ORI GIN AL PA PER

Flash flood occurrence and magnitude assessmentin an alluvial fan context: the October 2011 eventin the Southern Apennines

Antonio Santo1• Nicoletta Santangelo2

•

Giuseppe Di Crescenzo1• Vittoria Scorpio3

•

Melania De Falco2• Giovanni Battista Chirico4

Received: 8 March 2014 / Accepted: 20 March 2015� Springer Science+Business Media Dordrecht 2015

Abstract This study presents the analysis of flash floods triggered by an extreme rainfall

event that occurred on 7 October, 2011, over the Marzano carbonate massif (Southern

Apennines). The rainfall event reactivated alluvial fans built up at the outlet of two

mountain basins. Detailed geological surveys carried out immediately after the event al-

lowed the reconstruction of the main erosion and depositional processes that occurred both

in the drainage basin and in the fan areas. The volume of materials eroded in the basin and

deposited in the fan was evaluated by means of accurate topographic surveying and GPS

measurements. Morphological and morphometric properties of the basin/fan system as well

as the presence of human interventions and structures along the main channel and in the fan

area influenced flow propagation. The transported materials came mainly from debris and

gravels previously accumulated along the stream beds and mobilised by the flow during the

event. No significant evidence of landslide contribution to transported bed load was de-

tected. Extensive damage was done to buildings, river bank structures and agricultural

crops. Despite the existence of hundreds of similar alluvial/fan systems in the Southern

Apennines, few studies have been conducted to support adequate risk mitigation action in

these areas. Indeed, to our knowledge, this is the first study focusing on assessing the

magnitude of alluvial fan flooding in the context of the Southern Apennines. Studies like

the present one may help determine the volumes involved during flash floods whilst

providing support for detailed flood hazard zoning and for risk mitigation planning.

& Nicoletta Santangelonicsanta@unina.it

1 Dipartimento di Ingegneria Civile, Edile ed Ambientale (DICEA), Universita di Napoli Federico II,Piazzale Tecchio, 80125 Naples, Italy

2 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Universita di NapoliFederico II, Largo San Marcellino 10, 80138 Naples, Italy

3 Dipartimento di Bioscienze e Territorio, Universita del Molise, Contrada Fonte Lappone,86090 Pesche, Isernia, Italy

4 Dipartimento di Agraria, Universita di Napoli Federico II, Via Universita 100,80055 Portici, Napoli, Italy

123

Nat HazardsDOI 10.1007/s11069-015-1728-4

Keywords Flash flood � Debris flow � Alluvial fan � Event magnitude � Southern

Apennines

1 Introduction

Alluvial fans are common fluvial landforms linked to the activity of torrential streams

which can be subject to a variety of different flow processes. According to Costa (1988),

the processes occurring in the channel of small, steep watersheds can be divided into water

floods, hyperconcentrated flows and debris flows. In particular, ‘‘water floods are New-

tonian fluids with turbulent flow, non-uniform sediment concentration profiles, sediment

concentrations less than about 20 % by volume and shear strengths less than 100 dines/

cm2. Hyperconcentrated flows are approximated as moderately turbulent to laminar non-

Newtonian fluids with non-uniform to uniform sediment concentration profiles, sediment

concentrations ranging from 20 to 47 % by volume and shear strengths ranging from 10 to

40 N/m2. Debris flows are non-Newtonian visco-plastic or dilatant fluids with laminar flow

and uniform concentration profiles, sediment concentrations ranging from 47 to 70 % by

volume and shear strengths greater than about 40 N/m2’’. Hereinafter, we refer to this

terminology and these definitions to describe transport and deposition processes observed

in the study area.

Variations in flow processes are mainly caused by nature and concentration of sediment

entrained in the flow. Although each process has unique diagnostic effects and products, in

nature there exists a continuum of flow conditions and sediment concentrations. Identifi-

cation of the specific hydrogeomorphic process is thus a central problem to hazard

recognition because each process has different associated hazard characteristics (O’Brien

and Julien 1985; Costa 1988; Fema 2000; Jakob 2005). From this perspective, the in-

vestigation of floods, especially in small montane basins, requires proper identification of

the flow process occurring in the basin.

Recent studies have shown that relationships between morphometric attributes of al-

luvial fans and morphometric attributes of the corresponding upslope basins can be ef-

fectively exploited for classifying alluvial fans with respect to the two different dominant

flooding mechanisms, i.e. either debris flows (hereinafter referred to as Df alluvial fans) or

water floods (hereinafter referred to as Wf alluvial fans) (Kostaschuk et al. 1986; Sorriso-

Valvo et al. 1998; De Scally et al. 2001, 2010; Crosta and Frattini 2004; De Scally and

Owens 2004; Santangelo et al. 2012). Distinguishing between these two types of alluvial

fans is of primary interest for flood hazard mapping, because the corresponding hazard

levels differ by a few orders of magnitude (Kellerhals and Church 1990). The impact of a

debris flow is significantly higher than a water flood of the same volume and velocity,

given the higher bulk density of the debris flow (Aulitzky 1980; Welsh and Davies

2011).While alluvial fan hazards related to debris flows have been widely explored in

Alpine regions (e.g. Marchi and D’Agostino 2004; Berti and Simoni 2007; Cavalli and

Marchi 2008; D’Agostino et al. 2010), much less attention has been paid to similar hazard

scenarios in the Southern Apennines, except for a limited number of studies in recent years

(Zanchetta et al. 2004; Santangelo et al. 2011, 2012; Santo et al. 2002; Scorpio 2011).

Despite widespread urbanisation in this type of geomorphological setting since the

1960s, which has produced a considerable increase in exposure to such hazards, the

knowledge base for this type of phenomenon in the Southern Apennines is still too scant

Nat Hazards

123

for effective flood hazard mapping. These studies are even more impelling in the light of

the projected climate change scenarios, suggesting an increase in the frequency of violent

convective storms in Mediterranean areas towards the end of the century compared with

the present day (Romero and Emanuel 2013). Such storms produce rainfall patterns of high

intensity within small space–time scales (Piccolo and Chirico 2005) and are thus critical

for the initiation of flash floods (Marchi et al. 2010). As a result of these projected climate

change scenarios, an increased probability of major economic and social impacts is ex-

pected as the century progresses in such alluvial fans.

On 7 October, 2011, an extreme rainfall event occurred over the Marzano carbonate

massif which is part of the Southern Apennine chain in the region of Campania. The

rainfall event triggered flash floods which reactivated alluvial fans built up at the outlet of

two mountain basins known as Vadursi and Matrura close to the village of Teglia in the

comune of Buccino (Fig. 1). Extensive damage (evaluated equal to 2 millions of euro by

the local municipality) was done to buildings, river bank structures and agricultural crops.

