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The San Bernardino-Guvano landslide is one of the wider slope mass movements located along the eastern Ligurian coast between Vernazza and Corniglia, Cinque Terre National Park. It is an ancient and complex landslide that has been studied since 1853, when a catastrophic event occurred. This paper aims to describe the geomorphological evolution and monitoring of this coastal landslide: both geological and geomorphological field surveys supported by airborne imagery were carried out, as well as bibliographical research about past geotechnical investigations and topographical monitoring; furthermore, a new single-frequency Global Navigation Satellite System (GNSS) low-cost monitoring started in October 2015. Structural geology heavily influences the stability of this coastal slope: a fault cuts the landslide area N-S, as well as low-angle thrust fault planes with NE dip direction. The slope is affected by landslides with different intensity and kinematic evolution, in particular rockfalls and debris avalanches along the scarp and right flank and earth flow along the central sector and at the slope toe. Man-made structures are relevant and they mainly consist of retaining walls, drainage channels, buildings, hiking trails, roads and railway infrastructures. Data obtained by GNSS receivers have shown remarkable displacements during the last year, according to the results of previous topographical and geotechnical monitoring campaigns. Deep analysis of GNSS data, together with the support and maintenance of the actual monitoring program, will allow a better comprehension of the slope stability condition, essential for supporting the design of proper risk reduction interventions.
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AbstrAct:
Raso E., BRandolini P., Faccini F., REalini E., caldERa
s. & FiRPo M.
, Geomorphological evolution and monitoring of San Bernar-
dino-Guvano landslide (Eastern Liguria, Italy). (IT ISSN 0391-9838, 2017).
The San Bernardino-Guvano landslide is one of the wider slope mass
movements located along the eastern Ligurian coast between Vernazza and
Corniglia, Cinque Terre National Park. It is an ancient and complex land-
slide that has been studied since 1853, when a catastrophic event occurred.
This paper aims to describe the geomorphological evolution and monitor-
ing of this coastal landslide: both geological and geomorphological field
surveys supported by airborne imagery were carried out, as well as biblio-
graphical research about past geotechnical investigations and topographical
monitoring; furthermore, a new single-frequency Global Navigation Satel-
lite System (GNSS) low-cost monitoring started in October 2015. Structural
geology heavily influences the stability of this coastal slope: a fault cuts the
landslide area N-S, as well as low-angle thrust fault planes with NE dip
direction. The slope is affected by landslides with different intensity and
kinematic evolution, in particular rockfalls and debris avalanches along
the scarp and right flank and earth flow along the central sector and at the
slope toe. Man-made structures are relevant and they mainly consist of re-
taining walls, drainage channels, buildings, hiking trails, roads and railway
infrastructures. Data obtained by GNSS receivers have shown remarkable
displacements during t he last year, according to the resu lts of previous topo-
graphical and geotechnical monitoring campaigns. Deep analysis of GNSS
data, together with the support and maintenance of the actual monitoring
program, will allow a better comprehension of the slope stability condition,
essential for supporting the design of proper risk reduction interventions.
Keywords:
landslide, geomorphological hazard, monitoring activi-
ties, Cinque Terre, GNSS.
riAssunto
:
Raso E., BR andolini P., Faccini F., REa lini E., cal
-
dERa s. & FiRPo M
. Evoluzione geomorfologica e monitoraggio della frana
costiera di San Bernardino-Guvano (Liguria orientale, Italia). (IT ISSN
0391-9838, 2017).
La frana di San Bernardino-Guvano costituisce uno dei più estesi
movimenti di massa osservabili lungo la costa ligure orientale tra Vernaz-
za e Corniglia nell’ambito del Parco Nazionale delle Cinque Terre. È una
frana antica, di genesi complessa, che è stata oggetto di studi sin dal 1853,
quando avvenne un catastrofico evento di r iattivazione. Questo contribu-
to ha lo scopo di presentare l’evoluzione e le attività di monitoraggio di
questa frana costiera: è stato condotto un rilevamento geologico-geomor-
fologico integrato con la fotointerpretazione di immagini aeree e suppor-
tato da una ricerca d’archivio delle precedenti indagini geotecniche e dei
monitoraggi topografici svolti nell’area; inoltre è stata avviata una nuova
campagna di monitoraggio tramite l’utilizzo di sensori GNSS (Global
Navigation Satellite System) a partire dall’ottobre 2015. Le condizioni
geologico-strutturali condizionano fortemente la stabilità di questo ver-
sante costiero: una faglia con direzione N-S taglia l’area di frana unita-
mente ad un piano di sovrascorrimento con basso angolo di inclinazione
immergente verso NE. Il versante è interessato da movimenti franosi con
differenti cinematismi e intensità, con la presenza in particolare di frane
di crollo e di valanghe di detrito lungo la scarpata ed il fianco destro e di
colate di terra lungo il settore centrale ed il piede del versante. Rilevante
è l’interazione con le strutture antropiche, quali muri di contenimento,
canali di drenaggio, edifici, sentieri escursionistici e infrastrutture stra-
dali e ferroviarie. I dati ricavati dal monitoraggio GNSS hanno mostrato
significativi valori di spostamento, in accordo con i risultati ottenuti du-
rante le precedenti attività d i monitoraggio topografico e geotec nico. Una
profonda analisi dei dati raccolti unitamente ai futuri riscontri relativi
al prosieguo della campagna di monitoraggio in atto consentiranno una
migliore interpretazione delle condizioni di stabilità del versante, quale
indispensabile supporto ad un’adeguata programmazione degli interven-
ti di mitigazione del rischio geomorfologico.
termini chiAve
: frana, pericolosità geomorfologica, attività di mo-
nitoraggio, Cinque Terre, GNSS.
