Debris flow impact forces on bridge superstructures

Bridges form an essential part of the infrastructure system in developed countries, especially in the mountainous regions. However, bridges in those areas are exposed to various Alpine hazards, e.g. rockfalls, avalanches or mudslides. Debris flow impacts play a substantial role in the collapse of bridges in those regions. In recent years and decades, the horizontal impact forces for the bridge piers and abutments have been determined by miniaturized and real-size tests but debris flows can also exert forces on the superstructure of bridges, such as horizontal impact forces, upward pushing forces and friction. Total collapses or deteriorations of bridge superstructures after torrential events can be numerous observed in those regions. Partly similar effects on superstructures are e.g. known for ship impacts or tsunami waves. There is no basis for the design of the superstructure of bridges exposed to debris flow impacts. Within the scope of this project, the forces on the superstructure of bridges due to debris flow impacts are to be investigated experimentally. In contrast to the pier and abutment impacts, forces that push upwards and friction must be investigated in addition to the pure horizontal impact forces.

Bridges contribute significantly to the functioning of modern societies by maintaining the functionality of the infrastructure system. Their replacement value, especially in industrialised countries, is considerable. Due to the topography, the highest density of bridges is found in mountainous regions, where they are, however, at risk from a number of natural hazards such as rockfall, avalanches and debris flows. Climate change and increasing exposure suggest that bridges will be increasingly at risk in the future. This applies in particular to gravitational mass movements such as debris flows. These processes consist of fine and coarse sediments as well as water and prove to be very complex in nature due to their possible different composition. The flow behaviour of debris flows in the same catchment area can be completely different simply due to the different water content. This is highly relevant with regard to bridges, as coarser-grained debris flows lead to major damage or destruction, while more 'fluidised' debris flows can pass through a bridge without any danger. The effects of debris flows on bridge piers have already been investigated, but the effects on bridge superstructures were still unknown. Thus, a new, generally applicable load pattern was developed, identifying the different impact forces on bridge superstructures, and more than 100 tests were conducted out using an innovative 1:30 scale setup. The first part of the project focussed on the construction of the flume and the corresponding prototypical debris flow mixture. Our results show that the maximum pressure of such prototypical debris flows can be calculated based on the impact dynamics, allowing impact forces to be back-calculated based on documented impact heights. Further tests focussed on debris-flow impact on different bridge superstructures. For this purpose, 6 different superstructure profiles were modelled and subjected to experimental debris flows. Our results show that the shape of the bridge profile influences the magnitude of the frontal impact forces. The bridge pier only influences the direction of the frontal impact force, but not its magnitude. The results were also discussed in the context of similar processes and impacts on bridges in mountainous regions. The results were published as part of a comprehensive study of debris-flow events with bridge collapses and recommendations for the design of bridge superstructures that appear to be at risk from debris flows were developed. Final tests analysed the effect of woody debris in the debris-flow mixture. Particularly in forested areas such as Austria, the proportion of dead wood in some debris flows is a factor not to be underestimated. Preliminary results show that frontal impact forces on bridge superstructures are significantly influenced by woody debris - in addition to higher uplift forces due to tree trunks impacting the underside of the bridge.