sequence of instability processes triggered by heavy ......sequence of instability processes...
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Geomorphology 66
Sequence of instability processes triggered by heavy rainfall
in the northern Italy
Fabio Luino*
Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica, Sezione di Torino,
Strada delle Cacce 73, 10135 Torino, Italy
Received 3 June 2003; received in revised form 2 April 2004; accepted 4 September 2004
Available online 23 November 2004
Abstract
Northern Italy is a geomorphologically heterogeneous region: high mountains, wide valleys, gentle hills and a large plain
form a very varied landscape and influence the temperate climate of the area. The Alps region has harsh winters and moderately
warm summers with abundant rainfall. The Po Plain has harsh winters with long periods of subfreezing temperatures and warm
sultry summers, with rainfall more common in winter.
Geomorphic instability processes are very common. Almost every year, landslides, mud flows and debris flows in the Alpine
areas and flooding in the Po flood plain cause severe damage to structures and infrastructure and often claim human lives.
Analyses of major events that have struck northern Italy over the last 35 years have provided numerous useful data for the
recognition of various rainfall-triggering processes and their sequence of development in relation to the intensity and duration of
rainfall. Findings acquired during and after these events emphasise that the quantity and typology of instability processes
triggered by rainfall are related not only to an area’s morphological and geological characteristics but also to intense rainfall
distribution during meteorological disturbances. Moreover, critical rainfall thresholds can vary from place to place in relation to
the climatic and geomorphological conditions of the area. Once the threshold has been exceeded, which is about 10% of the
local mean annual rainfall (MAR), the instability processes on the slopes and along the hydrographic networks follow a
sequence that can be reconstructed in three different phases.
In the first phase, the initial instability processes that can usually be observed are soil slips on steep slopes, mud–debris flows in
small basins of less than 20 km2 in area, while discharge increases substantially in larger stream basins of up to 500 km2. In
continuous precipitation, in the second phase, first mud–debris flows can be triggered also in basins larger than 20 km2 in area.
Tributaries swell the main stream, which is already in a critical condition. The violent flow causes severe problems mainly along
valley bottoms of rivers with basins up to 2000 km2 in area. First bedrock landslides can occur, reaching a considerable area
density, with volumes from a few hundred up to about one to two million cubic meters. In continuous precipitation, in the third
phase, basins of more than 2000 km2 in area reach their first critical stage. River-bed morphology is extensively modified, with
erosional and depositional processes which can locally undermine the stability of structures and infrastructures. Waters overflow
levees, flooding villages and towns to various widths and depths and sometimes claiming casualties. Some days after an intense
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F. Luino / Geomorphology 66 (2005) 13–3914
rainfall period, large landslides involving the bedrock can still take place. These processes usually cause the movement of very
large rock masses. The total duration of rainfall usually has a greater effect on these landslides than does the number of short
periods of very intensive precipitation. This sequence cannot be divided into separate phases when the events occur simultaneously
because of the presence of intense rainfall pulses and the generation of very diffuse surface runoff. Such situations usually happen
during short-lasting heavy summer rainstorms or in late spring, when snow melt combines with intense rainfall. The three-phase
sequence has been identified in three severe events that are analysed in this paper: Valtellina (Lombardy) in 1987, Tanaro Valley
(Piedmont) in 1994 and Aosta Valley in 2000; but this sequence has also been observed during other events that occurred in
northern Italy: in Piedmont in 1968, 1977, 1978, 1993 and 2000; in Lombardy in 1983 and 1992; in the Aosta Valley in 1993.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Severe hydrological event; Instability processes; Sequence of development; Northern Italy
1. Introduction
In Europe, Italy ranks highest in the variety of
natural instability processes: landslides, glacier-related
phenomena, floods, earthquakes, subsidence and
volcanic eruptions. Throughout the country, these
processes claim victims and cause damage amounting
to billions of Euros every year. Historical research has
shown that 11,000 landslides and 5400 floods have
occurred in the last 80 years. The costs for these
processes are high: since 1980, the State has paid 42.4
billion Euros, or about 5.7 million Euros per day.
Since 1993, severe hydrological events have struck
northern Italy (Piedmont, the Aosta Valley and
Lombardy) five times, causing large floods, numerous
landslides, mud and debris flows. Even if the rate of
their occurrence appears to be increasing, these events
are evenly distributed over time. Historical research
demonstrates, for example, that over the last two
centuries Piedmont has been hit 101 times by such
events (one event every 24 months), causing damage
and often claiming victims. Such a distribution of
events demonstrates not an outright growth in
frequency but rather an expansion of the potential
for involving urban areas.
Human perception may fail to detect the natural
evolution of a hydrographic basin because it
proceeds by gradual, often imperceptible processes,
but brief violent episodes usually associated with
extraordinary hydrological events can sometimes
change that perspective. These events upset the
existing balance of conditions in each part of the
basin. The evolutionary processes triggered during
the events show different forms of development and
have different practical implications related to
morphological and topographical conditions and to
particular time intervals.
The objective of this paper is to highlight that,
during severe hydrological events in northern Italy, it
is possible to follow a time evolution of the natural
instability processes. This evolution corresponds to
increased risk and expected damage.
2. Geology and geomorphology
The study area includes Piedmont, the Aosta
Valley and Lombardy. Within the total area of
52,512 km2, 45.6% is mountainous landscape,
34.1% hills and 20.3% the Po plain. The geo-
morphology is strictly tied to its geological structure
and may be subdivided into four large regions,
roughly arranged in concentric crescents. Moving
along an imaginary line from Mont Blanc to the
Langhe Hills, the outer crescent is formed by the large
mountain chain, then a hilly belt of modest pre-alpine
ranges and amphitheatres of the valley mouths, and in
the center the large area of the Po Plain bordered on
the east by the structures of the Tertiary Piedmontese
Basin (Fig. 1).
The Alps are an important product of Tertiary
orogenesis, occupying an area of about 240,000 km2.
They constitute an extensive mountain system 800 km
long and 160 km wide that traces a large arc from the
Region of Liguria on the Mediterranean Sea and runs
along the borders between Northern Italy and SE
France and Switzerland eastward to Slovenia. The
western Alps rise as mighty massifs which, at some
points, soar to over 4000 m (Mont Blanc, 4810 m;
Mount Rosa, 4633 m; Gran Paradiso, 4061 m). Like
Fig. 1. Geomorphological regions of northern Italy, including Aosta Valley, Piedmont and Lombardy.
F. Luino / Geomorphology 66 (2005) 13–39 15
all mountain chains, the Alps are formed by great
volumes of rocks of different aspect, chemical
composition and genetic significance. Metamorphic
rocks are the most representative of the chain,
followed by sedimentary rocks, while, igneous rocks
(plutonic and volcanic) are least in subordinate
volume. Rocks have different mechanical properties
so that they behave differently during geomorphic
processes. In Piedmont, for example, about 16% of
landslides have occurred in the calcschistes, while few
occur in areas where granites, syenites and diorites
outcrop (Forlati, 1990).
The Alps are characterized by high crests and steep
slopes, with large, deep valleys. This morphology is
mainly the product of the Quaternary glaciations. Vast
ice masses moved through the valleys, transforming
them into deep troughs with steep walls; the overflow
of ice across the mountain divides shaped the passes.
Glacial deposits in the form of moraines dammed the
streams and rivers and produced many lakes. Only
summit regions above 3000 m are glaciated today,
about 2% of the total area (Schmidt, 2004). Peaks and
crests, however, rise above the ice as jagged shapes
(tooth-like horns, needles, and knife-edged ridges).
The post-glacial evolution of the area appears to be
greatly conditioned by instability processes, from
phenomena induced by gravity and running water.
The transition from mountainous regions to the
plain is characterized by a discontinuous belt of
morainic high ground (e.g. Rivoli and Ivrea amphi-
theatres), leaving the impression of a clear contrast
between the encircling mountains behind them and
the plain lying, in fact, bat the foot of the mountainQ.The morainic belt is bordered by valley mouths and
locally includes sectors of the plain, partially
occupied by dammed lakes or final stretches of the
great pre-alpine lakes.
The plain of northwestern Italy can be divided into
two areas: the upper plain close to the mountain
slopes (Cuneo, Mondovı and Saluzzo) and the lower
plain around Novara and Vercelli towards the East.
The Po Plain is a great Tertiary sedimentary basin
constituted by a thick blanket of alluvial deposits
carried by the Po River and its tributaries. In its
northern sector, the Po Plain is fed by the Alps, and its
southern sector by the Apennines. The detrital
contribution coming from the Alps contains coarse
and silty sediments, while that from the Apennines is
mostly clays. Along their course, the rivers of the Po
Plain differ in their geomorphological characteristics
considerably. They flow embanked in alluvial sedi-
ments, creating different orders of terraces, and
stretches in the lower plain, where the prevalence of
the sedimentary activity gives rise to elevated
riverbeds.
