Flood dynamics driven by multiple levee breaches: two-dimensional hydrodynamic–morphodynamic modelling of the 16–19 May 2023 Senio River event (Italy)

Abstract. The extreme rainfall event that affected the Emilia-Romagna region (Italy) between 16 and 17 May 2023 triggered widespread flooding and multiple levee breaches along the Senio River, leading to extensive inundation of agricultural and peri-urban areas. This study presents a high-resolution two-dimensional (2D) hydrodynamic model based on the parallel GPU-accelerated solver PARFLOOD, coupled with a morphodynamic model to explicitly simulate multiple levee breach formation and evolution during the May 2023 event. The model reconstructs the flood wave using observed hydrometric data and consistent hydraulic boundary conditions. A grid resolution of 1 m enables detailed representation of breach widening, crest lowering, erosion dynamics, and inundation processes. The modelling framework allows quantitative assessment of overflow volumes, flood extent, water depth distribution, and flood arrival times, and is supported by a comprehensive dataset including hydrometric records, high-resolution pre- and post-event topography, flood extent mapping, and eyewitness observations. Results show that morphodynamic feedback strongly controls flood volume partitioning and inundation timing. A substantial portion of the total flood volume is released through breaches; however, return-flow breaches are also observed, whereby part of the water previously spilled into the inundated areas re-enters the river channel. This behaviour leads to complex hydraulic interactions between the river and the surrounding areas, including non-monotonic discharge evolution along the river reach. The analysis highlights the importance of multiple simultaneous levee breaches, a condition that is rarely documented in the literature and challenges traditional flood hazard assessment approaches based on independent breach scenarios. The study shows that a physically based breach approach, coupled with a 2D shallow water model, provides a robust framework for flood reconstruction and offers new insights for residual flood risk assessment in embanked low-gradient river systems.