This study investigates the sedimentary deposits produced by Storm Alex (2 October 2020) in the Roya Valley (southeastern France), focusing on three sub-valleys (Dente, Consciente, and Cairos) to better understand the flow processes and their associated lithofacies. Lithology and morphometric parameters played an important role in controlling flow behaviour and sediment availability. In the Dente torrent, the remobilisation of glacial and colluvial deposits triggered debris flows that evolved into hyperconcentrated and bedload flows, forming sheet-flood deposits on the Vievola fan. In contrast, the Consciente torrent was dominated by bedload and hyperconcentrated flows with a strong sandy component, derived from the erosion of migmatitic formations of the Argentera-Mercantour massif, while debris flows were locally linked to landslide inputs. The Cairos River, characterised by gentler slopes and a wider valley floor incised into Wurmian terraces, was dominated by bedload processes, with localised debris flows from right-bank tributaries (from marly-limestones) enhancing sediment cascades and channel widening. A significant difference (∼5 wt%) in fine-grained content (clay + silt) was identified between debris flows in the Dente torrent and bedload/hyperconcentrated deposits in the Consciente and Cairos catchments, indicating distinct transport and depositional dynamics. In addition, slope breaks were key locations for the end-members of debris flows such as major channel widenings (width ratio > 3) favored their deposition. This case study highlights the complexity of flow transitions (from debris flows to bedload transport) and emphasises the role of lithological and topographic constraints in controlling sedimentary dynamics during extreme events. The findings emphasise the value of interdisciplinary, multi-scale approaches in documenting and understanding extreme hydro-sedimentary events in mountainous regions. • Significant variability in flow processes occurred during a single extreme event. • Slope, width, lithology and sediment supply governed diverse flow types, including debris flows, hyperconcentrated flows, and bedload transport. • An approximate ∼5% difference in fine-grained content distinguishes debris flows from hyperconcentrated and bedload flows, highlighting contrasting transport and depositional dynamics within the same event. • The type of lithology (unconsolidated or partially consolidated) and the role of channel confinement play a role in the widening of active channels. • Sediment accumulation varied between +1.5 m and + 9 m, emphasising the challenges of interpreting fossil deposits from extreme events.
Kerverdo et al. (2026) studied this question.