In July 2021, extreme flooding occurred in Belgium, The Netherlands and Germany, causing over 30 billion euro of damage. Numerous river structures were blocked by floating debris, increasing upstream water levels and thereby the extent of flooding. Moreover, debris accumulation frequently caused structural damage at bridges. This event forms both the motivation for the EMfloodResilience project and an opportunity to study floating debris accumulation better.

In this report, we present the results of the floating debris work package of the project. First, a database of floating debris accumulation during the 2021 floods is presented, documenting the geometry and characteristics of both the affected structures and the debris deposits. The data collection, performed by ULiege, RWTH Aachen and TUDelft teams, focuses on three rivers particularly affected by the flood: the Vesdre in Belgium, the Ahr in Germany and the Geul in The Netherlands. The resulting database includes 33 bridges in Belgium, 38 bridges in Germany and one culvert in the Netherlands, and encompasses around 60 parameters for each identified structure. Results highlighted the severity of the flood, with peak water levels reaching more than 1 m above the bridge deck for 40% of the bridges, 32 of the 71 bridges too damaged to be kept in service. The largest debris accumulations occurred at bridges with simultaneously a pier spacing of less than 10 meters and peak water levels at or above the deck. About 50% of the debris in both countries consisted of trees, the remainder predominantly of building rubble, construction wood, tanks and, in Germany, cars and caravans.

Based on these data, physical experiments were conducted at all three universities in order to determine how backwater rise (the increase of the upstream water level due to debris blockage) depends on the debris composition, hydraulic conditions and bridge and handrail design. The results of the experimental modelling were reported in a second database, which documents more than 280 tests. The presence of plates (i.e. flat objects) in the debris mixture resulted in higher backwater rise. At last, tests with modified bridge designs show that a reduction of the number of piers reduces the clogging probability. Without handrail, debris passed over the bridge sooner, resulting in lower backwater rise.

Based on the observed debris accumulations and experiments, the recommendation for bridge design is to use thinner bridge decks, large freeboard between bridge deck and water level, implement collapsible or foldable handrails and reduce the number of piers. Moreover, the effect of clogging should be implemented in flood hazard and flood risk maps as well as emergency plans, and measures to reduce the accumulation of debris at bridges should be integrated in river basin management.