Flood characterization based on forensic analysis of bridge collapse using UAV reconnaissance and CFD simulations.

Flash floods are common manifestations of extreme weather events and one of the most severe natural hazards. In Europe, they have been responsible for 359 fatalities and an economic loss totalling 67 million USD in the past decade (EM-DAT), while their increasing severity is linked to climate change. Nevertheless, flash floods remain a poorly documented natural phenomenon due to the lack of flow intensity data in many of the affected watersheds. Based on a thorough field investigation, including UAV-based 3D mapping and material characterization with on-site testing, we carry out a numerical study of a notable flood that caused the collapse of bridges and buildings in Central Greece, following a recent Mediterranean hurricane. Focusing on a carefully selected case study, we combine 3D modelling of flow-structure interaction with detailed mechanical modelling of the nonlinear structural response to reproduce the flood-induced fracture of a bridge abutment. Back-analysis of this failure responds to the fundamental problem of estimating the undocumented magnitude of this extreme event. The paper estimates a lower bound value of the flow velocity at the studied location. This can be valuable input for the interpretation of the extensive damage that took place downstream and for the re-assessment of flood risk in a region where similar events are expected to become more frequent because of climate change. The approach, where disaster forensics and engineering analysis are used to fill the gap of missing real-time measurements, can be implemented for the a posteriori estimation of flood intensity in similar events. The well-documented case study of a bridge failure due to extreme flooding can also be used for validation of future numerical and experimental methods and motivate investigations of the mechanisms governing flow-soil-structure interaction in river crossings.