The effects of secondary currents in tight bends on large wood transport in rivers: Lesson learned from Versilia flood in 1996

The presence and dynamics of large wood (LW) in river systems, particularly for streams flowing through forested areas, play a significant role in shaping river morphology and influencing flood events. This paper focuses on the numerical reproduction of observed LW trajectories during a flood event in the Versilia River in 1996 in Italy. The study reach, located near Pietrasanta in Tuscany, includes a tight river bend named S. Bartolomeo. During the event, more than two thousand cubic meters of transported wood were observed at this location. In addition, due to the collapse of the outer levee, six million cubic meters of water exited the river corridor, causing the flooding of Pietrasanta. The 2D Lagrangian model Iber-Wood was used for the numerical simulation of this event. In particular, two versions of the model were used: the original one (IB-NSC) and the enhanced version (IB-SC) which simulates the effects of secondary currents on LW dynamics. The results validated the applicability of the enhanced version to simulate real-world conditions and showed that the IB-SC model improves the agreement between observed and simulated LW trajectories by more than 20% compared to the IB-NSC model. Examining the influence of the breach on the LW trajectories, it appears that the presence of LW during the overtopping flow intensified and accelerated the erosion process, resulting in a rapid breach formation. This study therefore highlights the importance of including secondary current effects on LW transport for the development of flood risk assessments and river management strategies, particularly in channelized rivers with tight bends or meanders.