Slovenia suffers worst flooding in its history
In August, heavy rains lashed two-thirds of Slovenia, leading to historic flooding in the Alpine country, with damages estimated at 500 million Euros ($544 million USD) (Reuters, 2023). At least six people died as rivers overflowed, destroying homes, fields, roads, and bridges. Having asked for assistance from the European Union and NATO, Prime Minister Robert Golob described the event as the worst natural disaster in Slovenia’s 32-year history (Financial Times, 2023). The European Commission President Ursula von der Leyen visited the EU member state on 9 August to offer help with the rebuilding process (Independent, 2023).
Event Overview
Slovenia experienced heavy rain in July and rivers were already near capacity prior to 3 August. Rivers in Upper Carniola and Posočje began to flood on the evening of 3 August. From 3 to 4 August, 200mm of rain fell within 12 hours in central and northern parts of Slovenia – this is more than these places would usually receive during the whole of August, according to the country’s environmental agency ARSO. With multiple rivers breaking banks, including River Savinja, 4,000 people had to evacuate their households. Areas affected include Ljubljana, Maribor, Celje, and Nova Gorica (FloodList, 2023).
More than 1,000 weather-related incidents were reported across the country, and 16,000 homes suffered power outages due to the floods (FloodList, 2023). Seven bridges collapsed, and the road and energy infrastructure was particularly impacted (ReliefWeb, 2023). Countries including France, Germany, and the United States have offered their support to help with the rescue efforts (ABC News, 2023).
Flash floods also occurred in neighbouring Austria and Croatia whilst heavy rain and storms caused major damage to the east in Serbia, downstream of the swollen River Sava that flows from Slovenia.
Meteorological overview
According to an atmospheric circulation study by Milošević, D.D. et al. (2016), the north-western region of Slovenia experiences significantly wetter weather conditions than the rest of the country, with more than 2,400mm annual precipitation amount recorded in rainfall stations in the Alpine region. This is largely due to the presence of mountains in the Julian Alps and Dinaric Alps which allow orographic lifting of inflow moisture from the Adriatic Sea along the mountains’ windward side. The precipitation trends also reveal a significant increase in rainfall in central Slovenia from 2% to 4% per decade.
Studies also show that the Western Mediterranean Oscillation (WeMO) has a large positive influence on precipitation especially in central and eastern Slovenia. A positive phase of WeMO corresponds to an anti-cyclone over the Azores and low pressure in the Liguria Gulf, and thus winds would flow from south-west to north-east which potentially brings in a large volume of moisture from the western Mediterranean Sea (Lopez-Bustins, J.A. et al., 2020).
Historical events
A study on flood events in Slovenia between 1926 and 2014 notes that there have been 17 events that had extensive flooding exceeding 1-in-50-year intensity of which 10 resulted in loss of life (Špitalar, M. et al., 2019). Notable events with intense rainfall resulting in large-scale flash flooding have happened in Slovenia in 2007, 2010, and 2021.
On 18 September 2007, large amounts of rainfall resulted in flooding in Zelezniki leading to an estimated 100 million Euros worth of damage. The event is estimated to be more than 1-in-100-year with the maximum daily rainfall recording close to 200mm (Climate Change Post, n.d.).
In mid-September 2010, areas of Slovenia flooded after receiving 170-180mm of rainfall within 48 hours. This led to rivers breaking their banks and most notably Rive Krka; its water level at that time was estimated to be close to a 1-in-1,000-year volume of discharge (Climate Change Post, n.d.). The flood affected 137 of Slovenian’s 210 municipalities, flooding 8,241 buildings and damaging about 2,550km of roads. The direct damage from the 2010 floods was estimated at 251.3 million Euros which exceeds Slovenia’s damage threshold for "major disasters" by nearly 34 million Euros (Reliefweb, 2011). This is one of the extreme floods with average discharges equaling to a 1-in-100-year return period (Špitalar, M. et al., 2019).
On 29 September 2021, the capital of Slovenia, Ljubljana, was hit by floods when a record-breaking 94mm of rainfall fell over the city within one hour. The 2021 floods damaged more than 500 buildings which included important infrastructures like hospitals (FloodList, 2021). The Slovenian government reported this to be a 1-in-200-year event (AP, 2021).
With more than 30,000km of waterways, Slovenia is prone to flooding and has become more so in the last two decades. Recurrent flooding led to the start of a project in 2014 funded by the European Regional Development Fund to protect urban, industrial and agricultural areas near the Meža and Mislinja Rivers. The project aims to build 1-in-100-year protection for the area and reduce the number of buildings at-risk of floods by 755 (Euronews, 2021; European Union, n.d.).
Return period estimation
JBA has conducted Extreme Value Analysis on historic rainfall and streamflow data in order to estimate the return period of the event. We fit an exponential distribution to a 44-year daily rainfall climate data timeseries from NOAA (2023) to produce a rainfall exceedance probability curve near Slovenj Gradec in the north of Slovenia. The curve is shown by the solid orange line in Figure 2a, with the 70th percentiles given by the dashed lines. The observed 24-hour rainfall total of 115mm at the Šmartno Station on 4 August 2023 (shown by the blue line, ARSO 2023) suggests a greater than 1-in-200-year rainfall event at this location.
The exceedance probability of streamflow on the River Mura near Gornja Radgona in north-east Slovenia was calculated using daily mean watercourse discharge data from Slovenia’s Environmental Agency (ARSO, 2023). The data were fit with an exponential distribution to produce an exceedance probability curve, shown in Figure 2b. This location recorded a streamflow of 1,456 cubic metres per second with estimated return periods of around 200 years during the event.
According to maximum annual discharge recorded from 1895 to 2013, River Sava contributed to seven events with flood fatalities which includes those in 2000, 2007 and 2010. In 2010, Sava’s discharge was equivalent to a 1-in-100-year return period. Other rivers like River Savinja also saw more than a 1-in-100-year water discharge in 1990 and 1998. The most significant record comes from River Vipava which had a 1-in-1,000-year water discharge in 2010 (Špitalar, M. et al., 2019).
How will climate change affect flooding in Slovenia
Higher temperatures and greater instability of the troposphere is likely to increase the probability of intense rainfall in Slovenia. Increased rainfall intensity can result in higher volumes of rainfall in shorter periods, leading to a greater risk of flash floods and river flooding. Models suggest there will be a 20-30% increase in intense rainfall by the end of the 21st century in Alpine and sub-Alpine regions (Climate Change Post, 2023). The risk of flooding in (sub-)Alpine regions such as Slovenia is directly related to rising temperatures. Cooler temperatures in spring and autumn mitigate flood risk as precipitation falls as snow at higher elevations, but as temperatures rise the altitude of the rain/snow transition will increase, escalating the flood risk (IPCC, 2023).
JBA flood hazard maps
JBA has 30m resolution river and surface water flood hazard maps for every country in the world. Figure 3 shows an excerpt from JBA’s flood event footprint demonstrating that the river flood maps capture areas known to have flooded as a direct result of the August 2023 floods.
This report is accompanied by a flood footprint for the event - detailing extents and depths of the flooding in areas affected. Download it via our Client Portal or request a copy by emailing eventresponse@jbarisk.com.
If you are interested in any of our products or services to improve your management of flood risk, please get in touch for more information. This report is covered by JBA’s website terms – please read them here.
