Questioning the Design Storm Approach: Empirical Evidence for Small Basins from Emilia-Romagna’s Recent Catastrophic Floods

Between 2023 and 2024, Emilia-Romagna (Italy) was affected by highly anomalous, persistent, and spatially homogeneous frontal precipitation events that triggered disastrous floods. Analysis of small and very small Apennine catchments shows that maximum hourly areal rainfall was significant but never extreme, whereas aggregated rainfall over 6–48 hours reached exceptional values, with return periods exceeding 200 years based on records dating back to the early 1900s. The resulting flood discharges were extremely intense, challenging the traditional design storm approach: synthetic storms with durations equal to the hydrological response time provided very limited guidance on the frequency of the resulting flood, even for catchments with relatively low permeability. Continuous simulation, instead, demonstrated strong potential for flood estimation.
A parsimonious lumped hourly rainfall–runoff model, GR5H, was calibrated under conditions of scarce discharge data using hydrological signatures—Flow Duration Curve and cumulative probability of annual maxima (both Gumbel and GEV distribution were considered, conducting two parallel analysis)—rather than conventional performance metrics. Over 30-year long simulated hourly series were validated through temporal validation and further analysed through log-normal frequency fitting of AMS. Despite limited observations and complex antecedent conditions, the approach provided plausible estimates of extreme flood peaks, highlighting its effectiveness for small, data-scarce basins and its relevance for improving flood risk assessment under evolving hydro-climatic patterns.