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Introduction to the thematic issue: “Alluvial geomorphology in Italy”

Alessandro Fontana
p. 123-130
Traduction(s) :
Introduction au numéro thématique : « Géomorphologie fluviale en Italie »

Notes de la rédaction

Article soumis le 22 janvier 2011, accepté le 23 janvier 2012.

Texte intégral

1The Italian peninsula and the main Italian islands are worldwide famous for the existence of different landscapes and environments and for the various geomorphological and geological phenomena which made the country very attractive for tourists but also for geoscientists. The Dolomites, Etna volcano, Amalfi Peninsula, Maddalena Island and Chianti Hills are some paramount examples of this “Livre des Merveilles” of landscapes and geomorphology in which the Lagoon of Venice is the only flat environment that is generally mentioned. The Italian floodplains are virtually absent and this fact is mainly related to the little emotional impact that the view of flat landforms normally arouses in the common audience; little attention has been paid to these features even by geomorphologists, mostly attracted by more evident features as the mountain and coastal morphologies. These topics caught large part of research efforts even in the second part of 20th c., when the importance of morphological, pedologic and stratigraphic data in the management and environmental protection of alluvial plains (flood risk, geotechnics, hydrology and hydrogeology) became strongly evident. As mentioned by G.B. Castiglioni (1995) some noticeable “early” exceptions existed, as for example the works of M. Ciabatti (1966) and the French researcher P. Gabert (1962). Moreover the investigations about NE Italian floodplain by E. Feruglio (1925) and A. Comel (e.g., 1950; 1958) were very innovative for their period.

2A milestone in the Italian research dealing with alluvial environments was achieved with the publication of the Geomorphological Map of the Po Plain edited by G.B. Castiglioni (1997 a and b) and the related Illustrative Notes (Castiglioni and Pellegrini, 2001); their production involved many scientists from a number of universities and research centres (i.e., CNR and museums) for about 15 years, boosting the interest in alluvial geomorphology (e.g., Boenzi and Caldara, 1992; Castiglioni and Federici, 1995). The Map of the Po Plain had been preceded by some examples of morphologic and morpho-pedologic cartography (e.g., Castiglioni, 1982; Cavallin et al., 1987; Cremaschi, 1987; Gastaldini, 1987; Sorbini et al., 1994). Actually, recently few project of geomorphological cartography considered in detail large sectors of alluvial environments, mostly concentrating in NE Italy (e.g., Bondesan et al., 2004; Fontana, 2006). Among these, the Geomorphological Map of the Province of Venice, printed at scale 1:50,000 and at 1:20,000 in the digital version, represents the most advanced product. It covers about 3000 km2 below 10 m asl and used a specific legend for alluvial and lagoon and coastal environments (Bondesan and Meneghel, 2004; Bondesan et al., 2004). However, to be noticed that in the last years several other local administrations, as provinces and regions, funded some detailed geomorphological maps of alluvial plains, but these have not been published and, thus, they are internal reports.

3In the new Geological Map of Italy (CARG Project, see Pasquarè and Venturini, 2004), the geomorphological features are not generally represented in high detail but, in some of the sheets considering lowland areas the main alluvial landforms (i.e., palaeochannels, terraces, crevasse splays and fluvial ridges) were mapped in particular and they could allow to obtaining an effective derived map of floodplain morphologies (i.e., Stefani et al., 2003).

4With the term Po Plain (Pianura Padana or Val Padana in Italian), it is often described both the sector directly formed by the deposits of Po River (Po Plain strictu sensu) and its adjoining areas of the Venetian-Friulian Plain and Romagna Plain, which have not been actually built by the Po River (fig. 1). The Po Plain has a total extent of about 47,000 km2 and is considerably larger than the other Italian alluvial plains, moreover with its 20 millions of inhabitants, represents one of the largest alluvial environment of Europe, comparable to the Great Hungarian Plain in the Carpathian Basin and the Rhine-Meuse Delta; specially this latter has several similarities with the Po Plain due to its direct contact with the sea and the low gradient of the related marine shelf (see Berendsen and Stouthamer, 2000).

Fig. 1 – Digital Elevation Model (DEM) of Italy derived by the SRTM (Shuttle Radar Topographic Mission) with location of the main alluvial plains

Fig. 1 – Digital Elevation Model (DEM) of Italy derived by the SRTM (Shuttle Radar Topographic Mission) with location of the main alluvial plains

Elaboration by F. Ferrarese.