INTRODUCTION
Research on landslides has increased in Italy and world-
wide (Guzzetti, 2000; Crozier & Glade, 2006; Magri & alii,
2008; Salvati & alii, 2010; Corominas & alii, 2014) during
the last decades. The Geotechnical Society Commission,
on behalf of the United Nations and UNESCO, proposed
a catalogue of the world’s landslides (WP/WLI, 1993) and
established guidelines for the standardized description of
(*) DISTAV - Dipartimento di Scienze della Terra, dellAmbiente e
della Vita, Università degli Studi di Genova, Italy
(**) GReD - Geomatics Research and Development srl, Via Cavour 2,
22074 Lomazzo (CO), Italy.
* Corresponding author: E. Raso, emanueleraso.geo@gmail.com
Geogr. Fis. Dinam. Quat. DOI 10.4461/ GFDQ 2017.40.12
40 (2017). 197-210
EManuElE Raso
1
*, PiERluigi BRandolini
1
, FRancEsco Faccini
1,
EugEnio REalini
2
, stEFano caldERa
2
& MaRco FiRPo
1
GEOMORPHOLOGICAL EVOLUTION AND MONITORING
OF SAN BERNARDINO-GUVANO COASTAL LANDSLIDE
(EASTERN LIGURIA, ITALY)
198
landslides and classification criteria. A recent important
contribution in terms of classification of landslides is pro-
vided by Hungr & alii (2014).
With the National Law n. 267/1998, just enacted after
the disastrous landslide event of Sarno (Campania, South-
ern Italy), an inventory map of landslides was realized with
the aim of identifying and mapping landslides throughout
all the country. Altogether 480,000 landslides have been
recorded out of an area of 20,000 km2, representing ap-
proximately 7% of the national territory (ISPRA, 2008).
Landslides are the most frequent geomorphological
hazard in Italy and despite casualties and economic losses
are less than those caused by earthquakes, an higher fre-
quency of events has been detected since the early 2000s:
only in Liguria Region, many landslides were triggered by
heavy rainfall events, whose number is rising year after
year, such as those of 2000, 2002, 2007, 2010, 2011, 2013
and 2014 (Guzzetti & alii, 2005; Brandolini & alii, 2005;
Faccini & alii, 2005, 2015; Cevasco & alii, 2013, 2014; Galve
& alii, 2015; Raso & alii).
Liguria Region presents geological, geomorphological,
hydrological, climatic and anthropogenic features repre-
senting potential landslides predisposing factors: the In-
ventory of Italian Landslides (IFFI) highlights about 7,500
landslides in Liguria, approximately 8% of the whole na-
tional territory. Among these, about 20% are in state of
activity and characterized by recent displacements.
The presence of ancient and recent landslides along the
Ligurian coastal landscape is not very recurring, because
of an intense erosive activity of sea waves and running wa-
ters; the effects of landslides are often modified by human
activity and sometimes landforms associated with wide
slope mass movements are confused with marine terraces
(Fanucci & Nosengo, 1977). Coastal landslides therefore
represent an interesting research topic for their influence
on landscape and for their interaction with settlements and
infrastructure; consequently, they play a key role in the
geomorphological risk research.
The Ligurian coast is 345 km long and is characterized
by 190 km of rocky coastline, 41 km of beaches and 114
km of artificial structures. Along its eastern stretch, some
large coastal landslides are known in literature. They are
characterized both by very slow/extremely slow kinemat-
ics (i.e. Rodalabia & Guvano; De Stefanis & alii, 1978) and
by high intensity mechanisms, such as Monesteroli, the
Batternara landslide between Riomaggiore and Manarola
(Faccini & alii, 2015), along the “Via dell’Amore” and be-
tween Manarola and Corniglia (“Lama della Bansuola”, be-
low the village of Volastra). Under this aspect, the Cinque
Terre can be considered as one of the most outstanding
example of coastal landscape affected by high geomorpho-
logical risk, as dramatically confirmed by the catastrophic
events of October 25th, 2011 (Brandolini & Cevasco, 2015;
Raso & alii, 2016a, b).
The aim of this work is to evaluate the geomorphologi-
cal evolution of the San Bernardino-Guvano area, located
between Vernazza and Corniglia - Cinque Terre National
Park (Eastern Liguria); for this purpose, bibliographical
works dealing with the geological and geomorphological
evolution of the study area and with past geotechnical and
topographical monitoring campaigns were first examined;
secondarily, detailed geological and geomorphological field
surveys supported by airborne imagery were carried out,
as well as a new single-frequency Global Navigation Satel-
lite System (GNSS) low-cost monitoring campaign which
started in October 2015.
Climate change and its consequences on rainfall re-
gime are determining an increase in frequency of short and
intense precipitation events. Such phenomena triggered
several shallow landslides and caused diffuse land de-
gradation widely distributed throughout the Cinque Terre
catchments, consequently leading to a growth in sediment
transport and destructive power of flash floods and land-
slides in periurban and rural areas (Pieri & alii, 2016; Bran-
dolini & alii, 2016).
