Floods and Large Wood in Rivers: Exploring Their Dynamic Interactions Through Numerical Modelling and Field Data on the Allier River

Flood events are among the most devastating natural hazards, presenting multi-risks to infrastructure, ecosystems, and communities. Beyond the immediate impact of inundation, the entrainment and transport of large wood (LW) during these events amplify their destructive potential. Mobilized LW can obstruct critical infrastructure such as bridges, leading to increased backwater effects, exacerbating flooding, and causing structural damage. However, LW also plays a vital ecological role, contributing to habitat formation, nutrient cycling, and riverine biodiversity. As such, it cannot simply be removed without ecological consequences. Understanding the dynamics of LW entrainment, transport, and deposition is crucial for balancing flood risk reduction with the preservation of ecosystem functions.

This study addresses the challenge of modeling LW dynamics in rivers, with a specific focus on the Allier River in France. For this purpose, the study utilizes the ORSA2D_WT model, an Eulerian-Lagrangian two-way coupled approach, which integrates the two-dimensional Shallow Water Equations (SWE) with the Discrete Element Method (DEM). This hybrid model allows for a representation of entrainment thresholds, transport pathways, and inelastic collisions between LW elements and obstacles such as riverbanks and infrastructure.

This research integrates extensive field data, numerical simulations, and experimental findings to enhance predictions of wood mobilization during flood events. Field data collected from the Allier River, France (2020–2024), provides a robust basis for model improvement. This dataset includes Radio Frequency Identification (RFID)-tracked LW positions over multiple years, high-resolution Digital Terrain Models, granulometric sediment analyses and LW characteristics such as size, shape, density and burial conditions.

By combining numerical simulations with extensive field data, this study aims to refine the model’s ability to predict LW mobilization and transport across different flood scenarios, from moderate flows to extreme flood events. Furthermore, the study seeks to enhance the understanding of how environmental factors, such as LW properties and sediment dynamics, influence LW behavior during floods. The outcomes of this research will contribute to the development of a more accurate and reliable hydrodynamic model coupled with a LW transport model, offering insights into how the dynamics of LW affect riverine systems during flood events.