On 25 October 1964, a devastating flood of the River Sava struck Zagreb, SR Croatia, SFR Yugoslavia. High rainfall upriver caused rivers and streams in the Sava catchment basin to swell and spill over their banks in many places throughout Slovenia and northern Croatia. The worst of the flooding occurred in Zagreb. Sava floods were a known hazard in the city, having affected the development of the area since the Roman times, and the 1964 flood did not have the largest extent. However, it occurred following several decades of large-scale industrialisation and urban growth which had caused the city to expand into the most flood-vulnerable areas. The quality of building construction and flood defences in the floodplain was mostly low. Regulation of the Sava and its tributaries upriver from Zagreb cut off many natural detention basins, such as fields and pastures, which caused water to pile up ahead of the city. To make matters worse, the soil was already saturated from a mid-month episode of fairly high rainfall. A second episode of high rainfall, during 22–25 October upriver in Slovenia, produced a record-high water wave in the Sava. At Zagreb's Sava River gauge, the water crested at above zero level, exceeding the previous high water mark by more than half a metre (2 ft). This proved too much for the city's embankments. Around of the city was flooded, including most neighbourhoods on the western side of the floodplain.

The worst-stricken areas were the working-class suburbs of Trnje and Trešnjevka. A network of arterial roads raised on small embankments failed to hold the water. In some areas single-storey houses were submerged whole. The onset of the flood was quick, the warnings came late and the intensity and height of the flood was not communicated to the residents in time. Many were unable to save their belongings and ended up stranded in attics and on the rooftops. The brunt of the flood lasted from 25 to 28 October as the water progressed through the city from west to east, mostly on the more populous left (northern) bank. The neighbourhoods on the right bank only saw groundwater flooding for the most part. The railway embankment and sandbag dykes successfully protected the northern and eastern part of the floodplain, and Donji grad and much of Peščenica were spared. A total of 17 people died in the flood. The living quarters of 183,000 people were flooded; 40,000 of these lost their homes. The damage was estimated to 160 billion Yugoslav dinars, or, alternatively, over US$100 million.

The flood was followed by urban redevelopment, changing Zagreb's cityscape. Around 26,000 new flats and houses were constructed in the next four years. The challenge of housing flood refugees led to the creation of several new planned neighbourhoods. Existing neighbourhoods where much of the housing had been condemned due to water damage were rebuilt, often as high-rise housing estates. Several factories were moved eastward from the flood-prone central part of the city. The disaster also prompted a rework of the flood defences on the Sava. The Sava embankments in Zagreb were reconstructed, raised and lengthened, and the river was channelised throughout the city. Diversion canals were built around several cities in the Sava basin, including the Sava–Odra–Sava Canal bypassing Zagreb. As a result, although there have been several destructive Sava Valley floods since 1964, their effects in Zagreb were limited to groundwater flooding in a few streets. However, changes in the riverbed are believed to have somewhat decreased the effectiveness of the defences as of the early 21st century.

Background Geography and hydrology Located in northwest Croatia near the border with Slovenia, Zagreb's geography is largely defined by two features: the Medvednica Mountain whose foothills contain the city's historic core and northern suburbs, and the River Sava floodplain to the south, which contains most of the city, including the post-19th-century city centre, Donji grad. Originating in Slovenia, the Sava drains much of Slovenia and northern Croatia. It is the Danube's biggest tributary by discharge. The river exhibits a nivo-pluvial regime. The water level peaks with spring snowmelt and autumn rains, while the minimums occur in summer and winter. The autumn peak is more pronounced in the Sava's upper course. Due to the low permeability of the hilly and mountainous terrain of Slovenia and northwest Croatia, the river exhibits a high seasonal flow variability, exacerbating the risk of flooding. Before the river regulation works at the turn of the 20th century, Sava's flow in Zagreb was described as "torrential".

The average annual discharge at the Zagreb gauging station is around , while flows over occur on a yearly basis. Average yearly precipitation over the Sava's upper course in Slovenia measures , contributing the largest part of the Sava's water over its entire course. High rainfall events can increase water level in the Sava and its tributaries to flood levels within days. The location of the city on a transition from hilly to flat terrain increases flood risk. The Sava flows at an incline of 1 per cent through Zagreb, but between Rugvica (about east of the city core) and Sisak, the river is highly sinuous with sharp meanders, and flows on a nearly flat terrain with an incline of 0.05 per cent. The latter part sustains much lower flows than the former, which can cause the water to pile up upstream.