This, coupled with the presence of several exposed ele-
ments such as the Genova-Roma railway line, SP51 (Strada
provinciale 51) County Road, the “Sentiero Verde-Az-
zurro” (SVA), a coastal trail very popular among hikers,
and the village of San Bernardino at the top of the slope,
produces a high level of geomorphological risk. The San
Bernardino-Guvano landslide is therefore an interesting
case study for the assessment of geomorphological hazard
in order to mitigate geomorphological risk.
GENERAL OUTLINES
The S.Bernardino-Guvano coastal landslide is located
in Eastern Liguria (NW Italy), within the Cinque Terre
National Park (fig. 1), considered one of the most peculiar
examples of a terraced coastal landscape in the Mediterra-
nean region. In fact, during the last centuries most part of
the slopes, up to 400-500 meters a.s.l., have been terraced
with the purpose of cultivating olive groves and especially
vineyards, creating a highly unusual, man-made coastal
landscape fully integrated with the surrounding geomor-
phological environment (Terranova & alii, 2006). In some
cases, such as in the presented case study, the terraces de-
veloped themselves along coastal landslides and degrada-
tion scarps (Brandolini, 2017).
Previous studies
The landslide of San Bernardino-Guvano is historically
known from the second half of the nineteenth century, a
few years before the start of technical investigations related
to the construction of the Genova-Pisa railway.
First scientific research activity in this area was carried
out by a pioneer geologist, Gerolamo Guidoni, who de-
scribed in 1854 the catastrophic event concerning the re-
activation of the ancient landslide affecting the slope below
the village of San Bernardino the night between 26 and
27 December 1853 (Guidoni, 1854): the landslide crown
was about 200 m large and caused the collapse of several
buildings; remarkable displacement occurred until 1862
(Guidoni, 1902).
An extremely useful source of information about the
landslide is represented by the project realized by the King-
dom of Italy’s engineers in the middle of the XIX century
199
concerning the construction of the Sestri Levante - La Spe-
zia railway line between 1865 and 1874 (Direzione Tecnica
Governativa, 1880): in order to stabilize the landslide, a
complex shallow and underground drainage system was
realized, consisting in 26 galleries whose total length was
1129 m and 56 vertical wells with a total length of 709 m.
Furthermore, a massive, 300 m long concrete and stone re-
taining wall was built along the landslide toe with the aim
of protecting it by the erosional activity of sea waves (fig. 2).
In 1978 De Stefanis & alii realized the first geological
and geomorphological map of the landslide area at a large
scale, in which several geomorphological features (debris ac-
cumulations, edges of scarp, etc.) of the complex landslide
have been distinguished, as well as their state of activity.
Terranova (1984), in the framework of the first Geomor-
phological Map of the Cinque Terre Area (scale 1:25,000)
has presented an interesting geological-geomorphological
sketch map of the Guvano Area in which different land-
slide source areas related to the bedrock composition were
pointed out.
Federici & alii (2001) in the framework of the “Atlas of
urban areas affected by landslides in Liguria” produced a
very detailed geomorphological map at 1:5000 scale, based
on the guidelines proposed by the Italian Geological Survey
(Servizio Geologico Nazionale, 1994). Different kinemat-
ics (translational and flow types) of mass movement were
pointed out and the main effects caused by landslide dis-
placement and countermeasures to reduce it were described.
Cevasco (2007), within a wider analysis of rock mass in-
stability phenomena affecting the Cinque Terre coastline,
presented an updated synthesis of the Guvano landslide
features, supported by a systems data set.
Fig.
1 - S. Bernard ino - Guvano landslide location in the fra mework of geological sketch map of the Ci nque Terre National Park. Legend: 1. Urban area:
2. Landslide; 3. Sandstones; 4. Shales; 5. Limestones; 6. Cherts; 7. Marls; 8. Ophiolites.
200
Raggi & alii (2011), just after the dramatic flooding
event of October 25th, 2011, published a series of geomor-
phological sketch maps of the Cinque Terre area providing
a qualitative interpretation of the Guvano landslide body
thickness through several geological cross sections.
At last, Cevasco & alii (2013) describe the geomorpho-
logical effects of October 25th, 2011 high-intensity rainfalls
event, affecting a narrow area between Cinque Terre and the
Magra Valley. A total rainfall amount of 539 mm/24 hrs was
recorded by the Brugnato rain gauge, located approximately
10 km north of Vernazza, and a total rainfall amount of 382
mm/24 hrs was recorded along the coast by the Monterosso
rain gauge, located 3 km northwest of Vernazza. In Mon-
terosso, rainfall intensities of 90 mm/h, 195 mm/3 h and 350
mm/6 h were recorded between 9 and 15 UTC (A.R.P.A.L.-
C.F.M.I.-P.C., 2012). This event caused the triggering of
hundreds of shallow landslides and flooding phenomena
mainly along the catchments of Monterosso and Vernazza
(Cevasco & alii, 2014) and marginally also along the coastal
slope between San Bernardino and Corniglia: these high-in-
tensity landslides must be treated in a different way from the
well known Guvano complex Landslide.
Previous geotechnical investigations and monitoring activities
In 2000, severe rockfall events affected part of the SP51
County Road under the landslide crown; after the road
closure, in situ investigations and a topographical and geo-
technical monitoring program were predisposed in order
to obtain a detailed stratigraphy of the area.
Starting from June 2003 some field tests were carried
out by the “Comunità Montana della Riviera Spezzina”, fol-
lowed by a 12 months monitoring program ended in June
2004 (Eptaconsult, 2004). Tests included the realization of
5 soil borings and laboratory analysis on disturbed samples
(Eptaconsult, 2004); 4 borings were advanced adopting
rotary core barrels to obtain rock and soil samples while
one of them was advanced using a drill bit at the end for a
faster advance; laboratory tests on disturbed soil samples
were conducted to classify soils and determine their physi-
cal behavior.
