EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Nat. Hazards Earth Syst. Sci., 13, 559–573, 2013 Natural Hazards Natural Hazards www.nat-hazards-earth-syst-sci.net/13/559/2013/ doi:10.5194/nhess-13-559-2013 and Earth System and Earth System © Author(s) 2013. CC Attribution 3.0 License. Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Comparison between different approaches to modeling shallow Discussions Open Access Open Access landslide susceptibility: a case history in Oltrepo Pavese,Atmospheric Atmospheric Measurement Measurement Northern Italy Techniques Techniques D. Zizioli1, C. Meisina1, R. Valentino2, and L. Montrasio2 Discussions Open Access Open Access 1Department of Earth and Environment Sciences, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy 2 Department of Civil, Environmental, Territory Engineering and Architecture, UniversityBiogeosciences of Parma, Viale G. P. Usberti 181/A Biogeosciences 43100 Parma, Italy Discussions Correspondence to: D. Zizioli ([email protected]) Open Access Open Access Received: 6 September 2012 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: – Climate Revised: 18 January 2013 – Accepted: 21 January 2013 – Published: 5 March 2013 Climate of the Past of the Past Discussions Abstract. On the 27 and 28 April 2009, the area of Oltrepo of particularly intense and concentrated rainfall, they can be Pavese in northern Italy was affected by a very intense rain- densely distributed across a territory. Open Access Open Access fall event that caused a great number of shallow landslides. The susceptibility analysis of rainfall-induced shallow Earth System These instabilities occurred on slopes covered by vineyards landslides on a large scaleEarth is usually System performed using sta- or recently formed woodlands and caused damage to many tistical methods (Carrara et al.,Dynamics 1991; Bai et al., 2009; Cervi Dynamics roads and one human loss. Based on aerial photographs taken et al., 2010; Li et al., 2012). More recently, physically based Discussions immediately after the event and field surveys, more than models proved rather promising in evaluating shallow land- 1600 landslides were detected. After acquiring topographi- slide spatial susceptibility, starting from a distributed slope- Open Access cal data, geotechnical properties of the soils and land use, stability analysis (MontgomeryGeoscientific and Dietrich, 1994; Wu and Geoscientific Open Access susceptibility analysis on a territorial scale was carried out. Sidle, 1995; Iverson, 2000;Instrumentation Qiu et al., 2007; Baum et al., Instrumentation In particular, different physically based models were applied 2008; Lu and Godt, 2008; Simoni et al., 2008; Baum and to two contiguous sites with the same geological context but Godt, 2010). Encouraged byMethods the performance and of these phys- Methods and different typologies and sizes of shallow landslides. This pa- ically based models and withData the aidSystems of the widely available Data Systems per presents the comparison between the ex-post results ob- Geographic Information Systems (GIS), researchers devel- Discussions Open Access tained from the different approaches. On the basis of the ob- oped and applied methods that appear suitable forOpen Access determin- Geoscientific served landslide localizations, the accuracy of the different ing both the timing and localizationGeoscientific of shallow landslides in models was evaluated, and the significant results are high- response to rainfall on a regional scale (Salciarini et al., 2006, Model Development Model Development lighted. 2008; Godt et al., 2008a, b; Schmidt et al., 2008; Apip et al., Discussions 2010; Liao et al., 2011; Vieira et al., 2010; Arnone et al., 2011). With regard to the spatial susceptibility assessment of Open Access Open Access shallow landslides, differentHydrology methods of analysisand are not mu- Hydrology and tually exclusive. On the contrary, the use of different strate- 1 Introduction gies and the comparison ofEarth the obtained System results can help to: Earth System Sciences Sciences Rainfall-induced shallow landslides typically occur due to – Enhance the quality and reliability of each method. small failures of natural slopes that collapse catastrophically Discussions Open Access without warning. Although they generally involve small vol- – Highlight and identify the most important factorsOpen Access affect- umes of soil, due to their close proximity to urbanized areas, ing the phenomenon. Ocean Science they cause significant damage to structures and infrastructure Ocean Science Discussions and, sometimes, human losses. Moreover, as a consequence – Omit less influential aspects to simplify the models. Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions 560 D. Zizioli et al.: Comparison between different approaches to modeling shallow landslide susceptibility – Select the most appropriate methodology to achieve a April 2009 rainfall event (∼ 24 landslides per square kilome- specific goal. ter, equal to approximately 0.5 % of the entire area). How- ever, the land cover is quite different between the two study For these reasons, it is useful to apply different analysis ap- areas; specifically, area 1 is characterized by vineyards and proaches to a well-documented case history, such as for the woodlands (15 %) that correspond to vineyards abandoned event that occurred in the area of Oltrepo Pavese (Lombardy after the 1980s, and the land cover of area 2 is constituted Region, northern Italy) on the 27 and 28 April 2009. A partic- only by vineyards that have not changed over the last thirty ularly intense rainfall triggered a significant number of shal- years. low landslides (approximately 1600), which, in turn, caused The test site slopes are characterized by a medium-high damage to many roads and one human loss. The rich land- gradient and are composed of marls (S. Agata Fossili Marls), slide database, compiled on the basis of aerial photos and gravel, sand and poorly cemented conglomerates (M. Ar- field surveys, was useful in evaluating the results of different zolo Sandstones and Rocca Ticozzi Conglomerates). A small deterministic models in terms of the correspondence between portion of the area is characterized by the presence of the the landslides source area localization and the assessed un- Gessoso-Solfifera Formation (marls, sandy marls and vac- stable areas. uum limestone with lenses of gypsum-rudites containing Four physically based stability models, namely TRI- gypsum selenite) (Fig. 1). The strata dip east-northeast. Col- GRS (Transient Rainfall Infiltration and Grid-Based Re- luvial soils, derived from the weathering of bedrock, have a gional Slope-Stability Model; Baum et al., 2008), SHAL- thickness ranging from 0.5 m to 2 or 3 m at the bottom of the STAB (Shallow Slope Stability Model; Montgomery and Di- valleys. etrich, 1994), SINMAP (Stability Index Mapping; Tarboton, The climatic regime in the area is the Apennine– 1997; Pack et al., 1999) and SLIP (Shallow Landslides Insta- Mediterranean type, characterized by dry summers and cold bility Prediction; Montrasio, 2000; Montrasio and Valentino, winters, with a primary peak of rainfall in autumn and a 2008), were utilized to analyze the study area. secondary peak in spring. The mean annual temperature is These physically based models are characterized by dif- 12 ◦C. The average annual rainfall is approximately 700 mm ferent levels of complexity, and their results strongly depend in low-lying areas and 998 mm in the hills, with an increase on the quality and detail of the input data. Moreover, their in the rainfall amount from west to east. capability of providing reliable results within a reasonable Oltrepo Pavese is characterized by high density of land- computation time span is strictly linked to the extent of the slides. The mass movements often develop in clayey–marly study area. formations and are classified as complex deep movements Vegetation, land use and their change in time are widely triggered by prolonged rainfalls. More recently climate recognized as important factors influencing rainfall-triggered changes are responsible of extreme weather conditions, such landslides (Begueria, 2006a); therefore, the models were ap- as heavy rainfalls, that have triggered shallow landslides. The plied to two contiguous sites with the same geological con- event of April 2009 was the only documented case of rainfall- text, but different land cover change, which results in differ- induced shallow landslides that hit the Oltrepo Pavese since ent typologies and sizes of shallow landslides. the fifties. In the following section, the main qualities distinguishing On the 27 and 28 April 2009, the northeastern sector of the four models are outlined, and their predictive capabili- Oltrepo Pavese experienced an extreme rainfall event. On the ties are evaluated using a quantitative method known as re- 28 April, the Cigognola rain-gauge station recorded 150 mm ceiver operating characteristic, ROC (Begueria, 2006b). Fi- of rain in 48 h (20 % of the annual average amount), with nally, limits and key advantages of the different simulations a maximum rainfall intensity of 22 mm h−1 at 9 p.m. on are discussed, and comparisons