A Novel Index-Based Method for the Risk Assessment of Low-Intensity Natech Events caused by Floods

Natech events (natural events triggering technological accidents) are usually considered high-impact low probability (HILP) events involving the loss of containment of hazardous substances with potentially severe consequences such as fires, explosions, and toxic dispersions. However, in the last decade, environmental contamination due to the interaction of the water receding after a flood event and anthropic activities was reported in several Natech events, raising attention towards the so-called Low-Intensity Natech (LIN) events. Notably, to date, there are no approaches available to identify and assess the risk associated with these occurrences. This contribution proposes a novel index-based method to estimate the risk associated with LINs for a selected area addressing the environment as a target. The approach involves the characterization of the most relevant anthropogenic activities potentially impacted within the area (i.e., magnitude assessment) and the characterization of the flood hazard level in correspondence with each activity identified (i.e., frequency assessment). These two layers of information – magnitude and frequency assessment - are then integrated by means of GIS software into a LIN Risk Index, which provides a first quantification of the risk associated with LIN events targeting the environment. The proposed method was applied to two river basins with inherently different levels of industrialization in the North of Italy. The analysis of the results obtained confirms that the proposed risk index enables a straightforward assessment, and a comparison of the overall risk associated with LIN events in a river basin.