E3S Web of Conferences 415, 05024 (2023)
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https://doi.org/10.1051/e3sconf/202341505024

Rockfalls change the runout and frequency of debris flows atPunta Nera (Eastern Italian Alps).
Alessandro Simoni1*, Matteo Barbini2, Leonardo Battistel1, Martino Bernard2, Matteo Berti1, Osvaldo Cargnel3, PierPaolo Ciuffi1, and Carlo Gregoretti2
1University of Bologna, BiGeA Dept., via Zamboni, 67 – 40126 Bologna, Italy
2University of Padova, Land Environment Agriculture and Forestry Dept., 35020 Legnaro (PD), Italy
3Studio Associato Cargnel, 32100 Belluno, Italy

Abstract. In the Dolomitic region, many debris flow basins have headwaters characterized by extremely
steep slopes of bare outcropping rock. Cliffs are commonly incised by multiple chutes which rapidly deliver
runoff at their base where the channels incise scree deposits. Debris flows mobilize and transport sedimentalong such ephemeral channels following intense summer convective rainstorms. In the debris flow basinof Punta Nera, a series of rockfalls, which occurred in the upper part of the headwater between 2013 and2016, caused dramatic changes in the debris flow dynamics. Starting from the summer of 2014, the runoutof debris flow events increased so much that it reached the national road, which runs at the toe of the debris
flow fan, attracting media attention and prompting the adoption of protective measures. Here, we use
newspaper reports, direct observations, aerial and terrestrial photograph, monitoring data and topographic
surveys to document the rockfalls and the debris flow activity. The sudden increase in sediment availabilitychanged the magnitude of events, their runout and the critical conditions for debris-flow occurrence.

1 Introduction
The source of sediment that can be mobilized togenerate a debris flow can derive from landslides [1],progressive bulking of sediment operated by runoff
along steep mountain torrent [2] or en-masse failure ofchannel bed sediment [3]. The total debris flow volume
is largely controlled by the volume of material entrainedin the debris flow channel [4]. Consequently, the volume
of a new debris flow depends on the amount of available
loose material stored in channels.
Debris flow basins may be supply-limited ortransport-limited depending on the availability of loose
mobilizable debris in relation to climate forcing [5].
Sediment production is important in supply-limited
systems because it has strong influence on debris flow
frequency and magnitude. The process of channel
recharge is very often relevant for debris flow initiation.
In fact, the likelihood that a debris flow will be triggered
depends on the availability of sufficient precipitationand mobilizable sediment [6]. The second condition
depends on progressive time-dependent mechanisms ofsediment accumulation (dry ravel processes, channelbank failures, bed load, [2, 7]) or sudden episodes thatcan produce a large amount of sediment (upstream massfailures and rock falls, [8]). The remobilization oflandslide and rockfall deposits can feed debris flows formany years, as documented in the case of catastrophic
earthquakes-induced mass failures [9]. Many studies
have reported that debris flow frequency and magnitude
dramatically increases following upstream mass failures

and progressively decay toward background rates withina decade [10].
When sediment availability also changes the criticalconditions for debris-flow occurrence may change.
Debris flow-triggering rainfall thresholds may lower
significantly whereas more frequent and destructive
debris flows may occur [11, 12].
Research efforts to include the availability ofsediment and/or channel recharge rate in the predictionof debris flow magnitude and frequency have appearedin recent years [13, 14] but challenges and limitations
are still relevant for practical applications, therefore
direct observations are still in great need.
This short communication describes the abruptchanges affecting the sediment transport dynamics in asmall Alpine debris flow basin, during the last decade.
We document how a series of rockfalls occurred in theheadwater of a debris flow basin produced significant
changes to the magnitude and frequency of debris flow
events. The sudden increase in sediment availabilitychanged the critical conditions for the occurrence ofdebris flows and resulted in an increase in their runout
sufficient to repeatedly inundate the national road at thetoe of the fan.

