EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Timing of Decreasing Coastal Flood Protection Due to Sea-Level Rise

Tim Hermans1,2,3, Victor Malagón-Santos1, Caroline Katsman4, Robert Jane5, Dj Rasmussen6, Marjolijn Haasnoot7,8, Gregory Garner9, Robert Kopp10,11, Michael Oppenheimer6,12,13, and Aimée Slangen1
Tim Hermans et al.
  • 1NIOZ Royal Netherlands Institute for Sea Research, Department of Estuarine & Delta Systems, PO Box 140, 4400 AC Yerseke, The Netherlands
  • 2Department of Geosciences and Remote Sensing, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands
  • 3Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands (
  • 4Department of Hydraulic Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands
  • 5Department of Civil, Construction and Environmental Engineering, University of Central Florida, Orlando, FL, USA
  • 6Princeton School of Public & International Affairs, Princeton University, Princeton, NJ, USA
  • 7Deltares, Delft, Boussinesqweg 1, 2629 HV Delft, the Netherlands
  • 8Utrecht University, Department of Geosciences, Princetonlaan 8a, Utrecht, the Netherlands
  • 9GRO Intelligence, 505 Park Ave 12th Floor, New York, NY 10022, USA
  • 10Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA
  • 11Rutgers Institute of Earth, Ocean, and Atmospheric Sciences, Rutgers University, New Brunswick, NJ, US
  • 12Department of Geosciences, Princeton University, Princeton, NJ, USA
  • 13High Meadows Institute, Princeton University, Princeton, NJ, USA

Sea-level rise (SLR) amplifies the frequency of extreme sea levels as it raises their baseline height. Projections of the frequency amplification of extremes are often computed for arbitrary future years and relative to the historical centennial event, which is not necessarily meaningful locally. Consequently, such projections may not provide salient information to adaptation planners, as they do not indicate when certain flood risk thresholds will be crossed given the current degree of local coastal flood protection.

To better support adaptation planning, we introduce a framework that extends the emerging timing perspective on sea-level rise to the frequency amplification of extreme sea levels. Moreover, by relating amplification factors to local flood protection standards estimated with the FLOPROS modelling approach, we project the timing of decreases in the local degree of protection. The sea-level rise required for such decreases is derived from extreme sea-level distributions inferred from GESLA3 observations and combined with the relative sea-level projections of the Sixth Assessment Report of the IPCC until 2150 to compute the timing of these decreases at tide gauges globally.

Our central estimates indicate that the estimated degrees of protection will be exceeded 10 times as frequently within the next 30 years (the lead time that large adaptation measures may take) at 26 & 32% of the tide gauges considered, and annually at 4 & 8%, for respectively a low & high emissions scenario (SSP1-2.6 & SSP3-7.0). Even though our results are based on estimated degrees of protection, they highlight that at several locations substantial decreases in the degree of protection may occur before large adaptation measures can be completed. Furthermore, we find that under SSP3-7.0, the same decreases in the degree of coastal protection will occur substantially faster in the future as sea-level rise accelerates. Our projection framework adds a new perspective on the frequency amplifications of extremes that may help adaptation planners to assess the available lead time and useful lifetime of protective infrastructure, given unacceptable decreases in the degree of coastal protection.

How to cite: Hermans, T., Malagón-Santos, V., Katsman, C., Jane, R., Rasmussen, D., Haasnoot, M., Garner, G., Kopp, R., Oppenheimer, M., and Slangen, A.: The Timing of Decreasing Coastal Flood Protection Due to Sea-Level Rise, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4834,, 2023.