**1. Introduction**

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Heavy rainfall is one of the most important triggering causes of landslides - particularly in Mediterranean areas, that are characterised by moderate to low annual precipitation and, occasionally, by high precipitation intensity (up to >200mm/day)-. In agricultural or poorly vegetated hilly landscapes - particularly when characterised by clayey lithologies - heavy rainfall triggers very rapid geomorphological processes, such as floods, soil erosion (rill, gullies) and landslides (rapid earthflows) inducing strong erosion rates on the hilly landscape, sediment transport and sedimentation along the alluvial plains and at the mouths of rivers.

The distribution of geomorphological processes and landforms triggered by these events is variable and controlled by several geological, geomorphological, meteorological and landuse factors. In this work, we analyse the landforms triggered by heavy rainfall in three case studies from the Abruzzi region in Central Italy.

Over the last ten years, the Abruzzi region was affected by several heavy rainfall events. Three of them have had daily rainfall > 100 mm and >200 mm over few days: 1) on 23-25 January 2003 (in the whole region), 2) on 6-7 October 2007 (in a small part of the hilly and coastal Teramo area), and 3) on 1-2 March 2011 (in the hilly and coastal Teramo and Pescara area). These events have triggered different types of geomorphological instability: landslides, soil erosion and flooding. The distribution and types of instabilities and landforms is different in the three cases.

The 2003, 2007 and 2011, heavy rainfall events were analysed with regard to their meteorological aspects, and geological and geomorphological features, highlighting both common and distinct geomorphological effects on the landscape.

The meteorological aspects were studied by processing a >40 pluviometric station database. The data processing enabled the analysis and comparison of hourly rainfall intensity, cumulative rainfall, daily rainfall, monthly rainfall and previous monthly rainfall.

Geomorphological Instability Triggered

high angle normal faults.

common all over the hill slopes).

coastal slopes directly to the coastal plain.

and alluvial fan deposits join a narrow coastal plain.

valleys, by eluvial and colluvial, cover up to several meters thick.

by Heavy Rainfall: Examples in the Abruzzi Region (Central Italy) 47

rising within the piedmont area and rapidly reaching the coast. A fourth, secondary type but very important in heavy rainfall events - is given by small catchments flowing on the

The relief of the Apennine Chain is made up of carbonate ridges (NW-SE, NNW-SSE, N-S) separated by parallel valleys carved in terrigenous foredeep deposits or filled up with continental ones and by wide intermontane basins partially filled with Quaternary continental deposits. To the east, the relief abruptly slopes down into the piedmont area, where a hilly landscape is carved by cataclinal valleys (SW-NE) on arenaceous-pelitic thrusted and faulted successions and on a gently NE-dipping homocline of clay, sand and conglomerate deposits. Along the valleys and close to the coast, alluvial plains with fluvial

The lithologies of the Abruzzo area are made up of different units, mostly of sedimentary origin. In the recent official Geological map of Italy (CARG Project, Geological Survey of Italy, ISPRA, 2011) the lithological units are referable to pre-orogenic units (mostly marine Meso-Cenozoic carbonate rocks), syn-orogenic units (mostly Neogene arenaceous and pelitic rocks), and post-orogenic units (marine Plio-Pleistocene clay-sand-conglomerate rocks and Quaternary clastic continental deposits). These can be grouped in a limited number of units (Fig. 2). These units are mantled, particularly in piedmont slopes and

The structural setting of the chain area is defined by thrust ridges and faulted homocline ridges separated by tectonic valleys and basins. Main regional fault systems affect the chain area: Mio-Pliocene NW-SE to N-S low angle thrust faults, Pliocene NW-SE to NNW-SSE high angle normal faults, Quaternary (in some cases still active) NW-SE and SW-NE high angle normal faults. The piedmont area is defined in the inner part by thrust reliefs which are affected by regional NNW-SSE Pliocene thrusts and by minor high angle normal faults and, in the outer part, by a wide homocline slightly NE-dipping which is affected by minor

The geomorphological processes affecting the whole Abruzzo region are mainly fluvial slope processes and mass wasting. In the coastal areas, marine and aeolian processes are also very important, while in mountain areas karst landforms are present and the landform remnants of ancient Pleistocene glacial processes are preserved. These processes are frequently activated by the heavy rainfall events that affect the region. Fluvial processes affect the main rivers, alternating between channel incisions and flooding. The slope processes that are due to running water mostly affect the clayey and arenaceous-pelitic hills of piedmont and the coastal areas, generating outstanding landforms such as badlands (or "calanchi") and minor landforms, such as rills, gullies and mudflows (which are very

Mass wasting processes have induced the formation of a huge number of landslides and mass movement in the Abruzzi region, mostly affecting the hilly piedmont area as well as

This geological and geomorphological setting is the result of a complex geological and morphostructural evolution due to the Neogene compressional deformation of different Meso-Cenozoic paleogeographic domains and Neogene foredeep domains (that formed a NE-verging thrust belt) and to Quaternary extensional tectonics and uplift (that formed

the chain area and - locally - the coastal one (Fig. 2; D'Alessandro et al., 2003a, 2007).

Geomorphological effects of heavy rainfall were analysed through a field surveys, aerial photo analysis and inventories and technical reports, mapping the distribution of the landslides, soil erosion and flooding.

This work allowed us to highlight that these types of methods, investigations and data are basic in applied studies for the stabilisation and management of slopes and minor or major drainage basins, and for general land management. Only a high level of knowledge of geomorphological instability, connected to drainage, geological-geomorphological and morphostructural features and to meteorological events - particularly when joined to geotechnical data - allows effective stabilisation and management plans.

Finally, these types of studies are basic and complementary to recent methods of the investigation and mapping of land sensitivity to such geomorphological processes as landslides, soil erosion and desertification, etc. They allow us to define the future scenarios which sustainable land planning and management should be based on - by taking into account the specific destination of different areas and contributing to the identification of proper sites for quarries, dumps and purification plants, or else proper areas for industry, urban expansion, thereby generally supporting the process of creating an urban plan.
