Assessment of pre-simulated scenarios as a non-structural measure for flood management in case of levee-breach inundations

Levee breach inundations can entail large flood losses due to the high concentration of exposed assets in levee-protected floodplains and, sometimes, to the inadequacy or absence of early warning systems for this type of events. Since real-time modelling is computationally expensive and presents several uncertainties, which might prevent obtaining a reasonably accurate forecast of the flood propagation, an alternative methodology for the prompt prediction of flooded area, maximum depths, and arrival times during a real event was proposed. The strategy is based on the use of a database of pre-simulated scenarios of levee-breach inundations, obtained adopting a high-resolution two-dimensional shallow water model. The paper aims at the a posteriori assessment of the usefulness of this strategy. To this end, the December 2020 event on the Panaro River (Italy) is thoroughly analyzed. In the study area, the strategy had already been implemented before the event, and pre-simulated scenarios were consulted during the emergency. Post-event observations are also available for the ex-post model validation. The database was obtained considering two inflow synthetic hydrographs and a discrete number of breach locations, and unavoidable differences between real events and hypothetical scenarios were to be expected. However, for this case study, the closest levee-breach scenario in the database (in terms of breach position and inflow) provided reliable predictions of flood extent and maximum depths for the actual inundation. The pre-simulated database also helped identifying some critical spots, where effective emergency operations (sandbagging) helped protecting an urban district during the event. As accurate real-time forecasts of levee-breach inundations are yet to come, a database of pre-simulated scenarios is proven as an effective “surrogate” method for civil protection purposes.