2nd Regional Symposium on Landslides in the Adriatic-Balkan Region
14 – 16 May, 2015, Belgrade, Serbia
S3 - ″General case studies″
ANALYSIS OF RAINFALL-TRIGGERED
EXTREME LANDSLIDE EVENTS IN SLOVENIA
IN THE LAST 25 YEARS
Nejc BEZAK, Mojca ŠRAJ, Matjaž MIKOŠ
University of Ljubljana – FGG, Slovenia
Introduction
 Post-event analysis of extreme weather events,
during which landslides and/or debris flows
occured, can be used to evaluate different
methodologies for rainfall estimation that could
be used as part of the early warning systems
(EWS).
 In the presented study we have analyzed several
extreme rainfall events in Slovenia from 1989 to
2013 that triggered deep-seated and shallow
landslides, which caused more than 10
casualties and more than 100 million Euros in
economic damage.
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Aims of the study
 To analyze the rainfall triggering conditions for
landslides and debris flows.
 To test the suitability of some commonly used
empirical rainfall-threshold curves for the
Slovenian meteorological conditions.
 To investigate the influence of the inter-event
(IE) time selection on the location of the event in
the rainfall intensity-duration (I-D) space.
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Seven case studies in Slovenia
 Short overview of the selected case studies
Location
Name
Year
Solčava
Macesnik
1989 or 1990
Log pod Mangartom
Stože
2000
Lokavec
Slano Blato
2000
SE Slovenia
SE area
2005
Železniki
Železniki
2007
Ljubljana
Ljubljana
2010
Lokavec
Stogovce,
Znosence
2010
Type of event
Deep-seated
landslide
Deep-seated
landslide &
debris flow
Deep-seated
landslide
Shallow
landslides
Shallow
landslides
Shallow
landslides
Deep-seated
landslide
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Location of the selected case studies
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Location of the analyzed rainfall stations
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Basic characteristics of the analyzed 16 rainfall
stations in Slovenia (1-hour rainfall data!)
Station
Solčava
Log pod
Mangartom
Rateče
Bilje
Godnje
Otlica
Podkraj
Celje
Lisca
Novo mesto
Bohinjska Č.
Davča
Kneške Ravne
Vogel
Črni Vrh
Ljubljana
1990-2013
Mean annual
precipitation
[mm]
1,575
Station
elevation
[m.a.s.l.]
658
Macesnik, SE area
1999-2013
2,371
648
Stože
1975-2013
1991-2013
1992-2013
1999-2013
1984-2013
1977-2013
1984-2013
1973-2013
2003-2013
1999-2013
1982-2013
1982-2013
1996-2013
1948-2013
1,475
1,446
1,400
2,343
2,130
1,130
1,178
1,148
2,203
1,818
2,834
3,077
1,653
1,368
864
55
320
820
799
244
943
220
595
960
752
1,535
830
299
Stože
Slano Blato, Stogovce
Slano Blato, Stogovce
Slano Blato, Stogovce
Slano Blato, Stogovce
SE area
SE area
SE area
Železniki
Železniki
Železniki
Železniki
Ljubljana
Ljubljana
Analysed
period
Case study
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Selected empirical rainfall-threshold curves
Author
α; β
Type
Range [h]
Caine, 1980
14.82; -0.39
Shallow
landslides and
debris flows
0.167<D<500
Clarizia et
al., 1996
10; -0.77
Soil slips
0.1<D<1000
2.2; -0.44
Shallow
landslides and
debris flows
0.1<D<1000
Guzzetti et
al., 2008
“Standard” empirical rainfallthreshold curves
Empirical rainfall-threshold
curves (Aleotti, 2004):
Variables used:
 I mean rainfall intensity [mm/h]
 D mean rainfall event duration [h]
 NAR normalized antecedent rainfall for 10 days before the event [%]
 NCR is the normalized critical rainfall that triggered the event [%]
 IMAP normalized mean rainfall intensity [h-1 * 100]
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Two different inter-event (IE) values were
tested
IE time=period without rainfall
Methodology: If the time period without rainfall between two consecutive
events is smaller than the selected IE value, these two events are joined into
one event. In opposite situation two separate events are considered.
Two IE values were selected :
 Subsample 1: Inter-event (IE) time was 6 hours
 Subsample 2: Inter-event (IE) time was 18 hours
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Different rainfall triggering conditions I
Figure shows maximum daily intensities, which were calculated
based on the accumulated rainfall amounts for duration up to
40 days before the date when the extreme event happened.
We want to define the rainfall situation (convective storms, long
duration frontal precipitation) that triggered different types of
landsliding: shallow landslides, flash floods, deep- seated
landslides.
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Different rainfall triggering conditions II
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Conclusion I
 The triggering mechanisms in the selected case
studies were different.
 The SE area 2005, Železniki 2007 and Ljubljana
2010 events were triggered by short duration (from
a few hours to one or two days) rainfall events,
which caused flash floods and triggered several
shallow landslides.
 The Slano Blato 2000, Log pod Mangartom (Stože)
2000 and Stogovce 2010 deep-seated landslides
were triggered by longer duration rainfall events
(up to one week or a few weeks).
 Probably also deep-seated Macesnik landslide was
initiated due to a long duration rainfall event,
however the exact date of triggering is not known.
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Post-event analysis of the selected events I
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Post-event analysis of the selected events II
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Post-event analysis of the selected events III
Rainfall duration and mean rainfall intensity values using daily
data for 1989 for the Solčava rainfall station (Macesnik).
Several (more than 15!) events are located above the Guzzetti
and Clarizia curves.
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The IE value versus the I-D space I
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The IE value versus the I-D space II
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Conclusion II
 The inter-event time (IE) selection clearly has
significant influence on the rainfall analysis results.
 Independently of the selected IE value the empirical
rainfall-threshold curves proposed by Clarizia et al.
(1996) and Guzzetti et al. (2008) are not the most
appropriate for Slovenian climatological conditions.
 Why? - Convective storm events are frequent and we
found that therefore a relative large number of
rainfall events are located above the suggested
empirical rainfall-thresholds.
 Using such a threshold curve for an Early Warning
System would produce false alarms.
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Summing up
 The selected empirical rainfall thresholds can
be used as a part of an early warning system
(EWS); however this kind of system requires an
adequate rainfall measuring network with
suitable spatial density and selection of
appropriate rainfall-threshold curves.
 Inter-event (IE) time selection has significant
influence on the analysis of rainfall events that
triggered several extreme events in the last 25
years in Slovenia. Further analyses are needed.
 As expected, triggering mechanisms for deepseated and shallow landslides were different.
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Thank You for Your attention !
Hvala na pažnji !
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