  Extreme floods threaten lives, assets and ecosystems, with the largest
impacts occurring in urbanised areas. However, flood mitigation schemes
generally neglect the fact that urban floods carry a considerable amount of
solid load. In this study, we define Urban Flood Drifters (UFDs) as loose
objects present in the urban landscape that can become mobile under certain
flow conditions, thereafter blocking drainage infrastructure and endangering
both downstream and upstream communities. Based on 270 post-flood photographic
records from 63 major inundations of the past quarter-century across 46
countries, we provide a comprehensive analysis of UFDs and their flood-hazard
implications. We show that a variety of vehicles, furniture and a heterogeneous
mixture of drifters are present in post-flooding scenarios. Plastic,
construction debris and wood (natural or anthropogenic) dominate the statistics
of transported drifters in urban floods (with frequencies of roughly 50-60%
each), followed by cars (present in 31.5% of post-flood images). Other heavy
vehicles are readily observed in post-flood imagery and furniture such as bins,
garden sheds or water tanks also appear occasionally, therefore suggesting that
they can play a relevant role in extreme floods.
