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

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Abstract and Figures

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 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.
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Abstract
Northern Italy is a geomorphologically heterogeneous region: high mountains, wide valleys, gentle hills and a large plain
form a very varied landscape and influences 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 autumn.
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 trigger 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-two millions m³. 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 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.
Keywords: severe hydrological event, instability processes, sequence of development, northern Italy.
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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
11000 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 overtime. 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 distribution of the 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 the 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 km²,
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 the 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).
Fig. 1 – Geomorphological regions of the northern Italy, including Aosta Valley, Piedmont and Lombardy.
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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 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 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 calceschistes, while few occur in areas where granites, syenites, 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 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, both from phenomena induced by gravity and
running waters.
The transition from mountainous regions to the plain is characterised 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, “at the foot of the mountain”. 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, Mondovì 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 pensile riverbeds.
Another geomorphological area is the hilly sector of southern Piedmont, where there are outcroppings of
Cenozoic deposits of the Tertiary Piedmontese 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 rills
and gullies, showing very intense erosive 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° C in the plain (12.7° C in Turin, 12.9° C in Milan) which are 2-3° C higher than in places immediately
north of the Alps at approximately the same altitude (e.g. 9.6° C 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°
C in February 1956 and 41° C in August 2003. In the Alpine range, the annual mean 0° C 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, receives
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 Prealpine band.
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Fig. 2 – Map showing the variation in the mean annual rainfall (MAR) in northwestern Italy based on the 50 year normals
(1921-1970), of 501 stations.
Exceptions to this pattern are the western Aosta Valley and the Apennine zone, where the annual maximum
occurs in late autumn, and the intraalpine 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
high pressure 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 event and the mean annual rainfall (MAR) of the affected area
(Govi and Sorzana, 1980; Cannon and Ellen, 1987; 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;
Govi et al., 1979; CNR, 1983; 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.
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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).
Fig. 3 – Map of Valtellina showing isohyets (mm) of July 15-19, 1987 and the most important place names mentioned in
the text.
The first period began as brief showers between 5:00 and 9:00 on 15 July with locally varied total
accumulations ranging from 4-9 mm. Later that day rainfall ceased for several hours. In the early morning hours
of 16 July, about 18-22 hours 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), alternating with lighter rainfall or no precipitation over a period of 5-8 hours. These
conditions continued for 40-44 hours 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 hours.
Between 16:00 and 17:00 on 18 July, after 36 hours 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 both its banks, causing intense erosion and
violent flooding.
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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 ten people
(Fig. 4). The phenomena triggered after 85 hours of rainfall (total cumulated rainfall of about 243 mm), with 82
mm in the last 12 hours 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 hours 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.
Fig. 4. Tartano Valley (Lombardy), July 18, 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 ten people
(photo: Catenacci, 1992).
In the late afternoon hours of 18 July, after 90 hours 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 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),
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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 (1.5 x 106 m3) triggered on the 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 waters; 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 kilometres 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 extension 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 mid-lower 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 overflowings 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 Milan-Tirano 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 minutes. At 1:00 on 19 July, the violence
of the water flowing out 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 minutes and to back up towards the site of the levee breach. At around 10:00 on 19 July,
about 5 hours 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
footline 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 hours (Govi and Turitto, 1992). The inhabitants of the villages of
Morigone, San Antonio, Poz and Tirindrè were quickly evacuated.
At 7:24 on July 28, 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
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wave travelled 1000 m in about 30 seconds (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 Tirindrè 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 seconds 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 seconds and the
fall in 23 seconds.
Fig. 5. The Mount Zandila rock avalanche took place on July 28, 1987, ten days after the rainfall had stopped. The mass
movement totally covered the valley bottom with an estimated volume of about 40 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 cubic metres.
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 catastrophical: 44 victims, 2000
homeless, over 10 billion Euros of damage in all.
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 generally mild 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 prealpine
band. 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 raingauge stations in the provinces of
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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 characterised by modest, widely
distributed or intermittent rainfall that varied locally from 30 mm to 60 mm in places. During this phase, no
landslides or mud-debris flows were reported.
Fig. 6 – Map of Tanaro basin showing isohyets (mm) of November 5-6, towns and rivers mentioned in the text.
The second phase developed locally at various times between 4 and 6 November, with intensive rains lasting
24 hours 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 hours, 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 <1 m) occurred in the upper part of
the Bormida di Spigno river. Similar instability processes triggered 1-3 hours 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 floodings triggered in the small tributaries of the Tanaro (Armella and Pesino Creeks
at Ormea, areas of less than 20 km2), while further downvalley, along the Cevetta Stream (area, 62 km2), the first
10
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 surveyed in one
area alone (Luino, 1999).
