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19/12/2014 Quantitative hazard and risk assessment for slow-moving landslides from Persistent Scatterer Interferometry - Springer
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Landslides
Journal of the International Consortium on Landslides
© Springer-Verlag Berlin Heidelberg 201310.1007/s10346-013-0432-2
Original Paper
Quantitative hazard and risk assessmentfor slow-moving landslides from PersistentScatterer Interferometry
Ping Lu 1, 2 , Filippo Catani 3, Veronica Tofani 3 and Nicola Casagli 3
College of Surveying and Geo-Informatics, Tongji University, Siping Road 1239, Shanghai, China
Center for Spatial Information Science and Sustainable Development Applications, Tongji University,Siping Road 1239, Shanghai, China
Department of Earth Sciences, University of Firenze, Via La Pira 4, Florence, Italy
Ping LuEmail: [email protected]
Received: 24 October 2012
Accepted: 28 August 2013
Published online: 8 September 2013
Abstract
Preparation of reliable landslide hazard and risk maps is crucial for hazard mitigation and risk
management. In recent years, various approaches have been developed for quantitative
assessment of landslide hazard and risk. However, possibly due to the lack of new data, very
few of these hazard and risk maps were updated after their first generation. In this study,
aiming at an ongoing assessment, a novel approach for updating landslide hazard and risk
maps based on Persistent Scatterer Interferometry (PSI) is introduced. The study was
performed in the Arno River basin (central Italy) where most mass movements are slow-
moving landslides which are properly within the detection precision of PSI point targets. In
the Arno River basin, the preliminary hazard and risk assessment was performed by Catani et
al. (Landslides 2:329–342, 2005) using datasets prior to 2002. In this study, the previous
hazard and risk maps were updated using PSI point targets processed from 4 years (2003–
2006) of RADARSAT images. Landslide hazard and risk maps for five temporal predictions
of 2, 5, 10, 20 and 30 years were updated with the exposure of losses estimated in Euro (€).
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In particular, the result shows that in 30 years a potential loss of approximate €3.22 billion is
expected due to these slow-moving landslides detected by PSI point targets.
Keywords Slow-moving landslides – Landslide hazard and risk assessment – PersistentScatterer Interferometry – Remote sensing
Introduction
Landslide hazard and risk assessment is the common concern of researchers, communities
and administrators (Aleotti and Chowdhury 1999). A reliable hazard and risk mapping helps
to address further risk management activities such as mitigation works, early warning
systems and sustainable development planning. In recent years, various methodologies have
been developed using different statistical models for quantitative landslide hazard and risk
assessment. Overviews and summaries regarding these approaches can be found in previous
literature such as Aleotti and Chowdhury (1999), Guzzetti et al. (1999), Dai et al. (2002),
Glade et al. (2005), and van Westen et al. (2006, 2008). In addition, a guideline for landslide
susceptibility, hazard and risk mapping with a definition of general framework and uniform
terminologies was proposed by Fell et al. (2008).
However, with increasing case studies of landslide hazard and risk mapping, very few of
them were further updated. This is fairly contradictory to what landslide hazard and risk
assessment originally targets on, since a reliable hazard and risk mapping should be
considered as an ongoing work which is fundamentally needed to be updated as frequently as
possible (Aleotti and Chowdhury 1999). In particular, for those mapping results derived from
dynamic data, the assessment needs to be updated regularly and continuously (van Westen et
al. 2008). The current challenge to keeping landslide hazard and risk maps updated is often
due to the availability of new data, thereby making an update of previous assessment
difficult.
To some extent, remote sensing has the potential to solve this problem of lacking new data
owing to the scheduled revisiting time and orbit which ensure a frequent update of acquired
data. In practical terms, remote sensing has already shown its usefulness in landslide hazard
and risk assessment, particularly in landslide hazard identification, spatial extent prediction
and triggering factors detection (Metternicht et al. 2005). For rapid-moving shallow
landslides and debris flows, the hazard identification and inventory mapping can be carried
out using optical imageries by recognizing the removal of vegetation from spectral behaviors
(e.g., Sato et al. 2007; Martha et al. 2010; Lu et al. 2011). For slow-moving landslides, the
mass movement can be detected through image correlations of sequential high resolution
optical data (e.g., Casson et al. 2003; Delacourt et al. 2004, 2007). However, this method is
affected by weather conditions and illumination changes, and it has difficulty in operation
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during night (Travelletti et al. 2012).
