Extreme flood event and their depositonal signatures: the case of the Storm Alex in the Roya Valley

This study examines sedimentary deposits from Storm Alex (2 October 2020) in the Roya Valley, focusing on three different sub-valley (Dente, Consciente and Caïros) to understand flow processes and associated lithofacies. Key factors controlling sediment transport include lithology, slope, and sediment supply, which influence the occurrence of bedload, suspension, or debris flows. A notable ~5 wt% difference in fines (clay + silt) was observed between debris flows in the Dente sub-valley and bedload/hyperconcentrated flows in other areas.

In the Dente, extensive reworking of glacial and colluvial deposits triggered debris flows that transitioned into hyperconcentrated and bedload flows, culminating in sheetflood deposits on the Viévola fan. The Consciente sub-valley exhibited bedload and hyperconcentrated flows, with debris flows linked to lateral inputs from landslides. The Caïros sub-valley, characterised by gentler slopes and a wider valley floor, was dominated by bedload processes with localized debris flows originating from right-bank tributaries or natural dams.

Hydraulic reconstructions using empirical discharge, unit stream power, shear stress, and clast size estimates provided insights into event intensity, offering valuable reference points for understanding extreme hydro-sedimentary events. Spatial and temporal variability was significant, highlighting the challenges in interpreting fossil deposits without precise temporal context, since a single extreme rainfall event (>1000-year return period) produced a wide range of facies.

This case study underscores the complexity of flow transitions (debris flows to bedload) and the importance of lithological and topographic constraints. The findings emphasize the value of interdisciplinary, multi-scale approaches in documenting and understanding extreme hydro-sedimentary events in mountainous regions