Rainfall Extraordinary Extreme Events (EEEs) Frequency and Magnitude Assessment: The EEE Occurred on 14th–15th October 2015 in Benevento Area (Southern Italy)

The identification of suitable statistical models of rainfall maxima at regional scale is a key element for the definition of reliable flood and landslide risk mitigation plans and for the design and security evaluation of high hazard strategic engineering structures. The ability to develop such models is highly dependent on a rain gauge monitoring network able to observe the extreme events that occurred in a region for several decades. In Italy, the density of the monitoring network and the time series lengths are often inadequate to capture some of the rainfall extreme events (referred to as extraordinary extreme events - EEEs), characterized by very low frequencies and spatial extent scales much smaller than those of rainfall ordinary maxima. In recent years, new operational statistical approaches were proposed to properly retrieve the EEEs frequency from the available database. However, the meteorological patterns of the EEEs are still poorly known, due to the limited number of documented cases studies available. The post-event rainfall analysis of observed EEEs and the evaluation of the efficiency of the monitoring network in detecting their magnitude and spatial properties may certainly help to improve the interpretation of the phenomena and their probabilistic modeling. In this study, new insights about the characteristics of EEEs are retrieved by analyzing data collected by different automatic rain gauge networks operating in Campania region (Southern Italy) from year 2001 to 2020. In this time frame, the extreme rainfall event occurred on 14th–15th October 2015 in Benevento area is the only daily EEE observed. The analyses show the capability of different monitoring networks to observe the phenomenon and the impact of different statistical regional models of rainfall maxima in assessing its frequency.