Bridges in small basins with intense sediment transport and debris flow

In Italy a significant part of the bridge collapses are due to hydraulic causes. Despite this, the Italian technical construction standards NTC 2018 provide few indications about the design and verification criteria of bridges with respect to river processes. In 2022, a working group on the "Hydraulic Compatibility of Bridges" (sites.google.com/view/gii-ponti) was set up within the Italian Group of Hydraulics (GII - Gruppo Italiano di Idraulica), 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 "small basins" aims to provide analysis tools for small river basins: they have peculiar features, requiring the adoption of appropriate criteria for the analysis of forcing scenarios and safety measures to be implemented for the hydraulic compatibility of river-crossing bridges. Particular attention is devoted to climatic changes that, although gradual, can induce strongly non-linear responses. The present manuscript reviews the current best practice for analyzing the hydrologic response, the sediment balance, the flow propagation and the dynamic impact force against bridges in the case of mountain basins, pointing out limitations and possible future developments required in order to develop guidelines for bridge safety and flood hazard assessment.