The flood event of July 2021 in Western Europe was characterized by water levels far above the 100-year design flood and by billions of euros in damages. During the flood, voluminous debris accumulations of up to 4,000 m3 occurred at various bridges, causing backwater rise and increased inundation depths. These accumulations contained large amounts of man-made materials, in particular cars, building rubble and household items in addition to driftwood. Therefore, a multilaboratory test series was conducted in this study, aiming to quantify the effect of debris composition and bridge design on backwater rise. Two model scales and seven different debris compositions were studied with the help of flume experiments, carried out in parallel in three laboratories in Belgium, Germany, and the Netherlands. Based on postflood field observations, man-made debris was represented by plates and cubes, mixed with logs. Results showed similar correlations between debris composition and backwater rise in all three laboratories. Compared to debris mixtures with only logs, an increasing volume fraction of plate-shaped objects increased backwater rise due to their higher interlocking nature, while an increasing share of cuboid objects generated less backwater rise. In all cases, backwater rise increased with increasing Froude numbers above 0.13 for constant debris volumes. In contrast, relative backwater rise decreased with increasing initial water level and for a higher bridge blockage ratio. The design of the bridge deck influenced both clogging behavior as well as backwater rise at the bridge, and closed handrails led to higher backwater rise compared to configurations with porous or no handrails. When comparing the results from all laboratories, only minor differences were observed, showing consistency in the methodology. Finally, this study presents a predictive equation to determine backwater rise at bridges in narrow river sections considering debris composition, bridge design and hydraulic conditions.

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.