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.
During the afternoon and into the late evening, somewhat later than the soil slips, many rock block slides
triggered in the marly-silty and arenaceous-sandy alternances (range of thickness, 5-30 m). The first local rock
11
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 hours, starting
from the beginning of the first phase of the event (afternoon of 2 November). In several cases, rock block slides
were also surveyed during the morning of 6 November, after the rainfall event had begun to subside (Fig. 8).
Fig. 8. Rock-block slide on a slope near Murazzano. The big rocky clods moved about 80 m along a sliding
surface (11°-12°). 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.
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 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 countryside, 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:
- 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 hours, 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 hours and began to subside over 10 hours later;
12
- 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 hours.
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 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 water
courses.
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).
Fig. 9. Ceva, November 6, 1994. Floating materials blocked the bridge span; the floodwaters overtopped the
structure and levees upstream, invading a large urban area.
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 Carrù, comprising about 2.6 km where the Tanaro is spanned by 2 barrages and 3 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 7 roadway bridges and 3 railway bridges.
Further downstream, the events can be summarized as follows:
- in the area of Lequio Tanaro, between 22:00 and 23:00 on November 5, 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;
13
- 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 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 hectares; the
water level remained high until the left embankment of the bridge collapsed between 22:30 and 23:00;
- another backup developed (approx. 120 hectares 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 November 6. 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 water course rises in the massif of Mont Blanc and after crossing the Aosta Valley
inflows into the Po River after 160 km (Fig. 10).
The event started on 12 October, when a cold front, associated with wide 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.
Fig. 10 – Map of the hydrological event occurred in the Aosta Valley showing isohyets (mm) of 11-16 October 2000.
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) at the Champorcher raingauge station. Rainfalls triggered
14
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 hours and peaked at 23:00. The flood completely washed out
many sections of the main road along the riverbed 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 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 hour
(22:00-23:00).
On 14 October, rainfall grew heavier: 179.2 mm at Cogne (peak, 16.4 mm/h), 149.4 mm at Champorcher
(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° C in Aosta (565 m a.s.l.) and 9.7° C in Cogne (1495 m a.s.l.).
The Dora Baltea began to swell. At the Hone section, the hydrometric level rose from 4.91 m 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 right-sided 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 five hours, 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.
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
along the riverbed. 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 waterfloods 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 flood water
(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 hours
the level of the S. Barthélemy Stream (82.2 km²) had begun to rise dramatically due to the detritus and tree
trunks obstructing the Mazod Bridge. In the S. Barthélemy basin, tens of soil slips and debris flows had started,
associated with deep lateral erosion of the main water course. At 8:00, a violent debris flow of the S. Barthélemy
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 seconds
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 6 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 Comboé basin (16.2 km²) smashed into
buildings and gutted houses; seven lives were lost. An estimated volume of 150,000 m3 was left on the alluvial
fan (Tropeano et al., 2000).
At the same time, Buthier Stream waterfloods reached Aosta (area, 456.5 km²), where the stream level rose
one meter in 50 minutes. Because its exceptional discharge (> 500 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.
15
Fig. 11. During the 2000 event in the Aosta Valley, the Saint Barthélemy 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.
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
Hône 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 meters 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 hours) here was probably due to a temporary dam caused by the re-activation 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 broad surface cover alteration
deriving from an extremely tectonized and dislocated bedrock. Shallow landslides also occurred in the Rhêmes,
Cogne, Ayas and Lys valleys.
Re-activation of at least five large landslides (Pollein, Vollein, Chervaz, St. Rhémy-en-Bosses, Closellinaz)
were later surveyed. These landslides (from several tens of thousands to some millions of cubic meters) did not
collapse; however, they caused relevant 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-Hône. Thanks to the
hydrographical network of Regione Autonoma Valle d’Aosta, a general description can be summarized as
follows:
16
Fig. 12. October 15, 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.
- 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
one hour (6.7 ms-1). 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 waterfloods overflowed the banks only in some stretches. This sector is characterized by a well-
incised riverbed, with some islands and protected banks. The waterfloods flowed along 15 km in 60 minutes
(4.2 ms-1). In this reach, the contribution of two tributaries was 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 3h10’ (2.5
ms-1). 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 m to 15 m (near Montjovet)
where there are deep gorges. Also in this reach, the Dora Baltea waterfloods did not spread on the flood
plain, except in small areas.
- in the reach Champdepraz-Hône (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 ms-1, covering 10 km in 2h30’. In this reach, the valley bottom is
irregularly urbanized. The houses near the river bed (Verrès, Arnad, Hône) 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
waterfloods moved along 53 km in 6h40’ (2.2 ms-1).
