Author(s): Margherita Lombardo; Francesco Macchione

Linked Author(s): Margherita Lombardo, Francesco Macchione

Keywords: Flood hazard; Bridge clogging; 2D hydrodynamic modelling

Abstract: “Associated events” are events that should also been considered in flood modelling due to their importance for operational flood risk management. They include for example flood defense failures, flood mitigation strategies taking places during an event, backwater effects at a bridge, and a number of phenomena that can exacerbate the severity of a flood, but they are usually neglected in the context of flood hazard assessments and mapping. The inclusion of associated events can play an important role in the development of consistent scenarios for flood hazard analysis, in particular for such systems that are characterized by a rapid hydrodynamic response. The hazard scenario trajectories may include obstruction due to woody debris transport evolving sometimes in a subsequent dam-break flows. Moreover, this can impact on the upstream water levels with a backwater rise due to the bridge constriction and spanwise clogging, and downstream consequences on flow and depth hydrographs are even expected due to the sudden unblocking of the bridge. In this study we propose a simple decision tree for the inclusion of clogging and breakage in 2D flood modeling based on direct or predisposing factors and an original method for simulating the macroscopic effects of clogging and breakage at a bridge, particularly useful for the worst cases in which a “spanning jam” is likely to form during a severe flood event. The method relies upon setting an internal boundary condition that was capable to simulate both the clogging and the dam-break wave resulting from the instantaneous breakage. The method was validated against experimental tests and then applied to a well-documented case studio numerically reconstructed by means of a direct rainfall simulation, leading to reproduce the event dynamics in a more faithful way and in accordance of the witness reports. The study set new light on the possibility to account for such kind of associated events and evaluate their consequence on flow dynamics for the reconstruction of past events and “what if” (or probabilistic) scenarios for flood hazard assessment.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221091

Year: 2022