Author(s): Ashkan Pilbala; Tommaso Lazzarin; Davide Tognin; Federica Baldasso; Daniele Pietro Viero
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Keywords: Po River; Sand dunes; Dynamic bed roughness; Upper stage plane bed
Abstract: Conventional flood models rely on fixed-bed assumptions and assume that resistance coefficients remain constant regardless of any changes in hydrodynamic conditions during an event. Even though this approach has proven to be effective in relatively stable river systems, it may not be sufficient in situations characterized by marked changes in bedforms as a result of changes in hydrodynamic regime. In the terminal reach of the Po River, which is the largest river in Italy, large discrepancies have been observed in the stage-discharge relationship during extreme flood events (e.g., Dazzi et al., 2021). Using fixed-bed models with constant resistance coefficients, the water levels are typically underestimated for low-to-moderate flow rates and overestimated during high flow conditions. We identified changes in flow resistance induced by bedform evolution as one possible cause of this poor fitting. As is well known, dune height and length, as key determinants of the total flow resistance, can vary considerably according to the hydrodynamic regime. In recent studies (Naqshband et al., 2014; Naqshband, S., & Hoitink, 2020), it has been shown that transitions from sand-dune to upper-stage plane beds (USPBs) can reduce flow resistance and, in turn, increase river conveyance, even at low Froude numbers (i.e., F<0.8) which is the typical situation of lowland rivers as the Po. Thus, to produce correct water levels predictions from low to extreme stages in Po River, we enhanced an existing hydrodynamic model to account for the bedform dynamics and examined the role of transition from sand-dunes to USPBs.
Year: 2025