Sequence of instability processes triggered by heavy rainfall in the northern Italy

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

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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 In Europe, Italy ranks highest in the variety of natural instability processes: landslides, glacier-related phenomena, floods, earthquakes, subsidence and volcanic eruptions. Throughout the country, these processes claim victims and cause damage amounting 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 processes are high: since 1980, the State has paid 42.4 billion Euros, or about 5.7 million Euros per day. Since 1993, severe hydrological events have struck northern Italy (Piedmont, the Aosta Valley and Lombardy) five times, causing large floods, numerous landslides, mud and debris flows. Even if the rate of their occurrence appears to be increasing, these events are evenly distributed over time. Historical research demonstrates, for example, that over the last two centuries Piedmont has been hit 101 times by such events (one event every 24 months), causing damage and often claiming victims. Such a distribution of events demonstrates not an outright growth in frequency but rather an expansion of the potential for involving urban areas. Human perception may fail to detect the natural evolution of a hydrographic basin because it proceeds by gradual, often imperceptible processes, but brief violent episodes usually associated with extraordinary hydrological events can sometimes change that perspective. These events upset the existing balance of conditions in each part of the basin. The evolutionary processes triggered during the events show different forms of development and have different practical implications related to

morphological and topographical conditions and to particular time intervals. The objective of this paper is to highlight that, during severe hydrological events in northern Italy, it is possible to follow a time evolution of the natural instability processes. This evolution corresponds to increased risk and expected damage.

2. Geology and geomorphology The study area includes Piedmont, the Aosta Valley and Lombardy. Within the total area of 52,512 km 2, 45.6% is mountainous landscape, 34.1% hills and 20.3% the Po plain. The geomorphology is strictly tied to its geological structure and may be subdivided into four large regions, roughly arranged in concentric crescents. Moving along an imaginary line from Mont Blanc to the Langhe Hills, the outer crescent is formed by the large mountain chain, then a hilly belt of modest pre-alpine ranges and amphitheatres of the valley mouths, and in the center the large area of the Po Plain bordered on the east by the structures of the Tertiary Piedmontese Basin (Fig. 1). The Alps are an important product of Tertiary orogenesis, occupying an area of about 240,000 km2. They constitute an extensive mountain system 800 km long and 160 km wide that traces a large arc from the Region of Liguria on the Mediterranean Sea and runs along the borders between Northern Italy and SE France and Switzerland eastward to Slovenia. The western Alps rise as mighty massifs which, at some points, soar to over 4000 m (Mont Blanc, 4810 m; Mount Rosa, 4633 m; Gran Paradiso, 4061 m). Like

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Fig. 1. Geomorphological regions of northern Italy, including Aosta Valley, Piedmont and Lombardy.

all mountain chains, the Alps are formed by great volumes of rocks of different aspect, chemical composition and genetic significance. Metamorphic rocks are the most representative of the chain, followed by sedimentary rocks, while, igneous rocks (plutonic and volcanic) are least in subordinate volume. Rocks have different mechanical properties so that they behave differently during geomorphic processes. In Piedmont, for example, about 16% of landslides have occurred in the calcschistes, while few occur in areas where granites, syenites and diorites outcrop (Forlati, 1990). The Alps are characterized by high crests and steep slopes, with large, deep valleys. This morphology is mainly the product of the Quaternary glaciations. Vast ice masses moved through the valleys, transforming them into deep troughs with steep walls; the overflow of ice across the mountain divides shaped the passes. Glacial deposits in the form of moraines dammed the streams and rivers and produced many lakes. Only summit regions above 3000 m are glaciated today, about 2% of the total area (Schmidt, 2004). Peaks and crests, however, rise above the ice as jagged shapes (tooth-like horns, needles, and knife-edged ridges). The post-glacial evolution of the area appears to be greatly conditioned by instability processes, from phenomena induced by gravity and running water. The transition from mountainous regions to the plain is characterized by a discontinuous belt of

morainic high ground (e.g. Rivoli and Ivrea amphitheatres), leaving the impression of a clear contrast between the encircling mountains behind them and the plain lying, in fact, bat the foot of the mountainQ. The morainic belt is bordered by valley mouths and locally includes sectors of the plain, partially occupied by dammed lakes or final stretches of the great pre-alpine lakes. The plain of northwestern Italy can be divided into two areas: the upper plain close to the mountain slopes (Cuneo, Mondovı` and Saluzzo) and the lower plain around Novara and Vercelli towards the East. The Po Plain is a great Tertiary sedimentary basin constituted by a thick blanket of alluvial deposits carried by the Po River and its tributaries. In its northern sector, the Po Plain is fed by the Alps, and its southern sector by the Apennines. The detrital contribution coming from the Alps contains coarse and silty sediments, while that from the Apennines is mostly clays. Along their course, the rivers of the Po Plain differ in their geomorphological characteristics considerably. They flow embanked in alluvial sediments, creating different orders of terraces, and stretches in the lower plain, where the prevalence of the sedimentary activity gives rise to elevated riverbeds. Another geomorphological area is the hilly sector of southern Piedmont, where there are outcroppings of Cenozoic deposits of the Tertiary Piedmontese

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Fig. 2. Map showing the variation in the mean annual rainfall (MAR) in northwestern Italy based on the 50-year norms (1921–1970) of 501 stations.

Basin, a late-post orogenic episutural basin (Scambelluri et al., 2002). Within this area, D’Atri et al. (2002) have identified three great tectonic-sedimentary domains: the Langhe Basin, the Turin Hills and the Monferrato Hills. The hilly morphology of southern Piedmont is essentially tied to the nature and structure of the bedrock; for example, the particular asymmetry of the valleys (due to the isoclinal bedding of marly-silty and arenaceous-sandy alternances), and sectors characterized by gullies, showing very intense erosional activity.

3. Brief climatic framework of the study area The climate of Piedmont, Aosta Valley and Lombardy is strongly affected by various features of the Alpine and Apennine ranges surrounding the area on three sides. The mountain barrier forms a shield against winds, thus reducing the effects of cold Arctic or North-Atlantic air masses, with mean annual temperatures of around 12–13 8C in the plain (12.7 8C in Turin, 12.9 8C in Milan) which are 2–3 8C higher than in places immediately north of the Alps at approximately the same altitude (e.g. 9.6 8C in Geneva). The western end of the Po plain, which is less affected by maritime influence, shows a wide

temperature range between record high and low temperatures measured. In the past 50 years, the plain south of Turin has experienced temperatures between 25 8C in February 1956 and 41 8C in August 2003. In the Alpine range, the annual mean 0 8C isotherm is at a height of about 2300–2500 m. The orographic influence is markedly noticeable in the distribution of precipitation. Total annual rainfall varies from a minimum of 500 mm in the intraalpine cirque surrounding Aosta, well shielded from moist Atlantic and Mediterranean winds, to over 2500 mm in the mountain area above Lake Maggiore (Fig. 2). Moderate rainfall amounts of about 600 mm annually are typical of a small area in the upper Susa Valley and the southern Piedmont (basin of Alessandria). Other flatland areas receive 700–900 mm on average per year, while the Pre-alpine zones, which are more exposed to condensation of moist Mediterranean winds, receive 1300–1600 mm annually. A good part of this area of Italy has a sublittoral pluviometric regime, with the main pluviometric maximum in spring (April to May) and the minimum in winter (January to February), a pluviometric pattern typical of the Pre-alpine belt. Exceptions to this pattern are the western Aosta Valley and the Apennine zone, where the annual maximum occurs in late autumn, and the intraalpine

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valleys of upper Lombardy, where the rainiest part of the year is during the summer months; here the influence of a continental pluviometric regime typical of the upslope side of the Alps is perceived. Snowfall is irregular at low levels; in the plain, the mean winter snowfall cover is 20–40 cm, while in the Alps the annual amount of fresh snow is 250–300 cm at 1500 m and 600–700 cm at 2500 m, with a great variability due to the type of pluviometric regime and local positions more or less exposed to dominant moist air masses. Snowfalls of up to 100–150 cm in 2–3 days are not uncommon above 1500 m, when between late winter and early spring masses of Mediterranean air occur, particularly in the Alpine valleys near the plain which are more exposed to moist air inflows. At 2000 m, record ground covers of 5–6 m snow were measured in the Ossola basin valleys and in upper Lombardy in February 1951 and on Gran Paradiso in February 1972. Wind currents are highly influenced by the Alpine mountain range shielding the lower areas. Gusts are associated with foehn winds carrying mild and dry air down from the Alps. They are caused by an intensified flow of upper air masses from the west and the north. Wind gusts of over 80–100 km/h also occur on the plain during summer storms. Generally, however, wind movement is characterized by thermal breezes between the plain and the mountains, especially during summer afternoons. Little air motion, on the other hand, is also the cause of fog and accumulation of air pollution in the lower air levels during the winter months, when stationary highpressure conditions over the Alps and northern Italy persist for several consecutive days.