As a remote sensing product from active microwave sensor, Persistent Scatterer
Interferometry (PSI) is an Interferometric Synthetic Aperture Radar (InSAR) technique that
employs a multi-interferogram analysis of temporal Synthetic Aperture Radar (SAR) images,
for extracting long-term high phase stability benchmarks of coherent PSI point targets,
namely Persistent Scatterers (PS). PSI is not affected by weather conditions and illumination
changes. In the past years, several approaches have been developed to obtain these PSI point
targets. For example, the PSInSAR™ technique, which can estimate the phase stability of the
scattering barycenter with low differential atmospheric contribution and can extract PS
regardless of normal and temporal baseline, was firstly proposed by Ferretti et al. (2000,
2001) and further improved by Colesanti et al. (2003). Also, the Stanford Method for
Persistent Scatterers (StaMPS), which utilizes the spatial correlation of interferogram phase
to find persistent benchmarks, was introduced by Hooper et al. (2004) and further modified
by Hooper et al. (2007). Similarly, the Interferometric Point Target Analysis (IPTA), which
has advantage in finding stable benchmarks in areas of low coherence and can use large
baselines for phase interpretation, was presented by Werner et al. (2003) and Strozzi et al.
(2006). Besides, the Coherence Pixel Technique (CPT), which enables an estimation of linear
and nonlinear components of ground deformation, was reported by Mora et al. (2003) and
Blanco-Sanchez et al. (2008). Moreover, the Small Baseline Subset (SBAS), an approach
performed on small-baseline interferograms and combining unwrapped differential InSAR
(DInSAR) interferograms through the Singular Value Decomposition (SVD) method, was
described by Berardino et al. (2002), Lanari et al. (2004) and Casu et al. (2006). Furthermore,
the Spatio-Temporal Unwrapping Network (STUN) algorithm was proposed by Kampes
(2006) which combines displacement model with spatial network for phase unwrapping
based on single-master interferograms. Additionally, the Stable Point Network (SPN) was
suggested by Crosetto et al. (2008) and Herrera et al. (2011), focusing on pixels with stable
behaviors in SAR amplitude stability, interferometric coherence and spectral coherence.
More recently, Zhang et al. (2012) demonstrated a method of temporarily coherent point
(TCP) InSAR (TCPInSAR) which can estimate deformation signals without the need for
phase unwrapping.
Owing to the millimeter precision, PSI is suitable for studying slow-moving landslides in
several aspects. First, PSI can be used for detection of slow-moving landslides. Bovenga et
al. (2006) combined PS information with ground data and monitoring controls to detect
landslides in the Daunia Apennine Mountains in Southern Italy. Besides, landslides can be
potentially detected by the hotspot mapping of PS clustering (Bianchini et al. 2012; Lu et al.
2012). Second, PSI shows its usefulness in landslide mapping and inventory updating. In
particular, by integrating with optical images, ancillary maps and ground measurements, the
PSI information can be used to modify the existing inventory with an assessment of the state
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of landslide activity (Farina et al. 2006; Righini et al. 2012; Calò et al. 2012). Also, Cascini et
al. (2009) used PSI to check and update the existing landslide inventory at 1:25,000 scale and
to test the reliability of inventory based on the geomorphologic criteria. Third, PSI provides
an effective tool for landslide monitoring. This can be fulfilled by integrating PS with
leveling data and GPS for ground deformation monitoring (Colesanti et al. 2003). Greif and
Vlcko (2012) monitored the post-failure behavior of landslides in Central Slovakia using
transformation of line-of-sight (LOS) displacement rate to slope vector direction. Besides,
Herrera et al. (2011) detected and monitored the Portalet landslide in Spain using a
combination of X-band TerraSAR-X data and C-band ERS and ENVISAT data. The
monitoring capacity was further improved by Herrera et al. (2013) to figure out different
moving directions, measure different velocity patterns within the same moving mass and
identify triggering factors. Additionally, Bovenga et al. (2012) indicated that X-band sensors,
which have higher spatial resolution and shorter revisiting time, can estimate the surface
displacement using fewer images and the monitoring can be done in shorter time for high risk
cases. Fourth, PSI is valuable for landslide investigation. PS can be used for landslide
investigation at regional scale by combining visual interpretation of optical images (Farina et
al. 2006). PS can also refine the boundaries and the state of activity of landslides for
understanding the deformation pattern and relation with triggering factors (Tofani et al.
2013). Cascini et al. (2010) proposed a method for landslide feature investigation which can
be used for both full and low resolution analysis. Cigna et al. (2012) employed a PSI-based
matrix approach to evaluate the state of activity and intensity of slow-moving landslides.
