Bonacci et al. (2025) Understanding flash floods in a changing urban landscape: the Zagreb 1989 and 2020 events

Identification

• Journal: Natural Hazards
• Year: 2025
• Date: 2025-10-06
• Authors: Ognjen Bonacci, Ana Žaknić‐Ćatović, Tanja Roje-Bonacci, Tatjana Vujnović
• DOI: 10.1007/s11069-025-07704-3

Research Groups

• Faculty of Civil Engineering, Architecture and Geodesy, Split University, Croatia
• Department of Physical and Environmental Sciences, University of Toronto Scarborough, Canada
• Croatian Meteorological and Hydrological Service, Zagreb, Croatia

Short Summary

This study analyzes two major urban flash flood events in Zagreb (1989 and 2020) to characterize their drivers in a changing urban landscape, finding that retention basins are effective, and floods primarily occur during summer nights due to localized high-intensity rainfall and urbanization, rather than a long-term increase in daily precipitation totals.

Objective

• To characterize urban flash flood events in Zagreb by analyzing two major historical events (July 1989 and July 2020) to understand the contributing factors, evaluate existing mitigation infrastructure, and inform more effective flood protection measures in a rapidly urbanizing and topographically complex environment.

Study Configuration

• Spatial Scale: Zagreb city, including the Črnomerec stream catchment (4.54 km² upstream of Mikulići station, 6.70 km² upstream of Fraterščica 1 station), and the southern slopes of Medvednica Mountain. The urban area expanded from less than 100 km² to over 700 km².
• Temporal Scale: Analysis of two specific flash flood events (July 3–4, 1989, and July 24–25, 2020). Long-term historical precipitation series from 1862 to 2020 (158 years) for trend analysis.

Methodology and Data

• Models used: Linear regression and the non-parametric Mann–Kendall test were applied for trend analysis of precipitation time series.
• Data sources:
  • Croatian Meteorological and Hydrological Service (DHMZ): Hourly and daily precipitation records from four meteorological stations (Zagreb Grič, Zagreb Maksimir, Zagreb Airport Pleso, Puntijarka).
  • Croatian Meteorological and Hydrological Service (DHMZ): Hourly discharge measurements from two gauging stations on the Črnomerec Stream (Mikulići and Fraterščica 1).
  • Ombrographic measurements from Zagreb Grič and Zagreb Maksimir stations for 5-minute rainfall intensity analysis.

Main Results

• The Črnomerec retention basin effectively reduced downstream flooding during the July 1989 event, demonstrating the efficacy of strategically placed retention infrastructure.
• The July 2020 flood caused greater damage in central Zagreb despite lower total 24-hour rainfall at some stations compared to 1989, primarily due to exceptionally high rainfall intensity (e.g., 58.9 mm in one hour at Zagreb Grič, estimated as a 100-year return period event; 77.0 mm in two hours, exceeding a 400-year return period).
• Most flash floods in Zagreb predominantly occur during the warm season (June–August) and at night (between 21:00 and 01:00).
• No statistically significant long-term trend (1862–2020) in annual maximum 24-hour precipitation was detected at the Zagreb Grič station (p = 0.80).
• Urbanization, the position of convective cell centers, and the direction of storm movement are identified as key factors determining the location and severity of urban flash floods, rather than an increase in daily rainfall totals.
• Medvednica Mountain plays a crucial role in modifying Zagreb's precipitation patterns and contributes numerous torrential streams to the urban area, exacerbating flood risk.

Contributions

• Provides a detailed comparative analysis of two significant urban flash flood events in Zagreb, offering insights into the complex interplay of natural (Medvednica streams, convective storms) and anthropogenic (urbanization, infrastructure) drivers.
• Offers empirical evidence of the effectiveness of existing flood mitigation infrastructure (Črnomerec retention basin) in reducing downstream flooding.
• Identifies specific temporal patterns for flash floods in Zagreb (warm season, night-time) supported by long-term precipitation data, which is crucial for early warning systems and preparedness.
• Demonstrates that increased urban flood risk in Zagreb is primarily driven by rapid urbanization and localized high-intensity rainfall events, rather than a long-term increase in 24-hour precipitation totals, challenging common assumptions.
• Advocates for a paradigm shift in urban planning and drainage design, emphasizing adaptive, integrated flood management strategies (e.g., UNESCO’s Living with Floods) and the need for high-resolution, context-sensitive approaches in urban hydrology globally.

Funding

This research was partially co-financed through project KK.01.1.1.02.0027, co-financed by the Government of the Republic of Croatia and the European Union through the “European Regional Development Fund—the Competitiveness and Cohesion Operational Programme”.

Citation

@article{Bonacci2025Understanding,
  author = {Bonacci, Ognjen and Žaknić‐Ćatović, Ana and Roje-Bonacci, Tanja and Vujnović, Tatjana},
  title = {Understanding flash floods in a changing urban landscape: the Zagreb 1989 and 2020 events},
  journal = {Natural Hazards},
  year = {2025},
  doi = {10.1007/s11069-025-07704-3},
  url = {https://doi.org/10.1007/s11069-025-07704-3}
}