Another geomorphological area is the hilly sector
of southern Piedmont, where there are outcroppings
of Cenozoic deposits of the Tertiary Piedmontese
Fig. 2. Map showing the variation in the mean annual rainfall (MAR) in northwestern Italy based on the 50-year norms (1921–1970) of 501
stations.
F. Luino / Geomorphology 66 (2005) 13–3916
Basin, a late-post orogenic episutural basin (Scam-
belluri et al., 2002). Within this area, D’Atri et al.
(2002) have identified three great tectonic-sedimen-
tary domains: the Langhe Basin, the Turin Hills and
the Monferrato Hills. The hilly morphology of south-
ern Piedmont is essentially tied to the nature and
structure of the bedrock; for example, the particular
asymmetry of the valleys (due to the isoclinal bedding
of marly-silty and arenaceous-sandy alternances), and
sectors characterized by gullies, showing very intense
erosional activity.
3. Brief climatic framework of the study area
The climate of Piedmont, Aosta Valley and
Lombardy is strongly affected by various features of
the Alpine and Apennine ranges surrounding the area
on three sides. The mountain barrier forms a shield
against winds, thus reducing the effects of cold Arctic
or North-Atlantic air masses, with mean annual
temperatures of around 12–13 8C in the plain (12.7
8C in Turin, 12.9 8C in Milan) which are 2–3 8Chigher than in places immediately north of the Alps at
approximately the same altitude (e.g. 9.6 8C in
Geneva). The western end of the Po plain, which is
less affected by maritime influence, shows a wide
temperature range between record high and low
temperatures measured. In the past 50 years, the plain
south of Turin has experienced temperatures between
�25 8C in February 1956 and 41 8C in August 2003.
In the Alpine range, the annual mean 0 8C isotherm is
at a height of about 2300–2500 m. The orographic
influence is markedly noticeable in the distribution of
precipitation. Total annual rainfall varies from a
minimum of 500 mm in the intraalpine cirque
surrounding Aosta, well shielded from moist Atlantic
and Mediterranean winds, to over 2500 mm in the
mountain area above Lake Maggiore (Fig. 2).
Moderate rainfall amounts of about 600 mm annually
are typical of a small area in the upper Susa Valley
and the southern Piedmont (basin of Alessandria).
Other flatland areas receive 700–900 mm on average
per year, while the Pre-alpine zones, which are more
exposed to condensation of moist Mediterranean
winds, receive 1300–1600 mm annually. A good part
of this area of Italy has a sublittoral pluviometric
regime, with the main pluviometric maximum in
spring (April to May) and the minimum in winter
(January to February), a pluviometric pattern typical
of the Pre-alpine belt.
Exceptions to this pattern are the western Aosta
Valley and the Apennine zone, where the annual
maximum occurs in late autumn, and the intraalpine
F. Luino / Geomorphology 66 (2005) 13–39 17
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, espe-
cially 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 trig-
gered 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 (Cannon and Ellen, 1987; Govi and
Sorzana, 1980; Pierson et al., 1991).
Once the threshold has been exceeded, precipita-
tion usually triggers a series of effects on the
hydrographic network and the slopes. The effects
can be attributed to three different phases. In the last
35 years, this sequential type of phenomenon has been
observed in northern Italy during these large severe
events (Carraro et al., 1970; CNR, 1983; Govi et al.,
1979; Govi and Turitto, 1997; Luino, 1998; Tropeano
et al., 1999): in Piedmont in 1968, 1977, 1978, 1993,
1994, and 2000; in Lombardy in 1983, 1987 and
1992; in the Aosta Valley in 1993, and 2000.
This section analyses three of these severe events:
July 1987 in Valtellina (Lombardy), November 1994
in Tanaro Valley (Piedmont) and October 2000 in the
Aosta Valley.
4.1. The July 1987 event in Valtellina
A severe hydrologic event occurred in the second
half of July 1987 in Valtellina (Fig. 3): floods and
landslides caused catastrophical effects. Five vil-
lages were razed to the ground; roads, bridges,
railways were partially or totally destroyed, hun-
dreds of hectares flooded. In all, there were 53
victims and over 2000 million of damage (Govi
and Turitto, 1992).
On 15 July, critical meteorological conditions
began to brew as a vast, low pressure area over the
British Isles drew warm southerly winds along its
southern edge across northern Italy in a sweep
extending over 80 km from Lake Como to the
Camonica Valley. Along this front, various orographic
features and thermal contrast led to widespread,
locally intense rainfall that developed in three
consecutive largely similar periods (Brunetti and
Moretti, 1987).
The first period began as brief showers between
5:00 and 9:00 on 15 July with locally varied total
accumulations ranging from 4 to 9 mm. Later that day
rainfall ceased for several hours. In the early morning
hours of 16 July, about 18–22 h after the rain had
stopped, the second period began with rainfall
conditions that were similar on the Orobic side and
in the entire pre-lake Adda River basin and charac-
terized by brief intensive showers (up to 10 mm/h),
Fig. 3. Map of Valtellina showing isohyets (mm) of 15–19 July 1987 and the most important place names mentioned in the text.
F. Luino / Geomorphology 66 (2005) 13–3918
alternating with lighter rainfall or no precipitation
over a period of 5–8 h. These conditions continued for
40–44 h into the next day. More total rainfall was
recorded for the southern side of Valtellina (40–55
mm) than in the upper valley around the Bormio
cirque (16–25 mm). During this period, no soil slips
or mud debris flows were noted either. The third
period, which started on the afternoon of 17 July and
continued the following day, was marked by steady
rains. Starting from south to north, heavy showers
began at different times and continued to fall for the
next 48–50 h. Between 16:00 and 17:00 on 18 July,
after 36 h of rainfall (160 mm) with peaks of 51 mm/h
between 15:00 and 16:00, the initial effects of debris
flows in the upper Brembana Valley tributaries began
to occur. Almost simultaneously many soil slips
triggered. Shortly after 17:00, the Brembo Stream in
the area around Lenna (basin area, 307 km2) swelled
markedly and overflowed its banks, causing intense
erosion and violent flooding. Between 17:00 and
18:00 on 18 July, in the Tartano Valley, on the Orobic
side of Valtellina, numerous soil slips triggered. At
17:00, one of the largest struck an apartment building
and invaded a hotel, killing 10 people (Fig. 4). The
phenomena triggered after 85 h of rainfall (total
cumulated rainfall of about 243 mm), with 82 mm in
the last 12 h and a relatively intense episode (22.4
mm) in the last hour before the collapse. At 19:00,
with a total cumulated rainfall of 259.4 mm, a huge
debris flow triggered on the alluvial fan of Madrasco
Stream (28.7 km2), where the village of Fusine is
located. As rainfall continued throughout the evening,
many landslides occurred in the Madrasco Valley after
22:00. Meanwhile, between 20:00 and 21:00, Mallero
Stream at Sondrio cross section (area, 315 km2)
increased its discharge because of the remarkable
amounts of debris its tributaries had been bringing in
since 18:00. These conditions developed after 63 h of
rainfall (total cumulated, 100 mm), with a peak of 46
mm between 18:00 and 21:00. Just after 21:00, many
soil slips triggered in the Torreggio Valley.
In the late afternoon hours of 18 July, after 90 h of
light rainfall (total, 107 mm; peak, 30.4 mm between
16:00 and 18:00), the first impulsive debris flows
triggered between 17:30 and 18:00 along Vallecetta
Creek (4.6 km2), along the Mala Valley (2.2 km2) and
Presure Valley (5.2 km2) on the left orographic side of
the upper Adda valley. In the basins of the right side
of the valley, the Pola Valley (area, 1.7 km2) and the
Vendrello Valley (2.9 km2), similar torrential events
took place between 18:00 and 19:00 after incessant
rainfall (total, 117 mm). An estimated 600,000 m3 of
Fig. 4. Tartano Valley (Lombardy), 18 July 1987. At 17:00, a large
soil slip conveyed material into a small hollow incision, cut an
apartment building in two (black arrow) and invaded a hotel (white
arrow), killing 10 people (photo: Catenacci, 1992).
F. Luino / Geomorphology 66 (2005) 13–39 19
debris carried by the creek of the Pola Valley spread
out into the valley bottom, damming the Adda and
creating a basin upstream from the obstruction.
Between 19:00 and 23:00 (cumulated rainfall, 127
mm), very similar effects caused by the violent flood
of Massaniga Creek (9.7 km2) were surveyed 3 km
upstream. At 2:00 the following day, 19 July, the
discharge of the Adda flood increased on the main
valley bottom, when the waters breached the detritus
dam created by the Massaniga debris flow. The Adda
waters poured into the fields around S. Antonio
Morignone. That morning, between 9:00 and 10:00,
slightly later than the instability processes described
above, a large landslide (1.5�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 water; a huge volume of debris then spread in the
Mallero riverbed after having severely damaged a part
of the village. The paroxysmal phase of the flood
proceeding along the course of the Adda riverbed
begun the night of 18 July and continued to about
noon of the next day, when the river levels began to
drop.