Similar events were recorded in the same region at the beginning of the last century and

previously in the nineteenth century. Widespread urban development over the last 50 years

in this highly hazardous region testifies that local communities tend to lose collective

Fig. 1 Geological setting of study area (from Ascione et al. 2013, modified)

Nat Hazards

123

memory of the devastating impact of such events, which are triggered by extreme rainfall

events with return periods exceeding a human generation.

This study presents an accurate analysis of the 2011 event in Buccino. Our analysis was

conducted to gather major insights for the assessment of alluvial fan hazards in regions of

the Southern Apennines exhibiting geomorphological settings similar to those observed in

these two torrential basin/fan systems.

2 The study area

Mt. Marzano is located in the axial portion of the Southern Apennines chain (Fig. 1), a fold

and thrust belt that began to accrete in the Lower Miocene (Bonardi et al. 1988; Patacca

et al. 1990). The massif is a well-defined morpho-structure bounded by fault scarps, with a

maximum elevation of 1579 m. The bulk of the massif comprises a 2000-m-thick pile of

carbonate deposits extending from the Late Triassic to the Early Miocene (Pescatore et al.

1999). Outcrops of Late Miocene flysch deposits, Cretaceous-Oligocene clays and calci-

lutites (Sicilide thrust sheet), and shallow marine deposits of the initial part of the Middle

Pliocene occur sporadically in the massif (Amato et al. 1992).

In its southern sector, the massif includes two endorheic tectono-karst depressions,

called the Pantano di S.Gregorio Magno and Piana di Buccino. Outcrop and subsurface

data show that the two depressions are filled in with clays and silts that alternate with

pyroclastic sandy layers and change laterally into gravelly and sandy fan deposits. At least

two generations of alluvial fans can be identified at the periphery of the basins. The oldest

have been ascribed to the Middle–Late Pleistocene; the youngest include quiescent but

undissected fans, Late Pleistocene–Holocene in age (Ascione et al. 2003, 2013).

Fig. 2 a Studied basin–fan systems; 1 Inactive fan; 2 Active fan; 3 fan apex; 4 intersection point; 5 basin; 6feeder channel; 7 rain gauges locations. b Human interventions in the studied channels: 1 natural channel; 2main channel in previous centuries; 3 flooded area; 4 crossing structure; 5 weir; 6 artificial levee

Nat Hazards

123

The area is characterised by a humid Mediterranean climate, with an average annual

rainfall of 1250 mm and an average annual temperature of 8 �C, exhibiting marked sea-

sonality with very wet winters and very dry summers. The upslope area is dominated by

broadleaf forests and pastures, while cultivated lands (mainly olive groves) dominate the

plain area.

The studied basin/fan systems (Matrura and Vadursi, Figs. 2, 3) are included in the

southern sector of the Piana di Buccino endorheic basin. The main slopes are covered by

limited soil thickness with significant exposure of Mesozoic limestone bedrock to surface

weathering. Indeed, the main debris sources are represented by the fractured and weathered

portions of the carbonate succession.

Fig. 3 Geomorphological map based on 1984 air photos. 1 Slope with forested bedrock (limestone; soilthickness lower than 50 cm); 2 Slope with bedrock exposure (limestone; no soil cover); 3 Portion of slope orvalley bottom with cultivated area. Soil thickness between 50 and 100 cm; 4 drainage basin; 5 main channel;6 flat valley bottom; 7 gorge-like valley; 8 tributary stream with high mean gradient; 9 down-cutting stream;10 doline; 11 polye; 12 debris-slide or rock fall; 13 active fan; 14 inactive fan; 15 topographic apex; 16intersection point; 17 abandoned channel; 18 natural channel; 19 artificial channel

Nat Hazards

123

Based on interviews with local inhabitants, several alluvial events have affected the two

basin–fan systems in the last 150 years. For the Matrura system, elderly inhabitants recall

at least two floods of magnitude similar to that observed in 2011, based on their own direct

experience or on the accounts of their parents, occurring in 1870–1880, 8 September, 1929,

while a minor flood also occurred in 1977. For the Vadursi system, two episodes are

recalled: in 1929 and in September 1960. A minor flood also occurred on 13 October, 2006.

Some of these events did not generate casualties but only damages to agricultural lands like

vineyards and olive groves, since they occurred in environments which are much less

urbanised than nowadays. Unfortunately, we did not find any official report or other

documents which could have allowed us to retrieve an assessment of the damages.

3 Methods and materials

Detailed geomorphological and geological analyses were carried out both in the laboratory

and in the field. Aerial photographs taken before and after the event were examined to

make a preliminary assessment of the flooded area. Terrain data were processed to create a

5-m DEM of the two catchments. The values were calculated from the elevation points and

contour lines derived from 1:5000 numeric topographic maps. It was interpolated with the

regularised spline method by means of ArcGIS Spatial Analyst.

Field surveys were planned to obtain a detailed description of the event and to collect

stratigraphical (type of deposits, thickness measurements) and sedimentological observa-

tions (sedimentary structures and clasts shape and dimensions). Main erosion and depo-

sition processes occurring during the event were reconstructed by a field survey in the

upslope catchment, along the main streams and in the alluvial fan, with an evaluation of the

thickness of both the eroded and deposited debris. The main hydraulic works affecting the

dynamics of the surface flow in the river network were also identified.

Based on the 5-m DEM, the main morphometric attributes were assessed in order to

classify the studied systems in terms of expected depositional processes, following San-

tangelo et al. (2012). These authors examined a data set of 102 basin/fan systems located in

several carbonate massifs of Southern Apennines (Mediterranean humid geomorphic

systems). They found out that in this geological and geomorphological context, the best

discrimination between debris flow (Df) and water flood (Wf) processes is achieved by

means of two related variables, one for the basin (feeder channel inclination, Cg) and one

for the fan (fan length, Fl). A morphometric index of the alluvial fan (Fx) can be derived by

a linear equation applied to these two variables:

Fx ¼ �114:74� 0:34Cg þ 17:24 log Fl ð1Þ

The alluvial fan is water flood-dominated if Fx [ 0, while it is debris flow-dominated if

Fx \ 0.

The probability of belonging to Wf transport was evaluated by means of the equation:

P F xð Þ� �

¼eF xð Þ= 1þ eF xð Þ� �

ð2Þ

where a P value (P(F(x))) higher than 0.5 indicates that a basin/fan system belongs to group

‘‘Wf’’.

Information concerning the meteorological event and rainfall data measured at the local

rain gauges was obtained from the event report of the regional meteorological and hy-

drological service.

