Flood Frequency Analysis revisited under spatially and temporally Compound Flood Extremes: evidence from southern Valencia Metropolitan Area, Spain

Flood Frequency Analysis (FFA) is a fundamental tool for flood‐risk assessment and hydraulic design and constitutes the statistical basis of the flood hazard scenarios defined under the European Floods Directive and its national implementations. Classical FFA is typically applied under assumptions of temporal independence and spatial representativeness at individual gauging stations and within the framework of the design storm paradigm. However, these assumptions are increasingly challenged during more extreme compound hydroclimatic events, where rainfall and runoff responses occur synchronously across multiple connected catchments and in successive phases in time. The October 2024 flood in southern Valencia Metropolitan Area (Spain) offers a unique opportunity to revisit FFA under such conditions. Over the course of that day, a spatially extensive and temporally clustered rainfall event affected the five catchments draining in this area, producing an exceptional hydrological response. The accumulated hydrograph had a peak of 7,500 m3/s, but with significant delays between the individual hydrograpghs. The event was characterized by two distinct rainfall phases, with an initial episode in the morning modifying the antecedent hydrological conditions of the catchments, followed by an extreme afternoon-evening phase that induced a strongly non-linear runoff response. Several tributaries responded almost simultaneously, resulting in spatial compounding of peak discharges and unprecedented flow magnitudes at the basin scale. Such a response challenges the assumptions underpinning classical FFA and highlights the need for alternative frameworks capable of representing compound hydrological behavior.

Rather than relying solely on point-based discharge records, this study proposes an integrated approach that combines regional extreme rainfall analysis, stochastic weather generation, and distributed hydrological modelling to estimate discharge quantiles beyond the limitations imposed by short instrumental records and thee design storm hypothesis.

The results indicate that applying the proposed integrated framework leads to a substantial downward revision of discharge quantiles associated with fixed return periods when compared to classical point-based FFA. Flood frequency estimates derived exclusively from local discharge records are strongly influenced by limited sample sizes and by the extrapolation of the upper tail, which can result in unrealistically high discharge quantiles. By combining regional precipitation analysis, stochastic weather generation, and distributed hydrological modelling, the proposed approach better constrains the range and frequency of rainfall-runoff conditions capable of producing extreme flows. As a consequence, discharge magnitudes previously associated with very long return periods are shown to occur more frequently, implying lower discharge values for a given return period and a higher effective frequency of potentially damaging flows.

Overall, this study demonstrates that the proposed framework provides a more consistent and physically grounded basis for estimating flood quantiles under spatially and temporally compounding hydroclimatic conditions, and offers a robust foundation for the derivation of flood hazard maps within the context of current European and national flood-risk management frameworks.