sequence of instability processes triggered by heavy ......sequence of instability processes...

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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 km 2 in area, while discharge increases substantially in larger stream basins of up to 500 km 2 . In continuous precipitation, in the second phase, first mud–debris flows can be triggered also in basins larger than 20 km 2 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 km 2 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 km 2 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 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.09.010 * Fax: +39 11 343574. E-mail address: [email protected]. Geomorphology 66 (2005) 13 – 39 www.elsevier.com/locate/geomorph

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Page 1: Sequence of instability processes triggered by heavy ......Sequence of instability processes triggered by heavy rainfall in the northern Italy Fabio Luino* Consiglio Nazionale delle

www.elsevier.com/locate/geomorph

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

0169-555X/$ - s

doi:10.1016/j.ge

* Fax: +39 11

E-mail addr

(2005) 13–39

ee front matter D 2004 Elsevier B.V. All rights reserved.

omorph.2004.09.010

343574.

ess: [email protected].

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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

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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

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

stations.

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

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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),

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

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

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Fig. 4. Tartano Valley (Lombardy), 18 July 1987. At 17:00, a large

soil slip conveyed material into a small hollow incision, cut an

apartment building in two (black arrow) and invaded a hotel (white

arrow), killing 10 people (photo: Catenacci, 1992).

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

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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.

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Fig. 5. The Mount Zandila rock avalanche took place on 28 July 1987, 10 days after the rainfall had stopped. The mass movement totally

covered the valley bottom with an estimated volume of about 34 million m3, more than 2 km long. The average thickness of the accumulation

was about 30 to 60 m, with a maximum of 90 m. Within several days, the continuous inflow of water upstream from the huge accumulation

formed a lake (arrow); 30 days later, after another intense rainstorm, the basin filled to about 20 million m3.

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

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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

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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

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Fig. 8. Rock-block slide on a slope near Murazzano. The big rocks moved about 80 m along a sliding surface (11–128). At the end of the

movement, the surface appeared smooth like an inclined plane, sometimes showing the shallow tracks left by the sliding rock block.

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

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

invading a large urban area.

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)

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

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.

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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

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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

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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

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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

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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

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Fig. 16. Cumulative rainfall of the hydrological event: (a) Valtellina 1987 (Ta=Tartano, B=Brembana, To=Torreggio); (b) Tanaro Valley 1994

(R=Rodello, Ce=Ceva, Ca=Cairo M., Co=Cossano); (c) Aosta Valley 2000 (Ch=Champorcher, Co=Cogne). White asterisks show the 10%

threshold of the local MAR for each rain gauge; black arrows indicate the triggering moment of the first soil slips in the vicinity.

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

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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

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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.

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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

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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

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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.

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F. Luino / Geomorphology 66 (2005) 13–3938

References

Arattano, M., 2003. Monitoring the presence of the debris flow

front and its velocity through ground vibration detectors. In:

Rickenmann, D., Chen, C. (Eds.), Proc. of the Third Int.

Conf. on debris-flow hazards mitigation: mechanics, predic-

tion, and assessment, Davos, Switzerland, September 10–12,

2003, pp. 719–730.

ARPA Piemonte, 2003. Eventi alluvionali in Piemonte. Ages Arti

Grafiche. 327 pp.

Azzoni, A., Chiesa, S., Frassoni, A., Govi, M., 1992. The Valpola

landslide. Engineering Geology 33, 59–70.

Barbero, S., Graziadei, M., Rabuffetti, D., 2003. Evento alluvionale

regionale del 13–16 ottobre 2000: analisi idrometrica. Eventi

Alluvionali in Piemonte, ARPA Piemonte, Ages Arti Grafiche,

pp. 45–73.

Bonetto, F., Mortara, G., 2003. Fenomeni franosi in Valle d’Aosta.

Eventi Alluvionali in Piemonte, ARPA Piemonte, Ages Arti

Grafiche, pp. 303–309.

Brunetti, M., Moretti, A., 1987. Documentazione meteorologica

relativa all’alluvione in Valtellina del luglio 1987. Rivista di

Meteorologia Aeronautica XLVII (3–4), 199–213.

Campbell, R.H., 1975. Soil slip, debris flows, and rainstorms in the

Santa Monica Mountains and vicinity, Southern California.