5Along the Italian peninsula some other important coastal plains face the Tyrrhenian Sea between Tuscany and Campania regions: the Versilia, Arno, Maremma, Agro Romano, Agro Pontino, Fondi, Garigliano, Volturno, Sarno and Sele plains (fig. 2). Besides these zones, which in some cases are quite large but have a maximum area of ca. 1000 km2, there are some smaller ones which normally consist of narrow stretches of coastal plain, as for example the one facing the Gulf of Policoro in Basilicata, fed by Basento River (fig. 2). With regards to the main islands, the Plain of Catania can be cited for Sicily and in Sardinia the area of Arborea and Oristano, in the northernmost sector of the Campidano Graben. Moreover, considering the alluvial environments in Italy, the main Alpine and Apennine valleys have to be mentioned, where fluvial and/or fluvioglacial processes sometimes played an important role in their evolution (e.g., Nesci et al., this issue). Specially in the Apennines, where the staircase of terraces existing is some of the valleys and intramontane basins allows to extend the alluvial record up to Pliocene and to analyse the relationships existing among landforms, climate and tectonics.

Fig. 2 – DEM derived by the SRTM data of the Po Plain

Fig. 2 – DEM derived by the SRTM data of the Po Plain

Elaboration by F. Ferrarese.

6This Issue stem by the stimulating invitation of the Editor-in-Chief Gilles Arnaud-Fassetta and was planned with the ambitious aim to cover the various alluvial areas existing in Italy, highlighting different methods, problems and perspectives of the research. Obviously in this contribution only a partial vision of the whole picture can be supplied, but it is believed that the issue describes a significant part of the topics and of the zones in which alluvial geomorphology is mainly involved.

7This volume is mainly focused on the past evolution of the investigated areas, linking morphological and stratigraphic information; whereas, the recent and future dynamics of the river channels, presently included in the so-called fluvial geomorphology, are not considered. That topic has been strongly developed in Italy during last years (e.g., Rinaldi and Surian, 2008; Surian et al., 2009) and became almost an autonomous discipline of geomorphology.

8Due to the large extent of the Po Plain, three papers of this issue deal with northern Italy, while the other three are distributed along the peninsula. Besides their vast dimensions, Po and Venetian-Friulian plains differ from the other areas because they experienced the direct influence of glaciers during the late Quaternary glaciations. The alluvial megafans and fans which characterise the pede-Alpine fringe achieved their maximum extent and their typical internal differentiation during LGM (Last Glacial Maximum, ca. 27-17 ka cal. BP), when glacial fronts directly fed some of the main alluvial systems (Guzzetti et al., 1997; Castiglioni, 1999; Fontana et al., 2008). In the post-LGM large portions of the northern side of the Po Plain were abandoned by river activity and LGM surfaces still largely crop out due to the entrenchment that confined the sedimentary flux. This situation is explained in the paper by C. Ravazzi et al. (this issue) that studied some sections exposed along the terraces cut during Lateglacial by the rivers and exposing sediments with an age up to 40 ka ago. The study of these outcrops and of the related landforms allows to document different phases in the geomorphologic history of the central portion of the northern Po Plain; this study is an example of a muldisciplinary approach which characterised also the vegetational and sedimentary changes occurred in the area through pollen and petrographic analyses.

9Along the Apennine side of the Po Plain alluvial fans are smaller than along the pede-Alpine piedmont and some of them experienced important phases of sedimentation also during the last millennia, thus burying some soil sequences that document different Holocene landscapes (Cremaschi and Nicosia, this issue). Considering the long human presence documented in Italy and particularly the diffuse peopling occurred since Neolithic in the alluvial environments, the anthropic activity interfered with the geomophic and stratigraphic processes in the last 7500 years; the blooming of the interest in geoarchaeological perspective testifies the importance of the interactions between natural and human-induced processes (e.g., Arnaud-Fassetta and Landuré, 2003; Lespez, 2003; Ghilardi et al., 2009). In fact, all the papers of this issue somehow deal with methods and information related to archaeological remains and historical sources; these are used as stratigraphic markers, dating references, environmental and ecological indicators under the wide field of research investigated by the so-called alluvial geoarchaeology (Brown, 1997).

10Human impact left is footprint on the alluvial landscape at least since late Prehistory and this could be clearly documented by the paper from M. Cremaschi and C. Nicosia (this issue), in which techniques for investigating archaeological stratigraphies and palaeopedological properties were used between Parma and Modena. The human capability to shape the environment has significantly increased in the last centuries and one spectacular laboratory of this growing power is represented by the Lagoon of Venice and its mainland. Many rivers were artificially diverted since Middle Age to prevent the silting of the lagoon and now a huge project to save Venice from high tides with mobile barriers is going on (Ghezzo et al., 2010). The paper by A. Bondesan and P. Furlanetto (this issue) compares detailed geomorphological cartography and the analysis of historical topographic maps to describe the dynamics and the results that the huge hydraulic projects led in 17-19th c. by the Repubblica di Venezia had in the geomorphological setting of the Venetian Plain.