Different types of processes took place in relation
to the different morphotopographic characteristics of
the valley bottom. While erosion, which was intense
at certain sites, was prevalent along the first kilo-
meters of the river’s course between Bormio and
Tirano, diffuse overflowing accompanied by wide-
spread flooding started at Chiuro, 8 km upstream from
Sondrio. Flooding most often occurred at the con-
fluences with the already swollen Adda tributaries.
The extent of the areas submerged and the quantity of
sediment left by the waterfloods on the ground surface
testify to the impact of both the main river and its
tributaries. Because of damming of the upper valley,
the propagation wave along the entire river course to
its mouth at Lake Como (127 km) demonstrated
certain discontinuities as it flowed downvalley. The
developing times of the effects of the wave in the 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 overflows
upstream from the Albosaggia bridge occurred.
During the night between 18 and 19 July, after the
heaviest rainfall had ceased, the worst episode of the
Adda flooding took place. After a levee breached near
the village of Berbenno, the entire plain to the right of
the river was inundated. Between 23:30 and 24:00 on
18 July, when the discharge of the Ardenno segment
was more than 1000 m3/s and the hydrometric level
about 1 m below the edge of the levee, the waters
violently broke the levee in the Berbenno municipality.
The flow was initially contained by the intact levee to
the south and the railway embankment of the 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 min. At
1:00 on 19 July, the violence of the water flowing out
F. Luino / Geomorphology 66 (2005) 13–3920
of the 150-m-wide breach destroyed the railway and
road embankments 200 m ahead of it. The waters then
swept across the entire area of Piana di Selvetta. At
4:15 the waters reached Ardenno at the lower end of
the Piana di Selvetta, some 4.5 km away. In the area
around Ardenno, the flood wave was held back by the
right levee of the Adda and the left levee of the Masino
Stream. These obstacles caused the water to rise at a
rate of 6 cm every 5 min and to back up towards the
site of the levee breach. At around 10:00 on 19 July,
about 5 h later, the backup stretched 4 km upstream.
The water level on the Piana di Selvetta continued to
rise throughout the morning, submerging an area of
about 10 km2 on the valley bottom, with record levels
just over 4 m in low lying areas.
At 12:00 on 19 July, some inhabitants of Ardenno
destroyed the levee blocking the downvalley flow
direction of water into the Adda riverbed. The
discharge emptied through an opening (6 m wide, 2
m deep), allowing the water levels on the Piana to
decrease gradually (2–2.5 m in 30 h). Five days later,
the floodwaters has almost completely receded,
leaving behind a thick layer of mainly clayey-sandy
deposits measuring from 40 cm to 1 m thick in the
low lying areas near the levee opening. Because of
the breach in Piana di Selvetta and the breach
downvalley in the area of Talamona, the discharge
was considerably reduced, with less serious damage
to the area around Talamona and to areas further
downstream where the Adda waters, although they
overflowed the riverbanks, were held back by the
main levee that runs its final 15 km along the Adda
riverbed. The relative peak discharge was recorded 18
km downstream from Ardenno at 6:00 on 19 July.
The Fuentes gauge (2498 km2) a level equal to a
discharge of 1100 m3/s was observed.
On 25 July, several days after the critical period
had passed, a new state of emergency took place
when a discontinuous breach was sighted on the
eastern slope of Mount Zandila. The breach ran 60 m
along the scarp foot line at an altitude of 2200 m,
coinciding with the sliding surface of the old
landslide. In this area, the total cumulated rainfall
was 229 mm, with 124 mm of cumulated rainfall
measured on 18 July alone. From 26 to 27 July, the
breach widened to 900 m, forming a crescent-shaped
opening. On 27 July, several rockfalls on the eastern
slope triggered 98 falls in only 24 h (Govi and
Turitto, 1992). The inhabitants of the villages of
Morigone, San Antonio, Poz and Tirindre were
quickly evacuated.
At 7:24 on 28 July, a wide mass of rock (estimated
34 million m3) detached from the eastern slope of
Mount Zandila (Costa, 1991; Govi and Turitto, 1992).
The displaced mass, including the prehistorical slide
and the bedrock, moved in two short phases. The first
came down with a northward slide of the upper part
of the slope; the second, in a single rapid displace-
ment, spread eastward into the Adda valley bottom,
sweeping the village of Morignone away (Fig. 5).
The mass roared up the opposite slope of the valley to
about 300 m above the valley floor before splitting
into two parts, diverted upstream and downstream.
The downstream mass travelled almost 1400 m from
the impact point. The first plunged into the small
lake, shooting alluvial debris and muddy water 140 m
high. The impact unleashed a high wave that moved
quickly upstream. Eyewitnesses reported that the
wave travelled 1000 m in about 30 s (Govi and
Turitto, 1992). The mud marks surveyed at a
maximum height of 95 m near the source decreased
to 15 m northward at a distance of about 1300 m. The
villages of Poz, San Antonio and Tirindre were razed
to the ground. In the partly evacuated village of
Aquilone more than 2 km upstream 27 people
perished. Just before the wave impact, survivors
saw the bell tower of the San Antonio church shatter
from the violent blast, which also blew down trees on
the opposite slope over 300 m away. On the opposite
side of the valley and upstream to Massaniga Creek, a
dark dust cloud extending up to 2 km a.s.l. was seen
briefly before it disappeared about 20 s later (Azzoni
et al., 1992). No seismic activity was recorded before
the collapse; the seismogram indicated that the
detachment of the mass occurred in 18 s and the fall
in 23 s.
4.2. The November 1994 event in the Tanaro River
basin
On November 1994, a severe hydrological event
hit the Tanaro River basin (Fig. 6). Landslides and
large floods caused widespread damage to 38
urbanized areas. The effects were catastrophic: 44
victims, 2000 homeless, over 10 billion Euros of
damage in all.
Fig. 5. The Mount Zandila rock avalanche took place on 28 July 1987, 10 days after the rainfall had stopped. The mass movement totally
covered the valley bottom with an estimated volume of about 34 million m3, more than 2 km long. The average thickness of the accumulation
was about 30 to 60 m, with a maximum of 90 m. Within several days, the continuous inflow of water upstream from the huge accumulation
formed a lake (arrow); 30 days later, after another intense rainstorm, the basin filled to about 20 million m3.
F. Luino / Geomorphology 66 (2005) 13–39 21
During the first week of November 1994, a vast
low-pressure system over northwestern Europe
brought heavy rains to most of Piedmont (Mercalli
et al., 1995). The rains started on 2 November and
continued through the next day, with showers that
peaked in the Ligurian Alps (50 mm). Heavy rain
began to fall over nearly the entire area on 4
November, with intermittent showers that posed no
cause for alarm. However, the next day violent rainfall
developed and continued throughout 6 November,
particularly along the pre-alpine belt. On 4 and 5
November, over 200 mm of rain were recorded in the
upper and middle parts of the valley and in the upper
stretches of the Tanaro tributaries: the Belbo, Bormida
and Orba rivers. Precipitation reached a maximum
hourly intensity of 55 mm (Cairo M. station) and a
total cumulated rainfall of 264.4 mm in 24 h (Levice
station). The amounts of rainfall recorded at some
Tanaro basin rain gauge stations in the provinces of
Cuneo and Asti were particularly high. Previous
rainfall records were broken in 4 of the 42 stations
in 1 day and in 5 stations in 2 days. The first phase of
the event (50–60 h between 2 and 4 November) was
characterized by modest, widely distributed or inter-
mittent rainfall that varied locally from 30 to 60 mm
in places. During this phase, no landslides or mud–
debris flows were reported.
The second phase developed locally at various
times between 4 and 6 November, with intensive
rains lasting 24 h and varied total precipitation (from
150 mm to about 260 mm). This constituted the
critical phase of the event as it swept through the
entire upper Tanaro basin and the area between Alba
and Asti. During this phase (136 mm total rainfall in
10 h, with peaks of 109 mm recorded between 2:00
and 5:00 on 5 November), very fast soil slips of the
fluidified topsoil (mean thickness b1 m) occurred in
the upper part of the Bormida di Spigno river.
Similar instability processes triggered 1–3 h later just
north of Ceva (with peaks of 90.6 mm recorded
between 3:00 and 8:00). Meanwhile (morning of 5
November), the first torrential floods triggered in the
secondary hydrographic network, producing local
floodings along the upper valley courses of the
Bormida and Tanaro rivers.
At 8:30, local torrential flooding triggered in the
small tributaries of the Tanaro (Armella and Pesino
Creeks at Ormea, areas of less than 20 km2), while
Fig. 6. Map of Tanaro basin showing isohyets (mm) of 5–6 November, towns and rivers mentioned in the text.