USGS Professional Paper 851, 51 pp.

Cannon, S.H., Ellen, S., 1987. Rainfall that resulted in abundant

debris-flow activity during the storm. In: Ellen, S.G.F. (Ed.),

Landslides, Floods and Marine Effects of the Storm of January

3–5, 1982, in the San Francisco Bay Region, California, USGS

Prof. Paper, vol. 1434, pp. 27–33.

Carraro, F., Dal Piaz, G.V., Govi, M., Sacchi, R., 1970. Studi

geologici nel Vercellese e nella Valle Strona. Comitato Region-

ale per la Programmazione Economica del Piemonte, 113–182.

Catenacci, V., 1992. Il dissesto geologico e geoambientale in Italia

dal dopoguerra al 1990. Memorie della Carta Geologica d’Italia

XLVII, 301 pp.

Chiarle, M., Luino, F., 1998. Colate detritiche torrentizie innescate

dal nubifragio dell’8 luglio 1996 sul M. Mottarone (VB-

Piemonte). In: Luino, F. (Ed.), Proc. of the Int. Conf.:

Prevention of Hydrogeological Hazards: The Role of the

Scientific Research. Alba, Italy, 5–7 November, 1996, L’Artis-

tica, Savigliano, vol. II, pp. 231–245.

Chiarle, M., Mortara, G., 2000. Bosmatto (Gressoney Saint Jean), il

giorno del disastro. Nimbus (Torino) 21–22, 88–92.

CNR-Istituto di Ricerca per la Protezione Idrogeologica nel Bacino

Padano, 1983. Eventi alluvionali e frane nell’Italia settentrionale

(1972–1974). Torino, Fanton, 485 pp.

Costa, J.E., 1991. Nature, mechanism, and mitigation of the Val

Pola Landslide, Valtellina, Italy, 1987–88. Zeitschrift fqr Geo-morphologie 35, 15–38.

D’Atri, A., Dela Pierre, F., Gelati, R., Gnaccolini, M., Piana, F.,

Polino, R., 2002. Il Bacino Terziario Piemontese. In: Polino, R.

(Ed.), Il Sistema Alpino-Appenninico nel Cenozoico. 81a

Riunione Estiva della Societa Geologica Italiana: Guida alle

escursioni del 6–9 Settembre 2002. Litografia Geda, Nichelino

(TO), pp. 110–115.

Domınguez Cuesta, M.J., Jimenez Sanchez, M., Rodrıguez Garcıa,

A., 1999. Press archives as temporal records of landslides in the

North of Spain: relationships between rainfall and instability

slope events. Geomorphology 30, 125–132.

Eisbacher, G.H., Clague, J.J., 1984. Destructive mass movements in

high mountains: hazard and management. Paper-Geological

Survey of Canada 84-16, 230 pp.

Forlati, F., 1990. Restituzioni cartografiche prodotte: esempi e note

illustrative. In: Regione piemonte, Settore Prevenzione del

Rischio Geologico, Meteorologico e Sismico (Ed.) and CNR-

IRPI Torino bBanca dati geologicaQ, pp. 53–91.Govi, M., 1984. Les phenomenes catastrophiques d’origine

exogene. Proc. of the XXV Congres International de Geo-

graphie, bLes Alpesb, Paris, pp. 31–39.Govi, M., Sorzana, P.F., 1980. Landslide susceptibility as a function

of critical rainfall amount in Piedmont Basin (North-Western

Italy). Studia Geomorphologica Carpatho-Balcanica 14, 43–61.

Govi, M., Turitto, O., 1992. La frana di Val Pola del 1987 in alta

Valtellina. In: Vallario, A. (Ed.), Frane e territorio, pp. 392–414.

Liguori, Napoli.

Govi, M., Turitto, O., 1997. Recent and past floods in Northern

Italy. Proc. ICSU SC/IDNDR Workshop, Koblenz, pp. 13–32.

Govi, M., Mortara, G., Sorzana, P.F., Tropeano, D., 1979. Sintesi

dei dissesti idrogeologici avvenuti tra il 1972 e il 1974 nell’Italia

settentrionale. Bollettino della Associazione Mineraria Subal-

pina 16, 420–451.

