monitoring water discharge, suspend-
ed sediment and bedload transport has been operating
since 1986 in the Rio Cordon. Measurements are taken
by separating coarse grains (> 20 mm) from water and
fine sediments (l
enzi
et alii, 1999, 2004). The measur-
ing station consists of an inlet flume, an inclined grid
where the separation of coarse particles takes place,
a storage area for coarse sediment deposition, and an
outlet flume to return water and fine sediment to the
Fig. 2 - Plan view of the instrumented channel stretch-
es (3 and 4) in the Moscardo torrent (a, from
Marchi et al.2002) and of the Rio Cordon bed-
load measuring station (b, from Lenzi et al.,
2004). In the Rio Cordon, the grid separates
the coarse bedload (D>20mm), which accumu-
lates in a deposit area where ultrasonic sen-
sors monitor its volumetric growth, and the fine
sediments, which accumulates in a basin where
pressure transducers monitor their accumula-
tion
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LONG-TERM MONITORING OF BEDLOAD AND DEBRIS FLOWS IN TWO SMALL ALPINE BASINS OF DIFFERENT MORPHOLOGICAL
SETTINGS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
577
relationship between event magnitude and peak dis-
charge (M = aQ
p
b
), The coefficients and exponents are
a=356.7 and b=0.85 for the Moscardo and a=0.94 and
b=2.8 for the Rio Cordon. The coefficients of determi-
nation (R
2
) are 0.82 and 0.59, respectively. The expo-
nent of the regression is higher for the bedload events
in the Rio Cordon than for the debris flow events in
the Moscardo. This suggests a much higher rate of in-
crease of mobilized sediment volumes with discharge
for bedload streams than for debris flow channels. In
fact, unlimited sediment supply conditions character-
ize the Moscardo Torrent, where debris flow events are
not limited by a lack of available sediment. In contrast,
in the Rio Cordon there is a low correlation coefficient
of the flood peak and magnitude regression. This is due
to the reduced sediment supply in the Rio Cordon, due
to the presence of a low-gradient belt in the median
part of the basin which reduces the connection between
the upper and the lower part of the catchment area.
Also, a certain control on the availability of sediments
in the Rio Cordon has been exerted by the exceptional
September 1994 flood (l
enzi
et alii, 2004). Figure 4
confirms the much higher efficiency of debris flows in
sediment transport compared to bedload movement.
In fact, in the Rio Cordon only the September 1994
event reaches a magnitude comparable to those of the
smallest debris flows registered in the Moscardo Tor-
rent (around 1000 m
3
). At its very peak, the September
1994 flood likely approached hyper-concentrated flow
conditions, as inferred by observing the taped docu-
mentation of the event.
on the catchment slopes (Lenzi et al., 2004). Such a
high-magnitude, low-frequency event has represented
a geomorphic threshold for the Rio Cordon basin,
since it has altered the stream bed geometry (l
enzi
,
2001) and the sediment-supply characteristics of the
basin as a whole (l
enzi
et alii, 2004). Comparing the
bedload/flow rate relationship and the ratio between
bedload volume and effective runoff for the whole
floods, l
enzi
et alii (2004) demonstrated the increase
in sediment availability and the consequent increase in
bedload transport after the 1994 low-frequency event.
During “ordinary” flood events, bedload showed in-
tensities of up to 30 kg s
-1
(4.6 kg s
-1
m
-1
), but most
bedload rates ranged from 0.1 to 3 kg s
-1
(0.03–0.6 kg
s
-1
m
-1
). See l
enzi
et alii (2004) for a more detailed de-
scription of bedload intensities for different durations
and recurrence intervals floods..
MAGNITUDE–DISCHARGE RELATION-
SHIP
Debris flow magnitudes from the Moscardo and
bedload volumes from the Rio Cordon are plotted in
Figure 4 versus the associated peak discharges. Both
sediment volumes and water discharges have been nor-
malized by the basin areas of the respective study sites
(m
ao
et alii, 2009). Despite some differences in meas-
ured variables (peak discharge refers to the solid-liquid
mixture in the Moscardo and only to the liquid frac-
tion in the Cordon), a certain continuity is apparent be-
tween the two channels, even though the lack of over-
lap preclude any reliable assertion. Assuming a power
Fig. 3. Images of the Torrent Moscardo taken at the peak of a debris-flow occurred on July 8, 1996 (on the left, from
Marchi et al., 2002) and of the Rio Cordon taken at the peak of the bedload event occurred on September 14,
1994 (on the right)
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M.A. LENZI, L. MAO & M. CAVALLI
578
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
.