Secondarily, four boreholes were cased with inclinom-
eters and one with a piezometer: inclinometer probes were
used in order to detect displacement along the boring
length, while the groundwater level has been monitored
Fig.
2 - Interventions design to stabilize the S. Bernardino – Guvano landslide area realized by the “Ferrovie del Regno” Engineers (
diREzionE
tEcnica govER nativa
, 1880): a) Ruins of the retaining wall (landslide toe); b) Active scarp due to sea wave erosion (landslide toe). c) retaining wall
at the landslide toe.
201
along the piezometer equipped borehole and also along the
4 inclinometer boreholes.
A topographical monitoring was performed between
June 2003 and June 2004 to detect the displacements along
the landslide body: after 2004, the project was interrupted
and there has not been any further investigation until today.
Sand and mud layers constitute the upper part of the
sliding body while the underlying layers are composed of
a heterogeneous mix of mud and silt with boulders and al-
tered rock. The presence of marine sands at the bottom of
the S3 and S4 boring holes, strengthen the hypothesis of
the presence of a former beach already existing when the
1853 event occurred.
Half of the collected samples were classified as ML (low
plasticity mud), while the other half was classified as CL
(low to medium plasticity clays), according to the U.S.C.S.
soil classification system. Fine-grained fractions were com-
prised in a range between 35 and 45%, and the clay con-
tents were, in general, less than 20%. The coarse-grained
fraction consisted of angular pebbles up to 15 cm in diam-
eter. The Plasticity Index (PI) values were less than 15%,
with an increase in the lower portion of the landslide body.
Groundwater level was registered from June 2003 until
June 2004 between 6 and 9 m below the surface; water table
fluctuations were relatively small and range between 2 and
3 m. Results obtained by the 2003-2004 topographical and
geotechnical monitoring show displacement rates along the
crown area and in the central sector of the right flank. Incli-
nometers show remarkable displacement rates at the slope
toe, with the slip surface confirmed at 11 m of depth.
MATERIALS AND METHODS
Documents and notes describing slope instability phe-
nomena affecting the San Bernardino-Guvano area were
collected, as well as technical papers about railway works
realized between the end of the XIX century and the be-
ginning of the XX century along the lower portion of the
landslide.
A multi-temporal cartographic comparison was realized
to evaluate the morphological changes of the slope and the
evolution of the landslide during the last two centuries. In
particular, the Stati Sardi di Terraferma maps (1815-1822,
scale 1:9,450) have been used, as well as the IGMI (Istituto
Geografico Militare Italiano) maps dated 1878, 1904 and
1936 (scale 1:25,000), and two Regional Technical Maps
(CTR, scale 1:25,000) dated 1994 and 2010.
Detailed geological and geomorphological surveys were
carried out: the first one was focused on the relationship
between tectonic and lithological features and landslide
evolution. The latter was performed in order to detect the
landslide features: in particular, it was focused both on
typical landforms characterizing complex landslides (rock
rotational slides/earth flows) and on areas characterized by
high-intensity phenomena and possible first-failure land-
slides, i.e. crown area with a typical half-moon shape, overall
concave and convex forms related to the landslide scarps and
deposits, respectively, and the presence of back-tilted slope
facets due to the rotational movements (Carlini & alii, 2016).
Field survey has been supported by photo interpreta-
tion of aerial images provided by Regione Liguria (1973,
1984 and 2007 aerial surveys) and by Regione Friuli-Ve-
nezia-Giulia Geological Service (November 2011), a few
weeks after the October 25th, 2011 flooding event. Their
analysis allows to detect a wide distribution of gravita-
tional phenomena characterized by different intensity and
kinematics. Preparatory and triggering factors affecting
the whole slope were individuated in order to better es-
timate their relationship and influence with specific land-
slide types (Popescu, 2002). Standing on evaluations made
through the analysis of geological and geomorphological
features of the area and data obtained from field investiga-
tions, thickness of the sliding body could be verified and
compared with previous evaluations.
In 2015, the Vernazza Municipality, together with Soft-
eco S.r.l., GReD S.r.l. (Politecnico di Milano spin-off) and
the Department of Earth, Environment and Life Sciences
of University of Genova, started a new GNSS (Global Nav-
igation Satellite System) monitoring program based low-
cost technology.
A GNSS monitoring system is based on the continuous
calculation of the position of a set of GNSS receivers per-
manently installed on an infrastructure or area to be moni-
tored. To allow for measurements of deformation with mil-
limeter-level accuracy, relative positioning is typically used
(i.e., the positions of the moving nodes are known relatively
to one or a set of stable nodes), by means of dual-frequency
geodetic receivers. Such receivers, however, are costly (in
the order of 10,000 € for the receiver alone, not counting
the hardware deployment, data processing and mainte-
nance costs), and this severely limits their actual applica-
tion to operational landslide monitoring. The system used
in this work uses low-cost single-frequency receivers (hard-
ware cost: less than 2,000 € for each monitored point), with
advanced observation processing that allows to reach a
precision comparable to that of standard geodetic receivers
(Caldera & alii, 2016). It’s important to stress out that dis-
placements are typically evaluated in two ways: single-shot
or trend evaluation. The single-shot method consists in es-
timating the position of a monitoring point by processing
a specific dataset collected over a short range of time (e.g.
one hour, or one day). The trend is instead derived from a
statistical analysis of single-shot position time series and
provides a point-deformation model whose accuracy is es-
timated to be higher than the one of single-shot solutions.