4. Extraordinary hydrological events Since the end of 1960s, observation of the behaviour of northwestern Italian basins during extraordinary hydrological events has shown that the number and type of instability processes triggered by rainfall are not only related to the morphological and geological characteristics of the area where the rain falls, but also to the distribution of intense rainfall during the meteorological event. The critical precipitation threshold can change according to the relationship between the global

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event and the mean annual rainfall (MAR) of the affected area (Cannon and Ellen, 1987; Govi and Sorzana, 1980; Pierson et al., 1991). Once the threshold has been exceeded, precipitation usually triggers a series of effects on the hydrographic network and the slopes. The effects can be attributed to three different phases. In the last 35 years, this sequential type of phenomenon has been observed in northern Italy during these large severe events (Carraro et al., 1970; CNR, 1983; Govi et al., 1979; Govi and Turitto, 1997; Luino, 1998; Tropeano et al., 1999): in Piedmont in 1968, 1977, 1978, 1993, 1994, and 2000; in Lombardy in 1983, 1987 and 1992; in the Aosta Valley in 1993, and 2000. This section analyses three of these severe events: July 1987 in Valtellina (Lombardy), November 1994 in Tanaro Valley (Piedmont) and October 2000 in the Aosta Valley. 4.1. The July 1987 event in Valtellina A severe hydrologic event occurred in the second half of July 1987 in Valtellina (Fig. 3): floods and landslides caused catastrophical effects. Five villages were razed to the ground; roads, bridges, railways were partially or totally destroyed, hundreds of hectares flooded. In all, there were 53 victims and over 2000 million of damage (Govi and Turitto, 1992). On 15 July, critical meteorological conditions began to brew as a vast, low pressure area over the British Isles drew warm southerly winds along its southern edge across northern Italy in a sweep extending over 80 km from Lake Como to the Camonica Valley. Along this front, various orographic features and thermal contrast led to widespread, locally intense rainfall that developed in three consecutive largely similar periods (Brunetti and Moretti, 1987). The first period began as brief showers between 5:00 and 9:00 on 15 July with locally varied total accumulations ranging from 4 to 9 mm. Later that day rainfall ceased for several hours. In the early morning hours of 16 July, about 18–22 h after the rain had stopped, the second period began with rainfall conditions that were similar on the Orobic side and in the entire pre-lake Adda River basin and characterized by brief intensive showers (up to 10 mm/h),

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Fig. 3. Map of Valtellina showing isohyets (mm) of 15–19 July 1987 and the most important place names mentioned in the text.

alternating with lighter rainfall or no precipitation over a period of 5–8 h. These conditions continued for 40–44 h into the next day. More total rainfall was recorded for the southern side of Valtellina (40–55 mm) than in the upper valley around the Bormio cirque (16–25 mm). During this period, no soil slips or mud debris flows were noted either. The third period, which started on the afternoon of 17 July and continued the following day, was marked by steady rains. Starting from south to north, heavy showers began at different times and continued to fall for the next 48–50 h. Between 16:00 and 17:00 on 18 July, after 36 h of rainfall (160 mm) with peaks of 51 mm/h between 15:00 and 16:00, the initial effects of debris flows in the upper Brembana Valley tributaries began to occur. Almost simultaneously many soil slips triggered. Shortly after 17:00, the Brembo Stream in the area around Lenna (basin area, 307 km2) swelled markedly and overflowed its banks, causing intense erosion and violent flooding. Between 17:00 and 18:00 on 18 July, in the Tartano Valley, on the Orobic side of Valtellina, numerous soil slips triggered. At 17:00, one of the largest struck an apartment building and invaded a hotel, killing 10 people (Fig. 4). The phenomena triggered after 85 h of rainfall (total cumulated rainfall of about 243 mm), with 82 mm in

the last 12 h and a relatively intense episode (22.4 mm) in the last hour before the collapse. At 19:00, with a total cumulated rainfall of 259.4 mm, a huge debris flow triggered on the alluvial fan of Madrasco Stream (28.7 km2), where the village of Fusine is located. As rainfall continued throughout the evening, many landslides occurred in the Madrasco Valley after 22:00. Meanwhile, between 20:00 and 21:00, Mallero Stream at Sondrio cross section (area, 315 km2) increased its discharge because of the remarkable amounts of debris its tributaries had been bringing in since 18:00. These conditions developed after 63 h of rainfall (total cumulated, 100 mm), with a peak of 46 mm between 18:00 and 21:00. Just after 21:00, many soil slips triggered in the Torreggio Valley. In the late afternoon hours of 18 July, after 90 h of light rainfall (total, 107 mm; peak, 30.4 mm between 16:00 and 18:00), the first impulsive debris flows triggered between 17:30 and 18:00 along Vallecetta Creek (4.6 km2), along the Mala Valley (2.2 km2) and Presure Valley (5.2 km2) on the left orographic side of the upper Adda valley. In the basins of the right side of the valley, the Pola Valley (area, 1.7 km2) and the Vendrello Valley (2.9 km2), similar torrential events took place between 18:00 and 19:00 after incessant rainfall (total, 117 mm). An estimated 600,000 m3 of

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Fig. 4. Tartano Valley (Lombardy), 18 July 1987. At 17:00, a large soil slip conveyed material into a small hollow incision, cut an apartment building in two (black arrow) and invaded a hotel (white arrow), killing 10 people (photo: Catenacci, 1992).

debris carried by the creek of the Pola Valley spread out into the valley bottom, damming the Adda and creating a basin upstream from the obstruction. Between 19:00 and 23:00 (cumulated rainfall, 127 mm), very similar effects caused by the violent flood of Massaniga Creek (9.7 km2) were surveyed 3 km upstream. At 2:00 the following day, 19 July, the discharge of the Adda flood increased on the main valley bottom, when the waters breached the detritus dam created by the Massaniga debris flow. The Adda waters poured into the fields around S. Antonio Morignone. That morning, between 9:00 and 10:00, slightly later than the instability processes described above, a large landslide (1.5106 m3) triggered on the

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right slope of the Torreggio Stream, a tributary of the Mallero Stream in the central part of Valtellina. The dam blocking the Torreggio 1.5 km upstream from the village of Torre Santa Maria was rapidly ruptured by the water; a huge volume of debris then spread in the Mallero riverbed after having severely damaged a part of the village. The paroxysmal phase of the flood proceeding along the course of the Adda riverbed begun the night of 18 July and continued to about noon of the next day, when the river levels began to drop. Different types of processes took place in relation to the different morphotopographic characteristics of the valley bottom. While erosion, which was intense at certain sites, was prevalent along the first kilometers of the river’s course between Bormio and Tirano, diffuse overflowing accompanied by widespread flooding started at Chiuro, 8 km upstream from Sondrio. Flooding most often occurred at the confluences with the already swollen Adda tributaries. The extent of the areas submerged and the quantity of sediment left by the waterfloods on the ground surface testify to the impact of both the main river and its tributaries. Because of damming of the upper valley, the propagation wave along the entire river course to its mouth at Lake Como (127 km) demonstrated certain discontinuities as it flowed downvalley. The developing times of the effects of the wave in the midlower stretch between Piateda and Fuentes were later reconstructed. The Adda discharge at Ardenno (area, 2096 km2) was just under 500 m3/s between 18:00 and 19:00 on 18 July; meanwhile, the first overflows upstream from the Albosaggia bridge occurred. During the night between 18 and 19 July, after the heaviest rainfall had ceased, the worst episode of the Adda flooding took place. After a levee breached near the village of Berbenno, the entire plain to the right of the river was inundated. Between 23:30 and 24:00 on 18 July, when the discharge of the Ardenno segment was more than 1000 m3/s and the hydrometric level about 1 m below the edge of the levee, the waters violently broke the levee in the Berbenno municipality. The flow was initially contained by the intact levee to the south and the railway embankment of the MilanTirano line to the north. Within this 250-m-wide corridor, the flood current headed rapidly downvalley, covering a distance of about 2 km in 60–90 min. At 1:00 on 19 July, the violence of the water flowing out

