Interference between rainfall, municipal roads and geomorphologic stability in the territory of the Alto Astigiano Hills Community (Central Piedmont, Italy)

After the severe rainstorms of December 2008 and April 2009, the situation of slope instability along the hill roads in the territory of the Alto Astigiano Hills Community has become more and more worse. The Italian Government has recognised the natural calamity of national interest for the two events. For such landslide susceptibility, a constant monitoring activity in the Alto Astigiano territory (13 communities of the central Piedmont hills) had to be carried out by the public administration. A research finalized to the production of a guide to the correct land use has been realized with the cooperation of Alto Astigiano Hills Community, Regione Piemonte and Turin University (Dipartimento di Scienze della Terra). From a geologic point of view, the studied area is located in the Tertiary Piedmont Basin, composed of Upper Eocene to Messinian sediments In the northern part, the tertiary series forms anticlines more or less accentuated, repeating in a sub parallel way (directed NW-SE). The Eocene and Oligocene terrains result raised in large extent, in some cases sub vertical or strongly troubled. Southward, starting from the Miocene terrains, the slope decreases quite regularly from 40° to around 20°-10°, until, from the Upper Miocene, it reduces to 8°-5° up to vanishing in the Pliocene Deposits. Litologically, the oldest and more resistant terrains, situated in the northern sector, are represented by carbonate sequences of Cretaceous-Eocene and by sandstones formations of Oligocene. Miocene is characterized by alternative marls and sandstones (with lenses of anhydrite and gypsum in the marl formation of Upper Pliocene). In the central and southern part of the study area outcrop the Pliocene terrains, constituted by fine sediments at first (blue clays or Argille di Lugagnano Auct.) and the sands of Sabbie di Asti formation, representing the more widespread lithology outcropping on the territory of the Alto Astigiano Hills Community. The research activity has been articulated in the following steps: • enumeration of more than 50 cases of instability reported in the last years by the communities and homogeneously distributed on the territory; • investigation (technical-geological surveys, measurements, photos, etc.) on every site and finding of data relative to the individuated instability (eventual technical reports, photos just after the occurrence, testimonials, …); • drafting of a technical schedule for every case of individuated instability with the identification of the natural, anthropogenic, predisposing, and determining causes of the instability; • classification of the examined cases in typologies of recurring instabilities for the studied territory; • for every class of instability typology, individuation of preventive measures and intervention (right land use) for the attenuation of the geomorphologic risk; • indications for sector laws; • editing of the guide to the correct land use The examined cases of instability generally cover landslides both upward and downward the roadways but also flooding of the roads; instabilities occurred almost always because of the faulty drainage of the runoff. Some suggestion for the regular maintenance of the road sides and for the repair of the road bed and surface have been provided in the guide, as well as the temporary and definitive engineering interventions after a road break. But the principal efforts have focused on the importance of an effective runoff disposal for a correct management of the hill territory.