F. Luino / Geomorphology 66 (2005) 13–3922
further downvalley, along the Cevetta Stream (area,
62 km2), the first flood wave was generated at 10:00,
fed by an episode of increased rain intensity (120
mm recorded between 3:00 and 10:00 of the
morning of 5 November). Over the next hours of
the late afternoon, the rain front moved NNW into
the entire area of the Langhe towards West, where
widespread soil slips triggered in the saturated
superficial cover. Here the shallow landslides
occurred more often between 10:00 and 12:00 (10–
12 h of uninterrupted rainfall, with peak totals
between 80 mm around Alba and 110 mm around
Dogliani) (Fig. 7). As the rainfall continued into the
late evening, the number of soil slips increased
throughout the area up to 100 soil slips per km2 were
recorded in one area alone (Luino, 1999).
During the afternoon and into the late evening,
somewhat later than the soil slips, many rock block
slides were triggered in the marly-silty and arena-
ceous-sandy alternances (range of thickness, 5–30 m).
The first local rock block slides occurred between
12:20 and 18:00, with a major frequency between
18:00 and 23:00. Peak cumulated rainfall varied
locally from a minimum of 200 mm to just over 300
mm in some places. These rainfall amounts were
cumulated, although with certain brief interruptions,
over a time period of 70–80 h, starting from the
beginning of the first phase of the event (afternoon of
2 November). In several cases, rock block slides were
also recorded during the morning of 6 November,
after the rainfall event had begun to subside (Fig. 8).
The paroxysmal phase occurred between 5 and 6
November, with large-scale flooding along the upper
and middle basins of the Tanaro from Ormea to
Alba, nearly simultaneously with episodes of peak
rainfall intensity, whereas the lower river basin areas
(Asti and Alessandria) were to feel the effects of this
phase slightly later. Since the violence of the river
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.
F. Luino / Geomorphology 66 (2005) 13–39 23
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 recon-
structed from eyewitness accounts of the local
population. These data provided valuable informa-
tion about the passage of the flood wave as it moved
downstream through towns and villages. The infor-
mation also permitted the construction of a time line
of events and phenomena such as overflow processes
and flood propagation into the surrounding country-
side, with peak spreads of flooding and phases that
led to the destruction of important structures and
infrastructures along the river.
A general description of the downvalley translation
of the flood can be summarized as follows:
– in the upper Tanaro basin, up to the town of Ceva,
the first floodings occurred in the late morning of 5
November and reached the paroxysmal phase
during the late afternoon–early evening hours the
same day, with peaks between 18:00 at Garessio
and at 20:00 at Ceva. In both cases, evaluation of
the correspondence between the observed water
levels and the peak flood phase was influenced by
the effects of superelevation of the water levels and
formation of backwater due to obstruction by
bridges located in both towns and by accumulation
of detritus and tree trunks (Fig. 9);
– in the middle stretch of the river course (from Ceva
to Alba), the floodwaters started to overflow the
riverbanks during the early afternoon hours of 5
November, creating more violent phenomena after
21:00 (Niella Tanaro) and about 24:00 (Alba).
Pulsations in rising water levels occurred, with
local peaks sometimes earlier here than in stretches
further upstream or downstream. Generally, a rapid
retreat of floodwaters, often in 2–3 h, was
observed;
– along the lower stretch of the Tanaro (areas around
Asti and Alessandria), the flood reached its peak on
6 November. The first severe floodings (observed
at 2:00 at Asti and at 11:00 at Alessandria) reached
their peak levels in the two towns (Luino et al.,
1996) within 2 h and began to subside over 10 h
later;
– between 6 and 7 November, the abundant inflows
coming from the Tanaro and its tributaries caused
the water levels of the Po to rise rapidly. At the
Becca station, the closing point of the entire
western hydrographic network, a peak level of
7.65 m over hydrometric zero was measured at
11:00 on 7 November, a mere 20 cm below the
record high of 1951, with a rise of 2.65 m in less
than 20 h.
According to eyewitness accounts, in many towns
the flood did not invade the area in a single peak wave
but rather in a series of waves. However, the reasons
Fig. 8. Rock-block slide on a slope near Murazzano. The big rocks moved about 80 m along a sliding surface (11–128). At the end of the
movement, the surface appeared smooth like an inclined plane, sometimes showing the shallow tracks left by the sliding rock block.
F. Luino / Geomorphology 66 (2005) 13–3924
for such rises and falls cannot be completely
explained even when taking into account phase
differences of the waters brought by the main
tributaries of the Tanaro, as in the case of Corsaglia
Stream (area, 307 km2) whose flood flowed into the
Tanaro slightly before the Tanaro water levels peaked
due to the large size of the Tanaro basin (area, 503
km2) at the confluence of the two watercourses.
What emerged from surveys carried out during
the event and other information sources, particularly
in the stretch between Ceva and Alba, was evidence
of the widespread effect of partial or complete
obstruction of the flow back into the riverbed due
to road and railway structures (bridges, embankments
and approaches) and by damming due to the huge
amounts of floating materials (bushes, trees and
various other types of materials) blocked between
buildings. These obstructions impeded the water
from flowing back into river courses and led to the
rise in backups and overflows upstream from
bridges, often causing them to be washed out or
completely destroyed (Turitto et al., 1995).
The direct effect of these processes was the
generation of flood waves, as reported by eyewit-
nesses, 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 obstruc-
tion caused by a barrage near Clavesana. In this
stretch of wide meanders between the towns of
Clavesana and Carru, comprising about 2.6 km where
the Tanaro is spanned by two barrages and three
bridges, eyewitnesses reported that between 13:00 and
22:30 at least three flood waves had occurred. Slightly
further downstream, in the area around Farigliano, a
similar situation occurred that was characterized by
transient rapid rises and falls in water levels,
especially between 18:00 and 23:00, along this 7-km
stretch of meanders, where the river is spanned by
seven roadway bridges and three railway bridges.
Further downstream, the events can be summarized
as follows:
– in the area of Lequio Tanaro, between 22:00 and
23:00 on 5 November, the left bridge girder of the
first railway bridge was destroyed;
– in the area of Monchiero, a flood peak was reported
upstream from the approach embankment of the
provincial road bridge leading into the town at
about 21:00, just before a wide opening was torn
into the embankment;
– the effects of the unleashed backup floodwaters
were felt about 4 km downstream in the town of
Narzole, where a flood wave was observed just
Fig. 9. Ceva, 6 November 1994. Floating materials blocked the bridge span; the floodwaters overtopped the structure and levees upstream,
invading a large urban area.
F. Luino / Geomorphology 66 (2005) 13–39 25
after 22:00 at the road/railway bridge. Obstructed
by the bridge, the floodwaters backed up, tempo-
rarily invading the valley bottom and spreading
over about 90 ha; the water level remained high
until the left embankment of the bridge collapsed
between 22:30 and 23:00;
– another backup developed (approximately 120 ha
of the valley bottom) around the structures crossing
the Tanaro at Pollenzo. Here, between 23:30 and
24:00, floodwater accumulated behind the bridge
approach on the right riverbank, rising about 4 m
high from ground level of the low-lying area. At
about 1:00 on 6 November, the structure was
destroyed and the floodwaters spread 2500 m into
the right riverbed, where local morphotopographic
features forced the water back into the Tanaro
riverbed, causing erosion along the left bank,
which was already submerged by the runoff
coming out of drainage canals;
– in the area around Alba, 10 km downstream, the
peak water level along the Tanaro was observed
between 24:00 of 5 November and 1:00 of 6
November. This event occurred slightly earlier than
that at the Pollenzo bridge, and therefore has no
relationship with it. The city of Alba and the
surrounding area were invaded by floodwaters
(Luino and Turitto, 1998) from the Talloria and
Cherasca streams on 5 November several hours
before the flood wave generated along the Tanaro,
as reconstructed from evidence collected at Pol-
lenzo and Narzole.
4.3. The October 2000 event in the Aosta Valley
In October 2000, a severe hydrometeorological
event hit a large part of the Aosta Valley and the basin
of Dora Baltea River: the main watercourse rises in
the massif of Mont Blanc and after crossing the Aosta
Valley flows into the Po River after 160 km (Fig. 10).
The event started on 12 October, when a cold front,
associated with a wide, low depression over the
British Isles, reached the western Alpine rim, drawing
currents of moist unstable southwesterly air into the
Aosta Valley and bringing light rain to the areas
neighboring the region of Piedmont in the early
afternoon. During 13 October as the inflow of
southerly air currents into the Aosta Valley intensified,
the rainfall became widespread and heavier (Mercalli
and Cat Berro, 2001). Rising temperatures from
sirocco winds raised the freezing level from 2400 to
3000 m within a few hours. Such factors, together
with intense rainfall at high altitudes, melted the snow
that had fallen in late September.
