E3S Web of Conferences 415, 05001 (2023)
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Hydrologic-hydraulic modelling in the Vezza catchment (AlpiApuane, Italy): an area prone to flash floods and debris flows
Michele Amaddii1,2*, Giorgio Rosatti3, Daniel Zugliani3, Lutz Weihermüller4, Cosimo Brogi4, Mehdi Rahmati4,5, PierLorenzo Fantozzi2, and Leonardo Disperati2
1

Department of Earth Sciences, University of Florence, Florence, 50121, Italy
Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Università degli Studi di Siena, Siena, 53100, Italy
3
Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, 38123, Italy
4
Agrosphere Institute (IBG-3), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
5
Department of Soil Science and Engineering, Faculty of Agriculture, University of Maragheh, Maragheh, Iran
2Abstract. The Alpi Apuane (Italy) are located a few kilometres from the coast of the Ligurian Sea, andthey are characterized by peak elevations up to two thousand meters above sea level, as well as narrow,deeply incised valleys and steep slopes. Due to these morphoclimatic conditions, heavy rains are frequent,causing floods, landslides, and debris flows, particularly within the Vezza catchment. In this work weapplied two different hydrological-hydraulic models to this catchment, focusing on the catastrophic debris
flow event of June 19, 1996. Firstly, recent, well-documented rainfall events were used to validate theengineering geological model of the study area, then we began to analyse the rainfall-runoff and debris flow
event of 1996 in the Cardoso sub-catchment. As models, we used the FLO-2D and a novel experimental
model, developed by some of the authors and based on TRENT2D, in which the dynamic of a debris flow
is fully coupled with the rainfall-runoff response of a basin. Preliminary results show how the used approachallowed us to gain some insight into the hydrological behaviour and debris flows formation, erosion,transport, and deposition in the Cardoso sub-catchment.

1 Introduction
In recent decades, climate change has led to an increasein extreme precipitation and runoff events [1], which
can make flash floods and debris flows even larger andmore frequent than in the past [2]. In urbanized
mountainous areas, these types of events pose a major
threat to local environments, populations, andinfrastructures, leading to loss of life and livelihoods [3].
The study area selected for this work is the Vezza
catchment located in the Alpi Apuane (Italy), a region
particularly susceptible to intense precipitation events
due to peculiar morphoclimatic characteristics. The
Vezza catchment experienced severe flooding anddebris flows over the centuries, like the catastrophic
debris flow event that occurred on June 19, 1996, in theCardoso sub-catchment. Several authors have studied
the geological, geomorphological, hydraulic, andgeotechnical factors controlling the slope stability of thestudy area [e.g., 4-6]. However, specific rainfall-runoff
simulations, as well as modelling of erosion, transport,
and deposition rates by debris flows are lacking so far.
The objective of this paper is to model the debris
flow processes of the 1996 event through two different
numerical modelling approaches: the FLO-2D
hydrological-hydraulic model [7, 8] and a novel
experimental model, developed by the authors
belonging to the University of Trento, based on theTRENT2D model [9, 10]. In the latter, the dynamics ofdebris flows are fully coupled with the rainfall-runoff

response of a basin. Both models are based on physicalconservation principles.
For the 1996 event, rain gauges were not available
within the Cardoso sub-catchment, nor were direct
runoff measurements available. Hence, an in-depthhydrological analysis was performed in the Vezza
catchment by modelling some well-documented recent
rainfall events. For this task was used the FLO-2D
model applying the Green-Ampt (G-A) infiltration
approach [11] to a well-constrained engineering
geological model of the study area. The satisfactory
matching between simulated discharges of each eventand the discharges recorded by the hydrometric station
at the outlet of the Vezza, allowed us to check therobustness of the engineering geological model.
Both hydrological and debris flows modelling of the1996 event were then carried out, which allowed us tohighlight key processes which affected hillslopes andstreams, such as widespread erosional processes or
temporary deposition and subsequent erosion causingimpulsive waves typical of debris flows.

2 Overview of the study area
The Vezza catchment (27 km2) is located in northwestern Tuscany (Alpi Apuane, Italy) in the Versilia
River valley. The catchment area is characterized by thepresence of two main torrents (Cardoso and Vezza) thatconverge to form the Vezza River (Figure 1), which inturn forms the Versilia River further downstream.

* Corresponding author: michele.amaddii@unifi.it© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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Environmental and Physical Sciences of the Universityof Siena, to evaluate both the saturated hydraulicconductivity and the depth of the soil. Then,morphometric clustering methods [14, 15] provided thecontinuous maps of the above parameters used for thesubsequent numerical modelling.

