The role of residual risk on flood damage assessment: A continuous hydrologic-hydraulic modelling approach for the historical city of Rome, Italy

Study region: The municipality of Rome (Italy) and the Middle Tiber River valley. Study focus: Ordinary and residual flood risk assessment is a crucial topic in hydrology with a variety of practical implications for improving the extreme event management. Nowadays and in such context, the continuous hydrological-hydraulic modelling is an emerging approach and this study supports the conclusion that it can provide a very informative flood risk assessment. Moreover, when the study domain includes relevant and very expensive historical centers, like Rome, such methodology allows for a complete characterization of the risk providing the necessary information for decision-makers. New hydrological insights for the region: In the present work we illustrate an exhaustive flood risk assessment, quantifying the benefits of applying the hydrologic-hydraulic continuous modelling approach. Specifically, we verify that in the city of Rome flooding begins with events of 175 years return period which generates damage of hundreds of million euro. The flood-damage relationship as a function of return period, then, grows linearly up to floods with a return period of around 350 years, for which the majority of the historical town is flooded; then the estimated damages keep growing with the return period to reach a damage of about ten billion euros for 500 years flood events.