Construction and vehicle debris at the heart of the log jams caused by the 2021 floods
An international team of scientists documented and analysed the accumulations of debris observed at bridges during the July 2021 floods in Belgium and Germany. The results show that nearly half of the accumulated material was man-made, a finding that challenges conventional flood risk assessment models and clearly distinguishes this event from typical floods.
T
he catastrophic floods of summer 2021 struck Western Europe following rainfall of 150 to 250 mm over 48 hours in parts of Belgium, Germany and neighbouring countries. In addition to the direct damage caused by the exceptional water flows, debris carried by the floodwaters formed large accumulations in front of many bridges, locally exacerbating flood levels upstream and, in some cases, contributing to the structural failure of the structures. Researchers from the University of Liège (HECE / School of Engineering), in collaboration with TU Delft and RWTH Aachen, have documented this phenomenon systematically for the first time on this scale.
"The transport of floating debris during extreme floods is a long-known phenomenon," explains Sébastien Erpicum, a civil engineer at the University of Liège, "but until now it was mainly associated with natural elements such as tree trunks and branches. Flood risk analysis models in Belgium and neighbouring countries generally did not take this factor into account." The floods of 2021 highlighted the limitations of this approach.
The research team compiled a database covering 71 bridges affected by log jams – natural accumulations of debris – on six particularly affected rivers: the Vesdre and its tributaries the Helle and the Hoëgne in Belgium, as well as the Ahr, the Inde and the Vicht in Germany. Data was collected from several complementary sources, such as structural plans provided by public authorities, online mapping portals, field visits and photographs taken during and after the event. For each structure, more than sixty parameters were recorded, describing the bridge’s geometry, the hydraulic conditions during the flood and the characteristics of the debris accumulation.
One of the most striking findings of these studies concerns the composition of the debris. Unlike conventional logjams consisting almost exclusively of natural timber, the documented accumulations contained on average around 50% of man-made materials such as construction waste, timber, vehicles – and even caravans – and other objects swept away by the floodwaters from damaged buildings along the riverbank. This unusual proportion led to denser accumulations, which offered greater resistance to flow and caused a more significant rise in water levels upstream of the structures.
Furthermore, the analysis shows that on 85% of the documented bridges, the water reached or exceeded the deck. This finding highlights the often underestimated importance of deck and railing design in the risk of log jams, in addition to the piers, which have been the focus of most studies to date. The largest accumulations were observed at bridges where the pier s were spaced ten metres apart or less. “The 2021 flood, as dramatic as it was, provided a unique opportunity to collect valuable field data on log jams occurring at bridges,” adds Sébastien Erpicum. "This data paves the way for better incorporation of the phenomenon into flood mapping models, and thus for a more reliable risk assessment in Wallonia and beyond."
The database, freely accessible on the Zenodo platform, is intended to inform future research into the design of bridges less vulnerable to log jams, disaster response strategies, and hydraulic modelling incorporating floating debris. It thus serves as a reference for the international scientific community working on infrastructure resilience in the face of extreme floods. Finally, this study highlights that flood risk management must now take into account not only rising water levels but also the fate of objects likely to be swept away by the current—a factor that also places individual responsibility on riverside residents to prepare for floods.
Scientific references
- Erpicum, S., Poppema, D., Burghardt, L., Benet, L., Wüthrich, D., Klopries, E.-M. & Dewals, B. (2024). A dataset of floating debris accumulation at bridges after the July 2021 flood in Germany and Belgium. Scientific Data, 11, 1092. https://doi.org/10.1038/s41597-024-03907-8
- Poppema, D. W., Burghardt, L., Benet, L., Wüthrich, D., Klopries, E.-M., Dewals, B., & Erpicum, S. (2025). Bridge clogging in Belgium and Germany during the 2021 floods. Water Resources Research, 61, e2024WR039218. https://doi.org/10.1029/2024WR039218
Contacts
Partners and funding
- University of Liège - Hydraulics in Environmental and Civil Engineering (HECE)
- Delft University of Technology (TU Delft)
- RWTH Aachen University
This research was conducted as part of the Interreg EMfloodResilience project (project no. 228), co-funded by the European Regional Development Fund (ERDF)