2.1 Geological and morpho-climatic setting
The Versilia River valley is located in the southern
sector of the Alpi Apuane tectonic window, where ametamorphic complex (the “Autoctono” and Massaunits) and the Tuscan Nappe crop out [12].
The upper Versilia River catchment has a mountainmorphology characterized by narrow, deeply incisedvalleys, and steep slopes, clearly influenced by the
structural-geological arrangement of the Alpi Apuanearea [6]. In general, the higher portion of the basins isdivided by ridges composed of carbonate/dolomite
rocks, often with slopes greater than 60°. Usually, theseareas are rocky, or with a very thin soil layer, and areconnected to the lower slopes by talus and screedeposits. On the other hand, these slopes are moderately
steep (between 25° and 40°) and are often made up of
metamorphic sandstone and phyllite schist rocks, which
in turn are covered by thicker soils (0.3-2 m) where adense chestnut forest develops.
The morphoclimatic conditions of the Alpi Apuane,
which run parallel to the Tyrrhenian coast (LigurianSea), and the rapid rise of the relief (about 2,000 m a.s.l.
in less than 10 km) favour the rapid cooling of humid air
masses of Atlantic or Mediterranean origin [5], causing
intense precipitation and, in some cases, extreme rainsthat trigger floods, landslides, and debris flows.
2.2 Flood and debris flow events in the Vezza
catchment

Fig. 1. Location of the Vezza catchment and its morphology.
Red lines: borders between the sub-catchments of theCardoso and Vezza torrents. White boxes (1-3) and whitestar: rain gauges and hydrometric station, respectively.

During the last four centuries, numerous geohydrological events have occurred in the study area,some of which have been classified as "very highintensity" (catastrophic phenomena with very severe
damage and fatalities): May 7, 1636; September 27,1774; September 18-19, 1846; September 25-26, 1885;
October 11, 1902; and June 19, 1996 [13].
The most recent and catastrophic event occurred onJune 19, 1996, in the Cardoso sub-catchment. In thiscase, the observed maximum rainfall amount in 10 h was
460 mm, with a maximum hourly intensity of 160 mm.
Hundreds of shallow landslides and hillslopes and inchannel debris flows were triggered, mobilising
1,400,000 m3 of debris [5]. Most of the solid material
was deposited along the main Cardoso valley, partiallydestroying the village, and resulting in twelve fatalities.

3 Rainfall-runoff modelling of recent
rainstorms in the Vezza catchment
Ten rainfall events that caused a rapid increase in theflow of the Vezza River were selected (Table 1). These
events differ in the season they occur, duration, andintensity. In no case did flooding of inhabited areas orsignificant erosion processes occur.
Table 1. Maximum rainfall peak and associated maximum
peak discharge of the selected rainfall events recorded by thegauges in the study area. *Maximum rainfall peak andmaximum peak discharge of the 1996 debris flow event [4].
Event

2.3 Available data

19/6/201021-22/12/2010
25-26/10/2011
30/3/201321/10/2013
15-16/3/2018
1-2/2/2019
2-3/3/2020
4/6/2020
4-5/12/2020*19/6/1996

A hydrometric station (Ruosina station) is located at theoutlet of the catchment under study, while three rain
gauges are positioned at Retignano (1), Cardoso (2), andPomezzana (3) (Figure 1). Rain gauges 1 and 3 havebeen in operation since 1996, while gauge 2 since 1998.
Discharge data from the Ruosina hydrometric station
have been available since 2004.
In addition, a detailed engineering geological study
based on field measurements and laboratory
determinations was conducted in the Vezza catchment
by some of the authors of the Department of Earth,

Max Peak Rain
(mm/15’)
15
5.8
23
7.8
21.8
8.4
12.2
6.8
31
15.4
61.8

Max PeakDischarge (m3/s)
171
58
148
98
175
33
94
78
149
92
381

Numerical modelling of these events was performed
using the well-established software FLO-2D, a2

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hydrologic-hydraulic and distributed model that
continuously calculates rainfall-runoff volumes fromthe catchment area according to the digital elevation
model, rainfall amount and intensity, land use, and soil
type. Precipitation loss (interception) and infiltration are
also considered and spatially allocated within thecomputational domain. The G-A approach was used toestimate the infiltration processes. In this model
approach, the saturated wetting front is assumed tomove vertically downward from the ground surface as asingle piston-like displacement. Horizontal subsurface
flow is not considered in the FLO-2D model. The
matching between the simulated and measured
discharges was satisfactory, confirming the reliability ofthe engineering geological survey data and relatedmodel.

the G-A approach to determine runoff values along thehydrographic network.
Following the liquid forcing evaluation, a backanalysis of debris flow propagation and deposition wasperformed. Since FLO-2D is a fixed bed model, erosive
processes were not considered. For this reason, specificinflow sections of the flow mixture were selected along
the hydrographic network. In addition, various
rheological parameters and different amplification
factor formulas were evaluated to estimate thevolumetric concentration of sediment.
4.2 TheExperimental
TRENT2D-based,
Rainfall-Runoff Mobile Bed model approach
Initially, the rainfall-runoff model was used in its nonerodible bed mode, to evaluate the sole liquid runoff andresults were compared with those of FLO-2D. In thismode, the model uses a two-dimensional, coupledsurface and subsurface flow description in which thesurface flow occurs over a fixed bed. Like the FLO-2D
model, infiltration is based on the G-A equation while incontrast, the TRENT2D model considers sub-surface
flow and possible upwelling fluxes.
After this phase, we switched to the mobile bed
mode to study the impact of the debris flow formationon the runoff response of the basin. In this mode, themodel uses, as surface flow, a mobile bed debris-flow
model which interacts dynamically with the subsurface
model in terms of liquid flux exchanges and variation of
the relevant soil depth available for the subsurface flow.
Possible non-erodible zones can be considered as well.
The full model was applied in some sub-basins of theCardoso catchment, where erosion processes wereparticularly severe during the 1996 event, and where theimportance of the coupling was relevant in the formationof the debris flow and the runoff response of the basin.