Nat Hazards

123

The accumulated debris volume (Vf) was estimated at 136 points for the Vadursi fan

and at 101 points for the Matrura fan by means of a precision GPS system (Trimble

GeoXT). At each observation point, the thickness was assessed by identifying the base of

the event deposits in natural or artificial trenches visible after the flood. A thickness map of

the depositional areas of both fans was drawn by interpolating the observation points by

means of Natural Neighbour (Sibson 1981) and the Jenks Natural Break (Jenks 1967)

techniques within the GIS environment. Polygons were delineated corresponding to four

different thickness classes. The volumes of the debris deposited during the event were then

estimated as follows:

Vf ¼Xn

1

Afi hi ð3Þ

where Afi is the extent of the ith polygon, hi is the average thickness of accumulated

material within the ith polygon, and n is the total number of polygons delineated.

The volume of the eroded debris (Vb) was evaluated by applying the method proposed

by Hungr et al. (1984) which offers a standardised procedure, calibrated on British

Columbia debris flows, to determine the volumes of material eroded along the banks and

on the bed of a stream channel under examination. Our study constitutes an attempt to

apply the same methodology to the Southern Apennines. By means of an accurate field

survey, all the evidence of lateral and vertical erosion along the main streams (Fig. 4) was

mapped and measured, allowing precise reconstruction of volumes of materials eroded

during the event. Surveys were conducted at 62 cross sections of the Matrura channel and

32 cross sections of the Vadursi channel (Fig. 5). At each cross section, the thickness of the

eroded material was evaluated on both lateral banks and the average eroded thickness was

Fig. 4 Example of cross sections used for eroded debris volume evaluation. es: eroded thickness on the leftbank; ed: eroded thickness on the right bank; B: channel width

Nat Hazards

123

calculated. In this computation, a flat channel bed prior to the 2011 event was assumed in

the cross sections. This assumption implies slight overestimation of the eroded cross-

sectional area and of the eroded debris volume. Homogeneous sectors were then identified,

corresponding to sectors delimited by sections with similar average eroded thickness. The

total eroded debris volume (Vb) was evaluated as follows:

Vb ¼Xn

1

Abi ei ð4Þ

where Abi is the area of the ith sector, ei is the average thickness of eroded material of the

ith sector, and n is the number of homogeneous sectors. Five homogeneous sectors were

identified for Matrura and seven for Vadursi (Si in Fig. 5).

Fig. 5 Location of the homogeneous sectors used for eroded debris volume evaluation. 1 GPS points; 2watershed boundary; 3 homogeneous sector; 4 fan apex; 5 intramontane plain

Nat Hazards

123

4 Basin/fan morphometry and human intervention

The two basins have different morphometric and morphologic characteristics (Fig. 3;

Tables 1, 2) despite their similar geological context and their vicinity and are also subject

to different human interventions and management (Fig. 2b).

The Vadursi basin is larger (9.7 km2) than Matrura (5.4 km2) and shows a peculiar

longitudinal profile interrupted in its middle portion by the presence of a wide planar

segment (Vadursi plain, Fig. 6) with inclination ranging from 4� to 8�. Down valley of this

planar segment, the mean inclination is around 13� and the cross section is gorge-like. The

basin slopes are entirely made up of limestones and generally show the absence of soil

cover and vegetation as normally occurring in karst landscape. Presence of forested area is

controlled by slope exposure (north facing) and gradient (gorge flanks), and generally, the

soil thickness does not exceed 50 cm. The lack in a continuous vegetation cover associated

with a high degree of jointing make the carbonate successions cropping out in these basins

very sensitive to mechanical and chemical weathering. Crops and fields concentrate mainly

in the flat area of Vadursi plain, where a soil thickness ranging from 50 to 100 cm can be

estimated.

The Vadursi alluvial fan is entrenched, because the intersection point (Hooke 1967;

Bull 1968; Santangelo et al. 2012) between the main water course and the surface of the

fan does not coincide with the fan apex and the main channel is incised about 5 m below

ground level. Indeed, the fan apex is located at about 530 m a.s.l., while the intersection

point is at about 470 m a.s.l., about 1 km downslope (Fig. 3). As regards human inter-

vention (Fig. 2b), the Vadursi channel is confined within a river bed dug within the alluvial

fan and protected by gabion walls and check dams.

The upper portion of the Matrura basin is characterised by the presence of a little karst

polje (Melara plain in Fig. 3) which was recently captured by the regressive erosion of the

Matrura stream. The remnant portion of the longitudinal profile has an inclination of 16�and a gorge-like cross section. The basin slopes show bare, rounded crests with abundant

karst morphologies (dolines) and are characterised by forested soil cover in their lower

portion and along the flanks of the main gorge. The Matrura alluvial fan system is a

superimposed one, with the fan apex and intersection point overlapping at about 530 m

a.s.l. (Figure 3). According to oral witnesses, until the end of the nineteenth century, the

main channel was located along the western side of the fan, crossing the village. Then,

Table 1 Morphometric vari-ables for the Matrura and Vadursibasin–fan systems

Morphometric Variables Matrura Vadursi

Basin Area (km2) 5.4 9.7

Basin length (km) 3.7 4.8

Basin inclination (�) 24.1 26

Basin relief (km) 0.8 0.9

Feeder channel length (Km) 4.1 7.1

Feeder channel inclination (�) 13.1 11.2

Fan Area (km2) 0.6 1.5

Fan length (km) 0.96 1.5

Fan Inclination (�) 5.4 3.53

Melton Index 0.35 0.29

Nat Hazards

123

after a severe alluvial event occurred in 1889, it was diverted eastwards without any

defined receptor (Fig. 2b).

As the study area belongs to the same geological and geomorphological context anal-

ysed by Santangelo et al. (2012), we try to apply the method recently proposed by these

authors (see Sect. 3) in order to classify a basin/fan system in terms of dominant depo-

sitional processes (debris flow or water flood). The results obtained are listed in Table 2,

showing that the Matrura fan can be classified as debris flow-dominated with a probability

of 66 %, while the Vadursi fan is water flood-dominated with a probability of 99 %.

