Analisi storica e geomorfologica di fenomeni alluvionali in ambiente montano: casi di studio in Val Soana (Alpi Graie, Piemonte)

Historical and geomorphological analysis of alluvial phenomena in the mountain environment: case studies in the Soana Valley (Graie Alps, Piemonte)

Abstract

The mountain environment is often interested by alluvial phenomena affecting both the main fluvial river and the tributary alluvial fans. In both cases, the consequences of paroxysmal events can be very dangerous for people and anthropic structures. This study shows applications of an integrated methodology to support the geomorphological hazard assessment in a mountain environment. The methodology is based both on an historical and a geomorphological analysis of alluvial phenomena. The analysis of historical information defines the «historical/steady-state time» evolution of a phenomenon. The field analysis establishes the steps of the «geomorphological/dynamic time» and the spatial distribution of a phenomenon. So, the integrated analysis allows to get spatial/temporal information concerning processes and landform evolution of alluvial fans or mountain rivers. The study area is in the Soana Valley (Turin province), in the Graie Alps, a typical alpine region of the North-Western Piemonte, near the Aosta Valley. The historical information and geomorphological data collection put in evidence a lot of paroxysmal alluvial events in the past (1654 and 1845 A.D.) and in recent times (1993 and 2000). In particular, the ancient events data allowed to establish the reactivation time of the alluvial fans; also allowed a comparison between morphodynamic processes along the Soana mountain river and damage typology for man-made structures during the last two alluvial events. In a first case-study, the integrated methodology allowed to reconstruct the geomorphological evolution of the Soana river, pointing out that the river's abrupt changes towards a braided pattern is not an exception linked to the stronger alluvial events, but the natural tendency of the Soana river. In the second case this methodology enables the spatial and temporal analysis of geomorphological activity along tributary torrents: this leaded to the definition of frequency, intensity and spatial distribution of paroxysmal phenomena of debris flows in the Soana Valley. The integrated methodology resulted as a very important tool for a better definition of alluvial hazards and for a future evolution outlook on morphogenetic processes of the mountain environment.