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of the 150-m-wide breach destroyed the railway and road embankments 200 m ahead of it. The waters then swept across the entire area of Piana di Selvetta. At 4:15 the waters reached Ardenno at the lower end of the Piana di Selvetta, some 4.5 km away. In the area around Ardenno, the flood wave was held back by the right levee of the Adda and the left levee of the Masino Stream. These obstacles caused the water to rise at a rate of 6 cm every 5 min and to back up towards the site of the levee breach. At around 10:00 on 19 July, about 5 h later, the backup stretched 4 km upstream. The water level on the Piana di Selvetta continued to rise throughout the morning, submerging an area of about 10 km2 on the valley bottom, with record levels just over 4 m in low lying areas. At 12:00 on 19 July, some inhabitants of Ardenno destroyed the levee blocking the downvalley flow direction of water into the Adda riverbed. The discharge emptied through an opening (6 m wide, 2 m deep), allowing the water levels on the Piana to decrease gradually (2–2.5 m in 30 h). Five days later, the floodwaters has almost completely receded, leaving behind a thick layer of mainly clayey-sandy deposits measuring from 40 cm to 1 m thick in the low lying areas near the levee opening. Because of the breach in Piana di Selvetta and the breach downvalley in the area of Talamona, the discharge was considerably reduced, with less serious damage to the area around Talamona and to areas further downstream where the Adda waters, although they overflowed the riverbanks, were held back by the main levee that runs its final 15 km along the Adda riverbed. The relative peak discharge was recorded 18 km downstream from Ardenno at 6:00 on 19 July. The Fuentes gauge (2498 km2) a level equal to a discharge of 1100 m3/s was observed. On 25 July, several days after the critical period had passed, a new state of emergency took place when a discontinuous breach was sighted on the eastern slope of Mount Zandila. The breach ran 60 m along the scarp foot line at an altitude of 2200 m, coinciding with the sliding surface of the old landslide. In this area, the total cumulated rainfall was 229 mm, with 124 mm of cumulated rainfall measured on 18 July alone. From 26 to 27 July, the breach widened to 900 m, forming a crescent-shaped opening. On 27 July, several rockfalls on the eastern slope triggered 98 falls in only 24 h (Govi and

Turitto, 1992). The inhabitants of the villages of Morigone, San Antonio, Poz and Tirindre` were quickly evacuated. At 7:24 on 28 July, a wide mass of rock (estimated 34 million m3) detached from the eastern slope of Mount Zandila (Costa, 1991; Govi and Turitto, 1992). The displaced mass, including the prehistorical slide and the bedrock, moved in two short phases. The first came down with a northward slide of the upper part of the slope; the second, in a single rapid displacement, spread eastward into the Adda valley bottom, sweeping the village of Morignone away (Fig. 5). The mass roared up the opposite slope of the valley to about 300 m above the valley floor before splitting into two parts, diverted upstream and downstream. The downstream mass travelled almost 1400 m from the impact point. The first plunged into the small lake, shooting alluvial debris and muddy water 140 m high. The impact unleashed a high wave that moved quickly upstream. Eyewitnesses reported that the wave travelled 1000 m in about 30 s (Govi and Turitto, 1992). The mud marks surveyed at a maximum height of 95 m near the source decreased to 15 m northward at a distance of about 1300 m. The villages of Poz, San Antonio and Tirindre` were razed to the ground. In the partly evacuated village of Aquilone more than 2 km upstream 27 people perished. Just before the wave impact, survivors saw the bell tower of the San Antonio church shatter from the violent blast, which also blew down trees on the opposite slope over 300 m away. On the opposite side of the valley and upstream to Massaniga Creek, a dark dust cloud extending up to 2 km a.s.l. was seen briefly before it disappeared about 20 s later (Azzoni et al., 1992). No seismic activity was recorded before the collapse; the seismogram indicated that the detachment of the mass occurred in 18 s and the fall in 23 s. 4.2. The November 1994 event in the Tanaro River basin On November 1994, a severe hydrological event hit the Tanaro River basin (Fig. 6). Landslides and large floods caused widespread damage to 38 urbanized areas. The effects were catastrophic: 44 victims, 2000 homeless, over 10 billion Euros of damage in all.

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Fig. 5. The Mount Zandila rock avalanche took place on 28 July 1987, 10 days after the rainfall had stopped. The mass movement totally covered the valley bottom with an estimated volume of about 34 million m3, more than 2 km long. The average thickness of the accumulation was about 30 to 60 m, with a maximum of 90 m. Within several days, the continuous inflow of water upstream from the huge accumulation formed a lake (arrow); 30 days later, after another intense rainstorm, the basin filled to about 20 million m3.

During the first week of November 1994, a vast low-pressure system over northwestern Europe brought heavy rains to most of Piedmont (Mercalli et al., 1995). The rains started on 2 November and continued through the next day, with showers that peaked in the Ligurian Alps (50 mm). Heavy rain began to fall over nearly the entire area on 4 November, with intermittent showers that posed no cause for alarm. However, the next day violent rainfall developed and continued throughout 6 November, particularly along the pre-alpine belt. On 4 and 5 November, over 200 mm of rain were recorded in the upper and middle parts of the valley and in the upper stretches of the Tanaro tributaries: the Belbo, Bormida and Orba rivers. Precipitation reached a maximum hourly intensity of 55 mm (Cairo M. station) and a total cumulated rainfall of 264.4 mm in 24 h (Levice station). The amounts of rainfall recorded at some Tanaro basin rain gauge stations in the provinces of Cuneo and Asti were particularly high. Previous rainfall records were broken in 4 of the 42 stations in 1 day and in 5 stations in 2 days. The first phase of the event (50–60 h between 2 and 4 November) was characterized by modest, widely distributed or inter-

mittent rainfall that varied locally from 30 to 60 mm in places. During this phase, no landslides or mud– debris flows were reported. The second phase developed locally at various times between 4 and 6 November, with intensive rains lasting 24 h and varied total precipitation (from 150 mm to about 260 mm). This constituted the critical phase of the event as it swept through the entire upper Tanaro basin and the area between Alba and Asti. During this phase (136 mm total rainfall in 10 h, with peaks of 109 mm recorded between 2:00 and 5:00 on 5 November), very fast soil slips of the fluidified topsoil (mean thickness b1 m) occurred in the upper part of the Bormida di Spigno river. Similar instability processes triggered 1–3 h later just north of Ceva (with peaks of 90.6 mm recorded between 3:00 and 8:00). Meanwhile (morning of 5 November), the first torrential floods triggered in the secondary hydrographic network, producing local floodings along the upper valley courses of the Bormida and Tanaro rivers. At 8:30, local torrential flooding triggered in the small tributaries of the Tanaro (Armella and Pesino Creeks at Ormea, areas of less than 20 km2), while

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Fig. 6. Map of Tanaro basin showing isohyets (mm) of 5–6 November, towns and rivers mentioned in the text.

further downvalley, along the Cevetta Stream (area, 62 km2), the first flood wave was generated at 10:00, fed by an episode of increased rain intensity (120 mm recorded between 3:00 and 10:00 of the morning of 5 November). Over the next hours of the late afternoon, the rain front moved NNW into the entire area of the Langhe towards West, where widespread soil slips triggered in the saturated superficial cover. Here the shallow landslides occurred more often between 10:00 and 12:00 (10– 12 h of uninterrupted rainfall, with peak totals between 80 mm around Alba and 110 mm around Dogliani) (Fig. 7). As the rainfall continued into the late evening, the number of soil slips increased throughout the area up to 100 soil slips per km2 were recorded in one area alone (Luino, 1999). During the afternoon and into the late evening, somewhat later than the soil slips, many rock block slides were triggered in the marly-silty and arena-

ceous-sandy alternances (range of thickness, 5–30 m). The first local rock block slides occurred between 12:20 and 18:00, with a major frequency between 18:00 and 23:00. Peak cumulated rainfall varied locally from a minimum of 200 mm to just over 300 mm in some places. These rainfall amounts were cumulated, although with certain brief interruptions, over a time period of 70–80 h, starting from the beginning of the first phase of the event (afternoon of 2 November). In several cases, rock block slides were also recorded during the morning of 6 November, after the rainfall event had begun to subside (Fig. 8). The paroxysmal phase occurred between 5 and 6 November, with large-scale flooding along the upper and middle basins of the Tanaro from Ormea to Alba, nearly simultaneously with episodes of peak rainfall intensity, whereas the lower river basin areas (Asti and Alessandria) were to feel the effects of this phase slightly later. Since the violence of the river

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side, with peak spreads of flooding and phases that led to the destruction of important structures and infrastructures along the river. A general description of the downvalley translation of the flood can be summarized as follows:

Fig. 7. Cerretto Langhe (Langhe Hills). Coalescence of soil slips on a concave slope in November 1994. It is interesting to note the position of the old farmhouses on the ridges of the slope. The small road was buried, but the houses were spared, probably because the village elders knew where to build.

floodwaters destroyed the hydrographs installed along the Tanaro and swept away the staff gauges on several bridges, it was not possible to collect data on peak water levels or their chronology along the river’s course. The discharge was estimated by indirect reconstruction analysis of the marks the floodwaters left on the embankment terraces or structures. The flood dynamics were also reconstructed from eyewitness accounts of the local population. These data provided valuable information about the passage of the flood wave as it moved downstream through towns and villages. The information also permitted the construction of a time line of events and phenomena such as overflow processes and flood propagation into the surrounding country-

– in the upper Tanaro basin, up to the town of Ceva, the first floodings occurred in the late morning of 5 November and reached the paroxysmal phase during the late afternoon–early evening hours the same day, with peaks between 18:00 at Garessio and at 20:00 at Ceva. In both cases, evaluation of the correspondence between the observed water levels and the peak flood phase was influenced by the effects of superelevation of the water levels and formation of backwater due to obstruction by bridges located in both towns and by accumulation of detritus and tree trunks (Fig. 9); – in the middle stretch of the river course (from Ceva to Alba), the floodwaters started to overflow the riverbanks during the early afternoon hours of 5 November, creating more violent phenomena after 21:00 (Niella Tanaro) and about 24:00 (Alba). Pulsations in rising water levels occurred, with local peaks sometimes earlier here than in stretches further upstream or downstream. Generally, a rapid retreat of floodwaters, often in 2–3 h, was observed; – along the lower stretch of the Tanaro (areas around Asti and Alessandria), the flood reached its peak on 6 November. The first severe floodings (observed at 2:00 at Asti and at 11:00 at Alessandria) reached their peak levels in the two towns (Luino et al., 1996) within 2 h and began to subside over 10 h later; – between 6 and 7 November, the abundant inflows coming from the Tanaro and its tributaries caused the water levels of the Po to rise rapidly. At the Becca station, the closing point of the entire western hydrographic network, a peak level of 7.65 m over hydrometric zero was measured at 11:00 on 7 November, a mere 20 cm below the record high of 1951, with a rise of 2.65 m in less than 20 h. According to eyewitness accounts, in many towns the flood did not invade the area in a single peak wave but rather in a series of waves. However, the reasons

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Fig. 8. Rock-block slide on a slope near Murazzano. The big rocks moved about 80 m along a sliding surface (11–128). At the end of the movement, the surface appeared smooth like an inclined plane, sometimes showing the shallow tracks left by the sliding rock block.

for such rises and falls cannot be completely explained even when taking into account phase differences of the waters brought by the main tributaries of the Tanaro, as in the case of Corsaglia Stream (area, 307 km2) whose flood flowed into the Tanaro slightly before the Tanaro water levels peaked due to the large size of the Tanaro basin (area, 503 km2) at the confluence of the two watercourses. What emerged from surveys carried out during the event and other information sources, particularly in the stretch between Ceva and Alba, was evidence of the widespread effect of partial or complete obstruction of the flow back into the riverbed due to road and railway structures (bridges, embankments and approaches) and by damming due to the huge amounts of floating materials (bushes, trees and various other types of materials) blocked between buildings. These obstructions impeded the water from flowing back into river courses and led to the rise in backups and overflows upstream from bridges, often causing them to be washed out or completely destroyed (Turitto et al., 1995). The direct effect of these processes was the generation of flood waves, as reported by eyewitnesses, directly connected to the repeated invasion and retreat of the backed up floodwaters. This type of situation occurred between 18:00 and 19:00 on 5 November at the provincial road bridge near the town

of Bastia M., with repercussions 7–8 km downstream, exacerbating the pre-existing flood effects of obstruction caused by a barrage near Clavesana. In this stretch of wide meanders between the towns of Clavesana and Carru`, comprising about 2.6 km where the Tanaro is spanned by two barrages and three bridges, eyewitnesses reported that between 13:00 and 22:30 at least three flood waves had occurred. Slightly further downstream, in the area around Farigliano, a similar situation occurred that was characterized by transient rapid rises and falls in water levels, especially between 18:00 and 23:00, along this 7-km stretch of meanders, where the river is spanned by seven roadway bridges and three railway bridges. Further downstream, the events can be summarized as follows: – in the area of Lequio Tanaro, between 22:00 and 23:00 on 5 November, the left bridge girder of the first railway bridge was destroyed; – in the area of Monchiero, a flood peak was reported upstream from the approach embankment of the provincial road bridge leading into the town at about 21:00, just before a wide opening was torn into the embankment; – the effects of the unleashed backup floodwaters were felt about 4 km downstream in the town of Narzole, where a flood wave was observed just

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Fig. 9. Ceva, 6 November 1994. Floating materials blocked the bridge span; the floodwaters overtopped the structure and levees upstream, invading a large urban area.

after 22:00 at the road/railway bridge. Obstructed by the bridge, the floodwaters backed up, temporarily invading the valley bottom and spreading over about 90 ha; the water level remained high until the left embankment of the bridge collapsed between 22:30 and 23:00; – another backup developed (approximately 120 ha of the valley bottom) around the structures crossing the Tanaro at Pollenzo. Here, between 23:30 and 24:00, floodwater accumulated behind the bridge approach on the right riverbank, rising about 4 m high from ground level of the low-lying area. At about 1:00 on 6 November, the structure was destroyed and the floodwaters spread 2500 m into the right riverbed, where local morphotopographic features forced the water back into the Tanaro riverbed, causing erosion along the left bank, which was already submerged by the runoff coming out of drainage canals; – in the area around Alba, 10 km downstream, the peak water level along the Tanaro was observed between 24:00 of 5 November and 1:00 of 6 November. This event occurred slightly earlier than that at the Pollenzo bridge, and therefore has no relationship with it. The city of Alba and the surrounding area were invaded by floodwaters (Luino and Turitto, 1998) from the Talloria and Cherasca streams on 5 November several hours

before the flood wave generated along the Tanaro, as reconstructed from evidence collected at Pollenzo and Narzole. 4.3. The October 2000 event in the Aosta Valley In October 2000, a severe hydrometeorological event hit a large part of the Aosta Valley and the basin of Dora Baltea River: the main watercourse rises in the massif of Mont Blanc and after crossing the Aosta Valley flows into the Po River after 160 km (Fig. 10). The event started on 12 October, when a cold front, associated with a wide, low depression over the British Isles, reached the western Alpine rim, drawing currents of moist unstable southwesterly air into the Aosta Valley and bringing light rain to the areas neighboring the region of Piedmont in the early afternoon. During 13 October as the inflow of southerly air currents into the Aosta Valley intensified, the rainfall became widespread and heavier (Mercalli and Cat Berro, 2001). Rising temperatures from sirocco winds raised the freezing level from 2400 to 3000 m within a few hours. Such factors, together with intense rainfall at high altitudes, melted the snow that had fallen in late September. Champorcher Valley was the first area to receive intense precipitation. On 13 October, 176 mm was recorded (peak of 23 mm/h between 17:00 and 18:00)

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Fig. 10. Map of the hydrological event that occurred in the Aosta Valley showing isohyets (mm) of 11–16 October 2000.

at the Champorcher rain gauge station. Rainfalls triggered soil slips in several areas of the valley, severely damaging roads and houses. Near Champorcher, the Ayasse Stream (subtended area, 63.8 km2) rose 77 cm in 7 h and peaked at 23:00. The flood completely washed out many sections of the main road along the valley floor and a tourist recreation area (already damaged in 1994), and left a thick deposit of mud and sand on the local sports grounds. The storm then moved westwards into the Cogne Valley, where 83.8 mm of rainfall was recorded, with peaks of 9 mm/h. In the stretch between Lillaz and Champlong, a rock-block slide in glacial deposits (more than 100,000 m3) triggered on the left slope of the Urtier Stream (Bonetto and Mortara, 2003). The displaced mass moved on a gentle slope for some hundreds of meters, and then formed a temporary dam in the stream. Unlike the Champorcher Valley, the Cogne Valley witnessed no shallow landslides at this time. In the others valleys, record daily rainfall amounts of 20–40 mm were measured, with peaks of 8 mm/h. Several hours later than its tributaries, the Dora Baltea River rose 0.45 m in 1 h (22:00–23:00). On 14 October, rainfall grew heavier: 179.2 mm at Cogne (peak, 16.4 mm/h), 149.4 mm at Cham-

porcher (13.8 mm/h) and 116.2 mm at Valsavarenche (11.2 mm/h) were recorded. During the night, the temperature increased notably, reaching a maximum of 20.6 8C in Aosta (565 m a.s.l.) and 9.7 8C in Cogne (1495 m a.s.l.). The Dora Baltea began to swell. At the Hone section, the hydrometric level rose from 4.91 to 5.90 m between 06:00 and 18:00. Near Cogne, the first soil slips triggered at 18:00, blocking roadways and hindering traffic in the area. The Civil Defence closed many roads and bridges considered to be dangerous. In the night between 14 and 15 October, rainfalls gradually intensified, particularly around Cogne and Champorcher (1400 m). The maximum hourly rainfall amounts were 15.8 mm at Cogne (24:00–01:00) and 37 mm at Champorcher (02:00–03:00). All the righthand tributaries of the Dora Baltea reached high levels, causing general alarm among the local inhabitants. Early the next morning, the peak phase of the event took place. In 5 h, between 04:00 and 09:00, many soil slips and mud–debris flows triggered along the slopes and in the basins, followed by flooding of the tributaries and widespread inundation on the valley bottom of the Dora Baltea.