- 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 m3s-1 (Barbero et al., 2003), exceeding the
previous maximum of 1920 (2670 m3s-1).
- 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.
The effects of the October 2000 event, which severely affected about 60% of the Aosta Valley, were
particularly disastrouses due to the concurrence of the following factors:
17
Fig. 13. Dora Baltea valley bottom near Hône. Large sandy deposits delimited the flooded area: the asterisk show the
highway Torino-Aosta overflowed by the Dora Baltea waters on 15 October 2000.
- 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 two
weeks later.
- the wideness of the involved drainage basin 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 triggered on the slopes and 259 debris flows occurred along the tributaries,
flooding a total area of 5 km². On the 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.
5. Results and discussion
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 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).
18
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.
5.1. The first phase
A hydrological event, particularly in autumn and spring, usually starts with a period of light rainfall of some
millimetres 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 on 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 16° to
45°, involving a slope cover from 0.4 to 1 m in depth. So they are moderate in volume, ranging from a few
cubic metres to several tens of cubic metres. 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).
Fig. 15. Ceva (Tanaro
Valley) on November 5,
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.
19
In continuous precipitation, the soil slip volumes may be quite significant and may have a considerable area
density (Polloni et al., 1996; Luino, 1999). They are usually characterised 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 punctual, but the huge shock of the 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 ms-1 (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 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 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 hours 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 12.2% (Champorcher) and 16%
(Cogne) of the MAR, 2 and 12 hours, 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 (> 30 mm/h).
Mud-debris flows can start as a result of slope-related factors, and shallow landslides can dam stream beds,
provoking temporary water blockage. As the impoundments fail, a "domino effect" 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 ms-1 (Tropeano et al.,
1996; Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road
interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic
metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream
channel. Damage usually derive 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.
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 raingauge; black arrows indicate the
triggering moment of the first soil slips in the vicinity.
20
Fig. 17. Pollein (Aosta Valley).
The destroyed house testifies to the
devastating effects of the Comboè
debris flow over the urbanized area
of Chenaux village in the early
morning of 15 October 2000.
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 basins first debris flows triggered when total rainfall reached
11.9% of the MAR (107 mm/900 mm). During the 1994 event in Tanaro Valley, first debris flow occurred near
Ormea in the Armella Creek (area, 17.5 km2), after 45 hours of light rainfall (138 mm), 4 hours after reaching
the critical threshold. In October 2000, in the Aosta Valley, first mud-debris flows 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. River banks 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 river bed within 5 to 10 hours.
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 erosions 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 hours of light rainfall, when the value reached 14% of the basin MAR
(140/1000 mm).
21
Fig. 18 - Trino (near Gressoney – Aosta Valley), September 24, 1993. The Lys waters destroyed a house and the main road
located on the right side of the stream.
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 metres (Marchi and D’Agostino, 2004).
Villages and infrastructure located on alluvial fans may be partially or totally filled up by the debris (Govi et al.,
1979; Govi, 1984; Eisbacher and Clague, 1984; Chiarle and Luino, 1998; Luino, 1998; Tropeano et al., 1999;
Regione Piemonte, 1998; ARPA Piemonte, 2003; 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 three hours before the process began. The destructive flow triggered 13 hours 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 hours after reaching the critical threshold. The processes occurred when the mean
rainfall on the Saint Barthélemy 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 (1.5 x 106 m
3) 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 hours of rain, when the cumulative rainfall reached 17.6% of the local MAR
(176.4/1000 mm), 14 hours 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 metres up to 20-30 metres, while their sliding
surface was usually parallel to the dip of the slope and the inclination, which was often close to 11-12° (see Fig.
7). Since the landslide area ranged from a few tens to several thousands of m², the volumes varied from a few
22
hundred up to about one million m³. 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 decimetres to some hundreds of
metres per hour.
During the 1994 hydrological event, the greatest part of these landslides occurred after 55-72 hours of
rainfall. Largest landslides moved between 17:00 on 5 November and 10:00 on 6 November. They slided 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 hours 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 rock-block 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 centre 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. 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 m3s-1) occurred at 23:00 on 5 November, 12-14 hours 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. Lateral valleys are, in fact, located almost perpendicularly 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 man-made structures (Turitto et al., 1995; Regione Piemonte, 1998) that form a
series of obstacles to the natural flow (e.g. bridges with inadequate spans, 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 (Richards, 1982; Luino et al., 1996) and cause severe
damage. The ground is usually so saturated that large areas with stagnant waters can be still observed five or six
days after the paroxysmal phase of the inundation. Water floods usually leave widely spread silty-sandy
sediments ranging in depth from some decimetres to more than one metre.
