History often rhymes: evolution and reactivation of the Ciprnik complex landslide (Julian Alps, NW Slovenia)

The Ciprnik complex landslide in the Planica valley (NW Slovenia) triggered on 19. 11. 2000 and measured approximately 80,000 m³. Its movement was characterised by three distinct stages of transport mechanism: a translational landslide-debris flow-hyperconcentrated flow. It occurred due to local geological structure (highly fractured beds dipping parallel to the surface), lithology (alternation of Upper Triassic thin bedded carbonates and fine-grained clastics), and record-breaking monthly rainfall. On the night from 24. to 25. 10. 2023 a large part of the landslide was reactivated during an intense short-duration rainfall event. In this study we analysed surface changes of the area affected by the Ciprnik complex landslide following the 2000 event, performed a sedimentary analysis of the 2023 event and analysed the meteorological data to assess the triggering precipitation conditions. We created a surface-changes time-series spanning between 2006-2023 based on photogrammetrically derived Digital Elevation Models generated from publicly available aerial photographs and our own Unmanned Aerial Vehicle surveys. Granulometric analysis was performed on fourteen samples collected along the transport-depositional area of the 2023 event. Additionally, publicly available meteorological records were analysed. The results show that the area of the Ciprnik complex landslide remained unstable with an average annual erosion rate of 1,000 to 3,500 m³. The 2023 event measured 26,000 m³ and had identical transport mechanism stages as the initial one with very pronounced finning-down of the transported sediment. The proximal material of the debris-flow stage was composed of muddy sandy gravel with approximately 75% of gravel, 15% of sand and 10% of mud. Further down the slopes the granulometry of debris-flow stage fined down containing only 40% of gravel, 45% of sand and 15% of mud fraction. Only fine-grained material travelled in the hyperconcentrated flow stage composed of approximately 40% sand, 50% silt and 10% clay. Both events however differ in the duration and magnitude of the triggering precipitation. The 2000 event was triggered by long-duration low-intensity rainfall, which had record breaking quantities (613.6 mm in the month of the event). The 2023 event was triggered by intense short duration rainfall event (104.2 mm in 24 hours), which relatively commonly occurs in the study area. This case study of the Ciprnik complex landslide demonstrates the complexity of triggering thresholds in the aftermath of the main mass movement event. Even in the later events, which have identical transport mechanisms as the original event, the triggering precipitation can differ considerably in duration and magnitude.