Large wood recruitment and transport during a severe flash flood in Central Spain, September 2023.

In the face of escalating climate change and an expected increase in extreme precipitation events leading to extreme floods, this work addresses the understudied but critical aspect of woody material in river systems. We aim to understand the dynamics of large woody debris during a severe flood and the evolution of floodplain vegetation in the periods between floods. The study area is the River Alberche and its tributary, the River Perales (Tagus Basin, Central Iberian Peninsula). These were affected by an exceptional cut-off low weather situation (DANA in Spanish) in September 2023 producing heavy precipitation (up to 200 l/m2) and flash floods. This event flooded urban areas, damaged or destroyed four bridges, and resulting in two deaths. One of the damaged bridges had retained a significant deposit of large and fine woody material. After the flood, as usual, critical voices emerged from the affected population calling for the removal of woody material from the riverbeds. However, there are positive contributions of wood in rivers, enhancing hydro-morphological diversity and serving as a source of organic matter. Nevertheless, uncertainties remain regarding the dynamics and amounts of woody material, which warrant a comprehensive investigation. This research aims to fill existing gaps by investigating the dynamics of woody material transport under these exceptional flow conditions through a post-event forensic survey. In addition, it aims to understand river bed vegetation during non-extreme flood periods. The knowledge generated will contribute to the development of basin-scale models that integrate woody material, thereby improving the accuracy of flood risk assessments and enabling the formulation of effective mitigation strategies.