23
Inundations 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 governmental services, and expenditures 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 hours from the starting of the atmospheric disturbance and after 24 hours 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 28x106 m³.
In November 1994, the critical phase in the area of Alessandria occurred 75 hours 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
hours 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 ms-1. In the reach Ceva-Alessandria 55 railway and road bridges are
located, only 2 of which were completely 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).
Fig. 19. Asti during the
November 1994 event: the
Tanaro waters invaded the
streets of the town.
In October 2000, the critical phase for the valley in the final reach (Champdepraz-Hône) occurred after 62
hours after the start of the atmospheric disturbance in the upper part of the basin. In the reach Cogne-Hône, the
floodwaves moved along 73 km in 7h30’ (2.8 ms-1). 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 catastrophical effects in 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 re-activation of at least five great landslides
was surveyed. 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.
24
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 usually trigger when the rainfall 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 hours. In the second phase (12-24 hours) mud flows and debris flows in basins larger than 20
km² can be observed. This period is mostly characterised by floods in basins up to 2000 km² in area and bedrock
landslides of up to one-two million m³ in volume. Rainfall recorded are 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 hours after reaching the critical threshold of the basin. Some days after an intense
rainy period large landslides moving millions m³ 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 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; Eisbacher and Clague, 1984; Guzzetti
et al., 1994; Goytre and Garzón, 1996; Govi et al, 1998; Luino, 1998; Luino and Turitto, 1998; Dominguez
Cuesta et al., 1999; Luino et al., 2002; Wieczorek et al., 2002; Tropeano and Turconi, 2003). 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 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 an
urbanization was not governed by a carefully planned management of the territory, in relationship to the hazard
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.
25
Acknowledgements
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
reviewal of the manuscript. The author is grateful also to the 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.
26
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... The landforms in this region are relatively smoothed (see Figures A1a and A2b), with wide alluvial valleys occupying the lower areas and deep gullies present in the upper areas. The hilly morphology of southern Piedmont is primarily influenced by the nature and structure of the bedrock [22]. The slopes in this region are affected by significant colluvial deposits that rarely expose the underlying sedimentary bedrock. ...
... The rainfall patterns vary across the study area, with distinct differences between the southern and northern sectors. In the northern Monferrato hilly area, the typical annual rainfall is around 600 mm, whereas in the southern mountainous region, the typical annual rainfall is higher, ranging from 1300 to 1600 mm [22]. A recent study on rainfall in Piedmont covering the period 2004-2016 classified the study area as having a sublittoral pluviometric regime, characterized by maximum precipitation occurring in autumn and minimum in winter [35]. ...
... arenite, mudstone;(14) carbonate-rich mudstone, arenite; (15) arenite, siltstone; (16) arenite, impure limestone; (18) sand, gravel, clay/siltstone;(19) sand, gravel;(22) carbonate-rich mudstone, silt, sand; (23) conglomerates, arenite; (24) gypsum or anhydrite, limestone, clay; (25) gravel, sand (Figure 6).For the land use (USO) variable, we reduced the number of categories to seven based on our knowledge of the area: (1) urban fabric; (2) nonirrigated arable land; (3) vineyards; (4) complex cultivation patterns; (5) land principally occupied by agriculture, with significant areas of natural vegetation; (6) forest; (7) transitional woodland-shrub. ...
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Landslides triggered by heavy rainfall pose significant threats to human settlements and infrastructure in temperate and equatorial climate regions. This study focuses on the development of the Open Landslide Project (OLP), an open source landslide inventory aimed at facilitating geostatistical analyses and landslide risk management. Using a multidisciplinary approach and open source, multisatellite imagery data, more than 3000 landslides triggered by the extreme rainfall of autumn 2019 in northwestern Italy were systematically mapped (https://github.com/MicheleLicata/open_landslide_project). The inventory creation process followed well-defined criteria and underwent rigorous validation to ensure accuracy and reliability. The dataset's suitability was confirmed through multivariate correlation and Double Pareto probably density function. The OLP inventory effectiveness in assessing landslide risks was proved by the development of a landslide susceptibility model using binary logistic regression. The analysis of rainfall and lithology revealed that regions with lower rainfall levels experienced a higher occurrence of landslides compared to areas with higher peak rainfall. This was attributed to the response of the lithological composition to rainfalls. The findings of this research contribute to the understanding and management of landslide risks in anthropized climate regions. The OLP has proven to be a valuable resource for future geostatistical analysis.
... To compare these rainfall values with those recorded in the area in recently, we considered two automatic weather stations, currently measuring the values of rainfall and temperature. They are located in Arvogno (1, In the Supplementary File (table A1), the daily values exceeding 6% the Mean Annual Rainfall (MAR) (Luino, 2005) are also listed. According to Luino (2005), the 6% MAR overwhelming can be related to potential conditions of instabilities for debris flow in contexts that are analogous to the case study. ...