Govi, M., Mortara, G., Sorzana, P.F., 1985. Eventi idrologici e frane.

Geologia Applicata e Idrogeologia, 359–375 (Bari, 20, parte II).

Govi, M., Luino, F., Turitto, O., 1998. Successione di processi

evolutivi naturali in concomitanza ad eventi idrologici straordi-

nari. In: Luino, F. (Ed.), Proc. of the Int. Conf.: bPrevention of

Hydrogeological Hazards: The Role of the Scientific ResearchQ.Alba, Italy, 5–7 November, 1996, L’Artistica, Savigliano, vol. II,

pp. 261–278.

Goytre, M.J., Garzon, G., 1996. Analisis de las avenidas historicas

en el Rıo Jucar. Proc. Sexto Congreso Nacional y Conferencia

Internacional de Geologia Ambiental y Ordenacion del Terri-

torio, Granada, 22–25 de Abril de 1996, vol. III, pp. 29–41.

Guzzetti, F., Cardinali, M., Reichenbach, P., 1994. The AVI Project:

a bibliographical and archive inventory of landslide and flood in

Italy. Environmental Management 18, 623–633.

Luino, F., 1998. Study on some villages damaged by natural

processes in NW Italy. In: Moore, D., Hungr, O. (Eds.), Proc. of

the 8th Congress of the IAEG, Vancouver, 21–25 September

1998, vol. II, pp. 1065–1070.

Luino, F., 1999. The flood and landslide event of November 4–6,

1994 in Piedmont Region (North-West Italy): causes and related

effects in Tanaro Valley. XXII General Assembly European

Geophysical Society, Vienna (Austria). 21–25 April 1997, vol.

24. Elsevier, Amsterdam, pp. 123–129.

Luino, F., Turitto, O., in collaboration with Malcangi, V., 1998.

L’analisi storica quale elemento d’indagine per l’individuazione

delle aree soggette a rischio d’inondazione: il caso di Alba

(Italia nordoccidentale). In: Luino, F. (Eds.), Proc. of the Int.

Conf.: b Prevention of Hydrogeological Hazards: The Role of

the Scientific ResearchQ. Alba, Italy, 5–7 November, 1996.

L’Artistica, Savigliano, vol. II, pp. 289–300.

Page 27: Sequence of instability processes triggered by heavy ......Sequence of instability processes triggered by heavy rainfall in the northern Italy Fabio Luino* Consiglio Nazionale delle

F. Luino / Geomorphology 66 (2005) 13–39 39

Luino, F., Arattano, M., Brunamonte, F., 1996. Vulnerability of

urban areas to flooding: events in the North-West Italy,

November 1994. Proc. Sexto Congreso Nacional y Conferencia

Internacional de Geologia Ambiental y Ordenacion del Terri-

torio, Granada, 22–25 de Abril de 1996, vol. III, pp. 309–327.

Luino, F., Belloni, A., Padovan, N., in collaboration with Bassi, M.,

Bossuto, P., Fassi, P., 2002. Historical and geomorphological

analysis as a research tool for the identification of flood-prone

zones and its role in the revision of town planning: the Oglio

basin (Valcamonica-Northern Italy). 9th Congress of the

International Association for Engineering Geology and the

Environment. Durban (South Africa), 16–20 September 2002,

pp. 191–200.

Luino, F., Chiarle, M., Audisio, C., 2003. Mass movements in small

alpine watersheds: a common but underestimated risk. The case

of the Savarenche Valley (Aosta Valley) on June 23, 2002. In:

Picarelli, L. (Ed.), Proc. of the Int. Conf.: bFast slope Move-

ments Prediction and Prevention for Risk MitigationQ. Naples,May 11–13, 2003, pp. 325–331.

Marchi, L., D’Agostino, V., 2004. Estimation of debris-flow

magnitude in the Eastern Italian Alps. Earth Surface Processes

and Landforms 29, 207–220.

Mercalli, L., Cat Berro, D., 2001. L’evento alluvionale del 13–17

Ottobre 2000 nel bacino del Po: analisi pluviometrica. Nimbus

21–22, 33–40.