Moreover, the extended use of low cost single-fre-
quency GNSS receivers is possible in case of small-medium
size landslides monitoring (a few hundreds of meters to 1
or 2 km) because it allows a relative positioning with short
baselines (typically less than 5 km) which mitigates spa-
tially correlated errors (such as ionospheric disturbances)
and makes the use of L2 frequency unnecessary. Single-
frequency receivers have been successfully used in the past
for different landslide surveying tasks (Benoit & alii, 2015;
Heunecke & alii, 2011).
The monitoring system based on low-cost receivers that
was used in this study can therefore be described as a sum
of two main components: data acquisition and processing
202
followed by trend or single-shot displacements analysis.
Data obtained by GNSS sensors have been compared
with daily and cumulative rainfall amounts registered by
the ARPAL (Agenzia Regionale per la Protezione dell’Am-
biente Ligure) weather station located in Monterosso al
Mare (SP), 2 km far from San Bernardino.
The GNSS technique has already been tested as a pow-
erful tool in ground deformation analysis with high accu-
racy and reliability (Magri & alii, 2008; Benoit & alii, 2015).
The main objective of this program is to detect displace-
ments of the order of a few millimeters by relative position-
ing of low-cost receivers over a short baseline (about 2-3
km): the monitoring program has started in October 2015
when four GNSS sensors (also called GeoGuard Monitor-
ing Units – GMU – because they were originally designed
and developed by Softeco and GReD for the GeoGuard
monitoring service) were positioned along the landslide
body. The GMUs include low-cost single-frequency (L1)
hardware for both receiver and antenna. The receiver mo-
dule is a u-blox LEA-6T, providing GPS observations
Fig.
3 - Geological and geomorphological sketch map: MAC. Sandstone; ACC. Shales and limestone; RF. Rockfalls; SH. Shallow landslides triggered
by the October 25th, 2011 event; DA. Debris avalanches triggered by the October 25th, 2011 event; SHA. Area interested by several shallow landslides
and locally dry stone walls collapsed; CL. Complex landslide: active (CLa), inactive (CLb); DS. Beach deposits; 1. Edge of active marine scarp > 25 m;
2. Edge of active marine scarp < 25 m.; 3. Edge of active landslide scarp; 4. Edge of inactive landslide scarp. 5. N-S Fault; 6. Overthrust; 7. Normal fault.
8. Geological cross section. 9. Passive rockfall defence; 10. Active rockfall defence; 11. Bed attitude; 12. Beach deposit.
203
which are transferred by mobile connection to the control
center (GeoGuard Cloud) and processed by a customized
version of the free and open source software goGPS (Re-
alini & Reguzzoni, 2013). Single-shot displacement data
and trend analyses are then processed and managed by the
GeoGuard Cloud, which send it to the end-user service in-
terface.
RESULTS
Geological and geomorphological survey
The bedrock has a different composition in the lower
and upper portion of the slope: the lower one is mainly
composed of a turbiditic sandstone-siltstone flysch, with
coarse and medium-grained sandstone beds and thin in-
terbedded siltstones (Macigno Formation, MAC), belong-
ing to the Tuscan Nappe Unit; the upper part, shales with
limestones and silty sandstone turbidites (Canetolo shales
and limestones Formation, ACC), marly limestones and
calcarenitic turbidites (Groppo del Vescovo limestones
Formation) and fine-grained sandstone turbidites (Ponte
Bratica sandstones Formation) belonging to the Canetolo
Unit (APAT, 2006). The Tuscan Nappe and the Canetolo
Units are included in a wide overturned southwest-verging
antiform fold. These units are bounded to the Northeast by
a major normal fault (La Spezia Fault), beyond which the
Ligurian Units outcrop (Federici & alii, 20 01).
The area is characterized by the tectonic contact be-
tween the Canetolo shales and limestones and the Macigno
sandstones and siltstones (De Stefanis & alii, 1978; APAT,
2006); the N-S fault surface is only visible in proximity of
the landslide crown and along some ridge portions with
no vegetation. Shales and limestones outcrop in proximity
to the hamlet of San Bernardino and along the NE and
SE sector of the studied area, while significant outcrops of
sandstones and siltstones are detected along the NW sector
close to the landslide crown and westward from the land-
slide toe (figg. 3, 4, 5).
A detailed record on bedrock outcrops indicates
the rock strata dipping to SW (DIR°N 130°≈168°/
INCL°50°≈80°) along both the left and right flank of the
slope; a set of joints was found dipping to SE (DIR°N
55°≈60°/INCL°57°70°). Some measures were taken along
the depletion area, where the ACC formation clearly out-
crops, while some other outcrops (left flank) belong to
the eastern portion of the relict S. Bernardino landslide.
Degradation of rock mechanical properties due to fractur-
ing promotes and controls large rock slope failures: in this
case, such damage can be associated to the N-S fault line,
associated joint planes and bedding planes detected during
the geological survey. Furthermore, during rainy season,
the joints provide infiltration channels for the rainwater
into the inner slope (Xu & alii, 2016).
The main mechanism could be described as a retro-
gressive complex mass movement occurred along the
Fig.
4 - Geological cross section of S. Bernardino - Guvano landslide. 1. Medium/lower landslide body (silty and clayey sand); 2. Marine sands; 3.
