In the summer of 2021, devastating river floods occurred in Western Europe as a result of extreme rainfall. At numerous bridges, debris accumulations were observed, exacerbating flooding upstream by impeding waterflow and sometimes contributing to bridge failure. Due to widespread building damage and flooding of settlements along the rivers, these accumulations differed markedly from classic logjams, with substantial amounts of man-made objects. A new database of clogged bridges in Belgium and Germany – described in a separate data descriptor – was analysed to characterise bridge clogging and determine the effect of bridge design, bridge location and hydraulic conditions. Nearly half of the debris volume consisted of man-made materials, including building rubble, anthropogenic wood and vehicles. This created remarkably dense accumulations, highlighting the importance of further studying debris accumulations of mixed composition. Examination of the relations between bridge design and accumulation volumes found bridges with narrow pier spacing (≤10 metres) more susceptible to forming large accumulations. Blocking by the deck and railing also played a prominent role, in conjunction with blocking by the piers, as peak water levels at most bridges (85%) reached or exceeded the deck. These findings can help to better understand bridge clogging effects on flood conditions, to design bridges with lower debris accumulation risks, and to inform future flood hazard assessments, flood risk mapping, and disaster response strategies, especially in urbanised regions.