Impact of mean sea level rise on storm surges and tide-surge interaction in the German Bight

Liu, Xin; Gaslikova, Lidia; Weisse, Ralf

doi:10.5194/egusphere-2026-3871

Preprints

https://doi.org/10.5194/egusphere-2026-3871

Preprints

08 Jul 2026

| 08 Jul 2026

Impact of mean sea level rise on storm surges and tide-surge interaction in the German Bight

Xin Liu, Lidia Gaslikova, and Ralf Weisse

Abstract. Changes in mean sea level not only directly contribute to extreme water levels (EWLs) but also modulate tidal dynamics, storm surges, and their nonlinear interactions, particularly in shallow shelf seas. This study investigates how sea level rise (SLR) affects storm surges and tide-surge interaction (TSI) in the German Bight, with the aim of assessing the validity of linear approaches commonly used to estimate EWLs in coastal risk studies. A hydrodynamic model was used with a set of controlled experiments for the period 1981‒2010, explicitly separating tidal and atmospheric contributions and their interactions, additionally comparing present-day conditions with a +1 m SLR scenario. We demonstrate the importance of tidal phase in modulating the magnitude and timing of storm surges via TSI. The strength of this interaction increases with storm surge height, while its dependence on high tide magnitude and tidal range is comparatively weak. Under SLR, TSI weakens mainly because increased water depth reduces bottom friction, diminishing its dampening effect by about 25 %. Moreover, storm surge heights show a slight decrease, primarily due to phase shifts caused by faster wave propagation in deeper water and reduced wind stress efficiency acting upon a thicker water column. Overall, the nonlinear effects of SLR on storm surges and TSI are small, on the order of a few centimeters for 1 m SLR, relative to the direct contribution of SLR. This supports the use of linear superposition for many practical applications, while highlighting that nonlinear interactions introduce systematic, process-based adjustments relevant for precise EWL estimates.

Received: 30 Jun 2026 – Discussion started: 08 Jul 2026

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Xin Liu, Lidia Gaslikova, and Ralf Weisse

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-3871', Anonymous Referee #1, 19 Aug 2026
General comment
The present work addresses the impact of mean sea level rise on storm surges and tide-surge interaction in the German Bight. It relies on the use of a numerical hydrodynamic model to perform various experiments. German Bight as already been studied in terms of effect SLR on extreme water level. The main originality of the present manuscript is to investigate the sea-level rise (SLR) effect on surges and tide-surge interaction (TSI). The manuscript is well written, with clear figures, and clearly describes the experiments and results. However, before to be published, the manuscript required some complementary analysis to fully characterise the SLR effect on surge and TSI and the processes behind.

Major points to improve
Wave setup contribution is not taken into account in the present study. The paper should at least clarify that he neglects this term, and, if existing in the literature, provide some magnitude order of this contribution.

As the authors intend to study effect of TSI on EWL, then I think they should investigate skew surges (in addition to hourly surge or surge at high tide), as this is the skew surge that matters for EWL (rather than the surge at high tide).

Line 337-338, the authors suggest that stronger surges generate stronger currents and thus stronger nonlinear interactions. Perhaps, but this is not proved here. How looks like the tidal currents (of TIDE_0 runs) depending on the tidal phase? How looks like the current of the SURGE_0 runs? These elements are missing. Completing the present study with such complementary analysis would really reinforce the novelty of this work, as very few papers (even if based on shallow water modelling) show the patterns of the currents and discuss in a quantitative way the effect of the meteorological induced-current (and tidal current) on TSI.

Regarding the part on the effect of SLR on TSI (Line 365-370), I recommend extending it by analysing the various parts of the TSI changes (i.e. ITSI). Indeed, as it is written it suggest that these are just tide surge interaction changes, but this TSI changes result in fact from different processes (SLR effect on tide, SLR effect on surge, …). If I am not wrong we may write for instance ITSI = TSI_1 - TSI_0 = (WL_1 - TIDE_1 - SURGE_1) - (WL_0 - TIDE_0 - SURGE_0) = (TIDE_1 – TIDE_0) + (SURGE_1 – SURGE_0) + (WL_1 – WL_0) = (TIDE_1 – TIDE_0) + (SURGE_1 – SURGE_0) + (SLR_1 + RI), with RI the remaining interactions. Based on this equation the authors may investigate what drives the most TSI changes. This will probably change and deepen the process analysis on the TSI changes, compared to the present version of the manuscript. Such analysis would also make sense regarding the fact that the manuscript already investigate SLR effect on surges and tides.

“On-line” Remarks
Line 47: “SLR scales linearly with EWL” or “EWL scales linearly with SLR” ?

Line 117: why using FES2004 (22 years old) rather than the more recent products FES2014 or FES2022 than account for more tidal components and exhibit better skills (see e.g. Lyard et al., 2021)?

Line 118: remind the spatial resolution of the meteorological forcing dataset, and, if needed, discuss its suitability for the present study.

Line 136: specify if nodal correction has been activated or not (as it seem that U-tide is applied to 10 year time series)

Line 229: clarify if the 200 events are common to the three sites (i.e. same events).

Line 485-488: the authors may perform a tidal component analysis on there results to extract the effect of SLR on M2 amplitude and thus reinforce the discussion on the effect of SLR on tide and M2.

Line 541-550: not fully clear to me, with long sentences -> clarify / improve the readiness.

Line 557 to 561: this is a bit too general. The authors may provide examples of types of application for which linear addition may be sufficient, and for which it may not be sufficient.

Line 595-597: Clarify that the conclusion of the present paper on TSI is site specific, i.e. is valid for the German Bight study site.

