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

Abstract

Majority of the bridge collapses are due to hydraulic reasons. The Italian technical standards of construc-tions (NTC 2018) provide few indications about the design and verification criteria of bridges with respect to river processes. In the last year, a working group on the "Hydraulic Compatibility of Bridges" (sites.google.com/view/gii-ponti) was set up, with the aim of formulating proposals for good practices and guidelines for assessing the bridge hydraulic compatibility, as a basis for both bridge safety and flood risk analysis. The working subgroup on "floating debris" aims to define the methodology for assessing the impacts of wood accumu-lation on the bridge. Wood in rivers represents a key beneficial component of river systems from an environmental standpoint, whereas problematic consequences may be related to its transport and accumulation at bridges in case of high-stage events. The scientific community has been actively working in the last two decades to investigate wood transport dynamics, the formation of accumulation at bridges and their effects on hydraulics and structural stability. In a few cases, methodologies to assess wood-related risk have been included in the national legislation or in reports devoted to practitioners. Such implementations embrace different ap-proaches, shape of accumulation, and wood volume assessment methods, and typically include a restrict-ed number of parameters, in some cases estimated by qualitative evaluations only. This is due to make the estimation procedures feasible by practitioners, but also for the need to cope with the large complexity and high site specificity inherent in wood transport dynamics. This contribution aims at reviewing the factors which affect the formation of wood obstruction at bridges and the existing approaches for their assess-ment, with their pros and cons, in order to delineate a possible approach to be implemented at the national level.