Champorcher Valley was the first area to receive
intense precipitation. On 13 October, 176 mm was
recorded (peak of 23 mm/h between 17:00 and 18:00)
Fig. 10. Map of the hydrological event that occurred in the Aosta Valley showing isohyets (mm) of 11–16 October 2000.
F. Luino / Geomorphology 66 (2005) 13–3926
at the Champorcher rain gauge station. Rainfalls
triggered soil slips in several areas of the valley,
severely damaging roads and houses. Near Cham-
porcher, the Ayasse Stream (subtended area, 63.8
km2) rose 77 cm in 7 h and peaked at 23:00. The flood
completely washed out many sections of the main
road along the valley floor and a tourist recreation
area (already damaged in 1994), and left a thick
deposit of mud and sand on the local sports grounds.
The storm then moved westwards into the Cogne
Valley, where 83.8 mm of rainfall was recorded, with
peaks of 9 mm/h. In the stretch between Lillaz and
Champlong, a rock-block slide in glacial deposits
(more than 100,000 m3) triggered on the left slope of
the Urtier Stream (Bonetto and Mortara, 2003). The
displaced mass moved on a gentle slope for some
hundreds of meters, and then formed a temporary dam
in the stream. Unlike the Champorcher Valley, the
Cogne Valley witnessed no shallow landslides at this
time. In the others valleys, record daily rainfall
amounts of 20–40 mm were measured, with peaks
of 8 mm/h. Several hours later than its tributaries, the
Dora Baltea River rose 0.45 m in 1 h (22:00–23:00).
On 14 October, rainfall grew heavier: 179.2 mm
at Cogne (peak, 16.4 mm/h), 149.4 mm at Cham-
porcher (13.8 mm/h) and 116.2 mm at Valsavarenche
(11.2 mm/h) were recorded. During the night, the
temperature increased notably, reaching a maximum
of 20.6 8C in Aosta (565 m a.s.l.) and 9.7 8C in
Cogne (1495 m a.s.l.).
The Dora Baltea began to swell. At the Hone
section, the hydrometric level rose from 4.91 to 5.90
m between 06:00 and 18:00. Near Cogne, the first
soil slips triggered at 18:00, blocking roadways and
hindering traffic in the area. The Civil Defence
closed many roads and bridges considered to be
dangerous.
In the night between 14 and 15 October, rainfalls
gradually intensified, particularly around Cogne and
Champorcher (1400 m). The maximum hourly rainfall
amounts were 15.8 mm at Cogne (24:00–01:00) and
37 mm at Champorcher (02:00–03:00). All the right-
hand tributaries of the Dora Baltea reached high
levels, causing general alarm among the local
inhabitants. Early the next morning, the peak phase
of the event took place. In 5 h, between 04:00 and
09:00, many soil slips and mud–debris flows triggered
along the slopes and in the basins, followed by
flooding of the tributaries and widespread inundation
on the valley bottom of the Dora Baltea.
F. Luino / Geomorphology 66 (2005) 13–39 27
In the Cogne Valley, a soil slip near Epinel at 04:00
razed some houses. At 4:30, the inhabitants of two
small villages near Valpelline were woken by the
boom of a debris flow along Brison Creek (5.13 km2).
The mass movement buried the municipal road, a
square and the main road of the valley. At the same
time in the Cogne Valley, a debris flow of Arpisson
Creek (6.4 km2) struck the village of Epinel, levelling
all the houses adjacent to the river. Many were
invaded by mud and debris and some were completely
destroyed. At 5:00, in two small villages of Gressoney
Saint-Jean municipality, the Lys Stream floods under-
mined the foundation of an apartment building,
causing it to collapse but without claiming victims,
while a violent flood of a Lys tributary killed several
animals and damaged a farmhouse. At 6.15 in the Lys
Valley, near Issime, a rockfall in the basin of Rickurt
Creek (2.3 km2) augmented a debris flow that spread
onto the alluvial fan, causing damage. Displaced
materials damming the Grand’Eyvia Stream near
Cogne (60.6 km2) caused a backup of floodwater
(2.58 m) that peaked at 7:00. At 7:30, slightly
downstream from Valpelline, the Buthier Stream
overflowed, washing out the regional road. The
swollen waters headed towards the city of Aosta.
At Nus, on the left bank of the Dora Baltea, local
eyewitnesses reported that since the early morning
Fig. 11. During the 2000 event in the Aosta Valley, the Saint Barthelemy St
debris and sediments over an area of 0.45 km2 on the alluvial fan. The ph
many houses: 1288 people were temporarily evacuated. The arrow indicate
of the flow from the ordinary channel.
hours the level of the S. Barthelemy Stream (82.2
km2) had begun to rise dramatically due to the detritus
and tree trunks obstructing the Mazod Bridge. In the
S. Barthelemy basin, tens of soil slips and debris
flows had started, associated with deep lateral erosion
of the main watercourse. At 8:00, a violent debris flow
of the S. Barthelemy Stream burst across the Nus
alluvial fan, destroying buildings by the force of huge
masses of detritus (Fig. 11) deriving from the
hollowing of the alluvial fan body on the right side.
The flow lasted for several hours and left a deposit of
an estimated 200,000 m3 of detritus on the alluvial
fan. Along both sides of the main valley between
Aosta and Montjovet, many soil slips detached deep
sections of the topsoil at various elevations.
Around 8.30 a boom shook the village of Perron di
Fenis. According to eyewitness accounts, 10–15 s
later a debris flow of Bioley Creek (4.7 km2) invaded
several houses with several tens of thousands of cubic
meters, causing severe damage and claiming six lives.
Not only were newly built or restructured houses hit
by the mass, but also a 17th century chapel which in
its entire history may never have testified to the likes
of such an event (Tropeano et al., 2003).
At Pollein, near Aosta, at 9:00 a sudden mud–
debris flow in the Comboe basin (16.2 km2) smashed
into buildings and gutted houses; seven lives were
ream hit Nus village, spreading at least 350,000 m3 of mainly coarse
otograph shows the violence of the flow that destroyed and buried
s the house, at the apex of the alluvial fan that caused the deflection
F. Luino / Geomorphology 66 (2005) 13–3928
lost. An estimated volume of 150,000 m3 was left on
the alluvial fan (Tropeano et al., 2000).
At the same time, Buthier Stream floods reached
Aosta (area, 456.5 km2), where the stream level rose 1
m in 50 min. Because of its exceptional discharge
(N500 m3/s) (courtesy of L. Marchi), at 9:30, as
waterfloods overflowed the stream banks and inun-
dated the Dora quarter, 350 persons were quickly
evacuated (one victim) and vast areas were flooded,
leaving a remarkably thick deposit of mud and sand.
Between 11:00 and 11:30, the Dora Baltea began to
flood villages. At Donnaz, the river rapidly flooded
the old section of the village (one victim). The level of
the Dora Baltea continued to rise for several hours. At
the Hone gorge, it reached a maximum level of 8.73 m
on the hydrometric scale at 14:30. Some stretches of
the Turin-Aosta highway, the main communication
route through the Aosta Valley were washed out, even
though the embankment rises 2–3 m on the flood
plain. By the afternoon of 15 October, the first rescue
operations had reached the disaster area. Most roads
were interrupted and the valley bottom of the Dora
Baltea was covered by a vast sheet of water.
Arriving with considerable delay, a violent debris
flow occurred in Letze Creek (area, 1.02 km2) at
22:15 that night. Several houses of the Bosmatto
village (Gressoney Saint-Jean municipality) on the
Fig. 12. 15 October 2000. Letze alluvial fan: a violent debris flow razed to
Bosmatto village (near Gressoney). The debris flow submerged everything
than 10 m3 in volume were observed.
alluvial fan were completely razed to the ground
(Chiarle and Mortara, 2000). Compared with the
timing of the other debris flows in the area, the time
lapse (13–14 h) here was probably due to a temporary
dam caused by the reactivation of an old landslide on
the right slope of Letze Creek (Fig. 12). The rainfall
gradually let up over the later half of 15 October,
diminishing to between 1 and 6 mm/h. In the night
between 15 and 16 October, flood phenomena
subsided, ending in the afternoon of 16 October.
In the time period between 19:00 of 12 October
and 19:00 of 16 October, maximum rainfall amounts
were recorded at Champorcher (612.2 mm), Cogne
(456 mm), Valsavarenche (311.8 mm), Gressoney
(308.1 mm) and Aosta (262 mm). In these areas, the
rainfalls equalled from about 35–50% up to 65%
(Cogne) of MAR.
The soil slips were mostly concentrated along the
middle part of the main valley. This concentration
may be attributable to the geolithological features of
the sector, which is characterized by a broad surface
cover deriving from an extremely tectonized and
dislocated bedrock. Shallow landslides also occurred
in the Rhemes, Cogne, Ayas and Lys valleys.
