PhD thesis: Towards global-scale compound flood risk modeling

Published: 21 November 2022

Coastal deltas are prone to flooding due to their low elevation and dense population. Flooding in these areas can occur from several causes, including pluvial, fluvial, and coastal drivers. However, floods can also occur or be exacerbated from interactions between these drivers, which by itself do not have to be extreme. If these drivers are statistically dependent, their joint likelihood might be misrepresented if dependence is not accounted for. Dirk Eilander's (flood expert Deltares) doctoral research tomorrow morning at VU in Amsterdam, on coexistence in deltas, helps to better understand flood risks on a global scale.

At the global scale flood risk models still analyze each flood driver in isolation. At the local scale, state-of-the-art hydrodynamic models have been developed for compound flooding, but these are not globally applicable. The overall aim of the thesis is therefore, to improve our understanding and modeling capabilities of compound flood risk anywhere globally. Specifically, the objectives are: 1) to identify deltas globally that are prone to compound flooding; and 2) to develop and apply a globally-applicable computational framework for compound flood risk modeling.

To improve our understanding of compound flooding at the global scale, Dirk coupled a global riverine flood model with a global tide and surge model to obtain continuously simulated water levels in 3,433 deltas globally. In 19.7% of the deltas, compound flooding is the dominant driver of extreme water levels. These deltas generally have larger surge extremes and are located in basins with faster discharge response to rainfall, and/or flat topography. If compound surge-discharge events are ignored, flood depths are significantly underestimated for 30.7 million out of a total of 332.0 million (9.3%) of the annual expected population exposed to flooding in deltas.

Dirk developed a globally-applicable framework for compound flood hazard modeling. The framework is validated against two historical events in the Sofala province of Mozambique based on satellite-derived flood extents. Compare to a global flood model, the globally-applicable model generally has better skill. Furthermore, it results in more realistic flood depth maps due to better floodplain connectivity and provides a more holistic representation of flood processes as it includes direct coastal and pluvial flooding. However, the resolution of the globally-applicable model is not yet sufficient to capture fine grained channels, which play an important role in the drainage of precipitation, thereby, overestimating pluvial flooding.

Finally, Dirk extended the framework with an event-based approach for compound flood risk assessments from pluvial, fluvial, and coastal flood drivers and applied it to the same case-study. Here, rare events cause more impact if accounting for observed dependence between flood drivers compared to independence, e.g. 12% at the 100-year return period. The overall difference in risk of 0.55% in expected annual damage is however small as the physical interaction of drivers for frequent events is small.

Dirk’s thesis contributes to ongoing efforts to better understand and mitigate flood risks at the global scale. The coupled global models can be used to identify deltas prone to compound flooding, whereas the globally-applicable model framework can be used for comprehensive flood risk assessments in deltas.

PhD thesis live to follow

Dirk Eilander will defend his PhD thesis on Tuesday 22 November at 9:45 am in the auditorium of VU University Amsterdam.

Supervisor: Prof. Dr. P.J. Ward: Professor of Global Water Risk Dynamics at VU University Amsterdam.

Co-supervisor: H. Winsemius: Senior researcher/advisor hydrology at Deltares.

The thesis is also availabe online

Language: in English