Vehicles, pedestrians and flood risk: a focus on the incipient motion due to the mean flow

Floods are overflowing of water onto land, which is normally dry and are one of the costliest natural hazards. According to global scale reports, in the last decade floods affected the largest number of people with respect to other hazards such as earthquakes or droughts. Beside the damages to structures and infrastructures, floods also cause many fatalities and injuries. It has been demonstrated that the majority of fatalities occurs as a consequence of inappropriate high-risk behaviours like driving and walking in floodwaters. In fact, vehicles can lose stability also for very low water depths and may turn into deadly traps. For flood risk managers, people safety is the primary objective, but although vehicles are so crucial, very little is known about the critical conditions in which the onset of motion occurs. Besides, the existing instability criteria for pedestrians under water flow suffer from the large scatter of experimental pairs of critical water depth and velocity. As a matter of fact, the instability conditions of both vehicles and pedestrians are affected not only by flood parameters (i.e. water depth and velocity), but also by geometric and physical properties of the object. The main aim of this PhD research project is to better understand the instability mechanisms for pedestrians and vehicles, which are responsible for most of the casualties in order to introduce new hazard criteria capable of accounting for both flood and object characteristics. For this purpose, a comprehensive analysis of the current knowledge is firstly presented. Secondly, the forces acting on a partly immersed vehicle and human subject are examined and two dimensionless mobility parameters are introduced. The existing experimental data on vehicles and people instability are used to identify a dimensionless critical threshold of incipient motion. Thirdly, a 3D numerical model in the OpenFOAM framework is adopted to clarify the role of hydrodynamic forces and determine relevant dimensionless parameters and scaling numbers involved in the instability mechanisms, considering the mean flow properties. Then, the results of the numerical simulations for a selected vehicle, which reproduce a set of existing experiments, are analysed and discussed. Finally, two case studies are presented in order to demonstrate the applicability of the mobility parameters to the field scale and the advantages of hazard maps implemented with the proposed method.