A Comprehensive Probabilistic Flood Assessment Accounting for Hydrograph Variability of ESL Events

Flood characteristics caused by extreme sea level (ESL) events depend largely on the magnitude of peak water levels (WLs) and their temporal evolution. However, coastal flood risk is generally assessed based on only a limited number of potential peak WLs and a selection of past events or a design hydrograph. We address this gap and systematically estimate (a) spatial annual and (b) event-based flood probabilities by comprehensively accounting for both a wide range of peak ESLs and their temporal evolution, herein referred to as hydrograph intensity. We simulate flooding at the German Baltic Sea coast with the hydrodynamic model Delft3D. We produce probabilistic flood maps, which detail flood exposed areas together with annual probability of flooding. Additionally, we show how the flood extent changes, when accounting for upper, median, and lower quantiles of hydrograph intensities. Our results demonstrate that the relevance of the intensity is site and ESL dependent. While flood extents of some ESLs of the upper and lower intensity bounds indicate no differences, others differ by up to 45%. Further, we consider two ESLs (2.24 and 2.55 m) and simulate 100 intensities for each. Compared to intensity quantiles, this results in flood extents of up to 60% difference. Hence, we find that quantiles of intensity do not cover the full range when addressing uncertainty due to hydrograph variability. We, therefore, recommend accounting for a wide range of hydrograph intensities in addition to using a wide range of ESL in future flood risk assessments. Implementing and maintaining coastal protection to mitigate damages arising from flooding is a costly endeavor. With many possible solutions, it is difficult to identify what measures offer the best outcomes and where to deploy them. To inform coastal managers and to promote efficient planning, risk-based assessments of flooding are typically prescribed. Coastal flood risk has generally been assessed based on the impacts of a few past storm surge events. However, this is not sufficient for capturing the full picture of possible flooding, as storms can vary in temporal evolution and duration. In this study, we assess flooding at the German Baltic Sea coast under a very large range of potential extreme sea levels (ESLs) varying in magnitude and duration. We provide maps which detail flood extents and their associated probabilities. In addition to highlighting areas exposed to flooding, our results show that the duration of a storm surge event can significantly affect flood extents in certain areas, depending on the magnitude of the event. To better understand and prepare for coastal flooding, we recommend that a wide range of ESL magnitudes and storm surges in terms of duration and evolution should be considered in future flood risk assessments. Probabilistic flood maps from extreme sea levels (ESLs) and hydrograph variability for a local case study provide relevant information for coastal plannersSensitivity of flood extent due to hydrograph variability depends on considered ESL height and topographyHydrograph intensity quantiles do not cover the full picture of flood extent sensitivity to storm surge variability