Stony Debris Flows and Impact Forces on Bridge Piers: Insights from small-scale Laboratory Experiments

Debris flows are among the most devastating natural hazards in mountainous areas, posing a significant threat to infrastructure, particularly bridges that are crucial for regional connectivity. Climate change-induced increases in intense rainfall events have amplified both the frequency and magnitude of these sediment-laden flows. Consequently, bridge structures face growing exposure to extreme loading conditions. Bridge piers situated within active riverbeds are especially vulnerable, as debris flows generate highly impulsive forces that can surpass those accounted for in traditional design methodologies.

A reliable estimation of debris-flow-induced thrust on bridge piers is essential to improve existing design methodologies and to ensure resilience of infrastructure in debris-flow-prone environments.

To address this critical need, an innovative experimental apparatus has been developed to investigate the impact of stony debris flows under controlled laboratory conditions. This setup reproduces both the initiation and propagation phases of debris flows, enabling a more comprehensive analysis of their dynamics and impact forces.

Experiments were conducted in a tilting flume measuring 3 m in length and 0.3 m in width. The flume features an erodible granular bed, allowing debris flows to initiate and evolve through bed erosion, closely mimicking the mechanisms observed in natural settings. This design significantly enhances the realism of the experimental simulations.

Within this framework, particular attention is devoted to the investigation of debris flows propagating under subcritical flow conditions, a regime that has received comparatively limited attention in experimental studies but may be relevant for specific geomorphological and hydraulic contexts.

Debris flows are initiated by the controlled release of a predetermined water discharge, which induces sediment mobilization and subsequent flow development along the channel. The experimental setup is instrumented with pressure transducers, sonar sensors, and load cells to measure flow depth, velocity, and impact forces exerted on model bridge piers of varying geometries and dimensions.

A dimensionless analysis carried out to characterize the flow regime reproduced in the laboratory indicates that the experimental conditions successfully reproduce a stony debris flow in terms of flow composition and propagation dynamics.

Following the preliminary comparison between measured impact forces and those predicted by classical hydrostatic and hydrodynamic theoretical models presented at EGU 2025, an integrated hydraulic model that combines the two approaches is proposed. This model is used to interpret a set of experimental results that has now more than doubled in size. Model parameters are calibrated using an Orthogonal Distance Regression (ODR) procedure, which allows for the joint consideration of uncertainties in both experimental observations and theoretical predictions.

Overall, the findings provide novel experimental insights into debris-flow impact processes under subcritical conditions and demonstrate the capability of integrated modeling approaches in predicting debris-flow-induced forces on bridge piers. These results contribute to the validation and refinement of existing design models, while supporting the development of more reliable, physically based design criteria for bridges exposed to debris-flow hazards.