Storm Gloria (2020): joint probability of multiple flood hazards and compound events in a paradigmatic Mediterranean event

Abstract. Compound flooding arises when multiple hydrometeorological drivers co-occur and interact, amplifying risk beyond that expected from individual hazards. This work analyses Storm Gloria (19–25 January 2020, E Spain) as a paradigmatic compound event in the Mediterranean region. The coastal strip considered here extends for 1,609 km (from the French border to Málaga) and includes 8.4 million inhabitants in 12,167 km². The event affected 770 municipalities caused 13 fatalities and four missing persons and generated about EUR₂₀₂₂ 204 million in Spain's Insurance Compensation (CCS) payouts. The analysis combines daily precipitation (P₍₂₄ₕ₎; 662 AEMET rain gauges), maximum wind gusts (140 stations), and significant wave height (Hₛ; 50 SIMAR/CoExMed points) with municipality-level CCS compensation data. The event window is objectively defined (≥ 40 mm day⁻¹ at ≥ 2 stations), and hazard severity is mapped using operational thresholds (P₂₄ₕ = 40/100/200 mm; Hₛ = 2/5/7 m; wind gusts ≈ 70/90/100 km h⁻¹). Multivariate (river and sea flooding at the Tordera River mouth) and spatially compounding (flooding in the Júcar basin and sea storm in the Gulf of Jávea) cases are quantified through a two-sided conditional framework and copulas in two areas located in the northern and central part of the study region, respectively, whereas the temporally compounding case is evaluated empirically from heavy-rainfall episodes in the Segura basin, located in the southern part. Results indicate that, for the first case, the return period of the maximum recorded precipitation is estimated at 24 years; however, when the compound event – where wave action hindered the discharge of the Tordera River into the sea – is taken into account, the estimated return period increases to 85 years. In the second case, it goes from 1.5 years to 72 years, while in the third, the return period corresponding to the time sequence of three heavy rainfall events in the same basin, becomes 22 years. Overall, the largest compensation values tend to occur where hazard levels are highest, although exposure, vulnerability, and insurance coverage modulate the final impacts. This integrated workflow (objective event definition, threshold‑based hazard mapping, copula‑based joint recurrence, and impact linkage) provides a transferable, practice‑ready template for compound‑aware coastal risk assessment in Mediterranean settings and other coastal regions.