# Fragility Analysis Based on Damaged Bridges during the 2021 Flood in Germany
## 2 Materials and Methods
### Bridge Database
It can also be seen that the upper part of the river basin was affected by the greatest hydrologic loads (darker red on rain map: rainfall height \(\geq\) 90 mm), inducing high hydraulic force along the stream. The other highlighted areas represent the accumulated daily precipitation, with values ranging between 50 to 90 mm (red), 30 to 50 mm (lighter red region). Daily accumulated precipitation (combined microwave–IR) 0.1 deg. (GPM GPM\({}_{\text{-}}\)3 IMERGDF v06), 14–15 July 2021 [48]. Base map from NUTS250 [50], river shapefile from Waterbody-DE [51]. bridge should be investigated more broadly, as the riverbed on the upstream side of the bridge did not exhibit aggradation tendencies, suggesting long-term degradation. The situation is exacerbated by the flow contraction under the bridge, which locally increases the water velocity. Figure 5d shows an example of channel flanking scour on a bridge with deep foundation. The water eroded the soil adjacent to the bridge abutment, exposing the pile heads. This type of scour widens the channel, increasing the risk of riverbank instability [63]. In the specific situation, the bridge did not collapse, but in many other situations along the Ahr river, channel flanking led to bridge collapse. To summarize, scour caused significant damage to masonry arch bridges, while reinforced concrete structures with deep foundations were slightly affected by it. Figure 5: Surveyed scour mechanisms: (**a**) pit scour; (**b**) scour of pier; (**c**) scour of approach fill; (**d**) channel flanking scour. Pictures taken by the first author. The other recurrent failures were triggered by deck overtopping, with a key role played by clogged debris. The size of the hydrodynamic forces was magnified by wood logs and carried material, resulting in damming effects with severe consequences for both the structure and its surroundings, inducing backwater effects. Figure 6a shows the remaining instream pier of a wooden bridge, whose deck was found about 5 km downstream, while Figure 6b shows the steel deck of a footbridge dragged downstream a couple hundred meters from its original location. From these two examples, one can observe that only lightweight decks suffered from dragging, but evidence in the aftermath of the flood showed that mixed steel-concrete road decks and a steel deck of a railway bridge also suffered from dragging [46]. During the 2021 flood, the hydraulic force against decks caused damage to all structures, with a clear trend: beam bridges with simply supported spans showed a higher vulnerability to dragging when compared to arch bridges. Indeed, no arch bridges that were overtopped experienced a full collapse. Severe damage was observed for masonry arch bridges, including the removal of infill material, carriageways and railings. Nevertheless, their failure was eventually only triggered by scour, as seen in Figure 6d. Overtopped arch bridges built in reinforced concrete sustained minor damage compared to masonry bridges, mainly to railings and parapets, as seen in Figure 6c. Figure 6: Surveyed overtopping mechanisms: (**a**) remaining pier of a dragged wooden deck; (**b**) girder of a dragged steel deck; (**c**) damaged railing due to overtopping; (**d**) combination of overtopping and scour, with the latter being responsible for triggering the failure. Pictures taken by the first author. Concerning the wood log impact and clogging, experimental campaigns have provided insights into the governing forces [64; 65; 66]; log jams at bridges significantly reduced structural safety, due to an increased flow impact area. By looking at the collected evidence, a key role in collapses was played by driftwood for both arch and beam bridges, while damage caused by uplift was seen only in wooden structures, given the higher buoyancy of the material and lack of evidence within the investigated structures.