Geomorphology 66 (2005) 13–39 www.elsevier.com/locate/geomorph Sequence of instability processes triggered by heavy rainfall in the northern Italy Fabio Luino* Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica, Sezione di Torino, Strada delle Cacce 73, 10135 Torino, Italy Received 3 June 2003; received in revised form 2 April 2004; accepted 4 September 2004 Available online 23 November 2004 Abstract Northern Italy is a geomorphologically heterogeneous region: high mountains, wide valleys, gentle hills and a large plain form a very varied landscape and influence the temperate climate of the area. The Alps region has harsh winters and moderately warm summers with abundant rainfall. The Po Plain has harsh winters with long periods of subfreezing temperatures and warm sultry summers, with rainfall more common in winter. Geomorphic instability processes are very common. Almost every year, landslides, mud flows and debris flows in the Alpine areas and flooding in the Po flood plain cause severe damage to structures and infrastructure and often claim human lives. Analyses of major events that have struck northern Italy over the last 35 years have provided numerous useful data for the recognition of various rainfall-triggering processes and their sequence of development in relation to the intensity and duration of rainfall. Findings acquired during and after these events emphasise that the quantity and typology of instability processes triggered by rainfall are related not only to an area’s morphological and geological characteristics but also to intense rainfall distribution during meteorological disturbances. Moreover, critical rainfall thresholds can vary from place to place in relation to the climatic and geomorphological conditions of the area. Once the threshold has been exceeded, which is about 10% of the local mean annual rainfall (MAR), the instability processes on the slopes and along the hydrographic networks follow a sequence that can be reconstructed in three different phases. In the first phase, the initial instability processes that can usually be observed are soil slips on steep slopes, mud–debris flows in small basins of less than 20 km2 in area, while discharge increases substantially in larger stream basins of up to 500 km2.In continuous precipitation, in the second phase, first mud–debris flows can be triggered also in basins larger than 20 km2 in area. Tributaries swell the main stream, which is already in a critical condition. The violent flow causes severe problems mainly along valley bottoms of rivers with basins up to 2000 km2 in area. First bedrock landslides can occur, reaching a considerable area density, with volumes from a few hundred up to about one to two million cubic meters. In continuous precipitation, in the third phase, basins of more than 2000 km2 in area reach their first critical stage. River-bed morphology is extensively modified, with erosional and depositional processes which can locally undermine the stability of structures and infrastructures. Waters overflow levees, flooding villages and towns to various widths and depths and sometimes claiming casualties. Some days after an intense * Fax: +39 11 343574. E-mail address: [email protected]. 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.09.010 14 F. Luino / Geomorphology 66 (2005) 13–39 rainfall period, large landslides involving the bedrock can still take place. These processes usually cause the movement of very large rock masses. The total duration of rainfall usually has a greater effect on these landslides than does the number of short periods of very intensive precipitation. This sequence cannot be divided into separate phases when the events occur simultaneously because of the presence of intense rainfall pulses and the generation of very diffuse surface runoff. Such situations usually happen during short-lasting heavy summer rainstorms or in late spring, when snow melt combines with intense rainfall. The three-phase sequence has been identified in three severe events that are analysed in this paper: Valtellina (Lombardy) in 1987, Tanaro Valley (Piedmont) in 1994 and Aosta Valley in 2000; but this sequence has also been observed during other events that occurred in northern Italy: in Piedmont in 1968, 1977, 1978, 1993 and 2000; in Lombardy in 1983 and 1992; in the Aosta Valley in 1993. D 2004 Elsevier B.V. All rights reserved. Keywords: Severe hydrological event; Instability processes; Sequence of development; Northern Italy 1. Introduction morphological and topographical conditions and to particular time intervals. In Europe, Italy ranks highest in the variety of The objective of this paper is to highlight that, natural instability processes: landslides, glacier-related during severe hydrological events in northern Italy, it phenomena, floods, earthquakes, subsidence and is possible to follow a time evolution of the natural volcanic eruptions. Throughout the country, these instability processes. This evolution corresponds to processes claim victims and cause damage amounting increased risk and expected damage. to