2 The Punta Nera debris flow basin
The Punta Nera debris flow basin is located on thewestern slope of Mount Sorapiss (3205 m a.s.l.) in the
dolomitic region of the eastern Italian Alps. Steep* Corresponding author: alessandro.simoni@unibo.it© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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during the summer season by convective rainstorms [15,16].
The National Road SS151 is located at the toe of thefan and was repeatedly inundated during the years 2015
and 2016 before massive defensive structures wereprogressively built (completion in 2019).
From 2015 to 2018, we maintained a low‐costmonitoring station along the debris flow channel at anelevation of 1520 m. Here the channel is steep (about30°) and debris flow initiation occurs due to entrainmentof the channel bed material. The aim of the monitoring
was to gain knowledge of the channel runoff and debris
flow initiation by recording videos of the channel duringand after intense rainfalls. Measured parameters also
include rainfall and wind speed and direction. Whenevera rainfall intensity threshold (0.3mm/min) wasexceeded, the system powered the two cameras andacquired high‐frequency data (5s).

bedrock cliffs dominate the steep headwater whose area
measures 0.5 km2 approximately.
Debris flows represent the main sediment transportmechanism for the basin, whereas torrential activity ismostly absent. Debris flows are triggered by the surface
water runoff that steep rocky slopes concentrate into theheadwater [15]. Scree deposits drape the base of thedolomite walls. Here, at the fan apex, the debris flow
channel is incised into scree deposits while further
downstream its proneness to avulsions is demonstrated
by the considerable size of the fan.
Channel‐bed material accumulates along the channel
due to rockfall from dolomite cliffs and dry ravel fromsteep channel banks, promoted by drying and wetting
cycles. Debris flow activity evacuates accumulated
sediment, erodes the channel and underscours its bank,causing frequent localized bank failures feeding thecyclic process of channel refilling. Triggering rainfalls
consist of short and intense rainfalls typically produced

detachment

Fig. 1. Hillshade of the Punta Nera debris flow basin (left) derived from a 2015 lidar-derived DEM (1 m) and photograph (right)
taken shortly after the occurrence of the main rockfall event on May 20th, 2016.

detachment (Figure 1). The deposits spread along themain chute cutting through the headwater, covering an
area of about 14,000 m2. In this case, pre- and post-event
photographic documents indicate the detachment of a
thick pinnacle of dolomite. Assuming an average
thickness of 30 m, we obtained a total volume exceeding
100,000 m3, which is likely an underestimate.

3 Rockfalls in the headwater
The first evidence of rockfall occurring in the Punta
Nera basin dates back to October 2013. Similar eventsoccurred between October and November of thefollowing year. All rockfalls involved the dolomite
walls in the upper western part of the headwater (Figure1). Based on terrestrial photographs and direct
observations, we estimate volumes in the order of a fewtens of thousands of m3, although even the mapping inthe 2015 aerial photographs is problematic due toextreme steepness (>60°) causing shadowing.
All of the 2013 and 2014 events are likely precursorsof the main rockfall occurred on May 20th, 2016 in thesame area. In this case, aerial photographs and videos
taken from the helicopter on the following day, allowed
precise mapping of the area (4,000 m2 approx.) of the

4 Debris flow events
Before 2014, the debris flow catchment of Punta
Nera was not known by authorities and practitioners as
an hazardous debris flow basins. The neighbouring
Acquabona basin was better known for its debris flow
activity [17] as shown by the retention basin that
protects the national road SS51. Although the frequencyof events at that time at Punta Nera is unknown, there isno evidence of debris flows inundating the national road2

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earthmoving works, and the largest event of mid-June
event reached about 80,000 m3.
By comparing aerial photographs (2012, 2015, 2017,2018, 2020) and lidar surveys (2010, 2011, 2015, 2020)
we identified how the runout of debris flow events
increased abruptly during the years 2015 and 2016. The
path of the debris flow channel remained relatively
unchanged in the upper part of the fan while multiple
avulsions occurred further downstream with creation of
new channels reaching the national road and several
lobes of debris deposited in the lower fan (Figure 2).
During recent years (2017-21), the debris flow
activity at Punta Nera gradually returned to earlier levels(i.e., no active channel in the lower part of the fan). No
traffic disruptions were recorded thanks to the retention
basin but also because debris flow activity retreated in
the middle part of the fan (Figure 2). No subsequent flow
showed the mobility, nor the magnitude, of the large
events that occurred during 2015 and 2016.