Bovenga et al. (2013) integrated X-band COSMO-SkyMed, C-band ENVISAT and GNSS
measurements for landslide investigation in Assisi, Italy. Fifth, PSI has potential in landslide
mechanism understanding. For quantitative estimation, PS are suggested to be combined with
ground truth, field survey and analysis of acquisition geometry to understand landslide
mechanism (Colesanti and Wasowski 2006). The seasonality of ground acceleration revealed
by PS can be combined with the precipitation data to analyze the dynamics of slow-moving
landslide (Hilley et al. 2004). Zhao et al. (2012) indicated that by comparing PS velocity with
precipitation record, it can correlate landslide displacement with rainfall peak for further
investigation of landslide mechanism and defining rainfall threshold for early warning
purpose.
Although PSI has been extensively applied in landslide studies, none of the above-mentioned
works deals with the direct use of PS for quantitative landslide hazard and risk assessment. In
order to fill this gap, and also for an ongoing update of existing landslide hazard and risk
maps, a novel approach of quantitative landslide hazard and risk assessment using PSI point
targets is presented. The purpose is to quantify the potential hazard and risk resulting from
slow-moving landslides. The Arno River basin in central Italy was chosen as the study area
because most of the mass movements in the basin are slow-moving rotational landslides
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which are properly within the detection precision of PSI point targets (Lu et al. 2012). Catani
et al. (2005) have accomplished a hazard and risk mapping at catchment scale for all types of
landslides in the Arno River basin. This paper continues the work of Catani et al. (2005), but
will focus only on slow-moving landslides within the detection range of PSI. The novelty of
this paper is to provide a new application of PSI in quantitative landslide hazard and risk
assessment and to develop a new method to integrate PSI for an ongoing update of landslide
hazard and risk maps.
Study area and PS datasets
The Arno river basin is located in central Italy (Fig. 1), mostly within the Tuscany region.
The whole area of the basin is about 9130 km2. Since the basin is across the Northern
Apennines orogenic belt, 78 % of the area (ca. 7190 km2) is situated in mountainous and hilly
areas. The basin is strongly affected by landslides. More than 27,000 landslides were
previously mapped at a scale of 1:10,000, with a total affected area of more than 800 km2
(Catani et al. 2005; Farina et al. 2006). 74 % of these landslides are slow-moving rotational
slides, which can be periodically reactivated by prolonged and intense rainfall (Catani et al.
2005; Lu et al. 2012). These landslides pose great threat to human lives and vulnerable
elements considering the dense population (2.6 million inhabitants) within the basin, which
includes the major cities of Firenze, Pisa and Arezzo (Fig. 1).
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Fig. 1
The geographic location of the Arno River basin
The PS datasets were processed by Tele-Rilevamento Europa (TRE) on behalf of the Arno
River basin Authority using PSInSAR™ technique, which is only slightly affected by
temporal/geometric decorrelation and atmospheric disturbances as described by Ferretti et al.
(2000, 2001). 102 RADARSAT-1 SAR images (54 ascending and 48 descending scenes)
spanning from March 2003 to January 2007 were processed. These images were collected
with the beam mode of S3 which provides an incident angle ranging between 30° and 37°.
The orbiting tracks of the satellite are 54 for descending and 247 for ascending acquisitions.
These two tracks cover about 6,300 km2, approximately 70 % of the whole basin. With a
definition of coherence level above 0.60, more than 700,000 PS were identified. The
precision of displacement rates along the LOS varies from 0.1 to 2 mm/year and the
geocoding accuracy of PS position is within 5 m in the north–south and 10 m in the east–west
direction. The PS density is 54 points/km2 for ascending data and 60 points/km2 for
descending orbit. PS located in the flat area were masked out so that only PS situated in the
mountainous and hilly areas were used for further landslide hazard and risk mapping. The
point density decreased to 31 PS/km2 for ascending data and 32 PS/km2 for descending data
after the masking of flat areas.
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Methodology and results
PSI Hotspot and Clustering Analysis
The first step for landslide hazard and risk assessment from PSI is to employ the spatial
statistics approach of PSI Hotspot and Clustering Analysis (PSI-HCA) as proposed by Lu et
al. (2012). The purpose is to obtain a continuous estimation of PS distribution from their
spatially correlated velocities. PSI-HCA is composed of two spatial statistic approaches: (1)
Getis-Ord G i * statistics (Getis and Ord 1992) calculated for each single PSI point target and
(2) kernel density estimation (Silverman 1986) derived based on calculated G i * values. The
output of PSI-HCA is a PS hotspot map represented by the kernel density values. In this
study, PSI-HCA was performed on both ascending and descending PS datasets. Figure 2
shows part of the PS hotspot map in the Arno River basin. Pixels with positive kernel density
values are displayed with blue colors whereas pixels with negative kernel density values are
rendered on red colors. Both blue and red hotspots indicate where potential mass movements
exist, with deeper color indicating higher clustering level of high velocity PS. Full details and
explanations of PSI-HCA can be found in the work of Lu et al. (2012).
