During 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, revealing substantial amounts of man-made objects. A new database of clogged bridges in Belgium and Germany (described in a separate data descriptor) was analyzed to characterize bridge clogging and determine the effect of bridge design, bridge location and hydraulic conditions. Results showed that 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 that bridges with narrow pier spacing (≤10 m) are more susceptible to extreme clogging. Blocking by the deck and railing also played a prominent role, in conjunction with blocking by the piers, as peak water levels at 85% of the analyzed bridges reached or exceeded the deck. Altogether, 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 urbanized regions.

This study presents an analysis of debris accumulations at bridges and flume experiments, based on field data collected after the extreme flood event which hit Belgium and Germany in 2021. Post-flood photos were analyzed regarding bridge designs, debris accumulation volumes and debris compositions as well as flooding conditions. This showed that the voluminous debris accumulations contained a large share of anthropogenic materials characterized by various shapes. Based on averaged bridge data, prototype bridges were chosen for the experimental modelling, which was conducted in three laboratories in Belgium, Germany and the Netherlands. Thanks to this multi-lab approach, over 250 experiments were conducted, determining the effect of upstream hydraulic conditions, debris shape and bridge design on backwater rise. Compared to debris accumulations with only logs, backwater rise increased with larger shares of plates in the debris compositions, while decreasing with the same shares of cuboid elements. The number of piers and the geometry of the bridge deck showed a strong effect on the clogging behavior, and a closed handrail led to higher backwater rise compared to a porous or no handrail. As a result of various test set-ups and continuous comparisons, inter-lab differences could be determined and reduced, and therefore resulting in a more reliable dataset. On this basis, recommendations for future bridge design and operational flood protection measures were derived.