Bridge Failure and Consequences: the Existing Infrastructures Need of Mitigation Techniques

Bridges represent a critical issue in case of hazardous events, like extreme floods and debris flows, being their proper operation of fundamental relevance to avoid cascading effects on population. The reduction of the risk of failure of river crossings is fundamental to ensure the service given by the infrastructural network for the safety of the populations. The serviceability of the road and railway network must be guaranteed during hazardous events, when the efficiency of the infrastructure becomes fundamental to ensure the mobility of the rescue and a possible evacuation of the inhabitants during critical situations. Moreover, the safety of the bridge affects the surrounding environment with impacts on social and economic activities, representing a connection between different populations living along the sides of the river. Hydraulic phenomena are responsible of more than 50% of bridge failure (e.g. Montalvo et al., 2020; Wardhana & Hadipriono, 2003), and scouring around piers and abutments always causes serious damages if proper deepening of foundation is not provided in the design. These aspects are exacerbated due to the climate change that in the last decades increases the frequency of extreme events, occurring flood events more often than in the past (e.g. Seneviratne et al., 2021). Lacks in scientific and technical knowledges have led in the past to the realization of inadequate foundations, and this fact joined with the occurrence of hazardous events in the climate change context, amplifies the risk of failure of bridges. 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. Therefore, the effectiveness of the described technique to reduce the risk of failure needs to be investigated to establish its effectiveness. Here this is done by physical modelling of the sediment-flow-structures interaction, technique that leads the possibility to check the performances of countermeasures like riprap mattresses, investigating the size of the boulders, the lined area, and their durability over time. The experiments have been developed in a rectangular flume 1 m wide and 15 m long, using quite uniform sands (median grain size d50=0.4mm) to simulate the riverbed. Different pier geometries and water depths are considered in the experiments developed in steady state clear water conditions. According to the Froude and Shields similitudes, different arrangements and boulders size have been tested, evaluating the scour evolution in long time experiments. The effectiveness of the aforementioned maintenance techniques is analysed to understand the reduction of the risk of failure of bridges to limit the resulting cascading effects.