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Fig. 2
The landslide hotspot map covering the area between Volterra and Poggibonsi in the Arno River basin:a hotspot map derived from a kernel density estimation using ascending RADARSAT PS: red hotspotscorrespond to PS moving downwards and/or eastwards, whereas blue hotspots correspond to PSmoving upwards and/or westwards; b hotspot map derived from a kernel density estimation usingdescending RADARSAT PS: red hotspots correspond to PS moving downwards and/or westwards,
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whereas blue hotspots correspond to PS moving upwards and/or eastwards
Susceptibility mapping
The landslide susceptibility in the Arno River basin was previously mapped by Catani et al.
(2005). Since susceptibility assessment only concentrates on spatial probability of landslide
occurrences, regardless of temporal prediction, the susceptibility map created by Catani et al.
(2005) was also used in this study with the same area focused. Catani et al. (2005) described
the methodology of landslide susceptibility mapping in the Arno River basin in detail. Five
preparatory factors related to slope instability (slope angle, profile curvature, upslope
contributing area, land cover and lithology) were selected for the susceptibility analysis. The
slope angle, profile curvature and upslope contributing area factors were derived from a 10-m
DTM (created from topographic maps in 2002), classified into five, three and three classes,
respectively. The land cover factor, obtained from a 1:50,000 land cover map, was classified
into nine classes according to the legend of CORINE (Coordination of Information on the
Environment) land cover project (Heyman et al. 1994). The lithology factor, acquired from
the lithology map published by Canuti et al. (1994), was reclassified into eight classes. A
summary of these five susceptibility parameters and their classification algorithms is listed in
Table 1. The artificial neural network (ANN) was then deployed for statistical estimation of
landslide susceptibility because of its loose hypothesis on the variable distribution and the
possibility to involve mixed parameters (Ermini et al. 2005). The statistical prediction was
performed on the basis of unique condition unit (UCU), which is the subdivided
homogeneous terrain unit referring to the landslide preparatory factors. Those UCUs were
used to build a number of model vectors for training and testing of ANNs. For an accurate
ANN training, the training data were chosen with reference to the landslide inventory and
were validated in the field for accuracy and completeness. By checking the landslide
inventory, the percentage of UCU area subject to landslides was determined as predicted
variable. As ANN output, each UCU was assigned a degree of membership to a susceptibility
value from 0 to 100. The derived susceptibility map was then reclassified into four classes (S
0, S 1, S 2 and S 3, assorted with increasing susceptibility levels) by comparing the cumulative
density function of ANN outputs within mapped landslides with the cumulative distribution
of total ANN outputs, as proposed by Ermini et al. (2005) and modified by Catani et al.
(2005). The final output of the susceptibility mapping in the Arno River basin is illustrated in
Fig. 3.
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Table 1
The susceptible parameters and their classifications used for landslide susceptibility assessment in theArno River basin (Catani et al. 2005)
Susceptible
parameterClassification
Slope angle 0–5°, 5–10°, 10–20°, 20–33°, 33–90°
Profile
curvatureConcave; planar; convex
Upslope
contributing
area
0–1,000, 1,000–1,500, >1,500 m2
Land coverArtificially modified land; crops and permanent cultivation; forest; grassland; heterogeneous
cultivated land; rangeland; scrubland; wetland
LithologyCohesive soils; complex mainly pelitic units; granular soils; indurated rocks; marls and compact
clays; rocks with pelitic layers; weak rocks; weakly cemented conglomerates and carbonate rocks
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Fig. 3
The landslide susceptibility map of the Arno River basin (modified after Catani et al. 2005)
Hazard mapping
The landslide hazard map was accomplished based on the previously derived landslide
hotspot maps (Fig. 2). Five hazard levels (H 0, H 1, H 2, H 3 and H 4, assorted by increasing
hazard levels) were initially determined from the kernel density values of the hotspot maps.
Ascending and descending hotspot maps were analyzed separately for hazard assessment.