... They are located in Arvogno (1, In the Supplementary File (table A1), the daily values exceeding 6% the Mean Annual Rainfall (MAR) (Luino, 2005) are also listed. According to Luino (2005), the 6% MAR overwhelming can be related to potential conditions of instabilities for debris flow in contexts that are analogous to the case study. Indeed, they have already been considered for analysis in one of the Eastern Melezzo tributary basins (Bollati et al., 2018). ...
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On 7th-8th August 1978, a serious flood hit the Vigezzo valley, in the northeastern part of the Sesia Val Grande UNESCO Global Geopark (Northern Italy). The flood, strictly linked to the bedrock features, was triggered by heavy rains and made even worse by unsuitable urban sprawl. It left obvious scars in the landscape, which are now gradually being lost. The local population, as well as tourists, have a deep memory of the event, especially old people. Herein, we describe the Citizen Science Project, named The signs of the 1978 flood in the Vigezzo Valley: the population tells. Data collection started in 2021, through citizen collaboration (residents and tourists) and is still ongoing. With the support and patronage of many local entities (e.g. Municipalities), particularly the Regional Ecomuseum ‘Of the Soapstone and of the Stone-cutters’ (‘Ed Leuzerie e di Scherpelit’ in local dialect), in the Malesco Municipality, an electronic form/paper survey was distributed to collect information and pictures on the flood event, and to select the most meaningful sites in the valley, relating to the event. Thirty four sites were suggested by citizens and will be considered considered to create a community map and a trail of the flood event, to preserve and transmit this heritage to future generations. The Citizen Science Project is also discussed, as a contributor to different Sustainable Development Goals (mainly SDGs 4-9-11-13-17) of Agenda 2030 of the United Nations, in the framework of the Sesia Val Grande UNESCO Global Geopark.
... Gradually, these areas widen to the surrounding downstream slopes until, for a duration of 120 h, almost all the slopes, except the river, present a comparable lower rcrit. This result agrees with what has been observed in natural slopes where widespread shallow landslides often occur after prolonged rains that persist in an area for several days [99]. Conversely, if rain showers happen, only the steepest areas (the ridges) could experience localized instability, evolving sometimes into a debris flow [65]. ...
... Therefore, for ridges, short and intense rains such as showers or thunderstorms represent the most hazardous trigger (TR1h < TR120h), while for slopes, these are represented by prolonged and low-intense precipitation (TR120h < TR1h). These results are confirmed by historical surveys [15,99] and could be explained in these terms: intense and short rains can saturate of faster the thin soil covers across ridge areas while prolonged rains can have a greater impact on the local hydrogeology, which contributes significantly to progressive (but slower) terrain saturation of the terrain slope over time. According to [46,47,50,78], this dynamic is directly imputed to the characteristic of the hydrological model chosen for simulating subsurface flow. ...
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Powerlines are strategic infrastructures for the Italian electro-energetic network, and natural threats represent a potential risk that may influence their operativity and functionality. Geo-hydrological hazards triggered by heavy rainfall, such as shallow landslides, have historically affected electrical infrastructure networks, causing pylon failures and extensive blackouts. In this work, an application of the reworked version of the model proposed by Borga et al. and Tarolli et al. for rainfall-induced shallow landslide hazard assessment is presented. The revised model is called SLEM (Shallow Landslide Express Model) and is designed to merge in a closed-from equation the infinite slope stability with a simplified hydrogeological model. SLEM was written in Python language to automatise the parameter calculations, and a new strategy for evaluating the Dynamic Contributing Area (DCA) and its dependence on the initial soil moisture condition was included. The model was tested for the case study basin of Trebbia River, in the Emilia-Romagna region (Italy) which in the recent past experienced severe episodes of geo-hydrological hazards. The critical rainfall ratio (rcrit) able to trigger slope instability prediction was validated against the available local rainfall threshold curves, showing good performance skills. The rainfall return time (TR) was calculated from rcrit identifying the most hazardous area across the Trebbia basin with respect to the position of powerlines. TR was interpreted as an index of the magnitude of the geo-hydrological events considering the hypothesis of iso-frequency with precipitation. Thanks to its fast computing, the critical rainfall conditions, the temporal recurrence and the location of the most vulnerable powerlines are identified by the model. SLEM is designed to carry out risk analysis useful for defining infrastructure resilience plans and for implementing mitigation strategies against geo-hazards.