Fig. 6 Longitudinal profiles of Vadursi and Matrura streams. The letter ‘‘S’’ refers to homogeneous sectordefined in Fig. 5. The circle indicates the position of the fan topographic apex, the star the position of fanintersection point

Table 2 Morphometric variables and results of the discriminant equation proposed by Santangelo et al.(2012)

Basin/fan system Ipc (�) Lf (m) F(x) P(Wf type) Group assigned

Matrura 13.16 960 -0.63 3.4e-01 Df

Vadursi t 11.29 1500 ?7.72 9.9e-01 Wf

Nat Hazards

123

5 The rainfall event

During the morning and afternoon of 7 October, 2011, the region of Campania was

affected by very humid westerly airflows, which triggered convective storm cells, whose

intensity is likely to have been considerably enhanced by orographic uplift. Such events are

characterised by high spatial and temporal variability. As depicted in Fig. 2a, Contursi

Meteo (164 m a.s.l.) and Muro Lucano (596 m. a.s.l.) are the two rain gauges closest to the

flooded area. The actual rainfall on the Mt. Marzano plateau could have been significantly

greater than that recorded at the rain gauges, due to the large spatial variability of the

rainfall for this type of event. Of the two rain gauges, Muro Lucano is the most repre-

sentative for examining the rainfall event, as it is at higher elevation and closer to the Mt.

Marzano plateau and thus to the upslope areas of Vadursi and Matrura catchments from

which the floods were generated.

On analysing the temporal distribution of the rainfall at Muro Lucano, one can clearly

distinguish three major subsequent rainfall pulses of increasing intensity at 18:00, 19:00

and 21:00 (Fig. 7a). The duration of each rainfall pulse is about an hour, i.e. the same order

of magnitude as the time of concentration expected for the studied basins.

Fig. 7 a Hourly rainfall recorded at rain gauges (data kindly provided by Centro Funzionale della RegioneCampania); b Intensity–duration (ID) analysis and comparison with rainfall thresholds: dashed linesrepresent intensity–duration curves for different return periods; the bold line is ID threshold law suggestedby Paronuzzi et al. (1998) for NE Alps; the circles indicate the maximum values observed during the event

Nat Hazards

123

A probabilistic analysis was performed to assess the return period of the rainfall event at

different time intervals. A data set of annual maxima of the rainfall depth measured at

Muro Lucano for time intervals of 1, 3, 6 and 12 h was retrieved from event report of the

regional hydrological service (Centro Funzionale, Regione Campania). This report in-

cludes the data published on the hydrological annals of the former national hydrological

service for the period 1934–1987 and the data recorded by regional hydrological service

for the period 2003–2011.

Table 3 summarises the main statistics of the data set. For each rainfall duration, more

than 40 values of annual maxima were available. The maximum rainfall depth observed in

3 h during the event is the highest value of the observed 43 annual rainfall maxima since

1934.

The two component extreme value (TCEV) distribution was adopted for assessing the

return period (T) of observed rainfall depth for the four reference time intervals (Rossi

et al. 1984), since this is the most effective probability distribution for predicting the

frequency of the rainfall extremes in Mediterranean regions (Rossi and Villani 1994).

The return period of the maximum annual rainfall depth in a generic duration d, hd, with

mean value ld; is defined as follows:

T hdð Þ ¼1

1� exp �Kde�ghd=ld � K�K1=h�1 e�ghd=ld=h�

� � ð5Þ

The identification of the dimensionless parameters K� and h� involves the estimation of

statistical moments of order larger than two, which are subjected to high uncertainty when

it is based on site-specific data set of relatively small size, as in this case. For this reason,

parameters K� and h� are estimated on the basis of a regionalisation procedure (e.g. Preti

et al. 2011), which assumes K� and h� to be constant within a homogeneous region

including the study site. We took the parameter values K� ¼ 0:772 and h� ¼ 2:352 esti-

mated within the regional frequency analysis conducted for the Pulia region (Copertino and

Fiorentino 1994), which we evaluated to be the most representative for the site of Muro

Lucano rain gauge station. The other dimensionless parameters K1 and g can be also

assumed to be invariant with respect to the duration and, once K� and h� are identified, they

can be computed from the first- and second-order moment statistics of the available data. In

this case, we employed the method of L-moment (Hosking 1990) for estimating K1 and g,

since it provides estimates more robust than the method based on conventional moments.

We considered the three hours maximum rainfall depth as reference data, and we estimated

K1 ¼ 6:58 and g = 3.796.

Table 3 Statistics of the examined series of maximum annual rainfall depth at Muro Lucano, and estimatedreturn period T hdeð Þ of the maximum rainfall depth observed during the 2011 event by means of the TCEVdistribution

d (h) No. of data ld (mm) hde(mm) Ide (mm/h) T(hde) (year)

1 44 22.6 44.0 44.0 13

3 43 33.1 106.2 35.4 102

6 42 41.0 127.6 21.3 89

12 41 52.5 128.0 10.7 30

Nat Hazards

123

Table 3 summarises some relevant statistics for durations d equal to 1, 3, 6 and 12 h,

including the maximum rainfall depth and intensity observed during the event ðhdeÞ and the

corresponding return period T hdeð Þ. The maximum return period is achieved at 3 h, with a

value of 102 years.

In Fig. 7b, the maximum rainfall intensities observed during the event are compared

with intensity–duration curves estimated with the probability analysis outlined above. The

same figure contains the threshold intensity–duration law suggested by Paronuzzi et al.

(1989) for debris flow triggering in north-eastern Alps of Italy, in Friuli region. This law is

the one suggesting the highest intensity thresholds among those published for mountain

environments with geological settings comparable with the one herein examined (see

Guzzetti et al. 2008 for a review on this topic). The maximum observed rainfall intensities

are higher than the threshold values for durations of 3 and 6 h, while being very close to

the threshold value for duration of 1 h.

6 The flood event in the fan area

Based on field observations and local witness accounts, it may be argued that the flood

event at Matrura consisted of three waves occurring in the evening of 7 October, 2011,

between 18:00 and 22:00 (Fig. 8), corresponding to the three rainfall pulses mentioned

above. At the apical zone of the fan, the first flood wave occurred around 18:30 and it

Fig. 8 Distinguished phases of the flooding event of 7 October, 2011, at Matrura fan

Nat Hazards

123

mainly consisted of water, which produced significant erosion processes resulting in the

formation of a gully up to 2 m in depth. One hour later, at around 19:30, preceded by a

loud noise, a first debris lobe reached the alluvial fan, which filled the gully and buried part

of the buildings and the olive trees in the surroundings (Fig. 8). The third flood wave

occurred at around 22:00; in the apical zone of the Matrura fan, the water produced lateral

erosion and defined a valley bottom up to 30 m wide which was then filled by another

debris lobe. In the median and distal sectors of the fan, new erosion gullies were formed

and subsequently filled in by debris flows, also redistributing the materials deposited

during the previous flood wave. The differences observed in the dominant processes during

Fig. 9 Damage map of the 7 October, 2011, event. 1 Buried building; 2 Buried tree; 3 Uprooted tree; 4Flooded field; 5 Flooded street; 6 Eroded bank; 7 Damaged levee; 8 Topographic marker; 9 Referringnumber for photo location (see text for more details on photos); 10 Figure scale. Photos 5a and 5b refer tothe same place before (5a) and after (5b) the event

Nat Hazards

123

the three floods could also be related to the different intensities of the triggering rainfall

events.