Upper landslide body (boulders and debris); 4. Sandstone (MAC); 5. Shales and limestone (ACC); 6. Fault; 7. Mean water table (mwt); a1: Inclinometer
cumulative displacements profile during the 2003-2004 campaign time range; a2: Inset map showing borehole locations (S1-S5).
204
coastal slope between the village of S. Bernardino (350 m
a.s.l.) and the coastline, in correspondence of the tectonic
contact between the two main lithological formations pre-
viously described; the 600 m long landslide crown is eas-
ily recognizable just below Case Fornaci hamlet. Between
the main edge of scarp and the landslide toe in proximity
to the shoreline, the total length of the landslide body is
measured in 800 m. The upper portion has a width of 340
m, while a narrowing is detected along the middle portion
(90 m), and again a spreading along the toe (235 m). On
October 25th, 2011 heavy rainfalls have triggered several
shallow landslides of limited extent and volume, located
in areas characterized by high steepness rates. Rockfall
events occurred along the eastern sector of the main edge
of scarp and along the middle and lower portion of the
right flank, where the Macigno sandstones are severely
fractured and altered. From the middle to the lower por-
tion of the slope, the landslide body shows a progressive
width increase.
Various coastal landforms are detected in the area: ero-
sional edges of scarp often more than 25 m high border
the coastline between Vernazza and Corniglia, which is af-
fected by several rockfalls and debris avalanches. The sea
wave action at the landslide toe, mainly driven by south-
western winds (Libeccio), has created a steep earth cliff
composed by heterogeneous gravitational deposits and re-
fills, also affected by a diffuse rill erosion.
A short stream flowing N-S along the landslide body is
downcutting, especially along the lower and steeper sector
between the SVA and the shoreline.
Several anthropic interventions have been carried out
during the last two centuries: concrete and stone walls
along the higher and medium portion of the slope, drain-
age channels, wells and trenches and a massive retaining
wall surrounding the whole perimeter of the landslide toe
were built by the Italian Railways in the middle of XIX
century; concrete and natural boulders whose function was
to reduce wave energy were subsequently added during the
XX century. At present time, all human works are almost
completely destroyed; in particular, the retaining wall was
eroded by waves and the drainage channels now filled by
debris and vegetation along the middle portion of the slope.
The main reason of the abandonment and lack of main-
tenance of these structures is because of the relocation of
the railway line along a tunnel located 20 m North from the
former railway tracks: since then, slope maintenance was
no longer needed and erosional and gravitational processes
kept growing in intensity (Stanchi & alii, 2012).
Landslide evolution in XIX-XXI Century
The present-day situation represents the last step of
a complex slope evolution which started with an ancient
gravitational movement involving a wider area. Actual ev-
idences of that phase are the inactive edges of scarps and
the presence of residual gravitational deposits on the west-
ern and eastern boundaries of the landslide area.
The complex slope dynamics affecting the area dur-
ing the last 200 years can be reconstructed along six main
phases (fig. 6).
1. Beginning of the XIX century: the slope under the S.
Bernardino Church is a relict landslide cut by several
streams with N-S direction. The lower portion of the
slope is probably an inactive earth flow deposit. High
rocky cliffs are detected on the western and eastern side
of the slope.
2. End of XIX century: reactivation of the western portion
of the relict landslide from S. Bernardino to the coast-
line: the movement has a N-S component. The descrip-
tion of the 1853 event (Guidoni, 1854) helps to identify
it as a rock collapse. Another gravitational phenomenon
(active rockfall scarp) of the South-eastern side of the
Guvano Bay was probably generated by the construc-
tion of the Genova-Roma railway line at the end of the
XIX century. The construction of drainage channels,
horizontal and vertical wells took place, as well as the
realization of a retaining wall enclosing the slope toe.
3. Beginning of the XX century: geomorphological
dynamics is similar to the previous step, except for a
Fig
. 5 - Borehole stratigraphies: 1. Organic soil; 2. Silty and clayey sands;
3. Marine sands; 4. Fractured shales with silty interlayers; 5. Shales and
limestones; 6. Very fractured shales and limestones with silty interlayers.
wt1-wt2: range between maximum and minimum water table values reg-
istered between 2003 and 2004. SC = Soil Sample (see Table 1 for further
explanations).
205
reduction of the landslide width and the presence of
a NE-SW oriented stream. Sea waves erosion rates are
increasing.
4. Middle of the XX century: the 1853 landslide seems
to be inactive at that time, while active and dormant
scarps are present under S.Bernardino. The railway line
is transferred along a tunnel carved 20 meters north-
ward from the old tracks. The works realized at the end
of the XX century experience a continuous degradation
process.
5. End of the XX century: further slope modifications are
detected as the realization of the SP51 County Road,
the re-opening of the SVA, the complete abandonment
of the retaining wall at the slope toe and of drainage
wells and channels.
6. Beginning of the XXI century: the present day situa-
tion is illustrated in fig. 3: the main geomorphological
evidences are the retreat of the active scarp under San
Bernardino and the reactivation of the South-eastern
active rockfall scarp. Geotechnical and topographical
monitoring put in evidence the existence of displace-
ment rates.
Monitoring activities
The data collected by the GMU allowed estimating
continuous, high frequency 3D positions for a total of four
sensing points. Hereafter only the results of the horizontal
component, less noisy, are presented and discussed.
During this session, two GMU nodes (GUV3 and
Fig.
