Davide Wüthrich
Dam failures, impulse waves, tsunamis and storm surges are disasters that challenge humanity, often leading to massive casualties and important economic losses. These events generate highly unsteady flow conditions in the form of surges or turbulent bores, associated with extreme loading on infrastructure. In the past, the impact on these flows on buildings was considered too rare and wave induced forces neglected in the design process. However, recent events showed that some measures can guarantee human safety and reduce reconstruction costs. As such, vertical shelters able to withstand extreme loading are fundamental and an estimation of induced forces necessary. Field studies showed that buildings with openings perform better during wave impact and the objective of this research is to evaluate and analyse damage potential of resilient buildings hit by both surges and bores. In particular, the influence of openings as a mitigation measure on the loading process is addressed and quantified.
The research is based on an experimental approach. Wave formation is achieved through the vertical release of a water volume from an upper reservoir, generating dry bed surges and wet bed bores in the downstream horizontal channel. A variation of the released discharge results into waves with different hydrodynamic properties. A detailed methodology to hydraulically characterize the generated waves in terms of their water depths and flow velocities is presented and good agreement with the classical dam-break case for both dry surges and wet bed bores is demonstrated. Particular attention is given to wave front celerity and velocity profiles measured behind the wave front, showing some features typical of open channel flows.
Buildings were reproduced using aluminium structures installed on a force plate, providing a detailed time history of impact forces and moments. The impact was characterised by high splashes, followed by a quasi-steady flow around the building. For the impervious structures, dry bed surges resulted into horizontal forces proportional to the momentum flux. For wet bed bores, an attenuation of the peak force was constantly observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation.
In this study the effect of openings and overflow were tested. Four building configurations with seven opening values ranging from 0 to 84% were investigated using 12 standard waves. The flow through the structure reduced the upstream water depths, providing safer vertical shelters. In terms of loading, the openings produced a linear reduction of the maximum horizontal force, if compared to the impervious case. The configuration with an impervious back wall showed similar results to the impervious building. Openings on the side walls had no influence on the resulting maximum load. Analysis in terms of peak time, wave height at maximum force and impulse pointed out some key differences between forces exerted by surges and bores. The time occurrence of the maximum tilting moment corresponded to the maximum horizontal force and the resulting cantilever arm was constant for all opening configurations. In case of overflow, milder loading conditions were constantly measured.
Finally, new formulae to estimate the hydrodynamic load are introduced, taking into account the effect of openings within an adapted resistance coefficient, providing relevant information for the design of safer infrastructures.
EPFL2018
