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Abstract:
The disastrous consequences of the July 2021 flood in Western Europe have again demonstrated that current flood risk management is too strongly focused on design events. For instance, the 100-year flood is often used as the target safety level for flood defense, and events beyond such design scenarios are neglected. This disregard of ‘High-Impact / Low-Probability’ (HILP) events would not be advisable in a stationary system, but is even more inappropriate given the widespread climatic, environmental and socio-economic changes. We discuss methods to develop HILP flood scenarios, such as downward counterfactuals and perfect storms. Taking the Ahr catchment as an example, which experienced massive destruction and more than 130 fatalities during the 2021 flood, we demonstrate how a flood risk model chain can be used to develop HILP scenarios. The model chain, consisting of hydrological, hydrodynamic and damage models, is driven by a stochastic weather generator, which generates a very long time series of synthetic weather. In combination with modifying processes along the model chain, for instance, assuming failure of early warning, this setup allows (1) understanding how HILP events could evolve, and (2) generating a large range of flood scenarios beyond design events.