CONTRASTS IN CHANNEL MORPHOLOGY
Sediment supply conditions differ considerably
between the two study basins, with sediment avail-
ability in the Moscardo Torrent being higher than
in the Rio Cordon. As a consequence, the two main
channels feature a different degree of bed structur-
ing. In fact, the Rio Cordon displays fairly long step
pool sequences (Figure 1, l
enzi
, 2001; t
Revisani
et
alii, 2010). Step pool morphology usually forms
through selective transport and armouring processes
under high discharges and low sediment supply (C
Hin
& w
oHl
, 2005), and in presence of high jamming
ratios, i.e., larger clast size/channel width (C
HuRCH
& z
immeRmann
, 2007). Most importantly, step pool
morphology is typical of systems which are supply-
limited in terms of sediments (m
ontGomeRy
& b
uff
-
inGton
, 1997). Conversely, the Moscardo Torrent is
characterized by a poorly-structured bed profile – only
in part modified by the check dams built in its middle
segment – dominated by cascade morphology (Figure
1; m
ao
et alii, 2009). Such a bed morphology, typical
of steep coarse-bed streams, derives from the frequent
disturbances induced by the passage of debris flows
which leave lag deposits composed of large particles,
and by the lack of sufficiently strong and persisting
flows carrying small sediment fractions which would
be needed to arrange these clasts under a regular pat-
tern, such as in step pool architecture.
CONTRASTS IN SEDIMENT AVAILABILITY
Contributing area and slope gradient for each
cell of the main channels were derived from a 5 m
DEM grid by the D8 single flow accumulation algo-
rithm and the steepest descent algorithm (t
aRboton
,
1997). Longitudinal profiles of the Moscardo Torrent
and Rio Cordon main channels were extracted auto-
matically from the DEMs, and are showed in Figure
5. The Moscardo Torrent has a steeper and more lin-
ear longitudinal profile than in the Rio Cordon (Mao
et al., 2009). The average slope of the main channel
is about 37%, but the upper portion is much steeper
(>50%) and relatively regular. Conversely, the pat-
tern of the Rio Cordon’s longitudinal profile is more
complex, and its analysis allows the link between
channel morphology and domains of geomorphic
processes to be explored. In the upper part of the
basin the channel is colluvial, very steep (36%) and
exhibits cascade morphology. The channel than flat-
tens within a wide “hanging valley”, upstream of a
30 m-high waterfall located in the middle portion of
the profile. In the lower-gradient reach upstream of
the waterfall, transport capacity decreases thus lead-
ing to temporary sediment deposition. Further down-
stream the channel slope significantly decreases and
small tributary channels convey a significant amount
of sediments to the main channel, in the form of rela-
tively small debris and mud flows originating from
shallow landslides (l
enzi
et alii, 2004). Finally, in
the last 1000 m upstream of the measuring station,
Rio Cordon has a mean slope of about 14% and the
morphology is characterized by the alternation of
step pool and steep cascade reaches, which reflects
overall high transport capacity but supply-limited
conditions.
Fig. 4 - Relationship between peak discharge, debris
flow magnitude (Moscardo Torrent) and bed-
load volumes (Rio Cordon)
Fig. 5. - Dimensionless longitudinal profiles of the Mo-
scardo Torrent and Rio Cordon. The disconti-
nuity in the middle of the Rio Cordon profile
correspond to a geological knick point
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LONG-TERM MONITORING OF BEDLOAD AND DEBRIS FLOWS IN TWO SMALL ALPINE BASINS OF DIFFERENT MORPHOLOGICAL
SETTINGS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
579
lack of debris flows in autumn can be also attributed
to the temporary paucity of sediments in the initiation
zones after the occurrence of summer debris flows.
