Formation of wood obstructions at bridges: processes, related problems, and prediction tools

The primary cause of many bridge collapses is related to hydraulic issues, and the Italian technical standards for constructions (NTC 2018) offer limited guidance on the design and verification criteria for bridges, especially concerning river processes. Recognizing this gap, a working group dedicated to “Hydraulic Compatibility of Bridges” (sites.google.com/view/gii-ponti) was established in 2021. The group's objective is to develop proposals for good practices and guidelines that assess bridge hydraulic compatibility, providing a foundation for both bridge safety and flood risk analysis. Within this initiative, a working subgroup focused on floating debris is striving to define a methodology for assessing the impacts of wood accumulation on bridges. While wood in rivers is environmentally beneficial, its transport and accumulation at bridges during high-stage events can lead to problematic consequences. Over the past two decades, the scientific community has actively studied wood transport dynamics, accumulation formation at bridges, and their effects on hydraulics and structural stability. Although some methodologies to assess wood-related risk have been incorporated into national legislation or practitioner reports, these implementations vary widely in defining the shape of accumulation, the wood volume assessment and the number of parameters considered. In many cases, qualitative evaluations are used for estimation due to the complexity and high site specificity inherent in wood transport dynamics. This work aims to propose a comprehensive approach, based on the latest research findings, (i) to define the wood accumulation probability at a bridge, (ii) to estimate the size and position of the accumulation, and (iii) to suggest a methodology for the estimation of bridge scour induced by wood accumulation.