The data collected during the field survey immediately after the event (14 October,

2011) are summarised in a damage map (Fig. 9) and in a geomorphological map (Fig. 10).

Fig. 10 Geomorphological map of the 7 October, 2011, event. 1 Fan apex; 2 Intersection point; 3 Mainchannel; 4 Fluvial scarp and relative height (h \ 2 m; 2 m \ h\5 m; h [ 5 m); 5 Active fan; 6 location ofschematic section in Fig. 8; 7: Debris flow deposit (1: first lobe, 2: second lobe); 8 gravels; 9 gravels andsands; 10 Cut and fill channel; 11 lateral erosion; 12 Secondary channel

Nat Hazards

123

On the Matrura fan (Fig. 11a), it was still possible to observe some diagnostic features

of debris flow occurring. Depositional lobes up to 1 metre high and made up by very coarse

(clast up to 1 m in diameter) carbonate gravels are clearly visible in the apical zones of the

fan, and boulder train structures can be observed at the back of the main olive trees (photos

1 and 2 in Fig. 9). The debris covered part of the agricultural land and also accumulated

against house walls facing upslope (photos 3a and 3c in Figs. 9, 11b), with maximum

thickness of about 5 m. Moving to the centre of the alluvial fan, it was possible to observe

several depositional lobes, elongated in a radial manner downslope. In the lateral zones of

the main channel, a 10 cm thick level of mud with sub-rounded carbonate pebbles (4–5 cm

in diameters) was detected which can be referred to deposition by unconfined waters (sheet

flood deposits).

As regards the Vadursi stream, eye witnesses are less abundant, and event recon-

struction is less detailed. In the apical zone of the fan, the flood remained confined in the

river bed, causing mainly down-cutting and lateral erosion (photos 5a and 5b in Fig. 9): the

gabion walls collapsed in the upper sector (photos 5.6 and 5.7 in Fig. 9), being undermined

by excavation at the base. In the distal sector, where the slope is below 5 %, the flood

deposited finer (average size 1–2 cm) and less thick sediments than in the Matrura fan

(photos 8 and 9 in Fig. 9). Deposition was also enhanced by the backwater caused by a

Fig. 11 Aerial view of the Matrura fan immediately after the event

Fig. 12 Boulders at the Matrura fan apex (a) and gravel at the Vadursi fan apex (b)

Nat Hazards

123

bridge, which partially obstructed the river cross section (Fig. 9). The maximum water

height reached during the peak was 1 m.

The damage map clearly shows that the location and area of the damaged zone differ in

the two cases: in the case of the Matrura fan, depositional processes occurred mainly in the

apical zone, while in the Vadursi fan, it was the distal portion that experienced

sedimentation. This different distribution is strictly controlled by the different type (su-

perimposed in the first case and entrenched in the second) of the fan system. The sediments

mainly consisted of carbonate gravel (average size 1–5 cm) without matrix; in the apical

zone of the Matrura fan, the clasts reached diameters larger (60/80 cm) than in that of

Vadursi (5/10 cm) (Fig. 12a, b).

7 The flood event in the catchment area

Field investigations carried out after the event in the two catchments showed the occur-

rence of both erosion and deposition processes mainly due to the activity of running waters.

Erosion processes were mainly represented by stream vertical and lateral erosion which in

some cases produced damages to roads (Fig. 13a–c). A detailed survey along the main

Fig. 13 After event survey in the two catchments. a, c Evidence of erosional processes occurred in theMatrura basin; d, e, f Evidence of depositional processes in the Matrura channel; b, g, h, i) Evidence ofdepositional processes in the flat area in the mid-portion of the Vadursi catchment; n, m Little debris slidesin the Vadursi catchment

Nat Hazards

123

streams allowed the collections of all the evidence of down-cutting and lateral erosion

occurred during the event (see Sect. 3, Figs. 4, 5) which was successively used in the

calculation of eroded volumes (see Sect. 8).

Depositional processes (Fig. 13d, e, g) occurred along the main streams depending on

the average channel gradient (Fig. 7). In the case of the Matrura stream, the high mean

gradient facilitated the transport of a large quantity of sediments downslope which started

to accumulate in the final segment of the stream (S1 sector in Fig. 4) where a series of

weirs induced the deposition (Fig. 13f). In the case of the Vadursi basin, the presence of a

wide flat area in the mid-portion of the catchment (Fig. 7) enhanced depositional processes

upstream (Fig. 13l), reducing the bed load transport downstream. It is important to stress

that no significant gravity processes occurred during the event in either catchment, except

for small debris slides (Fig. 13n, m) Moreover, some of these slides were already visible on

air photo of 1984 used to draw up the geomorphological map of Fig. 3. After the present

event, they registered only a little enlargement.

In one case, a culvert narrowing the stream section caused a local increase in flow

velocity, the initiation of local bed scouring, the collapse of artificial bank protections and

an increase in bed load transport (Fig. 14a, b).

Basing on these observations, it can be affirmed that the debris which accumulated in

the fan area of Matrura and Vadursi mainly originated from previous channel bed sedi-

ments and from loose colluvial materials and weathered bedrock which accumulated

alongside the channels.

8 Assessment of the debris volume

The volume involved in the flood event may be determined by considering the volume

deposited in the fan area (Vf) or the volume eroded in the drainage basin (Vb).

After elaborating the field data collected in the fan area, we drew thickness maps

(Figs. 15, 16) and evaluated the total volumes involved during the event for each fan.

Despite the different area of the two catchments (9.7 km2 for Vadursi and 5.4 km2 for

Matrura), the largest volumes accumulated at the Matrura fan (about 36,800 m3) rather

than at Vadursi (average around 29,000 m3). This difference may be explained by con-

sidering that the main debris sources did not originate from the basin slopes but from

previously accumulated stream bed sediments, as ascertained by the field survey.