Reactivation of at least five large landslides (Pollein,
Vollein, Chervaz, St. Rhemy-en-Bosses, Closellinaz)
were later recorded. These landslides (from several
the ground one of the two twin apartment buildings (asterisk) of the
under 2–3 m of material; in front of the flow some rock blocks more
F. Luino / Geomorphology 66 (2005) 13–39 29
tens of thousands to some millions of cubic meters)
did not collapse; however, they caused morphological
effects, with serious implications for public safety
(Bonetto and Mortara, 2003).
Because Grand’Eyvia basin was probably the
watershed that influenced more the Dora Baltea
discharge, we can consider the downvalley translation
of the flood wave in the reach Cogne-Hone. Thanks to
the hydrographical network of Regione Autonoma
Valle d’Aosta, a general description can be summar-
ized as follows:
– in the Cogne Valley, a classical alpine valley
characterized by notable sways and many gorges,
in the reach between Cogne and Aymavilles (mean
channel slope of 4.4%), the Grand’Eyvia Stream
covered 20 km in 1 h (6.7 m/s). The violence of the
flow eroded long reaches of banks, producing
severe damage to the main road running on the
valley bottom.
– along the Dora Baltea riverbed, the flood wave
moved at different speeds depending on the
morphology of the valley bottom. In the reach
between Aymavilles and Brissogne (mean channel
slope of 0.56%), the Dora Baltea floods over-
flowed the banks only in some stretches. This
sector is characterized by a well-incised riverbed,
with some islands and protected banks. The floods
flowed along 15 km in 60 min (4.2 m/s). In this
reach, the contribution of two tributaries was
Fig. 13. Dora Baltea valley bottom near Hone. Large sandy deposits delim
overflowed by the Dora Baltea waters on 15 October 2000.
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 chan-
nel slope of 0.56%), the waters covered 28 km in 3
h 10 min (2.5 m/s). The Dora Baltea valley bottom
here is influenced by the presence of wide alluvial
fans on both flanks; in this reach the Dora Baltea
riverbed narrows from a maximum width of 90 to
15 m (near Montjovet) where there are deep
gorges. Also in this reach, the Dora Baltea floods
did not spread on the flood plain, except in small
areas.
– in the reach Champdepraz-Hone (mean channel
slope of 0.25%), the valley bottom is wide and flat.
The Dora Baltea spread out onto the flood plain,
which was almost totally inundated in some
stretches. For this reason, the flood wave reduced
its speed to 1.1 m/s, covering 10 km in 2 h 30 min.
In this reach, the valley bottom is irregularly
urbanized. The houses near the riverbed (Verres,
Arnad, Hone) were completely flooded. The
buildings on the other side of the highway
embankment were also overflowed (Fig. 13).
– in all, on the main valley bottom, from Aymavilles
to Hone (mean channel slope of 0.5%), the Dora
Baltea floods moved along 53 km in 6 h 40 min
(2.2 m/s).
ited the flooded area: the asterisk shows the highway Torino-Aosta
Fig. 14. Sequence of natural processes in northern Italy. Straigh
lines show the first emergence of each process during extraordinary
hydrological events. Dashed lines mark the possible evolution of the
process.
F. Luino / Geomorphology 66 (2005) 13–3930
– in the Aosta Valley the Dora Baltea discharge was
not measured. In Piedmont, at Tavagnasco station
(3313 km2), the peak discharge was indirectly
evaluated about 3100 m3/s (Barbero et al., 2003),
exceeding the previous maximum of 1920 (2670
m3/s).
– the Dora Baltea flood wave continuing downstream
caused heavy losses in different municipalities:
bridges, earthen approaches encroaching the flood
plain and river works were destroyed.
The effects of the October 2000 event, which
severely affected about 60% of the Aosta Valley, were
particularly disastrous due to the concurrence of the
following factors:
– the heavy rainfalls in the period from 28
September through 1 October, with more than
200 mm in the lower Aosta Valley. Some authors
(Mercalli and Cat Berro, 2001) have reported that
these precipitations, in addition to the partial
snow melting over the following days, might have
kept the soils and the underground hydrographic
network saturated, leading to a subsequent
increase in the instability processes that occurred
2 weeks later.
– the wideness of the drainage basin involved due to
the presence of a high freezing level (3000 m);
– the significant hourly increases of hydrometric
levels due to a short concentration time of the
tributaries caused by local regional morphology,
which is characterized by steep and relatively short
valleys (the average elevation of the Aosta Valley
is about 2100 m a.s.l., with 20% under 1500 m
a.s.l);
– the numerous mud–debris flows in the tributaries,
sometimes due to the collapse of landslides in the
middle-upper part of the basin, with subsequent
temporary damming and relative rapid outflow
when the displaced mass was demolished. Mud–
debris flows produced deep bank erosions, obstruc-
tion or destruction of bridges and huge spreading
on the alluvial fans, with severe damage and loss of
lives in the villages.
During the event, 385 landslides were triggered on
the slopes and 259 debris flows occurred along the
tributaries, flooding a total area of 5 km2. On the
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 sequen-
ces. These events have allowed us to identify a critical
threshold, which is about 10% of the local MAR.
Once the threshold has been exceeded, the instability
processes on the slopes and along the hydrographic
networks follow a sequence that can be reconstructed
in three different phases (Fig. 14).
5.1. The first phase
A hydrological event, particularly in autumn and
spring, usually starts with a period of light rainfall of
t
F. Luino / Geomorphology 66 (2005) 13–39 31
some millimeters per hour. When this cumulative
rainfall reaches the critical threshold above men-
tioned, the hydrological event begins. As the water is
no longer able to seep into the ground surface, it runs
off the slopes following natural or artificial drainage
paths (e.g. valley bottoms, hollows, roads). First
processes are usually soil slips (sensu Campbell,
1975), involving the saturated topsoil. The slip surface
forms along the irregular contact between the collu-
vium and the altered bedrock. Such movements
usually occur on slopes ranging from 168 to 458,involving a slope cover from 0.4 to 1 m in depth. So
they are moderate in volume, ranging from a few
cubic meters to several tens of cubic meters. Yet
despite their size, they start to produce problems:
displaced material can easily block roads and create
difficulties for drivers, but above all they impede the
work of rescue teams (Fig. 15).
In continuous precipitation, the soil slip volumes
may be quite significant and may have a considerable
area density (Luino, 1999; Polloni et al., 1996). They
are usually characterized by liquefied masses that
travel long distances (Govi et al., 1985). Common
underestimation of soil slips, deriving from the
scarcity of historical records and morphological
evidences, is due to the relatively low magnitude of
single events. The effects produced by these shallow
landslides are usually rapid, but the huge shock of the
Fig. 15. Ceva (Tanaro Valley) on 5 November 1994. A small soil slip in
retaining wall probably built to avoid just this kind of phenomenon.
mass added to event unexpectedness can also cause
severe damage (Luino et al., 2003). While their
movement starts as a shallow landslide, they can
sometimes evolve into a fast flow, particularly when
conveyed in small creeks or slope cuts. Although
small in mass, the flows are very dangerous because
they occur suddenly and travel at velocities of 2 to 9
m/s (Govi et al., 1985), producing high collision
forces.
The most significative hourly intensities triggering
numerous soil slips are those recorded in the last hours
just before the collapse. High hourly intensities
compensate for insufficient critical values of cumu-
lated rainfall or vice versa (Govi et al., 1985).
During the July 1987 event, the first soil slips in
the Torreggio Valley were triggered when cumulative
rainfall reached 9.9% of the MAR (128.8 mm/1300
mm), while this value was 10.7% in the Brembana
Valley (160 mm/1500 mm) and rose for the landslides
in the Tartano Valley (15.2% of the MAR) (Fig. 16a).
During the 1994 event in the Tanaro Valley, the first
shallow landslides occurred when, in different areas,
total rainfall reached 11% (Rodello), 12.3% (Ceva),
14.4% (Cossano) and 18.2% (Cairo M.), respectively.
The landslides triggered only 2–4 h after reaching the
critical threshold of 10% of the MAR (Fig. 16b). In
the 2000 event in the Aosta Valley, the first superficial
landslides occurred when cumulative rainfall reached
vaded the road. The mass was triggered on the flank of a concrete
Fig. 16. Cumulative rainfall of the hydrological event: (a) Valtellina 1987 (Ta=Tartano, B=Brembana, To=Torreggio); (b) Tanaro Valley 1994
(R=Rodello, Ce=Ceva, Ca=Cairo M., Co=Cossano); (c) Aosta Valley 2000 (Ch=Champorcher, Co=Cogne). White asterisks show the 10%
threshold of the local MAR for each rain gauge; black arrows indicate the triggering moment of the first soil slips in the vicinity.
F. Luino / Geomorphology 66 (2005) 13–3932
12.2% (Champorcher) and 16% (Cogne) of the MAR,
2 and 12 h, respectively, after reaching the critical
threshold (Fig. 16c).
