Multivariate design storms reveal misrepresentation of flood hazard by traditional approaches

Abstract

Design storms are widely used in hydraulic engineering to represent extreme precipitation and assess flood risk. Commonly used block-method design storms, such as Euler Type II, assign a single return period across all temporal scales of an event, neglecting dependencies in rainfall intensity across durations. This simplification may lead to unrealistic event representations. Here, we compare flood hazard from a traditional Euler Type II storm with storms generated using the multivariate REALISTIC framework, which accounts for joint return periods across durations. Using Zurich as a case study, we simulate 3-hour 100-year events and evaluate the resulting flood hazard with a hydrodynamic model. Results show that flood hazard is highly sensitive to the underlying intensity-duration profile. The block-method storm produces substantially higher flood depths and intensities than events whose profiles are consistent with observed joint return periods, which often result in limited flooding. These findings suggest that traditional design storms provide an incomplete estimation of flood hazard and highlight the need for multivariate approaches to support more robust risk assessments.