Validation of Synthetic Design Hydrographs through 2D hydrodynamic modelling

•Synthetic Design Hydrographs are derived on the basis of a long series of floods.•Historical floods and SDHs were routed in a river reach with a 2D-SWE numerical model.•Flood diversion into a paleochannel occurs for the most rare flood events.•The peak flows distributions of routed SDHs and historical floods compare well.•Few SDHs can be routed in place of a long series of historical floods. The procedure for the determination of Synthetic Design Hydrographs (SDHs), proposed in previous works, is validated by comparing the peak discharges obtained by routing a long series of historical floods and the synthetic floods at different stations along a complex river system. At this aim, the 60 km long terminal stretch of the Dora Baltea river (Northern Italy) has been modelled according to fully 2D high resolution hydrodynamic approach. The fluvial branch is of considerable complexity due to a strong contraction induced by the presence of a narrow Roman bridge, which, during the most important flood events, causes the reactivation of a paleochannel and the flooding of a part of the city of Ivrea. The hydraulic model has been calibrated on the basis of the main historical floods. Then, all the historical floods over a period of more than 80 years (1939–2020) and the SDHs derived by the same series have been routed. Historical and synthetic peak discharges at two downstream stations have been then compared in probability plots. The results show that the peak discharge distributions derived by routing the historical floods and the SDHs compare well. This suggests that SDHs construction procedure is reliable and has statistical significance.