In the first phase, violent mud–debris flows can
also be observed in small alpine watersheds of less
than 20 km2 (Fig. 17). Particularly in autumn and
spring, they usually develop when, after some hours
of light rainfall (3–6 mm/h), a violent shower occurs
(N30 mm/h). Mud–debris flows can start as a result of
slope-related factors, and shallow landslides can dam
Fig. 17. Pollein (Aosta Valley). The destroyed house testifies to the
devastating effects of the Comboe debris flow over the urbanized
area of Chenaux village in the early morning of 15 October 2000.
streambeds, provoking temporary water blockage. As
the impoundments fail, a bdomino effectQ may be
created, with a remarkable growth in the volume of
the flowing mass, which takes up the debris in the
stream channel. The solid–liquid mixture can reach
densities of up to 1.8–2 tons/m3 and velocities of up to
13–14 m/s (Arattano, 2003; Chiarle and Luino, 1998;
Tropeano et al., 1996). These processes normally
cause the first severe road interruptions, due not only
to deposits accumulated on the road (from several
cubic meters to hundreds of cubic meters), but in
some cases to the complete removal of bridges or
roadways or railways crossing the stream channel.
Damage usually derives from a common under-
estimation of mud–debris flows: in the alpine valleys,
for example, bridges are frequently destroyed by the
impact force of the flow because their span is usually
calculated only for a water discharge. For a small
basin (1.76 km2 in area) affected by a debris flow,
Chiarle and Luino (1998) estimated a peak discharge
of 750 m3/s for a section located in the middle stretch
of the main channel. At the same cross section, the
maximum foreseeable water discharge (by HEC-1)
was 19 m3/s, a value about 40 times lower than that
calculated for the debris flow that occurred.
During the July 1987 event, the first mud–debris
flows occurred in small alpine watersheds of the upper
Brembana Valley when cumulative rainfall reached
10.7% of the MAR, with a peak of 51 mm/h in the last
hour before the flows. Near Bormio, in several small
F. Luino / Geomorphology 66 (2005) 13–39 33
basins the first debris flows were triggered when total
rainfall reached 11.9% of the MAR (107 mm/900
mm). During the 1994 event in Tanaro Valley, the first
debris flow occurred near Ormea in the Armella Creek
(area, 17.5 km2), after 45 h of light rainfall (138 mm),
4 h after reaching the critical threshold. In October
2000, in the Aosta Valley, the first mud–debris flows
were triggered in Valpelline (Brison basin) at 13.1%
of the MAR, while in Cogne Valley (Arpisson Creek)
the processes occurred when the value reached 23% of
the MAR.
In the first phase, discharge increases substantially
in larger stream basins of up to 500 km2, as a
consequence of the mean rainfall fallen on a basin.
Riverbanks are severely eroded and streams begin to
threaten riverside structures and infrastructures (Fig.
18). The flow contains a remarkable volume of debris
and floating materials coming from the small tribu-
taries. 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 infra-
structures, sometimes owing to hundreds of uprooted
trees that obstruct part of the bridge span. This violent
flow may demolish bridges and road embankments by
side erosion. Usually, the floodwaters return to the
riverbed within 5 to 10 h.
Fig. 18. Trino (near Gressoney-Aosta Valley), 24 September 1993. The Lys
of the stream.
In July 1987, the Brembo Stream near Lenna (307
km2 in area) reached its first critical stage when the
mean rainfall on the basin, calculated by isohyetal
method (Wisler and Brater, 1959), was about 11% of
the local MAR. In November 1994, in the Tanaro
Valley, along the Cevetta Stream (area, 62 km2), the
first flood wave with erosion was generated at 10:00,
when the average precipitation on the basin was about
16.8% of the basin MAR (160/950 mm). In October
2000, the Ayasse Stream near Champorcher (area,
63.8 km2) overflowed its banks when the mean
precipitation was about 12.9% of the local MAR,
while the Buthier Stream inundated the town of Aosta
(area, 456.5 km2) after 57 h of light rainfall, when the
value reached 14% of the basin MAR (140/1000 mm).
5.2. The second phase
In continuous precipitation, during the second
phase, some violent flow phenomena can be observed
in alpine tributary basins larger than 20 km2 in area
(Govi et al., 1998; Tropeano et al., 2000). Processes
usually comprise hyperconcentrated flows (see Fig.
10) that can also convey large boulders. Measured
data have demonstrated a good relationship between
basin area and debris-flow magnitude; for the largest
watersheds the deposited mass can reach volumes of
hundreds of thousands of cubic meters (Marchi and
waters destroyed a house and the main road located on the right side
F. Luino / Geomorphology 66 (2005) 13–3934
D’Agostino, 2004). Villages and infrastructure located
on alluvial fans may be partially or totally filled up by
the debris (ARPA Piemonte, 2003; Eisbacher and
Clague, 1984; Chiarle and Luino, 1998; Govi et al.,
1979; Govi, 1984; Luino, 1998; Regione Piemonte,
1998; Tropeano et al., 1999; Tropeano et al., 2003).
During the 1987 event, the mud–debris flow of the
Madrasco Stream (28.7 km2) violently hit the village
of Fusine, when mean cumulative rainfall reached
18.5% of the basin MAR (259.4 mm/1400 mm), with
a peak of 38.4 mm in the last 3 h before the process
began. The destructive flow triggered 13 h after
reaching the critical threshold. In October 2000, in the
Aosta Valley, the first large mud–debris flows spread
on the Nus alluvial fan (Fig. 11), 12 h after reaching
the critical threshold. The processes occurred when
the mean rainfall on the Saint Barthelemy 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 land-
slides 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, endanger-
ing the villages and infrastructures located along its
banks.
During the July 1987 event, first remarkable
landslide (1.5�106 m3) was triggered on the right
slope of the Torreggio Stream. The mass movement
involved the granodioritic orthogneiss and phillite
schists bedrock. The landslide occurred after 100 h of
rain, when the cumulative rainfall reached 17.6% of
the local MAR (176.4/1000 mm), 14 h after reaching
the critical threshold.
In November 1994, the particular geomorphologic
setting of the Langhe hills, characterized by an
asymmetric slope profile due to the isoclinal bedding
of marly-silty and arenaceous-sandy alternances,
favoured many rock block slides. These landslides
involved the bedrock from depths of a few meters up
to 20–30 m, while their sliding surface was usually
parallel to the dip of the slope and the inclination,
which was often close to 11–128 (see Fig. 7). Since
the landslide area ranged from a few tens to several
thousands of square meters, the volumes varied from a
few hundred up to about one million cubic meters.
According to eyewitnesses, these slides occurred over
a period ranging from a few minutes to several hours,
starting from the appearance of the first cracks and
ending with the final collapse. During the peak phase,
the movements reached speeds varying from a few
decimeters to some hundreds of meters per hour.
During the 1994 hydrological event, the greatest part
of these landslides occurred after 55–72 h of rainfall.
The largest landslides moved between 17:00 on 5
November and 10:00 on 6 November. They slid in a
range of cumulative rainfall included between 19.9%
(Cerretto Langhe) and 28.6% (Gottasecca) of the local
MAR, in a period between 10 and 24 h after reaching
the critical threshold. Most of the landslides observed
in the Langhe Hills turned out to be reactivations of
landslides identified in the past. For the landslides that
occurred in the Langhe Hills in the 1970s, Govi et al.
(1985) identified a relationship between the critical
rainfall (which takes into consideration the rainfall
amount of the triggered event), the rainfall of the
previous 60 days and the monthly distribution of 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 vegeta-
tion is absent. Hydrographic stations are often swept
away by the violence of the water floods, so that the
discharges usually have to be evaluated indirectly.
The critical phase of a watercourse depends on the
distribution of rainfall on the basin. Rarely if ever
does a rainfall begin or end simultaneously over an
entire drainage basin, for usually the center of
disturbance is in motion. The direction in which the
storm travels across the basin with respect to the
direction of flow of the drainage system has a decided
influence upon the resulting peak flow and also upon
the duration of surface runoff. In the Tanaro Valley, in
November 1994, the first heavy rainfall hit the upper
part of the basin and the weather front then moved
northward approximately along the course of the
Tanaro River: so it was possible to follow the
translation of the flood waves along the main river.
F. Luino / Geomorphology 66 (2005) 13–39 35
In this case, also for a flood can be identifiable a
critical threshold, not local but for the entire drainage
basin. At Farigliano gauging station (area, 1522 km2),
the main peak level (3800 m3/s) occurred at 23:00 on
5 November, 12–14 h after the peak rainy period in
the upper part of the basin. Up to that moment the
mean rainfall over the basin, calculated by isohyetal
method, was 181 mm, namely the 16% of the MAR
(1130 mm).
