Modelling the effect of a vegetated mid-channel bar on large wood accumulation at bridge piers

Wood is a key-component of river ecosystems, but it is also regarded as a detrimental element that may increase the hydraulic risk. For example, large accumulations of wood and fine vegetation at bridge piers can reduce the bridge span and generate afflux, potentially extending flooded areas. Such vegetation is generally transported during floods, originating from landslides, debris-flow and bank erosion. Additionally, river re-naturalization and nature-based solutions like large wood addition or the building of vegetation patches, may inadvertently contribute to wood transport. Therefore, both natural events and human interventions can increase the amount of transported wood, potentially increasing associated hydraulic risks.

While several studies have addressed the risks related to wood accumulation at bridge piers, significantly less attention has been given to wood accumulation processes at natural structures, like vegetated bars. Similarly to bridge piers, stable vegetated islands can trap wood, fostering its accumulation, reducing or delaying its mobility and protecting the downstream areas.

The present contribution analyses the influence of a mid-channel vegetated bar on large wood transport in the Adda River (Italy) employing the two-dimensional hydrodynamic numerical model ORSA2D_WT, which includes large wood transport dynamics. The vegetated island is located just upstream of a four-pier bridge. Its effect in terms of trajectory deviation, accumulation at the bar, and wood-pier interaction is analyzed by simulating different scenarios of flow, and large wood abundance and positioning.

The results highlight that the presence of stable non-erodible vegetation on a bar upstream of the bridge reduces the interaction between the wood and the piers, thus reducing the probability of accumulation. In addition, the ORSA2D_WT model aids in identifying which piers are most subject to impacts from transported wood, thus facilitating maintenance strategies. The proposed approach could be applied to other natural or human structures, to assess their efficacy in sheltering downstream critical sections from wood accumulation.