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In the Cogne Valley, a soil slip near Epinel at 04:00 razed some houses. At 4:30, the inhabitants of two small villages near Valpelline were woken by the boom of a debris flow along Brison Creek (5.13 km2). The mass movement buried the municipal road, a square and the main road of the valley. At the same time in the Cogne Valley, a debris flow of Arpisson Creek (6.4 km2) struck the village of Epinel, levelling all the houses adjacent to the river. Many were invaded by mud and debris and some were completely destroyed. At 5:00, in two small villages of Gressoney Saint-Jean municipality, the Lys Stream floods undermined the foundation of an apartment building, causing it to collapse but without claiming victims, while a violent flood of a Lys tributary killed several animals and damaged a farmhouse. At 6.15 in the Lys Valley, near Issime, a rockfall in the basin of Rickurt Creek (2.3 km2) augmented a debris flow that spread onto the alluvial fan, causing damage. Displaced materials damming the Grand’Eyvia Stream near Cogne (60.6 km2) caused a backup of floodwater (2.58 m) that peaked at 7:00. At 7:30, slightly downstream from Valpelline, the Buthier Stream overflowed, washing out the regional road. The swollen waters headed towards the city of Aosta. At Nus, on the left bank of the Dora Baltea, local eyewitnesses reported that since the early morning

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hours the level of the S. Barthe´lemy Stream (82.2 km2) had begun to rise dramatically due to the detritus and tree trunks obstructing the Mazod Bridge. In the S. Barthe´lemy basin, tens of soil slips and debris flows had started, associated with deep lateral erosion of the main watercourse. At 8:00, a violent debris flow of the S. Barthe´lemy Stream burst across the Nus alluvial fan, destroying buildings by the force of huge masses of detritus (Fig. 11) deriving from the hollowing of the alluvial fan body on the right side. The flow lasted for several hours and left a deposit of an estimated 200,000 m3 of detritus on the alluvial fan. Along both sides of the main valley between Aosta and Montjovet, many soil slips detached deep sections of the topsoil at various elevations. Around 8.30 a boom shook the village of Perron di Fenis. According to eyewitness accounts, 10–15 s later a debris flow of Bioley Creek (4.7 km2) invaded several houses with several tens of thousands of cubic meters, causing severe damage and claiming six lives. Not only were newly built or restructured houses hit by the mass, but also a 17th century chapel which in its entire history may never have testified to the likes of such an event (Tropeano et al., 2003). At Pollein, near Aosta, at 9:00 a sudden mud– debris flow in the Comboe´ basin (16.2 km2) smashed into buildings and gutted houses; seven lives were

Fig. 11. During the 2000 event in the Aosta Valley, the Saint Barthe´lemy Stream hit Nus village, spreading at least 350,000 m3 of mainly coarse debris and sediments over an area of 0.45 km2 on the alluvial fan. The photograph shows the violence of the flow that destroyed and buried many houses: 1288 people were temporarily evacuated. The arrow indicates the house, at the apex of the alluvial fan that caused the deflection of the flow from the ordinary channel.

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lost. An estimated volume of 150,000 m3 was left on the alluvial fan (Tropeano et al., 2000). At the same time, Buthier Stream floods reached Aosta (area, 456.5 km2), where the stream level rose 1 m in 50 min. Because of its exceptional discharge (N500 m3/s) (courtesy of L. Marchi), at 9:30, as waterfloods overflowed the stream banks and inundated the Dora quarter, 350 persons were quickly evacuated (one victim) and vast areas were flooded, leaving a remarkably thick deposit of mud and sand. Between 11:00 and 11:30, the Dora Baltea began to flood villages. At Donnaz, the river rapidly flooded the old section of the village (one victim). The level of the Dora Baltea continued to rise for several hours. At the Hoˆne gorge, it reached a maximum level of 8.73 m on the hydrometric scale at 14:30. Some stretches of the Turin-Aosta highway, the main communication route through the Aosta Valley were washed out, even though the embankment rises 2–3 m on the flood plain. By the afternoon of 15 October, the first rescue operations had reached the disaster area. Most roads were interrupted and the valley bottom of the Dora Baltea was covered by a vast sheet of water. Arriving with considerable delay, a violent debris flow occurred in Letze Creek (area, 1.02 km2) at 22:15 that night. Several houses of the Bosmatto village (Gressoney Saint-Jean municipality) on the

alluvial fan were completely razed to the ground (Chiarle and Mortara, 2000). Compared with the timing of the other debris flows in the area, the time lapse (13–14 h) here was probably due to a temporary dam caused by the reactivation of an old landslide on the right slope of Letze Creek (Fig. 12). The rainfall gradually let up over the later half of 15 October, diminishing to between 1 and 6 mm/h. In the night between 15 and 16 October, flood phenomena subsided, ending in the afternoon of 16 October. In the time period between 19:00 of 12 October and 19:00 of 16 October, maximum rainfall amounts were recorded at Champorcher (612.2 mm), Cogne (456 mm), Valsavarenche (311.8 mm), Gressoney (308.1 mm) and Aosta (262 mm). In these areas, the rainfalls equalled from about 35–50% up to 65% (Cogne) of MAR. The soil slips were mostly concentrated along the middle part of the main valley. This concentration may be attributable to the geolithological features of the sector, which is characterized by a broad surface cover deriving from an extremely tectonized and dislocated bedrock. Shallow landslides also occurred in the Rheˆ mes, Cogne, Ayas and Lys valleys. Reactivation of at least five large landslides (Pollein, Vollein, Chervaz, St. Rhe´my-en-Bosses, Closellinaz) were later recorded. These landslides (from several

Fig. 12. 15 October 2000. Letze alluvial fan: a violent debris flow razed to the ground one of the two twin apartment buildings (asterisk) of the Bosmatto village (near Gressoney). The debris flow submerged everything under 2–3 m of material; in front of the flow some rock blocks more than 10 m3 in volume were observed.

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tens of thousands to some millions of cubic meters) did not collapse; however, they caused morphological effects, with serious implications for public safety (Bonetto and Mortara, 2003). Because Grand’Eyvia basin was probably the watershed that influenced more the Dora Baltea discharge, we can consider the downvalley translation of the flood wave in the reach Cogne-Hoˆne. Thanks to the hydrographical network of Regione Autonoma Valle d’Aosta, a general description can be summarized as follows: – in the Cogne Valley, a classical alpine valley characterized by notable sways and many gorges, in the reach between Cogne and Aymavilles (mean channel slope of 4.4%), the Grand’Eyvia Stream covered 20 km in 1 h (6.7 m/s). The violence of the flow eroded long reaches of banks, producing severe damage to the main road running on the valley bottom. – along the Dora Baltea riverbed, the flood wave moved at different speeds depending on the morphology of the valley bottom. In the reach between Aymavilles and Brissogne (mean channel slope of 0.56%), the Dora Baltea floods overflowed the banks only in some stretches. This sector is characterized by a well-incised riverbed, with some islands and protected banks. The floods flowed along 15 km in 60 min (4.2 m/s). In this reach, the contribution of two tributaries was

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relevant: (a) from the right slope, the Grand’Eyvia Stream; (b) from the left slope, the Buthier Stream (more than 500 m3/s), which invaded part of the city of Aosta and the nearby the steel plant industrial zone. – in the reach Brissogne-Champdepraz (mean channel slope of 0.56%), the waters covered 28 km in 3 h 10 min (2.5 m/s). The Dora Baltea valley bottom here is influenced by the presence of wide alluvial fans on both flanks; in this reach the Dora Baltea riverbed narrows from a maximum width of 90 to 15 m (near Montjovet) where there are deep gorges. Also in this reach, the Dora Baltea floods did not spread on the flood plain, except in small areas. – in the reach Champdepraz-Hoˆne (mean channel slope of 0.25%), the valley bottom is wide and flat. The Dora Baltea spread out onto the flood plain, which was almost totally inundated in some stretches. For this reason, the flood wave reduced its speed to 1.1 m/s, covering 10 km in 2 h 30 min. In this reach, the valley bottom is irregularly urbanized. The houses near the riverbed (Verre`s, Arnad, Hoˆ ne) were completely flooded. The buildings on the other side of the highway embankment were also overflowed (Fig. 13). – in all, on the main valley bottom, from Aymavilles to Hone (mean channel slope of 0.5%), the Dora Baltea floods moved along 53 km in 6 h 40 min (2.2 m/s).