Fig. 14 Channel narrowing (a) and downstream erosion phenomena (b) indicated by arrows

Nat Hazards

123

From the maps (Figs. 15, 16), it may also be noted that the thickness distributions in the

two fans differ substantially: in the Matrura fan, debris is mainly accumulated in the apical

zone (M3–M4 in Fig. 11), while in the case of Vadursi, the largest thickness (V3–V4 in

Fig. 16) was recorded along the distribution channel. Moreover, thickness exceeding 3.5 m

was reached at the Matrura fan, while in the case of Vadursi, the thickness reached 2 m

only in a limited number of cases. This difference can be easily explained by the different

morphology of the fan system: the Matrura fan is a superimposed system and sedimen-

tation occurred mainly in the apical zone, while in the case of Vadursi, the presence on an

entrenched distribution channel controlled the transfer and sedimentation of debris

downslope of the apex. The data and observations gathered suggest that events of a

different magnitude occurred, with high hazard levels recorded in the apical zone of the

superimposed Matrura fan.

Further considerations can be made on the basis of the estimated volumes. According to

the literature (Iverson et al. 1998; Jakob 2005; Rickenmann 2005; Berti and Simoni

2007), the relationship between area and volumes of inundated areas is expressed by the

formula:

A ¼ KV2=3 ð6Þ

where K is an empirically dimensionless coefficient which can be used to discriminate

among different transport processes as summarised in Table 4 and in Fig. 17.

Both the values, respectively, obtained for the Matrura (K = 52.1) and Vadursi

(K = 53.6) basins fall in the debris flows region of the diagram of Fig. 12. Moreover, both

Fig. 15 Assessment of deposited debris volume in the Matrura fan. a Flooded area and location of gps spotmeasure; b grid; c thickness map. 1 Thickness distribution, 2 thickness classes expressed in m; 3 gps spotmeasure; 4 houses buried by the flood event; 5 flooded area boundary

Nat Hazards

123

values are close to the line separating debris flows from water floods, evidencing the

uncertainty attached to this type of empirical approach for differentiating between these

two processes.

Fig. 16 Evaluation of total volumes involved during the event for both fans (V = Vadursi, M = Matrura).1 thickness classes (in metres); 2 houses buried by the flood event; 3 flooded area boundary; 4 apex; 5intersection point

Nat Hazards

123

The volumes of materials eroded during the event were reconstructed by applying the

method proposed by Hungr et al. (1984) (see Sect. 3 for method details), based on the

evidences of lateral and vertical erosion retrieved during the field survey along the main

streams.

The mean eroded volumes are around 35,100 m3 for the Matrura stream channel and

around 28,700 m3 for the Vadursi stream channel, including the planar stream segment of

the Vadursi plain (sectors 11 and 12 in Fig. 5). These data are in agreement with those

elaborated for the deposited debris volumes and thus confirm that the Matrura basin

recorded an erosion event of a higher magnitude than that of Vadursi. Moreover, and quite

unexpectedly, because of the different procedures and the exclusion in the computation of

other sources of sediments, the values of eroded and deposited debris volumes are very

similar.

Fig. 17 Area versus volume and coefficient K evaluation for the Vadursi and Matrura alluvial events

Table 4 Different values of coefficient K proposed in the literature

Process K range Region area Data set

Granulardebris flow

6.2 \ K\28.1 Alps (northern Italy,Switzerland),worldwide

Crosta et al. (2003); Scheidl andRickenmann (2010); Griswold andIverson (2008)

Debris flow 28.1 \ K \ 55 Taiwan; Alps (northernItaly, Switzerland,Austria)

Yu et al. (2006), Scheidl and Rickenmann(2010), Berti and Simoni, (2007)

Fluvialevent—debris flood

55 \ K \ 91.8 Alps (Austria) Scheidl and Rickenmann (2010)

Volcanic earthflow—Lahars

91.8 \ K \ 200 Alaska, USA, Colombia,Filippine

Waythomas et al. (2000), Iverson et al.(1998)

Nat Hazards

123

9 Discussion

The data we gathered allowed some interesting considerations about flood hazards in

alluvial fans.

9.1 Relationship between rainfall and flooding processes

Two points have to be highlighted in this study: on the one hand, the flood which occurred

was strongly controlled by the time distribution of the rainfall as testified by the strict

correlation of the three rainfall pulses with the three discharge peaks in the Matrura fan

area; on the other, despite the vicinity, the erosion and depositional processes varied

significantly between the two adjacent basin/fan systems, being influenced by their mor-

phological and morphometric characteristics.

9.2 In the studied case, morphological and morphometric propertiesof the basin/fan system have shown to be very useful in the identificationof alluvial fan flooding prone areas

The collected data clearly show that correct classification of the basin fan/system in terms

of fan type (superimposed or entrenched) by means of a simple terrain analysis can be

effectively used for predicting where flooding will occur. In the case of a superimposed

system (like that of Matrura), damage will be concentrated in the apical zone of the fan,

while in the case of an entrenched system (Vadursi), the flood-prone areas are located

along the entrenched stream channel and in the distal portion of the system. The same data

also confirm that the morphometric methods proposed by Santangelo et al. (2012) for the

carbonate massifs of Southern Apennines can be used in similar geological and geomor-

phological context (like that of Marzano Mt massif) to identify those basin/fan systems

which are more exposed to the occurrence of debris flow events. Several features observed

in the field (boulder trains, debris lobes) and the assessed volumes suggest that the Matrura

basin/fan system was indeed affected by the propagation of a debris flow with the depo-

sition of significant amounts of coarse material (mean diameter up to 80 cm, thickness up

to 3 m, estimated mean volume of 36,800 m3). This is in agreement with the morphometric

basin/fan classification as debris flow-dominated, which is obtained by applying the

morphometric discriminant equation proposed by Santangelo et al. (2012). All these results

suggest that the morphometric approach may be very useful in identifying process type and

therefore for alluvial fan flooding hazard mapping.

9.3 Influence of man-made structures on flood distribution

It has to be stressed that infrastructures in the apex zone (residential and livestock struc-

tures) affected the depositional processes, enhancing the accumulation of material against

the walls facing upslope. Similar processes, but at a smaller scale, occurred around the

olive trees, with debris lobes with a height up to 1.5 m. Roads and agricultural terraces also

influenced flood propagation in the external sector of the alluvial fan. In general, the

severity of the damage diminished according to the longitudinal distance from the apex and

the lateral distance from the channel. The event, though generating serious damage

(burying of buildings, cars transported by water, livestock loss, etc.), did not have the

impact force for destroying buildings (either in masonry or reinforced concrete). This

Nat Hazards

123

aspect is peculiar to alluvial fan flooding and represents a significant difference from other

kinds of events like rapid earth/debris flows (Cascini et al. 2008; Mavrouli et al. 2014).

9.4 Debris volume

The volumes in question confirmed that the two basin/fan systems recorded different

transport and deposition processes as suggested by morphometric analysis and field data. A

debris flow event with higher magnitude occurred in the Matrura basin involving

36,800 m3 of debris material. It has to be stressed that this evaluation does not represent

the estimate of the real eroded and deposited materials but represents a useful tool to define

the order of magnitude of the event. These data may be useful for planning risk mitigation

works in contexts with similar geological and geomorphological conditions.