This is due to the independent PSI processing approaches of long-term SAR images for
ascending and descending orbits, reflected by their differences in acquisition dates, master
images, reference points and coherence maps. For each orbit, blue and red hotspots,
indicating different moving directions of mass movements along LOS, were individually
analyzed for initial hazard zonation. The boundary and threshold for hazard zonation were
derived from heuristic determination by classifying hotspot maps into different levels, using
aerial photo interpretation and field surveys as supplementary references. A preliminary
hazard zonation was determined based on the Z-score values of hotspot map pixels. The Z-
score of 2.5 was chosen as the basic zonation unit since it corresponds to an approximate
99 % confidence interval of kernel density pixels. The kernel density values were divided
into five classes, with thresholds defined as 0, 2.5, 5 and 10 standard deviations for red
hotspots, and 0, 5, 10, 20 standard deviations for blue hotspots. The corresponding result for
assigning five hazard levels is summarized in Table 2. The selection of different zonation
conditions for red and blue hotspots is due to their different moving directions and
acquisition geometries. Red hotspots (negative kernel density) indicate the clustering of PS
moving away from the sensor, whereas blue hotspots (positive kernel density) suggest the
clustering of PS moving towards the sensor. Considering the incidence angle ranging
between 30o and 37o for RADARSAT, this can be interpreted as follows:
Table 2
The conditions of classifying hazard levels from kernel density values of hotspot maps
Ascending orbit
Red hotspot
H 4 Kernel density ≤ −280
H 3 −280 < kernel density ≤ −140
H 2 −140 < kernel density ≤ −35
H 1 −35 < kernel density < 0
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H 1 −35 < kernel density < 0
H 0 Kernel density = 0
Blue hotspot
H 4 Kernel density ≥ 560
H 3 560 > kernel density ≥ 280
H 2 280 > kernel density ≥ 70
H 1 70 > kernel density > 0
H 0 Kernel density = 0
Descending orbit
Red hotspot
H 4 Kernel density ≤ −200
H 3 −200 < kernel density ≤ −100
H 2 −100 < kernel density ≤ −25
H 1 −25 < kernel density < 0
H 0 kernel density = 0
Blue hotspot
H 4 Kernel density > = 400
H 3 400 > kernel density ≥ 200
H 2 200 > kernel density ≥ 50
H 1 50 > kernel density > 0
H 0 Kernel density = 0
– For the ascending orbit: red hotspots correspond to PS moving downwards and/or
eastwards, whereas blue hotspots correspond to PS moving upwards and/or westwards.
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– For the descending orbit: red hotspots correspond to PS moving downwards and/or
westwards, whereas blue hotspots correspond to PS moving upwards and/or eastwards.
For both ascending and descending orbits, blue hotspots correspond to PS moving upwards.
Although upward movements are typical features in lower portions of rotational landslides,
they are also possibly related to fluid injection (Doubre and Peltzer 2007), sedimentation of
rivers (Smith 2002) and tectonic uplift (Vilardo et al. 2009; Massironi et al. 2009; Morelli et
al. 2011). Therefore, the hazard levels for blue hotspots were more carefully defined with
larger standard deviations.
After, the hazard levels estimated from the hotspot maps were compared with the
susceptibility classes. For each pixel, if the initial hazard level is higher than the
corresponding susceptibility class, the new hazard level is determined by the former. Instead,
if the hazard level from the hotspot map is lower than the corresponding susceptibility class,
the final hazard level was assigned by the values of the latter. This is due to the fact that
underestimation of mass movements from PSI techniques possibly exists, resulting from a
lack of stable benchmarks with high coherence values.
For each of these five new hazard levels, a conventional recurrence time T was assigned (H 0:
10,000 years, H 1: 1,000 years, H 2: 100 years, H 3: 10 years, H 4: 1 year) as described by
Catani et al. (2005). The temporal probability was then calculated for each hazard level, using
the following algorithm (Canuti and Casagli 1996):
where T is the recurrence time, N is the time period considered for temporal probability
assessment which was calculated here for 2, 5, 10, 20, and 30 years, respectively. P{H(N)} is
the temporal probability of landslide occurrences in a given time span N. The result of
occurrence probability for each hazard level is listed in Table 3. It was assessed by five
classes (from H 0 to H 4), with each corresponding probability of occurrences (from 0 to 1)
over five periods. An example of a derived hazard (temporal probability) map over the period
of 30 years is displayed in Fig. 4.
