Predicting Mountain Bridge Damage from Increasing Frequency of Debris Flow Dynamic Impacts

Mountainous regions, due to their topographical characteristics, require a high density of bridges to connect road and railway infrastructures that support human activities. Climate change exacerbates the intensity and frequency of extreme events, seriously threatening bridge safety and infrastructure availability.

Among various hazards, debris flows are one of the most destructive phenomena in Alpine regions. Their destructive nature is due to the high velocities they achieve, the large volumes of sediment they mobilize, and the significant impact forces they exert on any obstacles in their path. When debris flows impact bridges, they often result in destruction. The unpredictability of these events, coupled with inadequate early warning systems and the huge amount of materials involved, frequently leads to loss of human life and significant economic damage.

Bridge piers, due to their location in stream beds, are extremely vulnerable to the impulsive nature of sediment flows and are subjected to the most severe impact forces. Understanding the thrust generated by debris flows on bridge structures is essential for designing works capable of withstanding their impact. This knowledge is crucial for defining appropriate design methodologies for bridges, accurately accounting for the dynamic forces exerted on piers.

This study presents a new experimental apparatus designed to provide accurate information on the dynamic thrust of stony debris flows on bridge piers. The debris flow is simulated in a tilted canal measuring 3 m in length and 0.3 m in width. A model bridge pier and associated impulsive force measuring equipment are installed at the channel's end section. Strategically placed sonar sensors along the canal, combined with pressure sensors and load cells, enable a comprehensive characterization of the debris flows that is triggered by suddenly releasing a pre-set water discharge onto a layer of previously saturated erodible material.

The results from the physical model experiments enabled a systematic study of the magnitude of dynamic forces acting on piers of various shapes and how these forces are influenced by the different physical parameters characteristic of debris flows. Analysis of the collected data provided insights that allow for an assessment of the predictive formulas proposed in the literature. This leads to a deeper understanding of the risk of mountain bridge damage, which is increasingly affected by debris flows in the present context of climate change.