6 - S. Bernardino - Guvano landslide evolution in the last two centuries: a. Talus flow; b. Complex landslide; c1. 1853 Landslide; c2. Rockfalls;
d. Edge of active scarp; e. Edge of inactive scarp; f. Limit of ancient landslide.
206
GUV4) were acquiring observations without any loss, and
one nodes (GUV2) worked discontinuously. One GMU
node (GUV1) was set up on a bedrock outcrop to avoid
small collapse effects and was used as reference for the
other three sensors.
In order to ensure the stability of the references and to
assess the real precision of the processing for site-specific
acquisition conditions, the baseline between the fixed re-
ceivers is processed.
Time series of horizontal cumulative displacements at
daily time resolution are computed for the GMU nodes
working during t he monitoring session, but only the March -
September 2016 results has been reported on figure 7 since
it includes data from all the sensors except GUV2.
A mesh of horizontal displacement rates (fig. 7) is ob-
tained for a 10-months period (2015/12-2016/09) in which
two GMU node (GUV1 – Sent.Azzurro and GUV4 – San
Bernardino) have been acquiring data, and for a 7-months
period (2016/04-2016/09) in which another GMU node
(GUV3 - Uliveto) acquired data. The GUV2 (Muro Sost-
egno) sensor has had a few data transmission issues and has
not worked adequately until now. The measured displace-
ment rates show a good spatial heterogeneity. Gradients
of displacement rates are observed in the north-south and
east-west directions. The displacement rates increase from
early-mid June to early July. Higher displacement rates are
recorded in the lower part of the landslide (a total of 2.5
cm displacement towards S-SW registered by GUV3) than
in the upper side (a total of 1.5 cm displacement towards S
registered by GUV4).
DISCUSSION
The San Bernardino-Guvano landslide must be con-
sidered as a relict or at least reactivated ancient landslide,
since its current situation represent the result of a sequence
of gravitational events along different morphoclimatic con-
ditions (Federici & alii, 2001), proven by several slope evi-
dences and by landslide deposits detected under the mean
sea level.
The primary factors which characterized this unstable
slope condition are the geological and structural features
and the neotectonic activities of Eastern Liguria coast. In
particular, an extensional tectonics characterized by a set
of high angle normal faults (NW-SE direction) and by a
horst and graben structure have strongly affected this area
starting from Upper Pliocene. Along these tectonic discon-
tinuities, displacements occurred until 18,000 years b.p.,
while all the region comprised between Eastern Liguria
and North-Western Tuscany was affected by tectonic up-
lift during the Pleistocene (Federici, 1980). Several strike-
slip faults are NE-SW oriented, and deeply influenced the
drainage pattern of Thyrrenian catchments.
A different approach was adopted to treat high intensity
phenomena (often described as first failure mechanisms/
high speed landslides) and the complex Guvano landslide
(low intensity and very slow to extremely slow movement)
in order to evaluate which elements at risk are subject to
these specific hazards in the whole area of San Bernardi-
no-Guvano: quick landslides could affect the high number
of hikers on the SVA trail of the Cinque Terre National
Park, cars and pedestrian on the County road SP51, while
low intensity movements endanger buildings and infra-
structures close to the scarp area and the railway tunnel at
the bottom of the slope (fig. 8).
High intensity phenomena
During the October 25th, 2011 rainfall events, a high
number of shallow landslides, debris avalanches and de-
bris - mud flows affected the Guvano area (fig. 3), causing
localized but significant damages.
These phenomena, even with a lower destructive power
compared to the ones occurring in the close villages of
Monterosso al Mare and Vernazza, were triggered by heavy
rainfall concentrated in a few hours: in the most part of
cases, they were surveyed as shallow landslides made by
fully saturated debris on a steep slope, without confine-
ment in an established channel, and debris slides evolved
into debris avalanches. These landslide types can be clas-
sified as extremely/very rapid shallow flows (speed range
between 3 m/min and 5 m/s, Hungr & alii, 2014).
The role of the existing road and trail network cutting
the landslide area as a preparatory factor of high intensity
landslides (i.e. debris flows and shallow landslides) was de-
termining.
The rockfall phenomena seems to be linked to the de-
cay of shear strength of rock masses and discontinuities
affected by deep weathering processes.
Low intensity phenomena
The San Bernardino-Guvano landslide can be consid-
ered as a very slow/extremely slow kinematic phenomenon
(Hungr & alii, 2014). The main predisposal factor is to be
found in the geological asset of the area: N-S direction fault
between Macigno and Canetolo shales and limestones for-
mation, both characterized by different geotechnical be-
havior, has been identified as the most important weakness
factor affecting the area.
Standing on data from GNSS monitoring, displacement
and strain rates show temporal variations and a total of
three different flow regimes can be identified: April 2016 -
mid-June 2016, mid-June 2016 - mid-July 2016 and mid-July
2016 - September 2016 (fig. 7). In order to investigate them,
displacement rates are calculated and compared with the
rainfall amount measured at Monterosso al Mare weather
stat io n (A R PAL).
The first regime corresponds to a period of rainfall av-
erage (60 days showing a total of 120 mm) in which scarce
evidence of displacement has been recorded.
The second regime corresponds to the start of a reacti-
vation of the sliding body: 1.75 cm towards SW from GUV3
and 1.5 cm towards SE from GUV4. The displacement is
triggered by a consistent rainfall amount (200 mm in 15
days, from 01/06 to 15/06). The time lag between the sliding
reactivation (20/06) and the start of heavy rainfall period
(01/06) can be explained by the complex hydrology of the
landslide with different response of the slope to rainfall.
