Numerical assessment of flood hazard risk to people and vehicles in flash floods

Flash flooding often leads to extremely dangerous and sometimes catastrophic conditions in rivers due to characteristics such as: short timescales, the limited opportunity for issuing warnings, and the frequent high average mortality. Many past extreme flood events have been accompanied by flash floods, and they have also been one of the main sources of serious loss of human life among the world’s worst natural disasters. Flash floods can also cause large loss of property, such as the recent floods in Pakistan and the damage to vehicles in the 2004 Boscastle flood in the UK. It is therefore desirable to be able to assess the degree of safety of people and vehicles during flash floods using numerical models. In the current study, an algorithm for assessing the flood hazard risk to people and vehicles has been integrated into an existing two-dimensional hydrodynamic model capable of simulating flash floods. In the algorithm, empirical curves relating water depths and corresponding critical velocities for children and adults, developed by previous researchers, are used to assess the degree of people safety, and a new incipient velocity formula is used to evaluate the degree of vehicle safety. The developed model was then applied to three real case studies, including: the Glasgow and Boscastle floods in the UK, and the Malpasset dam-failure flood in France. According to the analysis of model predictions, the following conclusions have been obtained: (i) simulated results for the Glasgow flood showed that children would be in danger of standing in the flooded streets in a small urban area; (ii) simulations for the Boscastle flood indicated that vehicles in the car park would be flushed away by the flow with high velocity, which indirectly testified the predictive accuracy of the incipient formula for vehicles; and (iii) simulations for the Malpasset dam-failure flood showed that the adopted method for the assessment of people safety was applicable, and some local people living below the dam would have been swept away, which corresponded well with the report of casualties. Therefore, the developed integrated model can be used to evaluate the flood hazard risk to people and vehicles in flash floods, and these predictions can be used in flood risk management.