billions of Euros every year. Historical research has shown that 11,000 landslides and 5400 floods have occurred in the last 80 years. The costs for these 2. Geology and geomorphology processes are high: since 1980, the State has paid 42.4 billion Euros, or about 5.7 million Euros per day. The study area includes Piedmont, the Aosta Since 1993, severe hydrological events have struck Valley and Lombardy. Within the total area of northern Italy (Piedmont, the Aosta Valley and 52,512 km2, 45.6% is mountainous landscape, Lombardy) five times, causing large floods, numerous 34.1% hills and 20.3% the Po plain. The geo- landslides, mud and debris flows. Even if the rate of morphology is strictly tied to its geological structure their occurrence appears to be increasing, these events and may be subdivided into four large regions, are evenly distributed over time. Historical research roughly arranged in concentric crescents. Moving demonstrates, for example, that over the last two along an imaginary line from Mont Blanc to the centuries Piedmont has been hit 101 times by such Langhe Hills, the outer crescent is formed by the large events (one event every 24 months), causing damage mountain chain, then a hilly belt of modest pre-alpine and often claiming victims. Such a distribution of ranges and amphitheatres of the valley mouths, and in events demonstrates not an outright growth in the center the large area of the Po Plain bordered on frequency but rather an expansion of the potential the east by the structures of the Tertiary Piedmontese for involving urban areas. Basin (Fig. 1). Human perception may fail to detect the natural The Alps are an important product of Tertiary evolution of a hydrographic basin because it orogenesis, occupying an area of about 240,000 km2. proceeds by gradual, often imperceptible processes, They constitute an extensive mountain system 800 km but brief violent episodes usually associated with long and 160 km wide that traces a large arc from the extraordinary hydrological events can sometimes Region of Liguria on the Mediterranean Sea and runs change that perspective. These events upset the along the borders between Northern Italy and SE existing balance of conditions in each part of the France and Switzerland eastward to Slovenia. The basin. The evolutionary processes triggered during western Alps rise as mighty massifs which, at some the events show different forms of development and points, soar to over 4000 m (Mont Blanc, 4810 m; have different practical implications related to Mount Rosa, 4633 m; Gran Paradiso, 4061 m). Like F. Luino / Geomorphology 66 (2005) 13–39 15 Fig. 1. Geomorphological regions of northern Italy, including Aosta Valley, Piedmont and Lombardy. all mountain chains, the Alps are formed by great morainic high ground (e.g. Rivoli and Ivrea amphi- volumes of rocks of different aspect, chemical theatres), leaving the impression of a clear contrast composition and genetic significance. Metamorphic between the encircling mountains behind them and rocks are the most representative of the chain, the plain lying, in fact, bat the foot of the mountainQ. followed by sedimentary rocks, while, igneous rocks The morainic belt is bordered by valley mouths and (plutonic and volcanic) are least in subordinate locally includes sectors of the plain, partially volume. Rocks have different mechanical properties occupied by dammed lakes or final stretches of the so that they behave differently during geomorphic great pre-alpine lakes. processes. In Piedmont, for example, about 16% of The plain of northwestern Italy can be divided into landslides have occurred in the calcschistes, while few two areas: the upper plain close to the mountain occur in areas where granites, syenites and diorites slopes (Cuneo, Mondovı` and Saluzzo) and the lower outcrop (Forlati, 1990). plain around Novara and Vercelli towards the East. The Alps are characterized by high crests and steep The Po Plain is a great Tertiary sedimentary basin slopes, with large, deep valleys. This morphology is constituted by a thick blanket of alluvial deposits mainly the product of the Quaternary glaciations. Vast carried by the Po River and its tributaries. In its ice masses moved through the valleys, transforming northern sector, the Po Plain is fed by the Alps, and its them into deep troughs with steep walls; the overflow southern sector by the Apennines. The detrital of ice across the mountain divides shaped the passes. contribution coming from the Alps contains coarse Glacial deposits in the form of moraines dammed the and silty sediments, while that from the Apennines is streams and rivers and produced many lakes. Only mostly clays. Along their course, the rivers of the Po summit regions above 3000 m are glaciated today, Plain differ in their geomorphological characteristics about 2% of the total area (Schmidt, 2004). Peaks and considerably. They flow embanked in alluvial sedi- crests, however, rise above the ice as jagged shapes ments, creating different orders of terraces, and (tooth-like horns, needles, and knife-edged ridges).