below. The analysis of aerial photographs demonstratethat the active channel was confined to the upper middle
part of the fan (Figure 2) while a dense coniferous forest
covered the lower fan.
After the end of the 2014 summer season, on
October 13th, a debris flow event reportedly caused
some flooding of the national road. After a few weeks
another similar event was recorded and, during thefollowing two summer seasons, more debris flows
caused traffic disruption due to roadway inundation withdebris (Figure 3). Fortunately, there were no victims or
major accidents. Relevant authorities intervened byexcavating ditches and building retention structures
aimed to protect the road. These were already partly
operating at the beginning of summer 2016. It was thatsummer, following the May rockfall, that the highest
number of events (Figure 3) and the largest volumes
were recorded. Debris flow events repeatedly filled theretention basins and inundated the road. Associatedvolumes were estimated by the road authority, based on

Fig. 2. Active debris flow channels on the Punta Nera fan during the summer seasons preceding the rockfalls (left), following therockfalls (centre) and following the evacuation of the rockfall deposits from the headwater (right). Channel mapping derives fromaerial photographs and lidar surveys.

filling from rock weathering and summer debris flow
sediment transport determines the amount of debrisavailable along the drainage network [6, 18]. Drainages,
with their low slope relative to the surrounding
hillslopes and rock slopes, act as a reservoir to retain
sediment until debris flow transport [18].
Sediment production and transport processes in a
debris flow basin may proceed relatively steady throughtime or change dramatically their rate in response to acatastrophic event. Most of the documentation of these
events describes the effects of large earthquakes andtheir consequences to sediment transport processes overlarge areas [9-11]. In the case of Punta Nera, the scaleof our observation is very different but the effects on asmall alpine debris flow basin appear similar. A seriesof rockfalls in the very upper part of the headwaterproduced a dramatic increase in the downslope sedimenttransported by debris flows. In our interpretation, one of
the key elements of such abrupt change is that therockfall deposited along the main chute of the channel
network present in the rocky headwater. This enhances
connectivity between the hillslope process (rockfall) andthe channel process (debris flow). Also, the slope of thechute (40 to 45°) plays an important role. On one side,Fig. 3. Number of recorded debris flow events per year androckfall episodes.

5 Discussion
Previous research has described the cycle of channel
filling, debris flows, and channel reloading [5, 6]. In
steep alpine landscapes with abundant exposed bedrock,the seasonal competition between winter/spring channel

3E3S Web of Conferences 415, 05024 (2023)
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10. X. Fan, H. Juang, J. Wasowski, R. Huang, Q. Xua,
G. Scaringi, C.J. van Westen, H.B. Havenith, Eng.
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Liu, S.Y. Lee, Eng. Geology 71, 49–61 (2004)
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McDonald, Hydrology 4, 7, (2017)
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15. M. Berti, A. Simoni, Landslides, 2, 171-182 (2005)
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Gregoretti, A. Simoni, T.L. Mote, A.M. Saylor, Nat.
Hazards and Earth Syst. Sc. 16, 2, 509-528 (2016)17. M. Berti, R. Genevois, A. Simoni, P.R. Tecca,
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the slopes are gentle enough to accommodate therockfall deposits, initially preventing further downslope
propagation. On the other, the slopes are extremely highfor the loose debris that could be rapidly destabilized
and eroded by water runoff. Our preliminary
observations, indicate that debris flows were extremely
efficient in evacuating the sediment along low-orderdrainages where exposed bedrock normally dominates.
The resulting debris flows showed strong erosive naturein the downstream fan, and mobilized overall volumes
that were never recorded in recent times at Punta Nera.
Higher flow magnitudes corresponded to longer runouts
and caused many problems to the national road at the toeof the fan. This geologic hazard may become more
frequent in the near future due to the expected increasein rockfall frequency in the alpine regions due to climatechange [19].
In addition to an increase of magnitude, the years
2015 and 2016 saw a notable increase in debris flow
frequency, because of the rockfalls. This indicates achange in the critical conditions required for debris flowinitiation. Although anomalies in rainstorm frequency
may also have played a role, we believe that debris
availability was the primary factor responsible for theincrease in frequency and volume of events observed
during the two-year period. Ongoing rainfall data
analysis is expected to reveal a temporary rainfall
intensity/duration ID threshold modification. Results ofprevious research [14] support the use of rainfall ID
thresholds across a range of decreasing sediment supplyconditions. In other cases [12] rainfall thresholds
showed substantial changes following a dramaticincrease of sediment availability across the landscape.
Future developments of the present study will
include the analysis of available digital terrain models todocument the landscape changes that occurredfollowing the rockfalls and subsequent debris flows.

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