4 Rainfall-runoff and debris flow
modelling of the June 19, 1996,
Cardoso event
Two numerical modelling approaches were considered
for the simulation of the 1996 catastrophic debris flow
event that destroyed the village of Cardoso (Figure 2),
namely the FLO-2D model and the experimental
TRENT2D-based, rainfall-runoff mobile bed model.
Their main features are presented in the followingsections.

5 Preliminary Results
The most relevant results that were obtained can besummarized as follows:• The hydrological analysis in the Vezza catchment
provided quantitative information on infiltration
and runoff processes in response to rainfall eventsof varying intensity and duration. Moreover, themodelled hydrographs were consistent with thoseobserved.
• The experimental rainfall-runoff mobile bed modelallowed us to simulate some key processes
connected to the widespread debris flows affecting
hillslopes, first-order streams, and the mainhydrographic network during the June 19, 1996
event in the Cardoso sub-catchment. Namely, it waspossible to model: (i) the variation of thehydrological response of the catchment caused bymorphological changes; (ii) the widespread debris
flows processes (an example is depicted in Figure3); (iii) the temporary deposition and subsequenterosion of solid material causing the developmentof impulsive waves typical of debris flows.

Fig. 2. Destruction of the Cardoso village caused by severe
debris flows deposition [16].

To validate the models, since no direct discharge
measurements were available for the Cardoso
catchment, the liquid discharge values were comparedwith those of [4], which were calibrated considering theamount of water flowing in and out of a dam near theCardoso catchment during the event. Moreover, tocompare the debris flow modelling results, we used themaps of both eroded and inundated areas obtained fromthe post-event orthophotos interpretation, as well as themaximum heights of deposition obtained by interpretingarchive photos of the event.
4.1 The FLO-2D model approach
As with the simulation of recent precipitation events, therainfall-runoff modelling of the 1996 event is based on3

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4.

R. Rosso, L. Serva, 19 giugno 1996 - Alluvione inVersilia e Garfagnana (in Italian) (ANPAARPAT, 1998)
5. G. D’Amato Avanzi, R. Giannecchini, A.
Puccinelli, Eng. Geol, 73, 215-228 (2004)
6. R. Giannecchini, D. Naldini, G. D’Amato Avanzi,
A. Puccinelli, Quat. Int. 171-172, 108-117 (2007)
7. J.S. O’Brien, P.Y. Julien, W.T. Fullerton, Hydrol.
Eng., (119), 244-261 (1993)
8. J.S. O’Brien, FLO-2D Users Manual (FLO-2D
Software Inc., 2018)
9. G. Rosatti, L. Begnudelli, Comput. Fluids, 71, pp.
179-195 (2013b)
10. G. Rosatti, D. Zugliani, J. Comput. Phys., 285, pp.
226-250, (2015)
11. W.H. Green, G. Ampt, J. Ag. Sci. 4:1-24 (1911)
12. L. Carmignani, P. Conti, L. Disperati, P.L.
Fantozzi, G. Giglia, M. Meccheri, Carta geologicadel Parco delle Alpi Apuane - Scala 1:50.000(Apuan Alps UGGp, S.EL.CA, 2000)
13. R. Giannecchini, G. D'Amato Avanzi, J. Phys.
Chem. Earth A/B/C 49, 32-43 (2012)
14. M.P. Papasidero, E. Trefolini, V. Vacca, F. Viti, L.
Disperati, Rend. Online Soc. Geol. It, 42, 34-37(2017)
15. T. Venturini, E. Trefolini, E. Patelli, M. Broggi, G.
Tuliani, L. Disperati, Rend. Online Soc. Geol. It.
39, 47-50 (2016)
16. A. Bartelletti, A. Amorfini, G. Ottria, Rains andRuins in the Apuan Alps (Apuan Alps UGGp,2017)

Fig. 3. Dynamics of erosion and deposition (m) debris flow
processes occurred on June 19, 1996, within a portion of theCardoso sub-catchment.

6 Concluding remarks
An adequate knowledge of the engineering geologicalframework, based on the integration of detailed mappingand specific field investigations, is a fundamental
prerequisite for further accurate rainfall-runoff anddebris flow modelling using physically basedapproaches.
Starting from this framework, the modelling tools
used allowed us to improve our knowledge of thehydrological behaviour and its modification connected
to debris flow processes in the Vezza catchment, an area
particularly prone to such extreme-precipitationinduced geo-hydrological events.

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