207
Fig.
7 - Monitoring results of
daily rainfall level registered
by Monterosso al Mare - AR-
PAL weather station and the
ground displacement rate
registered by GNSS sensors
from March to September
2016.
208
Fig.
8 - Slope dynamic sketch map: 1. GNSS sensors (2015-2016); 2. Inclinometers (2003-2004, after
EPtaconsult s.c.R.l., 2004
); 3. Topographic
landmarks (2003-2004, after
EPtaconsult s.c.R.l., 2004
); 4. SP51 County Road; 5. SVA; 6. S. Bernardino-Guvano coastal landslide; 7. Urban areas; 8.
Railway tunnel.
sample USCS S+G % M+C % wnat % LL % PI % f’ deg C’ kPa f’r deg
S1C1 ML 57 43 5.7 30 7 34 13 22
S2C1 ML 65 35 4.3 29 5 39 28 25
S2C2 CL 60 40 8.9 30 10 37 16 22
S3C1 CL 56 44 14 33 16 24 8 16
taB.
1. Physical and mechanical parameters derived from laboratory tests during the 2003-2004 monitoring campaign (Eptaconsult, 2004). SC = Soil
Sample; USCS classification: ML = silt, CL = low plasticity clay; S+G = sand + gravel fraction (percentage); M+C = silt + clay fraction (percentage);
wnat %: natural water content; LL % = Liquid limit (percentage); PI % = Plasticty Index (percentage); f’ (deg) = effective stress friction angle (degrees);
C’ (kPa) = soil cohesion (KiloPascal); f’r (deg) = residual friction angle (degrees).
209
The third rainfall regime is characterized by occasional
precipitation (a total 40 mm recorded in two months) and
shows a constant displacement rate at GUV3 (total of 0,75
cm towards South), while no relevant displacements have
been recorded by GUV4 at the same time.
The displacement field is in agreement with previous
knowledge (Eptaconsult, 2004), indicates a horizontal
gradient of speed rates of 30mm/year.
Higher displacement rates registered by GUV3 sensor
are directly related with residual strength: relationships
between the j’r (residual friction angle) and soil property
values are observable on table 1, where a direct correla-
tion between the decreasing of j’r with increasing of Liquid
Limit (LL), Plasticity Index (PI) and Clay Fraction (CF)
(Kimura & alii, 2014) is evidenced.
Therefore, kinematics is mainly controlled by:
a) Rainfall regimes causing infiltration of precipitation
and consequent changes in pore pressure and thus vari-
ations in excess shear stress controlling speed.
b) Intense landslide toe erosion caused by sea waves ac-
tion, especially when generated by southwestern winds
(Libeccio) which could reach a fetch length of 300 km
and wave height up to 6 m.
c) Poor geotechnical properties especially along the
middle-lower sector of the slope, where lower j’r cou-
pled with higher LL, PI and CF values contribute to
decrease the average residual strength of the sliding
body.
CONCLUSIONS
The San Bernardino – Guvano area is therefore af-
fected by landslides characterized by different size, geo-
materials and failure speed: high intensity phenomena
are concentrated along the road cuts and hiking trails, are
triggered by intense and concentrated rainfall events and
represent a threat for specific categories of elements at risk
(e.g. hikers, cars, pedestrians); low intensity phenomena are
here represented by the historical Guvano coastal landslide
and in this case a different study approach was needed: on
the basis of rainfall and surficial displacement monitoring
from October 2015 to September 2016, the sliding body re-
sponse to heavy rains and its effect on the slope movement
were analyzed.
A landslide reactivation started at mid-June 2016 was
observed after a prolonged rainfall period (200 mm cumu-
lative rainfall along the same period) and the movement is
still ongoing: the landslide is therefore to be classified as an
active, extremely to very slow rotational slide.
Horizontal measures provided an accurate and pre-
cious feedback about the sliding body kinematics, while
the vertical component of displacement has been affected
by external, unmodeled geophysical effects that influence
individual site positioning and depend also by the distance
between base and rover stations: to reduce the RMSE (root
mean squared error) affecting the positioning estimation
(currently of the order of 1-5 mm), the baseline length is
planned to be further shortened by installing a GNSS base
station outside the landslide mass.
Since the activity of the landslide has been assessed
through low-cost, GNSS monitoring, a network of wells
equipped with piezometers and pressure-type water level
gauges would be necessary to better understand the re-
sponse of groundwater table and water pressure to heavy
rainfalls and vertical infiltration of rainwater.
An alert system coupled with the 4 GNSS sensors
would be extremely useful for a potential evacuation of the
S. Bernardino village; further research will be carried out
on the high intensity landslides affecting the SP51 County
Road, the SVA and the relative elements at risk.
A proper risk mitigation program should be also car-
ried out through adequate environmental policies con-
sidering also climatic change and its impact on rainfall
regime. Restoring abandoned vineyard terraces and dry
stone walls maintenance is mandatory to prevent effects
caused by high intensity rainfall. In fact, terraced slopes
are not only the main issue of Cinque Terre landscape but
they represent moreover a fundamental factor to preserve
geomorphological equilibrium modified along the centu-
ries by human activities.
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the Kualiangzi landslide in Sichuan Basin, southwestern China. Land-
slides. 1-16 pp.
(Ms. received February 14, 2017; accepted May 12, 2017)
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