May and August 2023: the extreme landslide events in Slovenia, triggered by extreme rainfall

In 2023, Slovenia experienced two major natural disasters caused by prolonged and intense rainfall event. The first occurred in May 2023 with over 2,000 shallow landslides triggered by prolonged heavy rainfall that lasted for about three weeks (from May 5 to May 23). This event mainly affected the north-east of Slovenia. The landslides caused major material damage to agricultural land and infrastructure, and at least 10 houses were evacuated.

The most notable event occurred in August 2023 and was estimated to be one of the largest in the history of independent Slovenia. In the period between August 3 and 6, 2023, precipitation with heavy storms and intense rainfall covered almost all of Slovenia. The extreme rainfall led to widespread flooding and triggered numerous landslides.

We estimate that there were around 10,000 landslides across Slovenia, with a particularly high density in some areas. Due to hydro-meteorological conditions (increased water flow and rising groundwater levels), a large portion of the landslides turned into mud or debris flows and were deposited a few to several hundred meters away from the source area. The main reason for the extreme landslide disasters was the heavy rainfall and high soil moisture as a result of the rainfall in July.

Due to the large scale of the landslide disaster in August, a detailed damage assessment is still being carried out. Preliminary estimates by the Geological Survey of Slovenia (GeoZS) indicate that around 10,000 landslides occurred (with an area of 1,000 to over 75,000 m2) and caused damage of more than €3 billion, of which around 40% had a direct impact on the built environment.

In both cases, the GeoZS emergency service provided residents and the relevant authorities with landslide forecasts and warnings using the existing national MASPREM system (Slovenian Landslide Forecast and Warning System). Although the Slovenian LEWS is operational, the performance of the forecasts has shown that about 15 to 25 % of the warnings were false, which we attribute to the short period of antecedent percipitations (the current calculation takes into account 3 days of antecedent percipitations)  and lack of soil and hydrological related parameters (e.g. effective percipitations, soil moisture, etc.).

Fundings:

This research was funded by the Slovenian Research And Innovation Agency through research projects J6-4628 and programme P1-0419. Additional financial support was provided by the Ministry of Natural Resources and Spatial Planning, Ministry of Defence (through project MASPREM) and project “Development of research infrastructure for the international competitiveness of the Slovenian RRI space – RI-SI-EPOS” (co-financed by the Republic of Slovenia, Ministry of Education, Science and Sport and the European Union from the European Regional Development Fund).