CONCLUDING REMARKS
The quantitative comparison of sediment vol-
umes transported in the Moscardo Torrent and in the
Rio Cordon made it possible to outline several as-
pects of sediment dynamics in two small Alpine ba-
sins of comparable size and climate, but with highly
contrasting characteristics in terms of physiographic
setting and sediment supply. The magnitude-frequen-
cy relations can be deemed representative of bedload
channels with low to moderate sediment supply (Rio
Cordon) and of debris-flow torrents with unlimited
sediment availability (Moscardo Torrent). As it could
be expected, the latter displays much higher magni-
tudes for comparable peak discharge. The difference
in sediment supply conditions and sediment transport
behaviour between the two study basins is reflected
in the morphological diversity of the channels. The
Rio Cordon channel has lower average slope than
the Moscardo Torrent and displays an alternation of
high-gradient and low-gradient stretches (at both the
morphological unit and reach scales). This stepped
profile favours partial sediment deposition, so that
debris flows could happen (for very infrequent flood
events) only in some stretches of the channel and the
headwater portion of the basin provide a minor con-
tribution to the annual sediments yield. By contrast,
although deposition of small debris flows within the
basin has been observed in the Moscardo Torrent, a
MAGNITUDE-RUNOFF REALTIONSHIP
Sediment transport measured at the Rio Cordon
station has been compared wit
H
R
iCkenmann
s
(2001)
bedload formula, which has been developed for chan-
nel slopes up to 0.2 m m
-1
, and relates the total bedload
volume (M, in m
3
, with pore spaces) to the effective
runoff volume of the hydrograph (V
re
, in m
3
) and the
slope (S) as M = 1.95 V
re
S
1.5
. For the Rio Cordon the
effective runoff has been calculated as the water run-
off volume above the threshold discharge for bed load
on each flood hydrograph (l
enzi
et alii, 2004). Fig-
ure 6 shows that the bedload magnitude is reasonably
predicted only for the 14 September 1994 flood. The
considerable overestimation of bedload transported by
ordinary floods is mainly due to the sediment supply
limited conditions. Also, because of the high channel
gradient (> 0.05 m m
-1
) and the presence of step-pools,
the overprediction of bedload transport is likely due to
the additional form resistance (provided by immobile
boulders and step-pool sequences) as suggested by
f
eRGuson
(2007) and previously discussed by R
iCk
-
enmann
(2001) and R
iCkenmann
& k
osCHni
(2010).
As to the Moscardo Torrent, Figure 6 clearly shows
that the magnitude of transported sediments is sub-
stantial higher than in the Rio Cordon. Due to the lack
of direct measurements of the water content in the de-
bris flow mixture, in this case the effective runoff has
been calculated using the rainfall data collected within
the basin. Only the precipitation occurred during the
rainfall event that triggered the debris flow was used.
Even if this effective runoff value may not be straight-
forwardly compared with what used by R
iCkenmann
& k
osCHni
(2010) in their analysis of debris flow
occurred in Swiss Alps in 2005, it is worth noticing
that their debris flow formula (M = 16378 A
1.35
S
1.7
)
is in fair agreement with the volumes measured at the
Moscardo Torrent station. The maximum estimated
magnitude calculated using channel slope (S) and ba-
sin area (A) was not achieved during all debris flow
events, most likely due to the occurrence period (m
ao
et alii, 2009). Debris flows in the Moscardo Torrent
take place in the summer months. Although total pre-
cipitation in autumn is often very abundant, no debris
flows have occurred in October and November since
the torrent was instrumented. This could be due to the
infrequent occurrence of high-intensity storms during
these months. Although large amounts of loose debris
are present on the slopes of the Moscardo basin, the
Fig. 6 - Relationship between the magnitude of bed-
load and debris flow events and effective run-
off. The r
icKeNmANN
& K
oShNi
(2010) bedload
and the r
icKeNmANN
(2010) debris flow equa-
tions are plotted as well.
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M.A. LENZI, L. MAO & M. CAVALLI
580
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
high channel gradient and a regular longitudinal pro-
file, only slightly modified by the check dams built
in the middle part of the channel, make it possible for
most debris flows to reach the alluvial fan.
REFERENCES
a
Rattano
m. (1999) - On the use of seismic detectors as monitoring and warning systems for debris flows. Natural Hazards, 20
(2–3): 197-213.
b
laiR
t.C. (1999) - Sedimentology of the debris-flow dominated warm spring canyon alluvial fan. Death Valley, California.
Sedimentology, 46: 941-957.
b
ovis
m.J. & J
akob
m. (1999) - The role of debris supply conditions in predicting debris flow activity. Earth Surface Processes
and Landforms, 24 (11): 1039-1054.
C
Hin
a. & w
oHl
e. (2005) - Toward a theory for step pool in stream channels. Progress in Physical Geography, 29: 275-296.
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SETTINGS
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