Following the collapse of the Lenzino Bridge on October 3, 2020, generated by the collapse of the pier central pier in conjunction with the exceptional flooding of the Trebbia River, the modeling hydrological-hydraulic modeling of the watercourse assumed a key role in the design of the new bridge over the SS45. Estimation of design flow rates was carried out by hydrological analysis and modeling, while modeling of the hydrodynamic and morphological behavior of the river Trebbia was developed, for the stretch limited to the area under study, by means of a model two-dimensional computational model with a moving bottom and one with a fixed bottom. The following were thus determined the maximum values of undermining, hydraulic pulls and velocities for the different scenarios of the project: ACTUAL STATE with the presence of the temporary bridge and both piers of the old collapsed bridge; TRANSITION SCENARIO, with the presence of the new bridge, the temporary bridge and both piers of the old collapsed bridge; STATE OF PROJECT, with the presence of the new bridge and the restored pier of the old bridge collapsed (temporary bridge and pile in the hydrographic right-of-way removed). Particular attention was paid to studying the iteration between the new bridge and the old Lenzino bridge, which is subject to historical-monumental constraint, as well as the removal of the old sill, which was also damaged by the collapse. To calculate the stability of the works was then summed the generalized excavation determined by the modeling with a moving bottom with the localized one, the latter determined with empirical formulas available in the literature.
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