9.5 Efficiency of hydraulic works

The Vadursi channel is confined in a river bed dug within the alluvial fan and protected by

gabion walls and check dams. In this case, the flood event produced mainly erosion

processes in the apical and median zone of the fan while deposition took place in the distal

portion and involved lower volumes than in the Matrura basin. The data gathered and the

field observations suggest the occurrence of a water flood event with limited solid load

transport. In this case, the presence of bank protection along the channel limited flood

propagation even though in many places the protection was partly or completely destroyed.

In particular, near the apical zone of the Vadursi fan, the presence of a narrow culvert

caused a local increase in flow velocity and hence bed erosion at the base of the artificial

bank protection.

10 Conclusions

On 7 October, 2011, an extreme rainfall event generated flash floods which reactivated

alluvial fans at the outlet of two mountain basins called Vadursi and Matrura, in the area of

Buccino, southern Italy. The rainfall event involved a sequence of three rainfall pulses of

increasing intensity with duration similar to the concentration time of the basins. Erosion

and depositional processes were strongly influenced by morphological and morphometric

properties of the basin/fan system as well as by the presence of human interventions and

structures along the main channel and in the fan area. Transport processes were mainly

dominated by ‘‘debris flows’’ in the Matrura stream basin while by a ‘‘water flood’’ along

the Vadursi stream basin. The transported materials came mainly from debris and gravels

previously accumulated along the stream beds and mobilised by the flow during the event.

No significant evidence of landslide contribution to transported bed load was detected.

Events similar to that of Buccino have return periods exceeding a human generation,

and thus, local communities tend to forget their devastating impact, as testified by ex-

tensive urban development occurring in the last 50 years. In the light of the projected

climate change scenarios, suggesting an increase in the frequency of violent storms in

Mediterranean areas towards the end of the century, an increased probability of major

economic and social impacts has to be expected in such alluvial fans.

Despite the existence of hundreds of similar alluvial/fan systems in the Southern

Apennines, few studies have been conducted to support adequate risk mitigation action in

Nat Hazards

123

these areas. Indeed, to our knowledge, this is the first study focusing on assessing the

magnitude of alluvial fan flooding in the context of the Southern Apennines. Studies like

the present one may help determine the volumes involved during flash floods whilst

providing support for detailed flood hazard zoning and for risk mitigation planning.

Acknowledgments We are grateful to Dr. Mauro Biafore and Matteo Gentilella (Centro FunzionaleRegione Campania) for providing us with the rainfall data. We also thank the two anonymous reviewers.

References

Amato A, Cinque A, Santangelo N, Santo A (1992) Il bordo meridionale del massiccio del Monte Marzano ela valle del Fiume Bianco: Geologia e geomorfologia. Studi Geologici Camerti Special 1:191–200

Ascione A, Cinque A, Improta L, Villani F (2003) Late Quaternary faulting within the Southern Apenninesseismic belt: new data from Mt. Marzano area (Southern Italy). Quat Int 101–102:27–41

Ascione A, Mazzoli S, Petrosino P, Valente E (2013) A decoupled kinematic model for active normal faults:insights from the 1980, MS = 6.9 Irpinia earthquake, southern Italy. Geol Soc Am Bull125:1239–1259. doi:10.1130/B30814.1

Aulitzky H (1980) Preliminary two-fold classification of torrents. Proceedings international symposiuminterpraevent. Bad Ischl 4:285–309

Berti M, Simoni A (2007) Prediction of debris-flow inundation areas using empirical mobility relationships.Geomorphology 90:144–161

Bonardi G, Amore FO, Ciampo G, De Capoa P, Miconnet P, Perrone V (1988) Il ‘‘Complesso Liguride’’Auct.: stato delle conoscenze e problemi aperti sulla sua evoluzione appenninica ed i suoi rapporti conl’Arco calabro. Memorie Societa Geologica Italiana 41:17–35

Bull WB (1968) Alluvial fans. J Geol 16:101–106Cascini L, Ferlisi S, Vitolo E (2008) Individual and societal risk owing to landslides in the campania region

(Southern Italy). Georisk 2(3):125–140Cavalli M, Marchi L (2008) Characterisation of the surface morphology of an alpine alluvial fan using

airborne LiDAR. Nat Hazards Earth Syst Sci 8:323–333Copertino VA, Fiorentino M. (1994) Valutazione delle piene in Puglia, 286 p, La Modernissima, Lamezia

Terme, ItalyCosta JE (1988) Rheologic, Geomorphic and Sedimentologic differentiation of water floods, hypercon-

centrated flows, and debris flows. In: Baker RR, Kochel RC, Patton C (eds) Flood geomorphology.Wiley, New York, pp 113–122

Crosta GB, Frattini P (2004) Controls on modern Alluvial fan processes in the Central Alps, Nothern Italy.Earth Surf Proc Land 29:267–293

Crosta G, Cucchiaro S, Frattini P (2003) Validation of semi-empirical relationships for the defi nition ofdebris flow behavior in granular materials. In: Rickenmann D, Chen C (eds) Debris flow hazardsmitigation: mechanics, prediction, and assessment. Millpress, Rotterdam, pp 821–832

D’ Agostino V, Cesca M, Marchi L (2010) Field and laboratory investigations of runout distances of debrisflows in the Dolomites (Eastern Italian Alps). Geomorphology 115:294–304

De Scally FA, Owens IF (2004) Morphometric controls and Geomorphic responses on fans in the SouthernAlps, New Zealand. Earth Surf Proc Land 29:311–322

De Scally F, Slaymaker O, Owens I (2001) Morphometric controls and basin response in the CascadeMountains. Geogr Ann 83A(3):117–130

De Scally FA, Owens IF, Louis J (2010) Controls on fan depositional processes in the schist ranges of theSouthern Alps, New Zealand, and implications for debris-flow hazard assessment. Geomorphology122:99–116

FEMA (2000) Guidelines for determining flood hazards on alluvial fans. Federal Emergency ManagementAgency. http://www.fema.gov/mit/tsd/ft_alfan.htm

Griswold JP, Iverson RM (2008) Mobility Statistics and Automated Hazard Mapping for Debris-fl ows andRock Avalanches. US Geological Survey Scientific Investigations Report 5276. US Geological Survey,Reston, VA: 59

Guzzetti F, Peruccacci S, Rossi M, Stark CP (2008) The rainfall intensity–duration control of shallowlandslides and debris flows: an update. Landslides 5:3–17