... This period was influenced by the catastrophic natural event of July 1987, when prolonged and intense rainfall swept through the entire area of Prealps and Alps in North Italy. The consequences were exceptionally devastating, that is, flood, rockfalls, debris flows and shallow landslides, which provoked 53 casualties and a huge economic damage amounted to 2 billion euros (Blahut et al., 2012;Luino, 2005). This fact increased the sensitivity to natural hazards, leading the politicians to invest in the construction of TTCSs. ...
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... There is a long history of mournful events that have involved anthropic areas, causing victims and serious damage (Luino, 2005). Not only landslides are included among these well identifiable phenomena, but also other extremely rapid and dangerous processes that are almost always classified as landslides by the uninitiated. ...
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Climate change in the European Alps, in particular in the high-elevation environments, is causing an increase in mass movements and hazards. To learn more about relationships between mass movements and climate drivers, the location of the starting zone and date of the instability events need to be known. Nevertheless, not all existing inventories of mass movements are suitable for the purpose. For these reasons, we have implemented a specific inventory of mass movements which occurred in the Italian sector of the Alps at an elevation >1500 m. Currently, the inventory contains information relating to 772 mass movements. The most frequent types of processes documented are rockfall and debris/mud flows, with 279 and 191 cases respectively. The highest number of events occurred in 2022 (71 events), and an evident trend towards an increase over the years and during summer was found. This inventory is an excellent support tool for many activities that take place in and for the mountains, its consultation, both online and offline, makes the inventory suitable for use with different types of devices and can be used not only as a consultation tool on mass movements occurred in the past, but also to insert new events. This use can be particularly suitable for monitoring activities, managed by civil protection structures, municipalities, natural parks, environmental agencies, researchers, freelancers and so on.
... Alpine valleys have always had a central role in the history of populations, as they connect Southern and Central Europe (Freshfield, 1917). Human activities and geological evolution have always been tied in these territories, often not peacefully, as witnessed by the catastrophic events that involved settlements throughout the Alps (Guzzetti, 2000;Luino, 2005). A correlation between ongoing climatic variations and the rising of landslide events has been suggested in several studies in Alpine context (e.g., Crozier, 2010;Evans & Clague, 1994;Soldati et al., 2004;Stoffel & Huggel, 2012). ...
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Natural and anthropogenic mountain landscapes coevolve responding on different temporal scales to climate changes and geodynamics by a series of increments that cause the dynamic association of morphological stabilization surfaces, stratigraphic units and landforms. Understanding the incremental history of palimpsest landscapes helps to recognize and forecast the effects of climate change on the sensitive mountain environments, contributes to archaeological and historical reconstruction and supports management strategies for natural risks prevention and mitigation. The Italian Bregaglia Valley provides an excellent site to unravel the recent/historical increments of evolution of landforms and human settlement, permitting to map the paleo-digital terrain models (DTMs) corresponding to the relevant landscape turning points. After the last de-glaciation, two large-scale landslides reshaped the valley floor, both predisposed by deep-seated gravitational slope deformations and one surely triggered by intense rain-falls. The most recent and impacting event buried in 1618 the rich border town of Piuro, the ancient one occurred in the same area at least 1.5 ka before. Combining stratigraphic, geomorphological, topographic, archaeological and historical data, we drew the paleo-DTMs of the pre-and post-1618 settings of Piuro, sketching the landscape evolution. Since two millennia, human settlements took advantage of the decadal to secular most stable surfaces, represented by the inactive lobes of debris-flow fans, the highest trunk river terraces and the top of humps formed by the ancient landslide body in the valley centre. Stratigraphic relationships, archaeological findings and age determinations show that both landslides diverted the trunk river and covered the existing fan lobes. On a secular timescale, fan progradation and trunk river terrac-ing buried and reworked both the landslide bodies. The paleo-DTMs show their original areal extent and permit to compute their volume and to sketch the setting of the buried Piuro settlements, drawing the changes of the Mera trunk river course and the chronology of activity of the lateral debris-flow fan lobes.
... This flood greatly shocked the international community, especially because of the serious damage to art museums and the national library of Florence [37]. Lastly, we also remember the two great floods of 1994 and 2000 that hit Piedmont in particular, causing very serious damage, with 69 casualties (1994) [38] and 36 casualties (2000) [39,40]. ...
