A new modelling framework for regional assessment of extreme sea levels and associated coastal flooding along the German Baltic Sea coast
Abstract. Hydrodynamic models are increasingly being used in recent years to map coastal floodplains on local to continental scales. On regional scales, however, high computational costs and the need for high-resolution data limit their application. Additionally, model validation constitutes a major concern, as in-situ data are hardly available or limited in spatial coverage to small parts of the study region. Here we address these challenges by developing a modelling framework, which couples a hydrodynamic coastal inundation model covering the German Baltic Sea coast with a hydrodynamic coastal ocean model of the western Baltic Sea, to produce high resolution (50 m) regional scale flood maps for the entire German Baltic Sea coast. Using a LiDAR derived digital elevation model with 1 m horizontal resolution, we derive and validate the elevation of dikes and natural flood barriers such as dunes. Using this model setup, we simulate a storm surge event from January 2019, a surge with a return period of 200 years and two sea-level rise scenarios for the year 2100 (200-year event plus 1 m and 1.5 m). We validate the simulated flood extents by comparing them to inundation maps derived from Sentinel-1 SAR satellite imagery, acquired between 1.5 and 3.5 hours after the peak of the 2019 surge, covering a large part of the study region. Our results confirm that the German Baltic Sea coast is exposed to coastal flooding, with flood extent varying between 118 km2 and 1016 km2 for the 2019 storm surge and a 200-year return water level plus 1.5 m of sea-level rise, respectively. Hotspots of coastal flooding are mostly located in the federal state of Mecklenburg Western Pomerania. Our results emphasise the importance of current plans to update coastal protection schemes along the German Baltic Sea coast over the course of the 21st century in order to prevent large-scale damage in the future.
Joshua Kiesel et al.
Status: final response (author comments only)
RC1: 'Comment on nhess-2022-275', Anonymous Referee #1, 10 Mar 2023
- AC1: 'Reply on RC1', Joshua Kiesel, 10 May 2023
RC2: 'Comment on nhess-2022-275', Anonymous Referee #2, 31 Mar 2023
- AC2: 'Reply on RC2', Joshua Kiesel, 10 May 2023
RC3: 'Comment on nhess-2022-275', Anonymous Referee #3, 06 Apr 2023
- AC3: 'Reply on RC3', Joshua Kiesel, 10 May 2023
Joshua Kiesel et al.
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The paper by Kiesel et al. presents a modelling framework to assess the extreme sea levels and the extent of coastal flooding on the German Baltic Sea coast. The topic of the paper is highly relevant and is of great importance for the planning of the protection of the coastal regions. On the Baltic Sea coast, high sea level extremes are possible and will become more frequent with the global mean sea level rise. The paper is well written, the methods and results are clearly described. The quality of the presentation is high and the text is understandably written using good English.
I have only some minor comments for the authors to help them clarify some details.
Line 84 Explain what a hydrograph is, term might not be familiar to all readers
Line 167 Is the boundary condition the same for neighbouring boundary points? I understood from the text that the boundary point has as its boundary condition the hydrograph from the nearest boundary station. This could be explained in detail in the text.
Line 234 Has the land uplift due to postglacial rebound taken into account when SLR has been subtracted with a linear fit? You could mention the land uplift rate on the German Baltic coast and discuss whether it is relevant in this study.
Table 4 and Figure 5
It would be nice to have the station names of Table 4 in Figure 5 to be able to locate the stations in the map. The station could be given a number which is shown in Figure 5 to avoid too much text in the Figure. Figure 5 could also be larger, because it is one of the most interesting figures in the paper.
Line 357 You could discuss why the peak water levels are higher in SH than in MP in the 200-year event. Is it due to the shape of the coastline, does the bathymetry affect it?