Hooke RLB (1967) Processes on arid region alluvial fans. J Geol 75:438–460

Nat Hazards

123

Hosking JRM (1990) L-moments: analysis and estimation of distributions using linear combinations of orderstatistics. J Roy Stat Soc B 52:105–124

Hungr O, Morgan GC, Kellerhals R (1984) Quantitative analysis of debris torrent hazard for design ofremedial measures. Can Geotech J 21(4):663–677

Iverson RM, Schilling SP, Wallace JW (1998) Objective delineation of lahar inundation hazard zones. GeolSoc Am Bull 110(8):972–984

Jakob M (2005) Debris-flow hazard analysis. In: Jakob M, Hungr O (eds) Debris-flow hazards and relatedphenomena. Praxis Springer, Berlin Heidelberg, pp 411–438

Jenks GF (1967) The data model concept in statistical mapping. Int Yearbook Cartography 7:186–190Kellerhals R, Church M (1990) Hazard management on fans, with examples from British Columbia. In:

Rachocki AH, Chuch M (eds) Alluvial Fans, a Field Approach. Wiley, New York, pp 335–354Kostaschuk RA, MacDonald GM, Putman PE (1986) Depositional processes and alluvial fan drainage basin

morphometric relationship near Banff, Alberta, Canada. Earth Surf Proc Land 11:471–484Marchi L, D’Agostino V (2004) Estimation of the debris-flow magnitude in the Eastern Italian Alps. Earth

Surf Proc Land 29:207–220Marchi L, Borga M, Preciso E, Gaume E (2010) Characterisation of selected estreme flash floods in Europe

and implications for flood risk management. J Hydrol 394(1–2):118–133. doi:10.1016/j.jhydrol.2010.07.017

Mavrouli O, Fotopoulou S, Pitilakis K, Zuccaro G, Corominas J, Santo A, Cacace F, De Gregorio D, DiCrescenzo G, Foerster E, Ulrich T (2014) Vulnerability assessment for reinforced concrete buildingsexposed to landslides. DOI, Bull Eng Geol Environ. doi:10.1007/s10064-014-0573-0

O’Brien JS, Julien PY (1985) Physical properties and mechanics of hyperconcentrated sediment flows.Proceedings of ASCE specialty conference on the delineation of landsfides, flash floods and debris flowhazards in Utah, Utah Water Research Lab., Univ. of Utah at Logan, Utah, pp 260–279

Paronuzzi P, Coccolo A, Garlatti G (1998) Eventi meteorici critici e debris flows nei bacini montani delFriuli. L’Acqua, Sezione I/Memorie 6:39–50

Patacca E, Sartori R, Scandone P (1990) Tyrrenian basin and apenninic arcs: kinematic relations since lateTortonian times. Memorie della Societa Geologica Italiana 45:425–451

Pescatore T, Renda P, Schiattarella M, Tramutoli M (1999) Stratigraphic and structural relationships be-tween Meso-Cenozoic Lagonegro basin and coeval carbonate platforms in southern Apennines, Italy.Tectonophysics 315:269–286

Piccolo F, Chirico GB (2005) Sampling errors in rainfall measurements by weather radar. Adv Geosci2:151–155. doi:10.5194/adgeo-2-151-2005

Preti F, Forzieri G, Chirico GB (2011) Forest cover influence on regional flood frequency assessment inMediterranean catchments. Hydrol Earth Syst Sci 15:3077–3090

Rickenmann D (2005) Runout prediction methods. In: Jakob M, Hungr O (eds) Debris-flow hazards andrelated phenomena. Praxis Springer, Berlin, pp 305–324

Romero R, Emanuel K (2013) Medicane risk in a changing climate. J Geophys Res Atmos118(12):5992–6001

Rossi F, Villani P (1994) Regional flood estimation methods. Giuseppe Rossi, Nilgun Harmancioglu andVujica Yevjevich Coping with Floods Pag. Kluwer in cooperation with NATO Scientific AffairsDivision, Dordrecht, pp 135–169

Rossi F, Fiorentino M, Versace P (1984) Two-component extreme value distribution for flood frequencyanalysis. Water Resour Res 20(7):847–856. doi:10.1029/WR020i007p00847

Santangelo N, Santo A, Di Crescenzo G, Foscari G, Liuzza V, Sciarrotta S, Scorpio V (2011) Floodsusceptibility assessment in a highly urbanized alluvial fan: the case of Sala Consilina (southern Italy).Nat Hazards Earth Syst Sci 11:2765–2782. doi:10.5194/nhess-11-1-2011

Santangelo N, Daunis-i-Estadella J, Di Crescenzo G, Di Donato V, Faillace P, Martin-Fernandez JA,Romano P, Santo A, Scorpio V (2012) Topographic predictors of susceptibility to alluvial fan flooding,Southern Apennines. Earth Surf Proc Land 37:803–817. doi:10.1002/esp.3197

Santo A, Santangelo N, Benedice A, Iovane F (2002) Pericolosita connessa a processi alluvionali in areepedemontane: il caso di Castellamare di Stabia in Penisola Sorrentina. Il Quaternario 15(1):23–37

Scheidl C, Rickenmann D (2010) Empirical prediction of debris flow mobility and deposition on fans. EarthSurf Process Landforms 35:157–173

Scorpio V (2011) Analisi Geomorfologica dei sistemi bacino-conoide dell’Appennino campano: scenari disuscettibilita alluvionale. Tesi di Dottorato di Ricerca in Scienze della Terra. XXIV CicloUniversitadegli Studi di Napoli Federico II, pp. 182, Tavv. 5

Sibson R (1981) A brief description of natural neighbor interpolation, Chapter 2 in Interpolating Multi-variate Data, 21–36. Wiley, New York

Nat Hazards

123

Sorriso-Valvo M, Antronico L, La Pera E (1998) Controls on fan morphology in Calabria, southern Italy.Geomorphology 24:169–187

Waythomas C, Miller T, Beger JE (2000) Record of late holocene debris avalanches and lahars at Iliamnavolcano, Alaska. J Volcanol Geoth Res 106:97–130

Welsh A, Davies T (2011) Identification of alluvial fans susceptible to debris flow hazards. Landslides8:183–194

Yu F, Chen C, Chen T, Hung F, Lin S (2006) A GIS process for delimitating areas potentially endangered bydebris-flow. Nat Hazards 37:169–189

Zanchetta G, Sulpizio R, Di Vito MA (2004) The role of volcanic activity and climate in alluvial fan growthat volcanic areas: an example from Southern Campania (Italy). Sed Geol 168:249–280

Nat Hazards

123