Modeling the morphodynamic equilibrium of an intermediate reach of the Po River (Italy)

• We investigate the equilibrium bed profile of a reach of the Po River (Italy).
• Formative discharge is estimated though a 1D movable bed model and topographic surveys.
• Formative discharge is much smaller than the maximum annual discharge.
• The mean annual sediment discharge is estimated from the formative transport capacity.
• The river equilibrium state occurs under an average constant hydrological regime.

The Po River, in the last century, has undergone significant altimetrical and planimetrical changes, mostly induced by a progressively increasing human pressure. The extensive protection and regulations works carried out to reduce the risk of flooding, the narrowing of the river for improving the navigation, the local interruption of sediment transport caused by a large mobile barrage built for hydropower purposes and the intense sand mining caused huge alterations of the river morphology. These changes were initially very fast and determined a significant and generalized deepening of the middle water course. In the last few decades, however, the pressure induced by human activities on the river decreased significantly and, consequently, a dynamic equilibrium condition tended to be re-established along most of the reaches, as suggested by topographic surveys spanning a period of about twenty years. The present contribution investigates this equilibrium condition by means of a one-dimensional movable bed model, with reference to a 98 km long reach located between the confluence with the Oglio stream and the gauging section of Pontelagoscuro, for which an up to date stage-discharge relationship is available. Considering steady forcing conditions, we estimate the formative discharge producing the observed river topography and the corresponding sediment transport capacity. The field surveys of cross section geometry used to investigate the possible existence of an equilibrium morphology span a period (1982–2005) of about twenty years. In the presence of fixed banks, the rived bed morphology appears to be controlled by relatively moderate discharges, quite close to the mean yearly discharge and significantly smaller than both the ordinary flood discharge and the maximum annual discharge. Even though significant deviations from equilibrium are produced by the sediment waves triggered by larger floods, deposition occurring during lower stages and the continuous reworking of the bed due to less intense but more frequent discharges implies a tendency of the river to recover its equilibrium profile.