Existing Bridges: Risk of Failure in the Climate Change Context

In the last decades, climate change increases the frequency of extreme events, and floods have occurred more often than in the past (e.g. Seneviratne et al., 2021). Despite flooding affects large portion of areas interested by anthropogenic activities, the in-river structures, mainly bridges, seem to be over and over stressed. It is well known that more than 50% of bridge failure are related to hydraulic phenomena (e.g. Montalvo et al., 2020; Wardhana & Hadipriono, 2003), like scour around pier and abutments, that always causes serious damages if proper countermeasures, usually deepening of foundation, are not provided in the design. Lacks in scientific and technical knowledges have led in the past to the realization of inadequate foundations, and this fact joined with the increase in frequency of flood events, amplifies the compounded risk of failure. However, many bridges realized in the past are actually still working probably thanks to the ancient custom of filling the riverbed around bridge piers and abutments with stones and boulders after relevant flood events as an empirical maintenance technique. The bridge efficiency must be guaranteed above all especially in emergency conditions, like flood events, being fundamental the efficiency of the infrastructures to ensure the mobility of the rescue and guarantee the path for the evacuation of the inhabitants at risk (Salandin, 2021). Therefore, the effectiveness of the described maintenance technique needs to be investigated by adequate studies to establish their validity with the increasing frequency of flood events, and this is done by physical modelling of the sediment-flow-structures interaction. The experiments have been developed in a rectangular flume 1 m wide and 15 m long, using quite uniform sands (median grain size d50=0.34mm) to simulate the riverbed. Different pier widths and water depths are considered in the experiments developed in steady state clear water conditions. According to the Froude and Shields similitudes, proper scaled boulders placed at different times around the pier have been tested, evaluating the scour evolution in long time experiments. New insights about the effectiveness of the aforementioned maintenance technique are expected to understand their validity in the increased flood frequency conditions driven by climate change.