Seismic monitoring of sediment transport during flash floods: Case studies of Storms Alex and Aline on the Roya river, France

In mountain catchments, floods can mobilize large amounts of sediment, yet monitoring these events remains a major challenge. Understanding the processes governing sediment transport during extreme floods, such as flood waves and sediment pulses, is key to improving our understanding of rapid erosion dynamics. Environmental seismology offers a powerful approach to detect and quantify these processes remotely and continuously with high temporal resolution.

The Mediterranean basin is characterized by a climate and topography prone to flash floods. The objective of this work is to quantify the sediment transport that occurred during the extreme flooding associated with Storm Alex (October 2020), and the subsequent major flood caused by Storm Aline (October 2023), on the Roya River in southeast France.

To address this objective, we use seismic measurements from a single-component geophone (natural frequency of 4.5 Hz) installed at 5 m from the Roya River. We apply previously developed physical models describing the seismic power generated by river bedload transport (saltation model) and turbulent flow. Comparison with model predictions suggests that, at such short distances, the recorded seismic power is dominated by the bedload process, allowing us to focus on the saltation model.

To quantify the sediment transport during periods of peak seismic amplitude, we calibrate the parameters of the saltation model to the Roya River context. Although the resulting volumetric sediment flux is consistent in order of magnitude with theoretical and empirical estimates, the complexity of the physical environment calls for further investigation. Seismic parameters of the riverbed, realistic grain size distributions of transported sediments, and local hydrometric data remain difficult to constrain directly. We address these uncertainties through sensitivity analyses, which show that seismic medium parameters mainly control the shape of the seismic spectrum. We therefore explore the use of real data that we obtain from an active and passive seismic experiment to adjust those parameters

Water depth is another key parameter of the saltation model, as it controls the basal shear stress. We estimate flow depth using upstream discharge measurements and local discharge modeling with simplified theoretical relationships between flow depth, river width, and discharge. Ongoing and future work includes seismic array measurements and local hydraulic modelling to further constrain model parameters. Overall, our results highlight the potential of environmental seismology to quantify sediment transport during extreme flash floods and to improve process-based understanding of sediment transfer in steep Mediterranean river systems.