P{H(N)} = 1 − ,(1 − )1
T
N
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Table 3
The probability of landslide occurrence for different hazard levels and time spans
Recurrence
time T
(years)
P{H(2 years)} P{H(5 years)} P{H(10 years)} P{H(20 years)}
H
41 1 1 1 1
H
310 0.1900 0.4095 0.6513 0.8784
H
2100 0.0200 0.0490 0.0956 0.1821
H
11,000 0.0020 0.0049 0.0099 0.0198
H
010,000 0.0000 0.0005 0.0010 0.0019
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Fig. 4
The landslide hazard (temporal probability) map of the Arno River basin for 30 years, updated from PSIpoint targets
Landslide intensity
Landslide intensity can be measured from the kinetic energy of mass movement, which is
primarily considered as its volume and velocity, or a more complicated estimation of its run-
out distance (Hungr 1995). Due to the difficulty in measuring velocity of slow-moving deep-
seated landslides over large areas, the intensity is mainly measured from its estimated
volume. In the Arno River basin, Catani et al. (2005) measured the intensity of deep-seated
landslides from the estimation of landslide volume using the post-failure geometry based on
the assumption that the shape of landslide is half-ellipsoidal. In this study, the intensity was
additionally measured from the velocity of landslide, thanks to the technique of PSI which
enables a detection of slow movement of millimeters per year. Moreover, the PSI technique
provides the complete time series record of landslides velocity over the period of processed
SAR images, thus making a selection of maximum velocity of mass movement possible. This
is especially useful for landslide intensity estimation due to the fact that landslide intensity is
often determined by its maximum velocity instead of an average velocity over a period of
time (Hungr 1997).
In this study, for each single PS, in order to remove noise, the time series data of PS were
firstly smoothed using a moving average filtering with a smooth span of five consecutive
SAR acquisitions, namely five records of the time series data of each single PSI point target.
Since the PSI-derived velocities are calculated as averages of observations over a period, they
are found to be lower than the peak velocities of mass movements (Cascini et al. 2010; Cigna
et al. 2012). As a result, the maximum velocity was selected from the time series of velocity
for each single PSI point target for both ascending and descending orbits. The intensity field
was then interpolated from the maximum velocity of PS incorporating the geostatistical
approach of ordinary kriging (Stein 1999), firstly quantifying the spatial structure of PS and
subsequently performing a spatial prediction of other areas uncovered by PS data. The
kriging model employed the statistical relationships of spatial autocorrelation among the
measured maximum velocity of PS for a spatial prediction. This was done by calculating its
empirical semivariogram which estimated the squared difference between the velocity values
for all pairs of PS datasets. The kriging is an exact interpolator which preserves the PS
velocity at known locations. Considering the PS located in the Arno River basin are spatially
clustered (Lu et al. 2012), the kriging can also be used to compensate the spatial clustering
effect of those PSI point targets, assigning each single PS within a cluster less weight than an
isolated PS. The root mean square (RMS) standardized errors are 1.146 and 1.068 for
ascending and descending data, respectively. Both RMS standardized errors are close to 1,
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indicating the variability was assessed correctly in prediction. Besides, the mean standardized
prediction errors are 0 and 0.012 for ascending and descending data, respectively. Both mean
standardized prediction errors are close to 0, suggesting that prediction errors are unbiased.
The interpolated velocity field was then classified into four classes: v 4 (velocity
>10 mm/24 days), v 3 (10 mm/24 days > velocity > 4 mm/24 days), v 2 (4 mm/24 days >
velocity > 2 mm/24 days) and v 1 (velocity <2 mm/24 days). Here, the time span of 24 days is
the revisiting time of the RADARSAT satellite, namely the time interval between two
consecutive time series records. The reason 10 mm/24 days was selected as the threshold to
classify v 4 and v 3 is that it approximates the typical velocity of an active slow-moving
landslide (1.6 m/year), according to the classification reported by Cruden and Varnes (1996).
Similarly, 4 mm/24 days was chosen as the boundary between v 3 and v 2 due to the typical
velocity for differentiating very slow and extremely slow landslides (Cruden and Varnes
1996). Furthermore, 2 mm/24 days was defined as the typical velocity for extremely slow-
moving landslide to separate the velocity level of v 2 and v 1. These four classes of velocity
were used to define the new intensity levels by comparing with the initial intensity levels
(five classes from I 0 to I 4, with a significance of increasing intensity levels). The
comparison was based on a heuristic approach using the classification matrix indicated in
Fig. 5. The intensity classification was performed for both ascending and descending orbits,
which were subsequently merged into a unique intensity map based on the algorithm that
applies higher intensity classes if one pixel contains both values from two orbits. The final
derived intensity map is displayed in Fig. 6.
Fig. 5
The matrix for rendering new intensity levels based on the kriging-interpolated velocity level v and theinitial intensity level I mapped from the landslide inventory
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Fig. 6
The derived landslide intensity map in the Arno River basin
Vulnerability and exposure
The vulnerability is generally defined as a function of a given intensity, measured as the
expected degree of loss for an element at risk due to landslide occurrence, ranging between 0
(without damage) to 1 (full destruction) (Varnes and IAEG Commission on Landslides 1984;
Fell 1994). Exposure instead is related to the practical use of vulnerability, usually considered
as the number of lives or the value of properties exposed at risk (Schuster and Fleming 1986).