Fig. 13. Dora Baltea valley bottom near Hoˆne. Large sandy deposits delimited the flooded area: the asterisk shows the highway Torino-Aosta overflowed by the Dora Baltea waters on 15 October 2000.

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– in the Aosta Valley the Dora Baltea discharge was not measured. In Piedmont, at Tavagnasco station (3313 km2), the peak discharge was indirectly evaluated about 3100 m3/s (Barbero et al., 2003), exceeding the previous maximum of 1920 (2670 m3/s). – the Dora Baltea flood wave continuing downstream caused heavy losses in different municipalities: bridges, earthen approaches encroaching the flood plain and river works were destroyed.

valley bottom, the Dora Baltea River inundated an area of about 6.7 km2. The natural processes claimed 17 casualties and provoked damage to structures and infrastructures estimated at over 500 million Euros (Ratto et al., 2003). Considering the area involved, typology and intensity of phenomena, and damage, we must go back to 1846, October, to find a comparable case in the Dora Baltea basin: therefore, we can consider the 2000 event on a secular scale.

The effects of the October 2000 event, which severely affected about 60% of the Aosta Valley, were particularly disastrous due to the concurrence of the following factors:

5. Results and discussion

– the heavy rainfalls in the period from 28 September through 1 October, with more than 200 mm in the lower Aosta Valley. Some authors (Mercalli and Cat Berro, 2001) have reported that these precipitations, in addition to the partial snow melting over the following days, might have kept the soils and the underground hydrographic network saturated, leading to a subsequent increase in the instability processes that occurred 2 weeks later. – the wideness of the drainage basin involved due to the presence of a high freezing level (3000 m); – the significant hourly increases of hydrometric levels due to a short concentration time of the tributaries caused by local regional morphology, which is characterized by steep and relatively short valleys (the average elevation of the Aosta Valley is about 2100 m a.s.l., with 20% under 1500 m a.s.l); – the numerous mud–debris flows in the tributaries, sometimes due to the collapse of landslides in the middle-upper part of the basin, with subsequent temporary damming and relative rapid outflow when the displaced mass was demolished. Mud– debris flows produced deep bank erosions, obstruction or destruction of bridges and huge spreading on the alluvial fans, with severe damage and loss of lives in the villages. During the event, 385 landslides were triggered on the slopes and 259 debris flows occurred along the tributaries, flooding a total area of 5 km2. On the

The events just described occurred in 1987 (Valtellina), 1994 (Tanaro Valley) and 2000 (Aosta Valley), together with other severe hydrogeological events of the last 35 years, offered an opportunity to identify different kinds of processes induced by rainfalls and to determine their development sequences. These events have allowed us to identify a critical threshold, which is about 10% of the local MAR. Once the threshold has been exceeded, the instability processes on the slopes and along the hydrographic networks follow a sequence that can be reconstructed in three different phases (Fig. 14). 5.1. The first phase A hydrological event, particularly in autumn and spring, usually starts with a period of light rainfall of

Fig. 14. Sequence of natural processes in northern Italy. Straight lines show the first emergence of each process during extraordinary hydrological events. Dashed lines mark the possible evolution of the process.

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some millimeters per hour. When this cumulative rainfall reaches the critical threshold above mentioned, the hydrological event begins. As the water is no longer able to seep into the ground surface, it runs off the slopes following natural or artificial drainage paths (e.g. valley bottoms, hollows, roads). First processes are usually soil slips (sensu Campbell, 1975), involving the saturated topsoil. The slip surface forms along the irregular contact between the colluvium and the altered bedrock. Such movements usually occur on slopes ranging from 168 to 458, involving a slope cover from 0.4 to 1 m in depth. So they are moderate in volume, ranging from a few cubic meters to several tens of cubic meters. Yet despite their size, they start to produce problems: displaced material can easily block roads and create difficulties for drivers, but above all they impede the work of rescue teams (Fig. 15). In continuous precipitation, the soil slip volumes may be quite significant and may have a considerable area density (Luino, 1999; Polloni et al., 1996). They are usually characterized by liquefied masses that travel long distances (Govi et al., 1985). Common underestimation of soil slips, deriving from the scarcity of historical records and morphological evidences, is due to the relatively low magnitude of single events. The effects produced by these shallow landslides are usually rapid, but the huge shock of the

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mass added to event unexpectedness can also cause severe damage (Luino et al., 2003). While their movement starts as a shallow landslide, they can sometimes evolve into a fast flow, particularly when conveyed in small creeks or slope cuts. Although small in mass, the flows are very dangerous because they occur suddenly and travel at velocities of 2 to 9 m/s (Govi et al., 1985), producing high collision forces. The most significative hourly intensities triggering numerous soil slips are those recorded in the last hours just before the collapse. High hourly intensities compensate for insufficient critical values of cumulated rainfall or vice versa (Govi et al., 1985). During the July 1987 event, the first soil slips in the Torreggio Valley were triggered when cumulative rainfall reached 9.9% of the MAR (128.8 mm/1300 mm), while this value was 10.7% in the Brembana Valley (160 mm/1500 mm) and rose for the landslides in the Tartano Valley (15.2% of the MAR) (Fig. 16a). During the 1994 event in the Tanaro Valley, the first shallow landslides occurred when, in different areas, total rainfall reached 11% (Rodello), 12.3% (Ceva), 14.4% (Cossano) and 18.2% (Cairo M.), respectively. The landslides triggered only 2–4 h after reaching the critical threshold of 10% of the MAR (Fig. 16b). In the 2000 event in the Aosta Valley, the first superficial landslides occurred when cumulative rainfall reached

Fig. 15. Ceva (Tanaro Valley) on 5 November 1994. A small soil slip invaded the road. The mass was triggered on the flank of a concrete retaining wall probably built to avoid just this kind of phenomenon.

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Fig. 16. Cumulative rainfall of the hydrological event: (a) Valtellina 1987 (Ta=Tartano, B=Brembana, To=Torreggio); (b) Tanaro Valley 1994 (R=Rodello, Ce=Ceva, Ca=Cairo M., Co=Cossano); (c) Aosta Valley 2000 (Ch=Champorcher, Co=Cogne). White asterisks show the 10% threshold of the local MAR for each rain gauge; black arrows indicate the triggering moment of the first soil slips in the vicinity.

12.2% (Champorcher) and 16% (Cogne) of the MAR, 2 and 12 h, respectively, after reaching the critical threshold (Fig. 16c). In the first phase, violent mud–debris flows can also be observed in small alpine watersheds of less than 20 km2 (Fig. 17). Particularly in autumn and spring, they usually develop when, after some hours of light rainfall (3–6 mm/h), a violent shower occurs (N30 mm/h). Mud–debris flows can start as a result of slope-related factors, and shallow landslides can dam

Fig. 17. Pollein (Aosta Valley). The destroyed house testifies to the devastating effects of the Comboe` debris flow over the urbanized area of Chenaux village in the early morning of 15 October 2000.

streambeds, provoking temporary water blockage. As the impoundments fail, a bdomino effectQ may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid–liquid mixture can reach densities of up to 1.8–2 tons/m3 and velocities of up to 13–14 m/s (Arattano, 2003; Chiarle and Luino, 1998; Tropeano et al., 1996). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic meters to hundreds of cubic meters), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derives from a common underestimation of mud–debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin (1.76 km2 in area) affected by a debris flow, Chiarle and Luino (1998) estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1) was 19 m3/s, a value about 40 times lower than that calculated for the debris flow that occurred. During the July 1987 event, the first mud–debris flows occurred in small alpine watersheds of the upper Brembana Valley when cumulative rainfall reached 10.7% of the MAR, with a peak of 51 mm/h in the last hour before the flows. Near Bormio, in several small

