A comparative analysis of terrain stability models for predicting shallow landslides in colluvial soils
Introduction
Most of the slopes of the hilly areas of the Apennines are covered by colluvial soils originating from the weathering of the bedrock and down slope transportation. Shallow translational landslides are the most commonly observed failure mode involving colluvial slopes; they depend largely on the surface topography and are a recurrent problem. The soil types involved are quite varied, ranging from sand to clayey silt (Hutchinson, 1988). The triggering factor is represented by heavy rainstorms with high rainfall intensities and high duration. Several authors have studied these phenomena in order to determine the predisposing and triggering factors, to prevent future disasters (Van Asch et al., 1999, Antronico and Gullà, 2000, Campus et al., 2001, D'Amato Avanzi et al., 2003). Despite the modest volume involved, shallow landslides are very dangerous, due to the absence of incipient movement evidence and their high velocity.
Numerical models are often used for the preliminary identification of slopes prone to shallow landslide occurrence (Borga et al., 1998, Morrissey et al., 2001, Guimaraes et al., 2003). Several shallow landslide models have been developed on the basis of the infinite slope equation. SHALSTAB (Montgomery and Dietrich, 1994) and SINMAP (Pack et al., 1998, Pack et al., 2001, Pack and Tarboton, 2004) are two such models that combine steady state hydrology assumptions with the infinite slope stability model to quantify slope stability. Inputs are digital elevation data from which slope and drainage area are evaluated as well as hydrologic and soil parameters. These are two models with similar physical basis but they use different indices to quantify instability. SINMAP quantifies terrain stability in terms of the probability that the infinite slope stability model factor of safety is greater than one over uniform probability distributions quantifying the uncertainty in model parameters. SHALSTAB quantifies terrain instability in terms of the critical effective rainfall required to trigger pore pressure induced instability.
The aims of the study are:
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to apply and evaluate the different approaches of SINMAP and SHALSTAB for shallow slope instability analysis to a specific geological context (the Oltrepo Pavese), characterized by colluvial soils originating from the weathering of the clayey bedrock and down slope transportation;
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to compare the results of these analyses;
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to understand their respective potentials, with regard to their applicability and data demand, in the Apennines context.
The territory of S. Giuletta represents the test site. It is located in the Oltrepo Pavese (Northern Apennines, Italy) and it is characterized by clayey–silty colluvial soils on marly bedrock. In this area, representative of similar situations in Apennines, shallow landslides occurred on November 2002.
Section snippets
Weathered clay soils and shallow landslides
In most parts of the Italian Apennines the bedrock is represented by highly tectonized clays and by flysch consisting of alternating layers of clay, clay shales with calcareous arenaceous marls. Both represent structurally complex tectonized formations (Esu, 1977) having a predominant argillaceous component and they are very degradable material, also in temperate climate. The processes of chemical alteration and physical breakdown result in the weathering of the bedrock and the production of
The slope stability models
The mathematical models developed by Montgomery and Dietrich, 1994, Pack et al., 1998 available for studying shallow landslides, take into account the infinite plane slope stability model coupled with a steady state topographic hydrologic model. The term ‘steady state’ at this point does not refer to any long term, e.g. annual, averages, but to a critical period (event) of wet weather that is likely to trigger landslides. Both models are tools designed as an ArcView extension. Important
Geological and climatological setting of the study area
The Oltrepo Pavese is characterized by a complex geological and structural setting. The geology is dominated by sedimentary formations, ranging from the lower Cretaceous to the Pliocene, with a dominant clay component (Beatrizzotti et al., 1969, Braga et al., 1985). Silty and/or clayey deposits formed by weathering and down slope transportation cover the argillaceous bedrock units. The territory of S.Giuletta is located in the northern part of Oltrepo Pavese (Northern Apennines) (Fig. 2). The
The landslide inventory
A landslide inventory was completed in 1999 by aerial photograph interpretation at 1:10,000 scale (Fig. 3). The slopes are subjected to widespread instability and very large landslides developed in the years 1976–78, due to high intensity rainfalls that mobilized some old dormant slides. These events caused also the collapse of many houses. Different types of slope instability are present. The deepest kinds of movement, with a depth down to 40 m, concern the bedrock and are rotational slides,
The November 2002 event
On November 2002 intense rainfall triggered some shallow landslides. During the rainstorm of 25–26–27 November, roughly 140 mm of rain fell within a 72-h period. During the 10 days prior to the storm, antecedent rainfalls were about 90 mm (Fig. 2), increasing the soil moisture. The total November rainfall summed up to 249 mm, about three times the November average of 83 mm in normal years (Fig. 5) and was the highest since 1950. The rainfall during this event exceed the threshold for shallow
Pedological characteristics
The depth of a shallow landslide is generally not more than 1–2 m. Therefore the pedological map provides a useful basis for the study of the characteristics of these soils. Information on the soils are available from the soil maps at a scale of 1:10,000 covering the entire study area (ERSAL, 2001).
Following are the definitions of the soils (Fig. 7):
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MRI2 — Eutric Leptosol: silt and silt loam, up to 0.8 m thick, derived from the strong weathering of marls with silty intercalations (S. Agata
Application of SINMAP and SHALSTAB
The input data set of SINMAP and SHALSTAB consists of topographic slope and parameters quantifying material properties and climate (primarily a hydrologic wetness parameter). A Digital Elevation Model (DEM) provides the topographic basis for SINMAP and SHALSTAB study. The accuracy of output is heavily reliant on the accuracy of the DEM data input. Therefore the DEM was constructed from a 1:5000 scale contour map, with a resolution of the topographic data on the order of 5 m (grid size of 5 × 5 m).
Results and discussion
The stability index (SI) distribution in the study area is shown in Fig. 14. About the 53% of the study area is classified with SI below one (upper threshold and lower threshold classes), which refer to conditions that are highly unstable and thus critical. The areas most prone to instability are located on steep slopes on the contact between two different formations (Gessoso–Solfifera Formation and M. Arzolo Sandstones). The area around the village of Pizzolo (north-eastern sector of the study
Conclusions
The event of November 2002, which triggered many shallow landslides in the S. Giuletta territory, can be considered as an extreme pluviometric event. The greatest frequency of shallow landslides occurred near the village of Pizzolo in the eastern part of the area. The superficial covers involved are all of the order of 1–1.5 m and correspond to colluvial soils derived by the weathering of the bedrock and, above all, by the down slope transportation due to deep landslides that characterize the
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