Numerous torrential streams on the Medvednica Mountain pose an additional flood risk. The Medveščak stream caused the city's deadliest natural disaster in 1651, when a flash flood swept away houses in Tkalčićeva Street, killing 52 people. The most recent notable flash flooding event occurred in 2020. Floods of the Sava in Zagreb have been recorded since the antiquity. The development of Andautonia, a Roman town on the fringes of the modern-day city, was influenced by a devastating flood in early 2nd century AD. The first floods attested in modern records date back to 14th century. The first chronicled flood occurred in 1469. The twin cities of Kaptol and Gradec, which are located at the lower end of Medvednica foothills and form the core of modern-day Zagreb together with the adjacent Donji grad, controlled land in the floodplain on both banks of the Sava as early as the 13th century. This waterlogged area contained a few small villages and was mostly used for hunting, planting orchards and harvesting wood from willow trees as a construction material. The city was slow to expand since even the Donji grad area, which lies on the northern edge of the plain, was prone to flooding until it was artificially raised around the beginning of the 20th century. In the 19th century, floods would sometimes reach the bottom of the foothills, destroying houses in Ilica Street. Only with the 1850 unification of Kaptol and Gradec, which made financing flood defence projects possible, and the 1892 construction of the railway embankment which doubled as a flood barrier, did Zagreb's urban area begin to approach the river.

Suburban expansion into the floodplain

Industrialisation brought factories, whose number greatly increased after 1900. These were located near the railway, which divides the plain between Medvednica and the Sava into two parts. The land south of the railway had been largely owned by the Catholic Church until the land reform of 1918 which made it available for development, and it was settled in great numbers by factory workers, who migrated into the city from the countryside, constructing houses and shacks on small subdivided lots in the quickly growing suburbs of Trešnjevka, Trnje and Peščenica. Most dwellings were built without permits because underemployed factory workers could not afford the fees. Illegally constructed buildings were ineligible for insurance from flooding, which was a frequent occurrence in the new suburbs. Large floods occurred in 1876, twice in 1895, in 1923, 1925, 1926 and 1933, but smaller ones were almost a yearly occurrence.

In 1900, only 4,000 of Zagreb's 57,000 inhabitants lived in Trnje and Trešnjevka, while Peščenica had not yet been incorporated into the city. By the 1931 census, the population of Trešnjevka alone had grown to 21,350 residents, increasing the necessity and value of river regulation. The surroundings of the Sava, which had many meanders, oxbow lakes and islands, were partially protected when the river was channelised at the beginning of the 20th century. The oxbow lakes were fenced off by fascines. Work on flood defences was sporadic and of poor quality due to a scarcity of funding. An embankment protecting Trnje was partially built in 1924–25, but washed away by the November 1925 flood. By 1929 the central part of the suburbs was protected by an embankment, which was in the 1930s extended away from the river to Radnička Road in Peščenica in the east, and along the Črnomerec stream up to the northern border of Trešnjevka in the west. These defences were not enough to protect the city from the 1933 flood, when the Sava, at a height of , breached the embankments, causing flooding that lasted seven weeks. The embankments were mended and somewhat improved over the next several years, which were nevertheless marked by successive floods. Further works were carried out in 1945 and 1960–61 by youth work actions organised by the Communist government. Embankments were extended and flood defences improved in Trešnjevka, Trnje, and Peščenica. However, the embankments were still low and of poor construction quality, and protected only the central part of the city, leaving the flanks exposed.

Following World War II, the three suburbs were responsible for much of the city's high population growth. By 1961, Zagreb numbered 430,802 inhabitants; the population of Trešnjevka had surpassed Rijeka, Croatia's third most populous city. After a lull in construction following the war, late 1950s and early 1960s saw the creation of new neighbourhoods of apartment buildings, such as the prefabricated buildings in (Trešnjevka), Folnegovićevo naselje (Peščenica) and Trnjanska Savica, and the low-rise cohousing estate in Veslačka Street. The southern districts became home to new University of Zagreb faculty buildings, and industry complexes such as the Rade Končar factory in Trešnjevka and the Žitnjak industrial zone in Peščenica, while district heating was introduced with new thermoelectric and heat plants. However, living standards in general remained low; only 10 per cent of Trešnjevka's streets was paved in 1960. During Mayor Većeslav Holjevac's term, the city began to expand to the Sava's right bank, forming Novi Zagreb. Savski Gaj, the first planned neighbourhood in Novi Zagreb, was built in 1957. The Zagreb Fair was also moved from the city centre to its modern-day position in Novi Zagreb's Kajzerica near the Sava Bridge.

Flooding In late October 1964, the Sava's upper course in Slovenia was affected by heavy rains. Ljubljana saw of rainfall on 22–25 October; Sava Bohinjka's source received and Novo Mesto received . Although the rainfall totals were several times lower than in the 1933 flood, river levels increased rapidly. During the afternoon and evening hours of 24 October, the gauge station in Radeče near Zidani Most, Slovenia (roughly halfway between Ljubljana and Zagreb) crested at less than the 1933 maximum, but this was preceded by a rise of nearly in a matter of hours. This rise was likely accelerated by the saturation of the drainage basin by moderate rainfall the previous week, which had produced a smaller water wave that ended up cresting at in Zagreb on 16 October.

Timeline 25 October