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Many Italian cities and towns have been affected by geological or geohydrological processes. However, due to the loss of historical memory, lessons of the past have been ignored; new urbanized areas have expanded into the same zones where damage and casualties occurred in the past. Despite current practices, researchers are showing how historical data can be among the most valid tools for identifying the most affected and hazardous areas. When the completeness and quality of historical sources are sufficiently high, we can make useful statistical inferences regarding the spatiotemporal variations of natural processes. This information is of great importance for land use planning, as it makes us able to rely not only on the current state of the investigated areas but also on their dynamic evolutionary framework over time. In this article, we present a chronological review of past Italian works describing the occurrence of natural extreme events making use of historical data. Then, we present some Italian case studies in which the awareness of hazards gained by paying attention to past information would have ensured better management of the risk for the benefit of public safety. Finally, the authors stress the need to safeguard, manage, and enhance the large collection of historical data that constitutes Italy’s heritage.
... A hazardous event may trigger another, such as when an earthquake triggers a tsunami or a landslide. These kinds of hazard interactions are usually called cascades [8][9][10] or domino effects [11][12][13]. Multiple hazards may also occur at the same time because they are triggered by the same triggering event (e.g., storm surge and flooding occurring during a hurricane). ...
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Effective disaster risk management in a given area relies on the analysis of all relevant risks potentially affecting it. A proper multi-risk evaluation requires the ranking of analyzed risks and the estimation of overall expected impacts, considering possible hazards (and vulnerabilities) interactions as well. Due to their complex and challenging modelling, such interactions are usually neglected, and the analysis of risks derived from different sources are commonly performed through independent analysis. However, often the assessment procedures adopted for the analysis as well as the metrics used to express various risks are different, making results of single risk analyses hardly comparable. To overcome this issue, an approach that allows for comparing and ranking risks is presented in this study. The approach is demonstrated through an application for an Italian region. Earthquakes and floods are the investigated hazards. First, in order to select the case study area, the municipalities within the Veneto region where both risks could be highest are identified by adopting an index-based approach. Then, the harmonization of seismic and flood risk assessment procedure is performed. Sub-municipal areas are selected as scale of analysis and direct economic losses are chosen as common impact metrics. The results of the single risk analyses are compared using risk curves as standardization tool. The EAL (expected annual losses) are estimated through risk curves and the ratios between EAL due to floods and earthquakes are mapped, showing in which area risk is significantly higher than the other.
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This work presents the new model called CRHyME (Climatic Rainfall Hydrogeological Modelling Experiment), a tool for geo-hydrological hazard evaluation. CRHyME is a physically based and spatially distributed model written in the Python language that represents an extension of the classic hydrological models working at the basin scale. CRHyME's main focus consists of simulating rainfall-induced geo-hydrological instabilities such as shallow landslides, debris flows, catchment erosion and sediment transport into a river. These phenomena are conventionally decoupled from a hydrological routine, while in CRHyME they are simultaneously and quantitatively evaluated within the same code through a multi-hazard approach. CRHyME is applied within some case studies across northern Italy. Among these, the Caldone catchment, a well-monitored basin of 27 km2 located near the city of Lecco (Lombardy), was considered for the calibration of solid-transport routine testing, as well as the spatial-scale dependence related to digital terrain resolution. CRHyME was applied across larger basins of the Valtellina (Alps) and Emilia (Apennines) areas (∼2600 km2) which have experienced severe geo-hydrological episodes triggered by heavy precipitation in the recent past. CRHyME's validation has been assessed through NSE (Nash–Sutcliffe efficiency) and RMSE (root mean square error) hydrological-error metrics, while for landslides the ROC (receiver operating characteristic) methodology was applied. CRHyME has been able to reconstruct the river discharge at the reference hydrometric stations located at the outlets of the basins to estimate the sediment yield at some hydropower reservoirs chosen as a reference and to individuate the location and the triggering conditions of shallow landslides and debris flows. The good performance of CRHyME was reached, assuring the stability of the code and a rather fast computation and maintaining the numerical conservativity of water and sediment balances. CRHyME has shown itself to be a suitable tool for the quantification of the geo-hydrological process and thus useful for civil-protection multi-hazard assessment.
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9th Congress of the International Association for Engineering Geology and the Environment. Durban (South Africa) - 16-20 september 2002, 191-200
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Sono descritti in sintesi i fenomeni geomorfologici e gli effetti osservati in tutto l'areale maggiormente colpito dall'evento, esteso 1800 km2 e comprendente le valli alpine occidentali tra le valli Po e Toce. Sui versanti, centinaia di soil slip, taluni di eccezionale dimensione, hanno talora dato innesco a fenomeni di trasporto torrentizio in massa, con apporti in conoide di detriti grossolani in volumi anche molto elevati (Val Grande di Lanzo, Valle Orco, Valli aostane, Val Divedro); in una decina di casi, sintomi di riattivazione di grandi frane hanno richiesto particolari provvedimenti cautelativi. Sui fondivalle e nelle pianure, diffusi processi di erosione laterale e di fondo, riapertura di canali secondari, sovralluvionamento ed esondazione con deposito alluvionale hanno gravemente colpito, per decine di chilometri, le reti di collegamento viario, interessando in più casi zone abitate. Queste ultime hanno patito le più pesanti conseguenze in territorio montano, dove si è registrata la massima parte delle vittime (su un totale di 40) e la maggior entità dei danni.