The selection of the elements at risk for vulnerability assessment in this study was extracted
based on the regional digital topographic maps at the scale of 1:10,000, and an updated
CORINE land cover map of 2002 from European Space Agency at the scale of 1:50,000
(Heyman et al. 1994). A geodatabase of the elements at risks was then built, including the
exposure values and vulnerability as a function of intensity which was previously
determined. The elements at risk were classified into five categories: building, complex, road,
railway and land cover. Each category was further subdivided according to their practical
uses which render the exposure and vulnerability value for each element. For example,
complexes used for hospitals and schools are considered more vulnerable than sport facilities,
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thus receiving higher values for exposure and vulnerability. A detailed description of this
geodatabase regarding vulnerability and exposure can be found in the work of Catani et al.
(2005).
Quantitative landslide risk assessment
The quantitative risk assessment was performed with the direct application of the following
algorithm: Risk = Hazard × Vulnerability × Exposure, as suggested by Varnes and IAEG
Commission on Landslides (1984), Fell (1994), van Westen et al. (2006) and Remondo et al.
(2008). The calculation was performed on each pixel with a spatial resolution of 10 m,
completed for five different time spans of 2, 5, 10, 20 and 30 years, respectively. The final
output is a 10-m resolution landslide risk map with each pixel indicating the amount of
expected loss in Euro. An overview of landslide risk maps for 2, 5, 10, 20 and 30 years is
rendered in Fig. 7.
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Fig. 7
The landslide risk map estimated from PSI in the Arno river basin: a the shaded relief map, b–f riskmaps for 2, 5, 10, 20, 30 years, respectively. See the corresponding amount of losses in Table 4
The total estimated economic loss is summarized in Table 4, indicating the potential losses
(in Euro) of 2, 5, 10, 20 and 30 years. In particular, approximately €3.22 billion loss was
expected in the upcoming 30 years throughout the Arno River basin, due to the slow-moving
landslides within the detection range of PSI technique. The approximate losses for 20, 10, 5
and 2 years are €2.72 billion, €1.86 billion, €1.14 billion and €0.54 billion, respectively.
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Table 4
Landslide risks (losses estimated in Euro) in the Arno River basin calculated from PSI for five timespans
Time span (years) Expected economic losses (in Euro)
2 543,980,444
5 1,143,746,730
10 1,864,851,052
20 2,721,273,302
30 3,224,446,172
Discussion
To validate the result of risk assessment, some financial data concerning the expenses spent
on landslide prevention and risk mitigation in the Arno River basin during 5 years (2001–
2005) were collected. These expenses come from Italian national and regional funds,
allocated by two national laws, Department of Italian Civil Protection, regional decrees and
the landslide mitigation program in the Arno River basin. The detailed amount of expenses
from 2001 to 2005 is listed in Table 5. The financial data indicates that a total amount of
€0.52 billion was spent on landslide risk mitigation in the Arno River basin during these
5 years. The estimated economic loss for 5 years in this study is about €1.14 billion, which is
higher than the collected financial data. This difference between the actual funds for
mitigation measures and the expected loss is possibly due to two facts. First, the financial
data were not completely collected. Not all the public funding information was collected and
additionally none of the private expense was obtained. This renders potential underestimation
of actual expenses spent on landslide risk mitigation in the Arno River basin. Second,
mitigation measures were not built for all landslides in the basin during these 5 years. The
priority was given to landslides with higher intensity and more vulnerable elements at risk.
Therefore, the actual expenses spent may be lower than the estimated economic loss.
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Table 5
The amount of expenses used for landslide risk mitigation in the Arno River basin during 5 years (2001–2005)
Financial source Amount of expenses (€)
Italian National Law L. 183/89 4,068,176
Italian National Law D.L. 180/98 16,186,881
Department of Italian Civil Protection 185,283,204
Regional Allocation 201,470,084
Landslide Mitigation Program 109,072,488
Total 516,080,833
The result of risk assessment in this study was also compared to the previous study of Catani
et al. (2005). Similar to the observation of Catani et al. (2005), in this study the increase of
risk with time is nonlinear. However, compared to the previous risk mapping result, the risk
value in this study is significantly lower. Catani et al. (2005) expected ca. €6 billion loss for
30 years, whereas the estimation in this study is ca. €3.22 billion. The decrease of the
predicted risk is possibly due to the fact that the risk assessment performed by Catani et al.
