Ensuring safety of people in case of severe floods: feasibility and relevance of vertical evacuation strategies in high population density areas

Abstract

When a major flooding event is expected the authorities in charge of the crisis management often consider bringing people to safety by making them leaving temporarily the threatened area before the onset of the flood. This strategy is called “horizontal evacuation”. It has to be distinguished from “vertical evacuation”, which means that people reach a shelter above the wtaer level within the flood area. Vertical evacuation is often the spontaneousbehaviourof people who are surprised by the flood and are trying to reach a tree, a floor upstairs, a roof of a building etc. in order to get away from the rising water. But vertical evacuation could also be consideredas an alternative strategy to horizontal evacuation when moving outside the flood area is neither a faisible nor a relevant option, for example in high-population density areas. In order to be a credible alternative to horizontal evacuation, vertical evacuation has to be carefully planned. This paper aims to explain why horizontal evacuation is not always a suitable option in case of major flood and to explore under what conditions vertical evacuation can be a relevantalternative solution to horizontal evacuation. It also adresses some general recommendations about how to prepare a vertical evacuation strategy..