The effects of downstream boundary conditions on the hydraulic modelling of the flooding of estuarine cities: the Misa River (Senigallia, Italy)

This thesis investigates the influence of downstream boundary conditions on the hydraulic modelling of flood events in estuarine environments, with specific reference to the Misa River (Italy). Estuarine systems are governed by the interaction between fluvial discharge and marine forcing, which jointly control hydrodynamics, sediment transport, and erosion processes, particularly in the vicinity of hydraulic structures. The study focuses on the downstream reach of the river, where tidal dynamics play a key role in modulating flow behaviour. A one-dimensional hydraulic model was developed using HEC-RAS and calibrated against observed hydrometric and tidal data. A total of 26 numerical simulations were carried out, including 18 steady-state scenarios and 8 unsteady scenarios based on different combinations of upstream flood hydrographs and downstream tidal boundary conditions. The results show that, while tidal influence on water levels is limited during the most intense flood events, it significantly affects flow velocities in the downstream reach. In particular, low-tide conditions enhance flow velocities near the river mouth, increasing the potential for local erosion around bridge structures. Velocity variations associated with flood and ebb tidal phases are also evident under moderate flow conditions. These findings indicate that intermediate discharge events, often neglected in flood assessments, can play a critical role in sediment dynamics and local erosion when combined with unfavourable tidal conditions. Overall, the study highlights the importance of accurately representing downstream boundary conditions in hydraulic models of estuarine systems, even in microtidal environments. The results contribute to a better understanding of river–sea interactions and support improved flood risk assessment and management. Future work should consider fully coupled hydromorphodynamic modelling approaches to capture the feedback between flow and sediment processes.