Probabilistic Flood Hazard Mapping Considering Multiple Levee Breaches

Probabilistic methods are widely adopted for residual flood hazard assessment in flood-prone areas protected by levees. Such methods can consider various sources of uncertainty, including breach location and flood event characteristics, and allow for the quantification of the result confidence. However, the possible occurrence of multiple levee breaches during the same flood event is usually disregarded. This paper presents a probabilistic method based on levee fragility functions and basic probability rules to estimate the probability of selected breach scenarios, including multiple breaching events. The flood hazard classification is based on inundation variables calculated for flood events of different return periods. A combined 1D-2D hydrodynamic model is used for flood simulations. Probabilistic inundation extent maps and probabilistic flood hazard level maps are then created. Finally, probabilistic flood hazard estimates are summarized in a map of a suitable central tendency of the hazard level, coupled with a map of the Shannon entropy as an uncertainty indicator. This pair of statistical maps provides concise and effective information on a reference flood hazard along with the associated uncertainty. The method was applied to a region located along the middle reach of the Po River (northern Italy). Comparable central flood hazard estimates are obtained for the two sets of breaching events including or not including multiple breaches, with higher uncertainty if multiple breaches are considered. This result highlights the importance of considering multiple breaching events in flood risk management.