Mercalli, L., Paludi, S., Dutto, F., 1995. Alluvione del 5–6

Novembre in Italia NW: analisi pluviometrica. Nimbus III

(6–7), 25–32.

Pierson, T.C., Iverson, R.M., Ellen, S.D., 1991. Spatial and

temporal distribution of shallow landsliding during intense

rainfall, southeastern Oahu, Hawaii. In: Bell, D.H. (Ed.), Proc.

6th Int. Symposium on Landslides, Landslides, vol. 2. Balkema,

Rotterdam, pp. 1393–1398.

Polloni, G., Aleotti, P., Balzelli, P., Nasetto, A., Casavecchia, K.,

1996. Heavy rain triggered landslides in the Alba area during

November 1994 flooding event in the Piemonte region (Italy).

In: Senneset, K. (Ed.), Proc. of the VIIth Int. Symposium

on Landslides, Trondheim, Norway. Balkema, Rotterdam,

pp. 1955–1960.

Ratto, S., Sonetto, F., Comoglio, C., 2003. The October 2000

flooding in Valle d’Aosta (Italy): event description and land

planning measures for the risk mitigation. Int. J. River Basin

Manage. 1, 105–116.

Regione Piemonte, 1998. Eventi alluvionali in Piemonte: 2–6

Novembre 1994, 8 Luglio 1996, 7–10 Ottobre 1996. Tip.

L’Artistica Savigliano. 415 pp.

Richards, K., 1982. Rivers: Form and Processes in Alluvial

Channels. Methuen, London, 358 pp.

Scambelluri, M., Federico, L., Capponi, G., Crispini, L., Piccardo,

G.B., Rampone, E., Romairone, A., 2002. Subduzione alpina ed

esumazione di litosfera oceanica: l’esempio dell’Unita Erro

Tobbio delle Alpi Liguri. 81a Riunione Estiva della Societa

Geologica Italiana: Guida alle escursioni del 14 Settembre 2002.

Litografia Geda, Nichelino (TO), pp. 57–63.

Schmidt, E.T., 2004. Alps. MicrosoftR EncartaR Online Encyclo-

pedia 2004; http://encarta.msn.com n 1997–2004 Microsoft. All

Rights Reserved.

Tropeano, D., Turconi, L., 2003. Using historical documents for

landslide, debris flow and stream flood prevention. Applications

in Northern Italy. Natural Hazards 00, 1–17.

Tropeano, D., Casagrande, A., Luino, F., Cescon, F., 1996. Processi

di mud-debris flow in Val Cenischia (Alpi Graie). Supplement a

Geam, Torino 18 (33), 5–31.

Tropeano, D., Govi, M., Mortara, G., Turitto, O., Sorzana, P.F.,

Negrini, G., Arattano, M., 1999. Eventi alluvionali e frane

nell’Italia Settentrionale: periodo 1975–1981. L’Artistica, Savi-

gliano. 279 pp.

Tropeano, D., Luino, F., Turconi, L., 2000. Evento alluvionale del

14–15 ottobre nell’Italia Nord-Occidentale. Fenomeni ed effetti.

Geam, Torino XXXVII (4), 203–216.

Tropeano, D., Turconi, L., Rosso, M., Cavallo, C., 2003. The

October 15, 2000 debris flow in the Bioley torrent, Fenis, Aosta

valley, Italy: damage and processes. In: Rickenmann, D., Chen,

C. (Eds.), Proc. of the Third Int. Conf. on Debris-flow Hazards

Mitigation: Mechanics, Prediction, and Assessment. Davos,

Switzerland, September 10–12, 2003, pp. 1037–1048.

Turitto, O., Maraga, F., Luino, F., 1995. Impatto sulle infrastrutture

viarie prodotto da piene con inondazione. Geologia Applicata e

Idrogeologia 30, 75–88 (Bari, parte I).

Wieczorek, G.F., Larsen, M.C., Eaton, L.S., Morgan, B.A., Blair,

J.L., 2002. Debris-flow and flooding hazards associated with the

December 1999 storm in coastal Venezuela and strategies for

mitigation. USGS, Report 01-144, 40 pp., 3 tables, 2 appendi-

ces, 3 plates, 1 CD.

Wisler, C.O., Brater, E.F., 1959. Hydrology. Wiley, New York.

408 pp.