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basins the first debris flows were triggered when total rainfall reached 11.9% of the MAR (107 mm/900 mm). During the 1994 event in Tanaro Valley, the first debris flow occurred near Ormea in the Armella Creek (area, 17.5 km2), after 45 h of light rainfall (138 mm), 4 h after reaching the critical threshold. In October 2000, in the Aosta Valley, the first mud–debris flows were triggered in Valpelline (Brison basin) at 13.1% of the MAR, while in Cogne Valley (Arpisson Creek) the processes occurred when the value reached 23% of the MAR. In the first phase, discharge increases substantially in larger stream basins of up to 500 km2, as a consequence of the mean rainfall fallen on a basin. Riverbanks are severely eroded and streams begin to threaten riverside structures and infrastructures (Fig. 18). The flow contains a remarkable volume of debris and floating materials coming from the small tributaries. The water can breach the banks in places where they are particularly weak and it can invade the zones near the riverbed. This often happens, for example, along unprotected concave riversides or in the reaches upstream from bridges or other river-crossing infrastructures, sometimes owing to hundreds of uprooted trees that obstruct part of the bridge span. This violent flow may demolish bridges and road embankments by side erosion. Usually, the floodwaters return to the riverbed within 5 to 10 h.

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In July 1987, the Brembo Stream near Lenna (307 km2 in area) reached its first critical stage when the mean rainfall on the basin, calculated by isohyetal method (Wisler and Brater, 1959), was about 11% of the local MAR. In November 1994, in the Tanaro Valley, along the Cevetta Stream (area, 62 km2), the first flood wave with erosion was generated at 10:00, when the average precipitation on the basin was about 16.8% of the basin MAR (160/950 mm). In October 2000, the Ayasse Stream near Champorcher (area, 63.8 km2) overflowed its banks when the mean precipitation was about 12.9% of the local MAR, while the Buthier Stream inundated the town of Aosta (area, 456.5 km2) after 57 h of light rainfall, when the value reached 14% of the basin MAR (140/1000 mm). 5.2. The second phase In continuous precipitation, during the second phase, some violent flow phenomena can be observed in alpine tributary basins larger than 20 km2 in area (Govi et al., 1998; Tropeano et al., 2000). Processes usually comprise hyperconcentrated flows (see Fig. 10) that can also convey large boulders. Measured data have demonstrated a good relationship between basin area and debris-flow magnitude; for the largest watersheds the deposited mass can reach volumes of hundreds of thousands of cubic meters (Marchi and

Fig. 18. Trino (near Gressoney-Aosta Valley), 24 September 1993. The Lys waters destroyed a house and the main road located on the right side of the stream.

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D’Agostino, 2004). Villages and infrastructure located on alluvial fans may be partially or totally filled up by the debris (ARPA Piemonte, 2003; Eisbacher and Clague, 1984; Chiarle and Luino, 1998; Govi et al., 1979; Govi, 1984; Luino, 1998; Regione Piemonte, 1998; Tropeano et al., 1999; Tropeano et al., 2003). During the 1987 event, the mud–debris flow of the Madrasco Stream (28.7 km2) violently hit the village of Fusine, when mean cumulative rainfall reached 18.5% of the basin MAR (259.4 mm/1400 mm), with a peak of 38.4 mm in the last 3 h before the process began. The destructive flow triggered 13 h after reaching the critical threshold. In October 2000, in the Aosta Valley, the first large mud–debris flows spread on the Nus alluvial fan (Fig. 11), 12 h after reaching the critical threshold. The processes occurred when the mean rainfall on the Saint Barthe´lemy basin reached 13.4% of the local MAR. In hilly and mountainous regions, once the threshold of 10% of the local MAR has been exceeded, numerous landslides can take place. Mass movements interrupt road and railway networks by depositing debris on them. Landslides can temporarily dam the valley bottom, forming dangerous impoundments. Dam breaching can release a big wave along the riverbed, endangering the villages and infrastructures located along its banks. During the July 1987 event, first remarkable landslide (1.5106 m3) was triggered on the right slope of the Torreggio Stream. The mass movement involved the granodioritic orthogneiss and phillite schists bedrock. The landslide occurred after 100 h of rain, when the cumulative rainfall reached 17.6% of the local MAR (176.4/1000 mm), 14 h after reaching the critical threshold. In November 1994, the particular geomorphologic setting of the Langhe hills, characterized by an asymmetric slope profile due to the isoclinal bedding of marly-silty and arenaceous-sandy alternances, favoured many rock block slides. These landslides involved the bedrock from depths of a few meters up to 20–30 m, while their sliding surface was usually parallel to the dip of the slope and the inclination, which was often close to 11–128 (see Fig. 7). Since the landslide area ranged from a few tens to several thousands of square meters, the volumes varied from a few hundred up to about one million cubic meters. According to eyewitnesses, these slides occurred over

a period ranging from a few minutes to several hours, starting from the appearance of the first cracks and ending with the final collapse. During the peak phase, the movements reached speeds varying from a few decimeters to some hundreds of meters per hour. During the 1994 hydrological event, the greatest part of these landslides occurred after 55–72 h of rainfall. The largest landslides moved between 17:00 on 5 November and 10:00 on 6 November. They slid in a range of cumulative rainfall included between 19.9% (Cerretto Langhe) and 28.6% (Gottasecca) of the local MAR, in a period between 10 and 24 h after reaching the critical threshold. Most of the landslides observed in the Langhe Hills turned out to be reactivations of landslides identified in the past. For the landslides that occurred in the Langhe Hills in the 1970s, Govi et al. (1985) identified a relationship between the critical rainfall (which takes into consideration the rainfall amount of the triggered event), the rainfall of the previous 60 days and the monthly distribution of rockblock slides in the area of Tertiary rocks Piedmont Basin. Prolonged rainfall over large areas saturates both the drainage capacity of the slopes and the downflow capacity of the hydrographic network. The tributaries swell the main stream, which is already in a critical condition. An extremely hazardous part of this phase takes place mainly along the valley bottoms of rivers with basins up to 2000 km2 in area. The violent flow causes radical changes in cross-section, plan and gradient, particularly where stabilizing bank vegetation is absent. Hydrographic stations are often swept away by the violence of the water floods, so that the discharges usually have to be evaluated indirectly. The critical phase of a watercourse depends on the distribution of rainfall on the basin. Rarely if ever does a rainfall begin or end simultaneously over an entire drainage basin, for usually the center of disturbance is in motion. The direction in which the storm travels across the basin with respect to the direction of flow of the drainage system has a decided influence upon the resulting peak flow and also upon the duration of surface runoff. In the Tanaro Valley, in November 1994, the first heavy rainfall hit the upper part of the basin and the weather front then moved northward approximately along the course of the Tanaro River: so it was possible to follow the translation of the flood waves along the main river.

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In this case, also for a flood can be identifiable a critical threshold, not local but for the entire drainage basin. At Farigliano gauging station (area, 1522 km2), the main peak level (3800 m3/s) occurred at 23:00 on 5 November, 12–14 h after the peak rainy period in the upper part of the basin. Up to that moment the mean rainfall over the basin, calculated by isohyetal method, was 181 mm, namely the 16% of the MAR (1130 mm). The situation in the 1987 and 2000 events was different mainly because of the kind of hydrographic network involved. Where lateral valleys are located almost perpendicular to the main river, their contribution was very important and caused the main river levels to increase rapidly. In Valtellina, at Ardenno gauge (2096 km2), the peak discharge and relative first inundations on the floodplain occurred early, because the highest rainfall intensities hit mostly the Orobic Alps. The left tributaries emptied their waters into the Adda River some hours before the flow coming from upstream. Also in the Aosta Valley, in October 2000, tributary contribution rapidly raised the hydrometric levels of the Dora Baltea River. The first floods on the valley bottom were already recorded in the morning of 15 October, nearly simultaneously with the critical phase that was characterized by mud– debris and hyperconcentrated flows in the small basins. At Brissogne section (1900 km2), for example, the peak level was reached at 9:00, around the same time the violent processes on the alluvial fans hit Fenis and Pollein. 5.3. The third phase During the third phase exceptional discharges and large floods in the basins larger than 2000 km2 can be observed. The translation of a flood along a valley is influenced by many factors precedently described and for this reason it is difficult to follow a natural evolution of the process along the riverbed from the upper part of the basin to the mouth of the river. Different peak stages are recognizable: the time intervals between two consecutive surges cannot be considered merely as translation times of the peak stage, because they are conditioned by the presence of manmade structures (Regione Piemonte, 1998; Turitto et al., 1995) that form a series of obstacles to the natural flow (e.g. bridges with inadequate spans,