(2005) focused on all types of landslides in the inventory while in this study only the slow-
moving landslides were concentrated on. Although the whole basin is predominated by slow-
moving deep-seated landslides (ca. 74 %), and rapid-moving shallow landslides and debris
flows only account for 22 %, the consequence of rapid-moving landslides is more severe than
slow-moving landslides. Another reason of the lower estimated risk is possibly due to the
limitation of PSI technique in the areas without stable reflectors or coherent targets. This
causes the potential lack of PS for some landslides, thus bringing an underestimation of
landslide intensity and hotspot quantities. However, this limitation is expected to be largely
improved with the increasing uses of higher resolution X-band SAR sensors such as
COSMO-SkyMed (Bovenga et al. 2012) and TerraSAR-X (Prati et al. 2010) and new
processing approaches such as the SqueeSAR technique (Ferretti et al. 2011) and multi-
master interferograms strategy (Prati et al. 2010).
Recently, several studies have tried to represent PS velocity not only limited to LOS. For
example, Cascini et al. (2010) proposed an approach to project LOS deformation to the
steepest slope direction with the assumption that the mechanism of mass movement is
translational landslide. This approach was also applied in monitoring post-failure landslide in
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Central Slovakia (Greif and Vlcko 2012). In the Arno River basin, this projection is not used
because the dominant type of mass movement is slow-moving rotational landslides.
However, for the area susceptible to the translational landslide, it may provide an effective
approach for PSI-HCA and hazard map generation. On the other hand, if both ascending and
descending PS are available, the velocity vector can be represented on the East–
West–Zenith–Nadir plane (Lu et al. 2010). This approach has the advantage in combining
both ascending and descending orbits for PSI-HCA. However, for landslide studies, it has
significant difficulty in receiving stable radar targets from both ascending and descending
orbits due to the geometrical visibility and distortions (e.g., foreshortening, layover and
shadowing effects) in mountainous and hilly areas (Colesanti and Wasowski 2006; Cigna et
al. 2012).
Conclusion
Quantitative landslide hazard and risk assessment is essential for hazard mitigation and risk
management. For a sustainable development plan, the assessment is needed to be updated as
frequently as required. Possibly due to the lack of new data, currently very little attention has
been paid to updating previously mapped result. With scheduled revisiting time and orbits,
remote sensing products provide an important data source for a frequent update of landslide
hazard and risk assessment. In this paper, aiming at an update of previously mapped landslide
hazard and risk in the Arno River basin by Catani et al. (2005), a novel approach was
developed to evaluate the hazard and risk level of slow-moving landslides from PSI point
targets.
The quantitative risk assessment was based on the following algorithm: Risk = Hazard ×
Vulnerability × Exposure. Firstly, a susceptibility map completed by Catani et al. (2005)
using the ANN predictor was included in this study, subsequently combined with the kernel
density values of the hotspot map derived from PSI-HCA, for the generation of landslide
hazard maps for five temporal predictions of 2, 5, 10, 20 and 30 years. Moreover, a landslide
intensity map was determined by the velocity map interpolated from the maximum velocity
of PS time series data using the ordinary kriging method. With given intensity, elements at
risks were extracted from a regional digital topographic map and a CORINE land cover map.
The result of risk mapping was evaluated for 2, 5, 10, 20 and 30 years. In particular, an
expected loss of ca. €3.22 billion was estimated for the upcoming 30 years.
The estimated economic loss for 5 years in this study is higher than the collected financial
data indicating the actual expenses spent on landslide prevention and risk mitigation. This is
possibly because the collected financial data is incomplete and the mitigation works were not
built for all landslides in the Arno River basin. In addition, compared to the risk assessment
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by Catani et al. (2005), the mapping result from PSI technique shows a lower estimation of
potential losses. This is possibly due to the detection range of PSI which is primarily aiming
at slow-moving landslides. Also, PS processed from C-band RADARSAT images render
lower point density, especially in landslide areas without sufficient stable reflectors, thereby
making an omission of detecting slow-moving landslides, and accordingly an
underestimation of potential landslide risk level. Further improvement should include PS
products from X-band images (e.g., TerraSAR-X and Cosmo-SkyMed) as well as new PSI
processing techniques such as the SqueeSAR (Ferretti et al. 2011) and the multi-master
interferograms strategy (Prati et al. 2010).
Acknowledgement
This work was supported by National Natural Science Foundation of China (No. 41201424), 973
National Basic Research Program (No. 2013CB733203 and No. 2013CB733204), 863 National High-
Tech R&D Program (No. 2012AA121302) and Mountain Risks FP6 project of European Commission
(MRTN-CT-2006-035798). The authors are grateful to the staff of Tele-Rilevamento Europa, a spin-off
company of Politecnico di Milano owning the patent of PSInSAR™ technique, for the data processing
and software development. The authors also thank the Arno River Basin Authority for data sharing.
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