Urban Flood Drifters (UFDs): Onset of movement.

Despite their catastrophic implications in flood events, the mobilization and transport of large, loose objects -termed Urban Flood Drifters (UFDs)- are often overlooked in flood management. These objects are inherent to anthropogenic activities, but are not designed to remain stable under flooding conditions, nor are usually considered in flood risk studies. This oversight stems from our limited understanding of how flowing water interacts with these heterogeneous objects. To bridge this knowledge gap, we introduce a mechanistic stability model that predicts the onset of UFD mobilization across a diverse array of loose objects, from plastics to heavy vehicles. We further enhance the reliability of our model by incorporating a Monte Carlo-based probabilistic framework that accounts for uncertainties and interdependencies among the input parameters. Our results show that plastic and other litter are the most mobile objects found in urban setups, being subject to incipient transport under frequent floods. These are followed by wood (anthropogenic or natural) and urban furniture. Vans, caravans and recreational vehicles (RVs) can be more mobile than other light-weight vehicles in low-gradient areas, whereas trucks and buses remain considerably more stable; although more hazardous, when mobilized. Construction and metal debris are predominantly stable in low-slope areas. When integrated with flood maps or two-dimensional (2D) hydrodynamic models, our stability curves can guide urban planning efforts to predict and mitigate the impacts of UFDs during extreme flood events.