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riverbed narrowings). The propagation paths of an atmospheric disturbance with respect to the direction of the main river can also influence the space–time distribution of the flood effects along the valley (Luino, 1999). Riverbed morphology is extensively modified, with erosional and depositional processes in the alluvial deposits of the riverbed and substantial longitudinal and cross-profile changes in channel morphology. This can locally undermine the stability of bridge foundations, irrigation channels and flood control structures. Faults in structural defences (e.g. levee collapse) may also be revealed. Water overtopping the levees can flood towns and villages to various extent and depth (Luino et al., 1996; Richards, 1982) and cause severe damage. The ground is usually so saturated that large areas with stagnant waters can still be observed 5 or 6 days after the paroxysmal phase of the inundation. Water floods usually leave widely spread silty-sandy sediments ranging in depth from some decimeters to more than 1 m. The inundations that occurred in Valtellina in the Tanaro and Aosta valleys showed these characteristics, even if they were different in size, area inundated, duration depending on natural and certain manmade conditions. They resulted in losses to inhabitants including loss of life and property, hazards to health and safety, disruption of commerce and government services, and expenditure for flood protection and relief. In July 1987, at Fuentes gauging station (2498 km2) the peak discharge was recorded at 6:00 on 19 July, after 100 h from the starting of the atmospheric disturbance and after 24 h from the most intense rainy period in the upper basin. After a levee breached in the Berbenno municipality, more than 10 km2 of the plain to the right of the river was flooded, with record levels just over 4 m in low lying areas, and an evaluated total volume of about 28106 m3. In November 1994, the critical phase in the area of Alessandria occurred 75 h after the start of the meteorological event in the upper part of the Tanaro basin. The flood peak employed a lag time of about 20 h between the upper part of the basin (Garessio) and Montecastello gauge station (197 km). The flood crest moved with an average velocity of about 2.7 m/s. In the reach Ceva-Alessandria 55 railway and road bridges are located, only 2 of which were completely

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destroyed and 7 severely damaged. On the valley bottom, waters inundated 15 urbanized areas, affecting not only small villages but also large towns like Alba, Asti (Fig. 19) and Alessandria (Luino et al., 1996). On average, 30–50% of urban areas were flooded and up to 100% (three villages). In October 2000, the critical phase for the valley in the final reach (Champdepraz-Hoˆne) occurred after 62 h after the start of the atmospheric disturbance in the upper part of the basin. In the reach Cogne-Hoˆne, the flood waves moved along 73 km in 7 h 30 min (2.8 m/ s). The span of some bridges over the Dora Baltea River proved inadequate for so large discharge; the bridges were overtopped, creating many problems particularly for the houses located just upstream from the structure. Some days after a prolonged rainy period, large landslides involving the bedrock can still take place. These phenomena usually cause the movement of very large rock masses and can cause catastrophic effects in

Fig. 19. Asti during the November 1994 event: the Tanaro waters invaded the streets of the town.

case of collapse. The total duration of rainfall usually has a greater effect on these landslides than does the number of short periods of very intensive precipitation. The delayed response depends mainly on the lithological conditions of the bedrock and on the level of the water table. For example, in July 1987, the great rock avalanche of Mount Zandila occurred after 10 days from a violent rainy event that struck the Valtellina. In October 2000, some days after the end of the hydrological event that hit the Aosta Valley, the reactivation of at least five great landslides was recorded. These landslides (from several tens of thousands to some millions of cubic meters) did not collapse, but provoked remarkable relevant morphological effects, with serious implications for public safety.

6. Conclusions Historical studies have demonstrated that in northern Italy the highest risk of instability processes is related to meteorological events of high intensity or extended duration. Throughout this section of the country, landslides, mud and debris flows and floods have caused serious losses in property and lives once every 2–3 years on average over the last two centuries. In studies the CNR-IRPI of Turin has carried out since 1970 on severe hydrogeological events in northwestern Italy, the number and typology of rainfall-triggered instability processes have proven to depend not only on the local lithological and morphological characteristics, but also on the quantity and the time distribution of instability processes during a rainfall event. When rainfall exceeds a critical threshold, a certain percentage of the mean annual rainfall (MAR), which may vary depending on the instability process and the hydrological conditions prior to the triggering event, instability processes on slopes and along hydrographic networks follow a sequence that can be reconstructed fairly reliably. Analysis of hydrological events over the last 35 years has identified that once a critical threshold has been exceeded (10% of the MAR), the sequence of the instability processes may be roughly divided into three different phases. During the first phase, shallow landslides, mud and debris flows in small watersheds and floods in basins less than 500 km2 can easily occur. These processes are usually triggered when the rainfall

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has reached a value equal to 10–20% of the local mean annual rainfall. This generally happens after continuous and heavy rainfall up to 10–12 h. In the second phase (12–24 h) mud flows and debris flows in basins larger than 20 km2 can be observed. This period is mostly characterized by floods in basins up to 2000 km2 in area and bedrock landslides of up to one to two million cubic meters in volume. Rainfall recorded is usually equivalent to 15–30% of the local MAR. The third phase is characterized by large floods involving basins at least 2000 km2 in area. That generally occurs after more than 24 h after reaching the critical threshold of the basin. Some days after an intense rainy period large landslides moving million cubic meters of rock can take place in mountainous areas. During some of the events studied, the sequence could not be divided into separate phases because the events occurred simultaneously. This was mainly due to the presence of intense rainfall pulses and the generation of very diffuse surface runoff. Such situations usually occur during brief, heavy summer rainstorms or in late spring, when snow melt combines with intense rainfall. Usually, it is not uncommon for the person in charge to devote an incredibly short time to the determination of the evolution and magnitude of the natural process. For this reason, when a severe meteorological event is about to occur, the ability to foresee in which sequence the instability processes may be triggered can prove to be very important. Advance knowledge of the phases and their development could permit the timely preventive evacuation of risk areas and the start of rescue actions when and where necessary. In order to forecast instability processes, the knowledge of recent phenomena needs to be integrated with comprehensive information about the effects of past events (CNR, 1983; Domı´nguez Cuesta et al., 1999; Eisbacher and Clague, 1984; Govi et al., 1998; Goytre and Garzo´n, 1996; Luino, 1998; Luino and Turitto, 1998; Guzzetti et al., 1994; Luino et al., 2002; Tropeano and Turconi, 2003; Wieczorek et al., 2002). By utilizing these data, statistical studies can be conducted on the frequency of instability processes in time and space. The same frequency forecasts can be extrapolated for the future, assuming that the probability of a given event will not change over reasonably short time intervals. The collection of historical data is very important but is insufficient to

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predict instability in absolute terms and to ensure a permanent safety level across wide land areas. Even though the effects connected to the hydrological event are often disastrous, it is necessary to underline that the extent of the damage is mainly due to the extreme vulnerability of the territory that has been undermined by intensive and unorganized urbanization, which has taken place mostly since the post-war period. Such urbanization was not governed by a carefully planned management of the territory, in relationship to the hazards of natural processes. The lesson to be learned from these events is that strict caution should be taken when operating on the land, not only in rebuilding operations, especially with the aim of preventing risk in areas of future urban expansion. In Italy, in these years, Civil Protection is working full-time to prevent risks related to the development of instability processes by control systems based on meteorological forecasting and monitoring systems. With a dense network of instruments in operation, Civil Protection Units can receive real-time recording and transmission of data (e.g. rainfall, temperature, wind, water levels). These values, rapidly analysed by complex mathematical models and managed by a GIS, need to be compared with the data on past events, and with critical rainfall thresholds and hydrometric levels in particular. After identification of the at-risk areas, a detailed weather report can be compiled and sent to local authorities so that rescue teams can be dispatched in a timely fashion; but these efforts must be necessarily supported by large prevention campaigns to create public awareness of environmental risks and to teach people to coexist with such risks before, during and after an emergency.

Acknowledgments The author would particularly like to thank the IRPI colleagues M. Govi and O. Turitto for allowing me to use their data on Valtellina; D. Tropeano, G. Mortara, M. Chiarle and S. Silvano for their useful indications and review of the manuscript. The author is grateful also to friends D. Cat Berro, F. Bonetto, C.G. Cirio, M. Giardino, W. Giulietto, F. Guzzetti and S. Ratto. A particular thanks to D. Alexander. All the photographs, without further specification, belong to the CNR-IRPI Turin Archive Department.

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