E3S Web of Conferences 415, 05028 (2023)
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https://doi.org/10.1051/e3sconf/202341505028

Debris flows and debris avalanches initiation and runout
susceptibility assessment in Campania region (Italy)
Rita Tufano1, Davide Mazza1, Francesco Maria Guadagno1, Pantaleone De Vita2, Giacomo Russo2 and Paola
Revellino1*
1Department of Sciences and Technologies, University of Sannio, Benevento, Italy
2 Department of Earth, Environmental and Resource Sciences, Federico II University of Naples, Naples, Italy

Abstract. Sectors of the Campania region (southern Italy), where pyroclastic airfall deposits covercarbonate slopes, have frequently experienced debris avalanches and debris flows. These landslides aretriggered by heavy rainfall and in the last century caused severe destruction and hundreds of deaths. Theiroccurrence is controlled by the interaction of several factors, both natural and human induced. The initial
failure is linked with the presence of morphological discontinuities such as rocky scarps and road cuts, whichinterrupt the stratigraphical continuity of an “infinite slope”. Here the main criteria for susceptibility analysis
are defined trying to develop a procedure for identify highly-susceptible sources, runout and invasion areas.

1 Introduction
Landslide hazard of the Campania carbonate slopes(southern Italy) covered by pyroclastic deposits ismainly related to the occurrence of high-velocity debris
avalanches and debris flows. Historical analyses haveshown that landslides are spatially and temporally very
common and remarkably similar to each other in termsof style, morphology, and materials involved. The initiallandsliding process is controlled by the interaction ofnumerous factors, both natural and human induced.
Geomorphological settings play a decisive role inlocating the source failures of landslides. On this basis,landslide hazard can be assessed by the following
crucialapproaches: a) recognition ofthegeomorphological controlling factors in terms of sourceareas and flows paths; b) definition of landslide intensity
parameters in terms of velocities, volumes and runout
distance. An approach combining geomorphology,
slope stability analysis and numerical runout modellingis here adopted with the aims to identify possible
landslide source areas and simulate landslide intensity
scenarios. Moreover, to predict the runout behaviour ofpotential debris avalanches-debris flows by using a
dynamic model previously calibrated on observed
events.

2 Landslide features
2.1 Geological predisposing factors
The Campania Apennines are characterised bycalcareous-dolomitic mountains formed by monoclinal
series which are mantled by a thin veneer of air-fallmaterial, such as ash, lapilli and pumice, and ignimbritetype flows, principally linked to cyclic explosive volcanic
activity of the Phlegrean Fields and Somma-VesuviusVolcanoes. This results in a typical geomorphologicalsetting where the pyroclastic soils overlay on thebedrock at angles close to the frictional angle of thematerial. Even though the pyroclastic materials strongly
smooth slope irregularities, the morphology of thecalcareous bedrock is manifest by scarps and deepgullies.
The fertility of the pyroclastic mantle has constantly
encouraged the use of slopes for agricultural andforestry purposes, so that mechanised forest
management practices have favoured the growth of adense track network, significantly unsettling the delicateequilibrium of slopes.
Detailed surveys carried out demonstrate the role ofthese morphological discontinuities both on water
circulation and on the stability of masses [1].
2.2 Landslide mechanisms
The failure phenomena occurring in the pyroclastic soil
exhibits a distinctive general behaviour. Analogies
concern landslide type (initial translational slides
evolving into viscous, fluid flows), material involved,
confinement in gullies or steep stream channels if
present, high velocity and high erosive capacity.
Moreover, they are generally triggered by heavy andprolonged rainfalls whose effects on slope stability arecontrolled by antecedent soil hydrological conditions[2]. The initial movement consists of a translationalslide of thin but relatively wide segments of thepyroclastic cover (thickness from some decimetres up to2 m). The initial mass is able to destabilize the* Corresponding author: paola.revellino@unisannio.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/).

E3S Web of Conferences 415, 05028 (2023)
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pyroclastic cover downslope. As a result, initial slides
soon transform into extremely rapid, fluid debris
avalanches [3], increasing in volume by eroding theslope soil cover. The landslide material can flow along
the open slopes or become channelled into gullies
where, if incorporating water, can transform in debris
flows and hyperconcentrated flows.
In most landslide cases, initial translational slides
were triggered at the edges of morphologicaldiscontinuities (natural scarps or road cuts) (e.g. [4]) andthey were linked to kinematic freedom conditions of the
pyroclastic masses in such morphological contexts (Fig.
1). A less-frequent scenario in the region consists of
instabilities occurring immediately downslope of cutsfor tracks and involving road-fill materials. Typicalmorphological settings recognised in landslide sourceareas are shown in Fig.1.
Both surficial and groundwater circulation play afundamental role in triggering the phenomena. Field
observations show that along the slopes water canconcentrate and infiltrate, inducing the development ofpore pressure for the initial instabilities and supplying
water for fluidification of masses.

geomorphological origin of the slope (for examples fault
or dip slopes, open slopes, etc.); iii) occurrence of
pyroclastic deposits and their lithological andgeometrical characteristics; iv) occurrence andgeometricalcharacteristic
ofmorphologicaldiscontinuities of the bedrock (e.g. natural scarps); v)
occurrence and characteristic of human-induced
morphologies; vi) possibility and/or evidence ofconcentrated, superficial water flows.

