An extreme May 2018 debris flood case study in northern Slovenia: analysis, modelling, and mitigation
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Nejc Bezak I Jernej Jež I Jošt Sodnik I Mateja Jemec Auflič I Matjaž Mikoš
An extreme May 2018 debris flood case study in northern Slovenia: analysis, modelling, and mitigation
Abstract Debris floods can cause large economic damage and endanger human lives. This paper presents an extreme May 2018 debris flood that occurred in northern Slovenia near the Krvavec ski resort and caused large economic damage. The debris flood was initiated by an extreme rainfall event with a return period of over 50 years. There were large differences in the measured rainfall amounts using different equipment. The estimated volume of the debris material during the event was 4000 m 3 /km 2 for the Brezovški graben. In order to mitigate the risk due to future debris flood and debris flow events, a check is planned to be constructed. The part of the design process is presented in this paper. Additionally, RAMMS model was used to validate the empirical equations that were used in the process of the check dam stability design. The model was calibrated using information about the deposition area. Two adjacent torrents were modelled, and we were not able to find a common RAMMS parameter set that would yield adequate simulation performance in both cases. Keywords Debris floods . Hyperconcentrated flows . Slovenia . RAMMS . Numerical modelling . Mitigation measures Introduction Slovenia is among the European countries where different types of mass movements such as debris flows, shallow landslides, or deepseated landslides can occur relatively frequently (e.g., Mikoš et al. 2004; Mikoš et al. 2005; Sodnik and Mikoš 2006; Petkovšek et al. 2011; Jemec Auflič et al. 2016; Bezak et al. 2019a), and the density of active landslides in the Slovenian national database is more than three landslides per 10 km2 (Herrera et al. 2018). Most often, this kind of mass movements in Slovenia is rainfall-induced, i.e., triggered by extreme rainfall events (e.g., Mikoš et al. 2004; Bezak et al. 2016; Jemec Auflič et al. 2016; Bezak et al. 2019a), and less frequent they are earthquake-induced (Mikoš et al. 2013). Extreme events can be either of short duration with very high rainfall intensities (e.g., Železniki case study) or of prolonged duration with smaller rainfall intensities where antecedent conditions are also important (e.g., 2000 Log pod Mangartom debris flow) (e.g., Bezak et al. 2016). Other triggering mechanisms are rarer. Shortduration storms with extreme intensities can also often lead to flash floods where sediment transport (i.e., bed and suspended load) is very intense (e.g., Bezak et al. 2017) and can similarly as in the case of debris flows or deep-seated landslides lead to large economic damage. A transitional process between water (i.e., flood) and debris flow is debris flood (Hungr et al. 2014) respectively hyperconcentrated flow (e.g., Pierson 2005; Calhoun and Clague 2018). Hungr et al. (2014) have updated the Varnes classification of landslide types, and in this modified classification, they defined, among other types, also “debris floods”:
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