11The influence that sea-level variations occurred in the last centuries had on the final tract of the rivers is often difficult to detect or almost invisible; in fact sea-level changes had a low magnitude and they have generally not been clearly testified in the alluvial geomorphology. On the contrary, if we consider longer periods, including the post-LGM transgression, the changes in the sea-level position could be considered one of the main factors driving the sedimentation, style and morphology of the river systems. An example is given by the paper by V. Rossi et al. (this issue), that compares the geomorphological data with the subsoil information collected through mechanical cores, and documents the incision and filling phases of the buried valleys which once characterised the plain of Pisa. Due to the faint evidence that often alluvial landforms present, in their study it was often necessary to merge together the information related to the surface morphologies, the sediments which they are formed by and the soils covering them. Thus, in many researches dealing with alluvial environments a clear separation among geomorphology, geology and pedology does not exist and an integrated approach is needed.

12The incised valleys are particular features that often strongly connote the landscape of alluvial plain or connoted it in the past; in the mountain areas the valleys could be characterised by terraces which also document the shifting from alluvial aggradation to incision phases that could have alternated several times. In Italy there are several examples of this complex evolution. A review of the Quaternary evolution of the valleys in the Marche region is presented (Nesci et al., this issue). This is considered representative for many rivers draining the eastern sector of central Apennines, during an interval of time which encompasses millions of years and that allows to appreciate the role played in the alluvial evolution by the 100-ka climatic glacio-eustatic cycles and the tectonic forcing.

13Another case study dealing with the history of the alluvial filling of a mountain catchment considers the valley of Basento River, in Basilicata (southern Italy), where the semi-arid climate of the last millennia has been conditioned by the alternation of wet and dry periods, which strongly controlled the vegetation cover and, therefore, indirectly also the sedimentary supply (Piccarreta et al., this issue).

14Since the availability of the Regional Technical Cartography, introduced in Italy between the ’70s and ’80s of the 20th c., the possibility to create detailed topographic maps of the alluvial plains has been supported and, from these, DEMs (Digital Elevation Model) could be derived. These products quickly became a key tool in the detection of low-relief landforms (Castiglioni, 1995, 1997b) and now they are a common method of analysis and representation in alluvial geomorphology (e.g., Ravazzi et al., this issue). In the last years the use of LiDAR (Ligh Detecting And Ranging) topographic surveys introduced a new generation of DEMs and DSMs (Digital Surface Model) allowing the possibility to appreciate even more faint morphologies (fig. 3) and to run automated morphometric analyses (Berendsen and Volleberg, 2007; fig. 3). Currently LiDAR topographic surveys cover only some floodplain areas of Italy, but is expected that in a few years it will be available for the whole Country and a new exciting perspective is offering to the researchers.

Fig. 3 – DEM derived by the LIDAR topographic survey of the area near the stadium of Padua

Fig. 3 – DEM derived by the LIDAR topographic survey of the area near the stadium of Padua

The incised trace of a meandering channel, abandoned by the Brenta River about 6500 years ago, is clearly evidenced; elevation of the natural topographic surface ranges between 9 to 15 m asl.

Modified from Ninfo et al., 2011.

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Table des illustrations

Titre Fig. 1 – Digital Elevation Model (DEM) of Italy derived by the SRTM (Shuttle Radar Topographic Mission) with location of the main alluvial plains
Légende Elaboration by F. Ferrarese.
URL http://geomorphologie.revues.org/docannexe/image/9792/img-1.jpg
Fichier image/jpeg, 1,8M
Titre Fig. 2 – DEM derived by the SRTM data of the Po Plain
Crédits Elaboration by F. Ferrarese.
URL http://geomorphologie.revues.org/docannexe/image/9792/img-2.jpg
Fichier image/jpeg, 1,9M
Titre Fig. 3 – DEM derived by the LIDAR topographic survey of the area near the stadium of Padua
Légende The incised trace of a meandering channel, abandoned by the Brenta River about 6500 years ago, is clearly evidenced; elevation of the natural topographic surface ranges between 9 to 15 m asl.
Crédits Modified from Ninfo et al., 2011.
URL http://geomorphologie.revues.org/docannexe/image/9792/img-3.jpg
Fichier image/jpeg, 802k
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Alessandro Fontana, « Introduction to the thematic issue: “Alluvial geomorphology in Italy” », Géomorphologie : relief, processus, environnement, 2/2012 | 2012, 123-130.

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Alessandro Fontana, « Introduction to the thematic issue: “Alluvial geomorphology in Italy” », Géomorphologie : relief, processus, environnement [En ligne], 2/2012 | 2012, mis en ligne le 02 novembre 2012, consulté le 11 février 2016. URL : http://geomorphologie.revues.org/9792

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Auteur

Alessandro Fontana

Università di Padova – Dipartimento di Geografia “G. Morandini” - Via del Santo, 26 - 35123 Padova - Italy (alessandro.fontana@unipd.it).

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