On the night of 24–25 July 2020, after 21:00 local time (CEST, 19:00 UTC), Zagreb, Croatia, was struck by a storm which produced one of the worst flash floods in the city's history. An episode of intense rainfall dumped more than of rain within two hours. The torrential streams of the mountain overlying Zagreb surged and overwhelmed the city's combined sewer system, causing flooding. The effects were worst felt in the inner city, notably Ilica, Donji grad and Trnje, though many suburbs were also affected, including Buzin, Črnomerec, Jankomir, and Maksimir. Public transport and automobile traffic ceased in many streets, while trapped people had to be rescued from flooded cars and apartments. A firefighter died at an intervention. No financial estimate of damages was released by the authorities. The city government reported receiving over 2,000 damage claims during an eight-day filing period.
Zagreb lies on the foothills of Medvednica mountain, with the inner city squeezed on a floodplain between the mountain and the River Sava. Both the mountain streams and the river have produced deadly floods in the past. A system of flood defences has since been created to prevent flooding, and is under the jurisdiction of the state water management company, Hrvatske vode. However, in spite of flash flood hazard warnings issued by Croatian and international agencies in the preceding 48 hours, the flood defences were not activated. A group of Trnje residents broke into the empty building of the Kuniščak dam, which functions as the outlet to many of the torrential streams, and opened the dam to drain the flood waters from the city. In the following days, a controversy ensued about who was responsible for the dam being closed, and its control building locked and unstaffed. Hrvatske vode, the mayor, and the city's water and sewer utility company all denied responsibility for the flood and faced criticism from the public. Additionally, a local man broke in and operated the dam controls was arrested by the police in an incident described by some as a case of police brutality, prompting a public outcry. Hrvatske vode filed a police complaint for breaking and entering against the group of residents. The man was exonerated of all charges in May 2021.
Background
Zagreb faces flooding hazard from two sources: the River Sava, which caused the devastating flood of 1964; and the so-called Zagreb streams of Medvednica Mountain, on whose southern side the city lies. There are several dozen streams on the Zagreb-facing side of Medvednica. They are known for torrential flow during rainstorms. A particularly violent flash flood occurred in July 1651, when the Medveščak stream overflowed during a night-time storm, washing away numerous houses and trees in Potok Street and causing 52 deaths. Flood defences were built around Medveščak as early as the 17th century. The last major flooding event in the city was the overflowing of Zagreb streams during record rains of 3–4 July 1989, causing flooding in the area from the Zagreb Zoo in Maksimir Street to Žitnjak, which prompted the construction of retentions and sluices to control the drainage of storm waters from Medvednica foothills, as well as channelling and covering of Zagreb streams. As of 2004, 19 out of planned 39 retentions had been built. No new retentions have been constructed since. Risk of flooding from Zagreb streams is exacerbated by the city's geography. The terrain transitions abruptly from hills to impermeable highly urbanised river lowland, occasionally overwhelming the sewer system during heavy rainfall events.
On 23 July at 02:00 CEST the European Flood Awareness System issued a flood warning for Central Croatia. The Croatian Meteorological and Hydrological Service (, DHMZ) had issued a red Meteoalarm thunderstorm warning, the highest level, for 24 July in Central Croatia, specifying a danger of flash flooding and power failures, and expected rainfall over . The European Storm Forecast Experiment issued a Level 2 alert for an area including Zagreb, warning of possibilities of large hail, severe wind gusts and tornadoes, and noting that "an excessive rainfall risk could arise over parts of Slovenia and Croatia during the overnight hours."