Fig. 1. The morphological setting of the main source area types
for the Campania landslides: (a) above natural scarp; (b) above
man-made cut; (c) involving fills, in correspondence of"sagging rope" segment of pathways; and (d) involving fills in
the road bends.

4.1 Micro-morphological factors map

4 Study area
The selected test area is located in the southern sector ofMt. Sole close to Salerno town, wheregeomorphological factors controlling landslide
susceptibility were identified. The test area comprises
the Olivieri Valley incised by deep gullies that flow into
the main N-S direction valley floor at about 200 m a.s.l..
Quaternary and Holocene deposits consisting mainly of
in-situ and reworked pyroclastic soils and colluvium,
discontinuously cover the carbonate bedrock. Theirthickness quite varies depending on the slope angle andthe morphological setting of the bedrock (Fig. 2),
according to [5,6]. In particular, the greatest thickness(>2 m) is found in the lower part of the slope where theslope angle ranges from 0° to 20°, while thicknessbetween 0.5 and 2 m characterizes the intermediate
sector of the slope, not interested by gullies, for which
the slope angle ranges from 20° to 35°. Conversely, theoutcrop of the bedrock and gullies are characterized bya slope angle greater than 45° and the absence of cover.
Similar conditions are also recognized at the base of theslope where man-made cuts occur. Finally, trackways(i.e. man-made road cuts) in both the upper and middle
sectors of the test area further entail discontinuity in thecover.

Based on the procedure calibrated by [7] on areas
involved by landslides in a similar geo-morphologicalcontext (i.e. Avella Mt.s), micro-morphological factors,intended as local morphological discontinuities
collected through a detailed field survey, wererecognized because identified as controlling landslide
susceptibility at the source areas (Fig. 2). Suchprocedure was selected because of the topographic,
geometric, morphological and lithological data available
for the sector of slope potentially susceptible oflandslide triggering. At this aim, identified
morphological discontinuities (scarps) of the carbonate
bedrock were grouped into two classes on the basis of
their height: a) H > 5 m; and b) H < 5 m. Scarp height isa morphological criterion because controlling the impact
energy of masses. It should be noted that along scarps
rock falls may also occur, which can have similar
destabilizing effects. Artificial cuts were carefully
identified and mapped, since they represent one of themost important details influencing landslide occurrencein the pyroclastic soils of the Campania region [1]. Since
they act as preferential lines of water infiltration into themore permeable pyroclastic layers, the most critical

3 Criteria in susceptibility maps
Analyses carried out on historical and recent debris
avalanches/flows in Campania region permit to selectsome criteria in locating the source areas of potential
future occurrences. It is also true that a single triggeringmodel is not enough to explain the initial landslide
mechanism. A complex variety of models should rather
be needed, making susceptibility mapping quite difficult
and uncertain.
Generally speaking, prediction of the starting pointsof potential landslides should be made through fieldobservations, by identifying critical locations within the
typical morphological settings recognised in the failure
areas. Often, such critical locations can be recognizedby open cracks, points of preferential water run-off orseepage, soil masses displaced forward and overhanging at the scarp edges. In particular, the criteria
should be referred to: i) slope angles; ii)

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s = - θe (ua – uw)

locations were identified at bends and along "saggingrope" segments of the zigzag pathways. Finally, bymeans of field inspections, gully segments weredistinguished based on the presence or absence of debris
material deposited on the gully floors. The existence ofdebris filling the gullies leads to more hazardous
situations, since they can increase the magnitude of apotential landslide event by entrainment during motion.

4.3 Runout susceptibility
In order to define the most susceptible to runout areas,
the geometry of the potential source areas was defined
based on the minimum and maximum values of apex
angle as estimated by [1] for each typical micromorphological factor (i.e. natural scarps, man-maderoad cut, and material filling). Subsequently,
propagation modeling was completed using DAN-3D
software [14] following the procedure described in [15].
DAN-3D is based on a continuum numerical solution of
the depth-averaged Lagrangian equations of motion,
which allows the propagating landslide mass to be
simulated as a hypothetical material governed by asimple rheological relationship. In this case, therheological model selected is the Voellmy model. Suchmodel considers the resisting stress at the base of the
propagating flow as:

4.2 Initiation susceptibility
Stability analysis of pyroclastic cover was conductedbased on the spatial distribution of micro-morphologicalfactors most critical for landslide triggering. Due to thestratigraphic setting of the pyroclastic cover, stability
analysis was carried out by infinite slope approach (e.g.
[8,9]). In particular, the factor of safety (FoS) was
estimated considering the unified effective stresscriterion incorporating the effect of partially saturated
soil conditions (e.g. [10]) for which shear strength is afunction of both normal stress and suction contribution[11]. Accordingly, the failure criterion can be written as:(1)

𝜏 = 𝛾𝐻 (𝑐𝑜𝑠𝛼 +where  is the shear strength, c’ is the effective
cohesion,  is the total stress, ’ is the friction angle andσs is the suction stress, equivalent to pore-waterpressure, uw, for saturated conditions:s = -(ua – uw)

for ua – uw ≤ 0(3).

