Bridges influence large wood trapping efficiency during large floods: insights from the Francolí River flood in 2019

Recent large floods across Europe, including those in Belgium and Germany in 2021 or, more recently, in Italy in October 2022, showed that major obstructions of bridges due to the mobilized large wood (LW) significantly influenced the flood-related damages. However, in principle, none of the dangers posed by wood was inherent to the wood itself but to the obstacles and infrastructures that were not designed to allow the wood to pass. Understanding this legacy effect on wood in rivers due to the increased artificial trapping efficiency of river structures (bridges, dam reservoirs) still needs to be completed.

The Francolí River in Catalonia, NW Iberian Peninsula (853 Km2 area and 59 km length) underwent a major flash flood on October 22, 2019, that caused six fatalities. The rainfall recorded in the NW basin was 293 mm in 24 hours. Consequently, significant bio-geomorphological changes occurred; a large amount of sediment was eroded, transported and deposited, and many trees were damaged or uprooted with subsequent large wood (LW) supply and transport. In addition, infrastructures were severely damaged (e.g., three bridges collapsed).

The legacy effects on instream large wood related to the human infrastructures in river systems is an essential factor to consider when assessing the effects of floods and potential risks. Therefore, this study's main objective was to evaluate the influence of bridges on large wood accumulation during floods. 

We analyzed a reach of 30 km along the Francolí River in which there were 23 bridges. The reach was split into 52 sub-reaches based on their morphological characteristics (i.e., the width of the valley bottom, slope, and sinuosity), the presence of infrastructures, or lithologic and anthropic knickpoints, and the junction with tributaries. The 52 sub-reaches were grouped into four main typologies based on statistical segmentation and clustering.

Individual pieces of LW and accumulations were digitalized using post-flood high-resolution orthophotos (i.e., 0.10 m resolution). They were characterized using four attributes: orientation with respect to the channel (parallel, perpendicular, oblique), transported (yes or not), location (active channel or floodplain), and length. Average Nearest Neighbour, Spatial Autocorrelation (Global Moran's I test) and Density were computed and revealed the depositional pattern of LW along the study reach.

Preliminary results showed that morphological characteristics favoured LW trappings: wide valley bottoms and sinuous bends. In addition, the standing vegetation and other in-channel obstacles were crucial to trap wood. The most significant aspect, however, was the presence of bridges. A significantly more considerable amount of wood (i.e., the highest density observed, ranging between 33 and 101 pieces/ha) was trapped upstream from bridges, where wood was deposited at significantly higher elevations. Further analyses will explore the characteristics of the bridges and upstream sub-reaches.

This study will provide crucial information to understand large wood accumulation at bridges during floods and will inform flood-hazard assessments and river management.