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 [email protected]
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
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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
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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)
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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)
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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
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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
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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.
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