Mapping direct and indirect fluvial hazard in the Middle Calore River valley (southern Italy)

Fluvial environment is characterized by delicate equilibriums, constantly threatened by human disturbances. An incorrect management of such environment can increase hazard and associated risks. In recent years, and especially in the second half of the last century, many rivers underwent substantial geomorphological changes (Liebault and Piegay, 2002; Rinaldi, 2003; Surian and Rinaldi, 2003, Surian et al., 2009; Perşoiu and Rădoane, 2011). These changes mainly consisted of channel narrowing and riverbed degradation and were mostly related to human interventions. They had consequences also in terms of fluvial hazard and associated risks, as they produced new geomorphological landsurfaces, hanging few meters above the water level and often cultivated or anthropized, which were subject to over-flooding during flood events (Allamano et al., 2009). Floods are constantly and directly associated to the fluvial dynamics; thus, this kind of hazard is to be considered as a form of direct fluvial hazard. The landsurfaces produced by channel narrowing and riverbed degradation are always bordered downslope by steep fluvial scarps. These latter are subject to undercutting processes, that increase the probability of landslides to occur. Differently from floods, landslides are not constantly associated with fluvial dynamics, so the related hazard cannot be considered as a direct fluvial hazard. However, in this particular case, landslides are induced by a typical fluvial process, i.e. undercutting. Thus, the hazard associated to this particular kind of landslides can be considered as a form of indirect fluvial hazard. The probability of a given landsurface to be overflooded during a particular flood event is unquestionably controlled by the increase of the water level during the event, but also by the nature and the elevation of the landsurface above the riverbed (Domeneghetti et al., 2013; Mazzorana et al., 2013). Thus, a geomorphological approach can provide useful data in the framework of flood hazard assessment. Similarly, the hazard associated with landslides induced by undercutting along the fluvial scarps is affected by the frequency and intensity of undercutting processes. Fluvial scarps that directly border the active channel are constantly subject to undercutting. Thus, the landslide hazard will be here higher. Also in this case, data derived from detailed geomorphological survey are fundamental in assessing hazard and risk. GIS are extremely useful to process, quantify and graphically represent data and results derived from geomorphological analysis (Diakakis, 2011). In this note, the results of a detailed geomorphological analysis were processed in GIS environment to map both landsurfaces with different degrees of flood hazard, on a relative scale, and fluvial scarps with different hazard associated to undercutting-induced landslides. These data have been interpreted in the framework of the degree and type of anthropization of the landscape to produce corresponding relative risk maps. The method was applied to a International Conference Analysis and Management of Changing Risks for Natural Hazards 18‐19 November 2014 l Padua, Italy AP5 ‐ 2 7.7 km2-wide sector of the middle Calore River alluvial plain (southern Italy) (Fig. 1). In the second half of the last century, this river underwent remarkable geomorphological changes, which greatly affected the morphology of the alluvial plain (Magliulo et al., 2013). In particular, a mean narrowing up to 66% and an incision generally ranging from 2 to 4 meters took place and concurred to transform the channel morphology from transitional to singletread