Information about high-intensity precipitation of short duration and the corresponding return values is of great importance when designing urban drainage systems. In this study, an extreme rainfall event in summer 2020 which caused an urban flood in the city of Zagreb was analyzed from the hydro-meteorological point of view. To put this event in a broader climate context, first, a general climatology of the short-duration (from 5 to 120 min) rainfall amounts and trends in the associated extremes was analyzed for the Zagreb-Gric station in the period 1908-2020. For the flood event analysis, time series from additional two stations, a suburb and mountain one, were used covering the period 1959-2020. The generalized extreme value distribution (GEV), both stationary and non-stationary, was employed to estimate the return value curves. The results revealed more frequent short-duration wet events (rainfall amounts larger than the corresponding 10-year return values) occurring after the 1970s. Trend results showed a general increase in the amounts of precipitation with a range of duration from 30 to 120 min with the rate up to 2.9%/10 years. Combined with the analysis of occurrence of extremes during year and correlogram analysis, such results imply changes in the occurrence of corresponding weather types responsible for heavy rainfall episodes. The case study analysis showed that the 120-min extreme precipitation event in the summer 2020 over the city center was an exceptional one with the return period higher than 100 years. According to all available data, measurements and numerical weather prediction models, all ingredients for deep moist convection were present before the event, together with a deep layer shear. In addition, formation of cutoff low, the presence of an intense positive potential vorticity anomaly and the surface convergence line appear to be the main processes which further enhanced triggered deep convection, consequently also heavy precipitation. The hydrological analysis revealed that the sewerage system of Zagreb city could not drain the stormwater from the streets, pointing out the need for its redesign and adaptation to climate change.
Nimac, I; Kalin, KC; Renko, T; Vujnovic, T; Horvath, K
The analysis of summer 2020 urban flood in Zagreb (Croatia) from hydro-meteorological point of view
Natural Hazards