The main physical properties (porosity n = 53%,
natural unit weight  = 15 kN/m3, natural water content
w = 13%) of the pyroclastic layer were determined inlaboratory on representative samples. The assumed
shear strength parameters, determined by means ofdirect shear tests on saturated undisturbed samples, were
’ = 37° for friction angle and c’ = 2,4 kPa for cohesion,
are in good agreement with parameters reported by[12,13] for similar pyroclastic formations. The water
retention curve implemented in the numerical code wasrelated to the grain size distribution of the pyroclasticlayer. Suction was assumed constant with depth andequal to 36 kPa, corresponding to an effective degree ofsaturation Sr = 0.7 on the water retention curve,corresponding to the representative value of the
pyroclastic soils in the area. Calculated FoS, also
depending on slope angle and cover thickness of thepossible sources area, was reclassified to define
different susceptibility classes. In particular, four
susceptibility classes from the higher to the lower FoS
values are marked for the Olivieri Valley test area (Fig.
3). Results identify highly-susceptible source areas as
mostly localized above and long trackways, in themiddle and upper part of the slope above natural scarps
and in the eastern middle and upper part of the slope,
probably linked to the possible superficial water flow.
Susceptible areas to initial landsliding are
considered sources of potential initial debris slides
which trigger the following avalanches-debris flow
stages. The runout susceptibility of the latter is further
estimated.

Fig. 2. Map of micro-morphological factors controlling
landslide susceptibility in the test area of the Oliviero Valley:(1) road; (2) trackway; (3) landslide source area; (4) gully; (5)watershed; (6) railroad; (7) highway; (8) natural scarp; (9)measured cover thickness; (10) bedrock outcrop/cemented
cover area.

 = c’ + [( – ua) – s] tan’for ua – uw > 0𝛼𝑐𝑔)𝜇 + 𝛾𝑣2
𝜉(4)

where τ is the resisting stress at the base of the flow, γ isthe unit weight of the flowing material, H is the flow
depth, α is the slope angle, μ the dynamic friction
coefficient related to the total normal stress at the baseof the flow, αc is the centrifugal acceleration resulting
from the vertical curvature of the flow path, and ξ is aturbulence coefficient. The dynamic friction coefficientand turbulence coefficient were set according to thosedetermined by [13] (μ=0.07; ξ=200 m/s2) in(2)while it corresponds to the product of effective
saturation degree (θe) and the matric suction (ua−uw) for
partially saturated conditions:3

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geomorphologically similar areas (i.e. Sarno-Quindici
and Cervinara Mt.s), estimated through back-analysis ofpast events using a trial-and-error procedure.
The thickness of the initiating debris slides anderosion depth downslope from the source area was set
according to pyroclastic cover thickness measured
during field surveys. Moreover, two different scenarios
were considered for each source area: 1) maximum
intensity scenario, for which source area was defined by
the maximum values of apex angle and by the total
entrainment of the slope and gully material along thelandslide path considering a maximum soil thickness; 2)
medium intensity scenario, for which source area wasdefined by the minimum values of apex angle and by thetotal entrainment of the slope and gully material along
the landslide path considering a medium soil thickness.
The outcomes of the dynamic analysis in terms ofrunout distance, maximum landslide velocity, anddeposit thickness for each modeled event in the selected
test area, allow the zonation of runout susceptible areas.
To this aim, four susceptibility classes were defined forwhich the maximum intensity scenario was associatedwith high susceptibility, while the medium intensity
scenario to the very high susceptibility class (Fig. 3).

defining different intensity scenarios of events useful inthe evaluation of possible impacts to structure andinfrastructure. It is important to note that the dynamic
model applies only to the specific conditions of theCampania debris flows and debris avalanches due to thepeculiar characteristics of pyroclastic deposits and theirgeomorphological setting. The described procedure isreproducible in similar geomorphological frameworks
for which data about geological, geomorphological, andgeotechnical features are available.
Finally, if initiation susceptibility mapping appears
to be fundamental in the land-use planning context, thedefinition of kinematic and dynamic characters ofevents also appears as a priority for the safeguard ofstructures and infrastructures potentially involved inlandslide phenomena.

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Fig. 3. Initiation and runout susceptibility map of debris
avalanches and debris flows in the Oliviero Valley test area.

5 Conclusions
This paper presents a hybrid procedure for initiation andrunout susceptibility to debris avalanche-debris flow
assessment based on geomorphological and slope
stability analysis with the aim to identify possible
landslide source areas, and numerical modeling forpredicting the runout behavior of potential instability.
Especially, dynamic modeling conducted with
DAN-3D software using Voellmy rheological model
enables the evaluation of runout distance, maximum
landslide velocity, and deposit thickness that allow

4