Evaluation of shallow landslides susceptibility in Alto Monferrato Hills (North-western Italy)

Shallow landslides generally trigger as a consequence of high intensive and long duration rainfalls. Their kinetic energy and spatial distribution frequency make them hazardous and their prevision necessary for a correct land-planning in relief territories. A physically based model has been applied in order to evaluate shallow landslides susceptibility in the south-eastern sector of Alto Monferrato Hills (central Piedmont - north-western Italy). The study area is located between Piola and Scrivia creeks, East of the town of Ovada (Alessandria district), in a hilly zone interested by a generalized fluvial erosion phenomenon linked to a lowering of the fluvial network base level. The area belongs to the Tertiary Piedmont Basin, a Oligo-Miocene sedimentary basin, which forms there a large north-westward dipping monocline (8°-15°) of alternating marls and sandstones centimetric to decimetric layers. The main morphological features are the asymmetrical profiles of cuestas. In the past Century, severe rainfall events occurred over the Alessandria district (May 1926, August 1935, September 1948, November 1968, October 1977, November 1994) In the note, the severe rainfall event of October 1977 has been considered. Critical values of 50-60 mm/h were registered. The Piancastagna meteorological station (high Orba Valley) recorded a 24 hours rainfall of 432 mm. On that occasion, hundreds of shallow translational slides, and some deeper and larger translational and rotational slides affected the watersheds of the Creeks Erro, Visone, Orba, Piota and Lemme. Around 600 shallow landslides have been evidenced using aerial photographs Starting from a hyetogram similar for duration and intensity to those occurred during the severe events of 1977 the shallow landslides susceptivity of the study area has been assessed by applying a simplified model that takes into consideration both the rainfall infiltration in the soil and her influence on the slope stability. For the infiltration analysis the Green-Ampt method (1911) is utilized in the model. It considers the downwards advancement of a saturation front from the ground surface in consequence of a rainfall. The saturated soil thickness (h) is then used as input information for the stability analysis according to the infinite slope equation: F=(f∙γt∙h∙cosα∙cosα+c)/( γt∙h∙cosα∙senα); where F is the Safety Factor, f is the friction coefficient (f = tanφ), φ is the friction angle, γt is the soil specific weight, h is the thickness value of saturated soil, α is the slope angle and c is the cohesion value. The rainfall used in the model is connected in that way with the landslides triggering through the h value. In that way, a graphic representation of the territory where F < 1 in consequence of the rainfall used in the model has been carried out, using a DEM 10 realized by ARPA Piemonte. In order to validate the model the shallow landslides susceptibility map obtained has compared to the landslide map relative to the 1977 rainfall event. Fifty-one percent of landslides transferred in the landslides map falls in cells considered instable after the application of the model. If a graphic error is considered in transposition of the triggering zone and a buffer of one cell is applied in the comparison, the agreement percentage arise to seventy-five percent. Furthermore, another five percent of shallow landslides falls at less than 10 meters from human activities as slope cuttings for roads, constructions, etc. whose sharp slopes have probably hidden by the DEM. If also these slides should considered, the superposition with the susceptibility map might further on arise to eighty percent. For assessing the shallow landslides hazard, hourly precipitation data of neighbouring meteorological stations have been processed according to Gumbel’s probabilistic method: the probabilistic map relative to the rainfall used in the model has revealed return times ranging in the study area between 100 and 500 years.