Modeling Two-Pulse Convective Storms Induced by Orographic Forcing: A Meteo-Hydrological Approach for Urban Flood Risk Assessment

Extreme rainfall events in coastal environments are often affected by local orography on the larger-scale atmospheric circulation, which triggers convective storms and influences the rainfall occurrence in the area. To assess the effects of this phenomenon on the water systems design and management, the previously undocumented role of Mount Epomeo (789 m a.s.l.) located on Ischia Island was considered to investigate its relevance in generating two-pulse convective storms that entail flash floods in the Phlegraean Fields (Naples, Italy). Namely, it was assumed that the mountain’s triangular-based pyramidal morphology provokes a two-stage uplift of moist air, in turn, entailing successive convective cloudbursts separated by a short rainfall lull. To test this hypothesis, twenty-three years of high-resolution rainfall data from the Pozzuoli rain gauge (2001–2023) were assessed by identifying the occurrence of characteristic two-pulse heavy precipitation events (HPEs). The dynamic interaction between Mt. Epomeo and the atmospheric flow was thus modeled, and local-scale Depth-Duration-Frequency (DDF) curves were derived and compared to regionalized approaches. The EPA-SWMM model was then applied to assess the hydraulic response of the urban drainage system serving a portion of the Phlegraean Fields under both historical and DDF-derived rainfall events. Results revealed that orographic forcing enhances local rainfall intensity by up to about 25–30% higher than regional estimates, thus inducing significant flood events. Linking pyramidal orography, two-pulse convective storms, and urban flash flood allowed providing a novel conceptual and technical insight into urban flash flood generation in Mediterranean areas.