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The Valtellina in Northern Italy was deluged by more than 600 mm of rainfall in mid-July, 1987 during a period of exceptionally warm temperatures. Runoff from melting glaciers and the unusual rainfall produced numerous small landslides, and floods with recurrence intervals in excess of 100 years. Between July 18-19, a debris flow from one small tributary (Val Pola) dammed the Adda River, forming a lake with an estimated volume of 50 000 m³ and a depth of about 5 m. On July 28, 1987, a rock avalanche with a volume of 35 million m³ collapsed into the Adda River valley, catastrophically displaced the water in the Val Pola debris-dam lake, and ran up 300 m on the opposite valley side. The avalanche sent a wave of water and sediment 2.7 km upstream, damaged three villages, claimed 27 lives, and dammed the Adda River valley. -from Author
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Flooding in Western Italian Alps, 14-15 October 2000. On October 14-15, 2000, ten major valleys in the Northwestern Italian Alps (Piedmont and Aosta Valleys) and related floodplains, accounting for 8,500 sq. km, were affected by huge flooding and landslides, which resulted in death of 40 persons, 32,000 evacuated, hundreds of houses severely damaged and partly destroyed, many kilometres of roads inundated, road interruptions (by hundreds, included highways and railways) and bridge destruction (by tens). Industry, farming, school and business activities were generally hindered during the 1-2 days throughout Western Italy due to the collapse of structural linkages. Not only were villages (by dozens), and isolated houses flooded, but also urban quarters of Torino, Aosta, Ivrea, Susa, Casale Monferrato, which were invaded by floodwaters of the major rivers that cross them. An official balance of structural damages was estimated at 5.5 billion Euro. The area most affected by the event extends from the Cottian to the Lepontine Alps, belonging to metamorphic and igneous rocks spanning from the Permian to the Cretaceous, all throughout covered by Quaternary glacial and terraced alluvial deposits. Within 2 days, as much as 400-600 mm rain fell on high-altitude slopes due to the rise of the hysotherm 0 above 3000 m. The IRPI-CNR meteorological station located at 2200 m a.s.l. in the Cenischia Valley (one of the epicentres of the event), recorded 400 mm in 48 hours with a shower of 39 mm in one hour. Such values are absolutely unusual for that area. Stream discharges were exceedingly high as well. The Po River (the Ticino and Adda Rivers outflow respectively Lake Maggiore and Lake of Como which largely inundated the adjoining towns and villages), discharged as much as 13,000 m3/s during its long peak stage down to the Adriatic Sea six days later. In the valleys, the most commonly surveyed processes were the partial re-activation of large historical landslides, as well the occurrence of soil slips (attaining a local density of about 100/km2) part of which had triggered 100s of debris flows. Some of these issued from narrow, steep gorges, attained huge proportions in the Aosta Valley: e.g. the Saint Barthélemy alluvial fan, roughly accounting for newly-brought 200,000 m3 of gravel and boulders which partially destroyed and buried the Nus village; the Comboé torrent discharged over its fan some 100,000 m3 of debris, destroying houses in the Chenaux village near Pollein; the Bioley torrent near Fenis carried a muddy debris flow (accounting for more than 50,000 m3) into the settled area of Pleod. Torrential flood also brought severe destruction in the Ronco Canavese and Locana communities in the Soana and Orco valleys respectively. Other remarkable processes surveyed have been: streambank erosion and slope undercutting; re-mobilization of coarse bottom deposits (accounting for a sediment transport energy for given cross-sections of up to 300 m3/s of bouldery deposits along the channels); channel wandering and extensive flooding, with deposits of loamy sand over several sq. km (e.g. along the lower course of the Dora Baltea River, in vicinity of the Montjovet and Bard villages. The valleys of Western Alps were already affected by floods in the past. In the last two centuries, events which at least locally had produced comparable morphological effects with the present flood, occurred in 1810, 1839, 1846, 1857, 1868, 1900, 1920, 1948, 1957 and, late, in 1993 and 1994. This supports a widespread opinion about a tendencial increase in the frequency of “recent” flood events. Certainly, the time span which usually occurs between one disastrous event and the following in a particular area of land (20-40 years) decreases the historical memory. The need for re-activating the use of land and the progressive increase of the goods over the territory, steadily leads to an increase of the damages. A wise policy of land management, however, has been applied since some years to prevent or mitigate danger and damage.