The situation in the 1987 and 2000 events was
different mainly because of the kind of hydrographic
network involved. Where lateral valleys are located
almost perpendicular to the main river, their contri-
bution was very important and caused the main river
levels to increase rapidly. In Valtellina, at Ardenno
gauge (2096 km2), the peak discharge and relative
first inundations on the floodplain occurred early,
because the highest rainfall intensities hit mostly the
Orobic Alps. The left tributaries emptied their waters
into the Adda River some hours before the flow
coming from upstream. Also in the Aosta Valley, in
October 2000, tributary contribution rapidly raised the
hydrometric levels of the Dora Baltea River. The first
floods on the valley bottom were already recorded in
the morning of 15 October, nearly simultaneously
with the critical phase that was characterized by mud–
debris and hyperconcentrated flows in the small
basins. At Brissogne section (1900 km2), for example,
the peak level was reached at 9:00, around the same
time the violent processes on the alluvial fans hit
Fenis and Pollein.
5.3. The third phase
During the third phase exceptional discharges and
large floods in the basins larger than 2000 km2 can be
observed. The translation of a flood along a valley is
influenced by many factors precedently described and
for this reason it is difficult to follow a natural
evolution of the process along the riverbed from the
upper part of the basin to the mouth of the river.
Different peak stages are recognizable: the time
intervals between two consecutive surges cannot be
considered merely as translation times of the peak
stage, because they are conditioned by the presence of
manmade structures (Regione Piemonte, 1998; Turitto
et al., 1995) that form a series of obstacles to the
natural flow (e.g. bridges with inadequate spans,
riverbed narrowings). The propagation paths of an
atmospheric disturbance with respect to the direction
of the main river can also influence the space–time
distribution of the flood effects along the valley
(Luino, 1999).
Riverbed morphology is extensively modified,
with erosional and depositional processes in the
alluvial deposits of the riverbed and substantial
longitudinal and cross-profile changes in channel
morphology. This can locally undermine the stability
of bridge foundations, irrigation channels and flood
control structures.
Faults in structural defences (e.g. levee collapse)
may also be revealed. Water overtopping the levees
can flood towns and villages to various extent and
depth (Luino et al., 1996; Richards, 1982) and cause
severe damage. The ground is usually so saturated that
large areas with stagnant waters can still be observed
5 or 6 days after the paroxysmal phase of the
inundation. Water floods usually leave widely spread
silty-sandy sediments ranging in depth from some
decimeters to more than 1 m. The inundations that
occurred in Valtellina in the Tanaro and Aosta valleys
showed these characteristics, even if they were
different in size, area inundated, duration depending
on natural and certain manmade conditions. They
resulted in losses to inhabitants including loss of life
and property, hazards to health and safety, disruption
of commerce and government services, and expendi-
ture for flood protection and relief.
In July 1987, at Fuentes gauging station (2498
km2) the peak discharge was recorded at 6:00 on 19
July, after 100 h from the starting of the atmospheric
disturbance and after 24 h from the most intense rainy
period in the upper basin. After a levee breached in
the Berbenno municipality, more than 10 km2 of the
plain to the right of the river was flooded, with record
levels just over 4 m in low lying areas, and an
evaluated total volume of about 28�106 m3.
In November 1994, the critical phase in the area of
Alessandria occurred 75 h after the start of the
meteorological event in the upper part of the Tanaro
basin. The flood peak employed a lag time of about 20
h between the upper part of the basin (Garessio) and
Montecastello gauge station (197 km). The flood crest
moved with an average velocity of about 2.7 m/s. In
the reach Ceva-Alessandria 55 railway and road
bridges are located, only 2 of which were completely
F. Luino / Geomorphology 66 (2005) 13–3936
destroyed and 7 severely damaged. On the valley
bottom, waters inundated 15 urbanized areas, affect-
ing not only small villages but also large towns like
Alba, Asti (Fig. 19) and Alessandria (Luino et al.,
1996). On average, 30–50% of urban areas were
flooded and up to 100% (three villages).
In October 2000, the critical phase for the valley in
the final reach (Champdepraz-Hone) occurred after 62
h after the start of the atmospheric disturbance in the
upper part of the basin. In the reach Cogne-Hone, the
flood waves moved along 73 km in 7 h 30 min (2.8 m/
s). The span of some bridges over the Dora Baltea River
proved inadequate for so large discharge; the bridges
were overtopped, creating many problems particularly
for the houses located just upstream from the structure.
Some days after a prolonged rainy period, large
landslides involving the bedrock can still take place.
These phenomena usually cause the movement of very
large rock masses and can cause catastrophic effects in
Fig. 19. Asti during the November 1994 event: the Tanaro waters
invaded the streets of the town.
case of collapse. The total duration of rainfall usually
has a greater effect on these landslides than does the
number of short periods of very intensive precipitation.
The delayed response depends mainly on the litho-
logical 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 hydro-
logical event that hit the Aosta Valley, the reactivation
of at least five great landslides was recorded. These
landslides (from several tens of thousands to some
millions of cubic meters) did not collapse, but
provoked remarkable relevant morphological effects,
with serious implications for public safety.
6. Conclusions
Historical studies have demonstrated that in north-
ern 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 land-
slides, mud and debris flows in small watersheds and
floods in basins less than 500 km2 can easily occur.
These processes are usually triggered when the rainfall
F. Luino / Geomorphology 66 (2005) 13–39 37
has reached a value equal to 10–20% of the local mean
annual rainfall. This generally happens after continu-
ous and heavy rainfall up to 10–12 h. In the second
phase (12–24 h) mud flows and debris flows in basins
larger than 20 km2 can be observed. This period is
mostly characterized by floods in basins up to 2000
km2 in area and bedrock landslides of up to one to two
million cubic meters in volume. Rainfall recorded is
usually equivalent to 15–30% of the local MAR. The
third phase is characterized by large floods involving
basins at least 2000 km2 in area. That generally occurs
after more than 24 h after reaching the critical threshold
of the basin. Some days after an intense rainy period
large landslides moving million cubic meters of rock
can take place in mountainous areas.
During some of the events studied, the sequence
could not be divided into separate phases because the
events occurred simultaneously. This was mainly due
to the presence of intense rainfall pulses and the
generation of very diffuse surface runoff. Such
situations usually occur during brief, heavy summer
rainstorms or in late spring, when snow melt
combines with intense rainfall.
Usually, it is not uncommon for the person in charge
to devote an incredibly short time to the determination
of the evolution and magnitude of the natural process.
For this reason, when a severe meteorological event is
about to occur, the ability to foresee in which sequence
the instability processes may be triggered can prove to
be very important. Advance knowledge of the phases
and their development could permit the timely pre-
ventive 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 inte-
grated with comprehensive information about the
effects of past events (CNR, 1983; Domınguez Cuesta
et al., 1999; Eisbacher and Clague, 1984; Govi et al.,
1998; Goytre and Garzon, 1996; Luino, 1998; Luino
and Turitto, 1998; Guzzetti et al., 1994; Luino et al.,
2002; Tropeano and Turconi, 2003; Wieczorek et al.,
2002). By utilizing these data, statistical studies can
be conducted on the frequency of instability processes
in time and space. The same frequency forecasts can
be extrapolated for the future, assuming that the
probability of a given event will not change over
reasonably short time intervals. The collection of
historical data is very important but is insufficient to
predict instability in absolute terms and to ensure a
permanent safety level across wide land areas.
Even though the effects connected to the hydro-
logical 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 urbaniza-
tion, which has taken place mostly since the post-war
period. Such urbanization was not governed by a
carefully planned management of the territory, in
relationship to the hazards of natural processes. The
lesson to be learned from these events is that strict
caution should be takenwhen operating on the land, not
only in rebuilding operations, especially with the aim
of preventing risk in areas of future urban expansion.
In Italy, in these years, Civil Protection is working
full-time to prevent risks related to the development of
instability processes by control systems based on
meteorological forecasting and monitoring systems.
With a dense network of instruments in operation, Civil
Protection Units can receive real-time recording and
transmission of data (e.g. rainfall, temperature, wind,
water levels). These values, rapidly analysed by
complex mathematical models and managed by a
GIS, need to be compared with the data on past events,
and with critical rainfall thresholds and hydrometric
levels in particular. After identification of the at-risk
areas, a detailed weather report can be compiled and
sent to local authorities so that rescue teams can be
dispatched in a timely fashion; but these efforts must be
necessarily supported by large prevention campaigns
to create public awareness of environmental risks and
to teach people to coexist with such risks before, during
and after an emergency.
Acknowledgments
The author would particularly like to thank the IRPI
colleagues M. Govi and O. Turitto for allowing me to
use their data on Valtellina; D. Tropeano, G. Mortara,
M. Chiarle and S. Silvano for their useful indications
and review of the manuscript. The author is grateful
also to friends D. Cat Berro, F. Bonetto, C.G. Cirio, M.
Giardino, W. Giulietto, F. Guzzetti and S. Ratto. A
particular thanks to D. Alexander. All the photographs,
without further specification, belong to the CNR-IRPI
Turin Archive Department.
F. Luino / Geomorphology 66 (2005) 13–3938
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