References
Lyard et al., 2021 : https://doi.org/10.5194/os-17-615-2021
Citation: https://doi.org/10.5194/egusphere-2026-3871-RC1
RC2: 'Comment on egusphere-2026-3871', Anonymous Referee #2, 22 Sep 2026

General comments
This study systematically examines the extent to which non-linear processes are relevant to the estimation of extreme water levels in the German Bight at present-day mean sea level and in the case of a sea level rise of 1 m. The scenario of rising sea level, in particular, is a highly relevant issue. This article provides a comprehensive insight into how non-linear processes increase or decrease extreme water levels. The study design, which involves carrying out various well-designed simulations and systematically defining and analysing the various factors that contribute to extreme water levels, is well thought out.
Overall, the article is well written. However, I have some general comments for improvement. The manuscript could benefit from being structured more systematically. Furthermore, the method used and the definitions employed should be described in more detail in some places. A paragraph should be added to the manuscript discussing the limitations of the methods and any potential drawbacks.
The study is of scientific significance. I recommend publication after revision. A detailed list of comments follows below.

Specific comments
Structure of the manuscript:
I believe that here and there, the distinction between the ‘Methods’, ‘Results’, and ‘Discussion’ sections could be made clearer. Some methodological explanations in the ‘Results’ section could be moved to Section 2. I feel that the comparison of the results presented here with the literature, on the other hand, belongs in the ‘Discussion’ section. The ‘Discussion’ section, like the ‘Results’ section, contains results. Please consider moving the results presented (i.e. Fig. 14 and Fig. 15) to the ‘Results’ section.
Discuss limitations of the study:
In the ‘Discussion’ section, a paragraph on potential limitations of the results presented is missing. Please discuss the drawbacks of the model used. Its spatial resolution of 1.6 km is not fine enough to resolve local tidal inlets, channels, and intertidal flats. How would you rate the robustness of your results if you were to use a higher model resolution? Do you believe that your results can be generalised to locations other than the three analysed? Fig. 14b shows, the changes in values of WL changes due to SLR near the coast appear to be larger. Are these values reliable? Please discuss the possibility that the results might differ in estuaries, particularly in regions where water levels are influenced by river discharge.

Specific comments by text location:
L 31: It would be helpful for the reader, when the term ‘storm surge’ first appears, to briefly explain how this term is defined in the context of this article.
L 116: consider including reference to Figure 1a: “The model domain covers the North Sea and adjacent parts of the northeast Atlantic with 12.8 km spatial resolution (Figure 1a), with …”
L151: The discrepancies between the model and the observations could, in part, also be due to the use of hourly water levels.
L230-331: This sentence might easily slip the reader’s mind. Please note, in all figures where appropriate, that the data relate to Helgoland.
Figure 3a and L270-273: In Figure 3a, the NTR values are color-coded according to the tidal phase. However, the method used for this grouping is not explained until the introduction of Figure 4. Please ensure that all methods used are explained before referring to their results.
Line 290: Delete “for example”, as the results displayed relate to Helgoland and no other results are shown.
Line 291: How long is the averaging period for “rising tide” and “falling tide? I assume it refers to the period between high tide ±1 hour and low tide ±1 hour. Please clarify.
Line 365: Why “could”? Suggestion: “SLR effects on TSI can be quantified with our simulations.”
Line 396: Please check whether the reference to Fig. 3a is correct. Shouldn’t it actually be Fig. 3b?
Figure 10: Please, give a short explanation for the two larger red spots in Figure 10.
Section 3.4: This section needs to be revised or removed. As this section does not contribute to the main findings of the study in terms of the argument put forward it is a nice addition, but it is not strictly necessary.
Line448-449: Why were only the highest 200 storm surge events analysed? As tide-only simulations are analysed here, storm surge events are not relevant to the analyses.
Line 452-454: Comparing changes in high tides with the M2 amplitude is flawed. This would only be feasible on the assumption that the low tides remain unchanged in the simulation with SLR. As an alternative to directly comparing the M2 amplitude, it would also be possible to compare the tidal range with the M2 amplitude.
L483-488: The final sentence of this section notes that the metric of high tide and M2 amplitude are not equivalent. Yes, which is why it makes little sense to compare them. I recommend revising this section in the light of my comments to L452-454.
L515-L516: As far as I understand, the increase in water level beyond SLR due to the weakening of the negative TSI could be higher, were not for the countervailing effect of the reduction in SURGE caused by SLR (as described in L434-435). This should be mentioned here in order to provide a complete picture.
L521-531: This paragraph is important. However, it appears rather suddenly. Please reconsider its placement in the article. It definitely belongs in the ‘Discussion’ section. Maybe it seems a little out of place because the ‘Discussion’ section is very similar in style to the preceding ‘Results’ section (see also general comment on structure).
L533-534: Please provide a clear definition of ‘WL changes due to SLR’. Is it the difference: WL_SLR – WL – SLR?
Line 541-543: In view of the results presented above, it seems surprising at first glance that the difference between WL_SLR and WL+SLR is only 1%. However, this is the case because the value refers to the total water level and not to the SLR used. Please consider whether, in general, a table showing the key values might be helpful. It would, for example, clarify this point and bring all the values together in one place – effectively as a summary of the results.
L548: The term “non-surge conditions” suggests, that surge conditions are excluded from the analysis. However, this does not appear to be the case, as the caption for Figure 15 states that ‘all flood levels’ were analysed. Please clearify.

Technical corrections
Figure 1b: The Figure is too small. The text in particular is difficult to read. Labelling of the Y-axis: Which height reference system is used? Metres above mean sea level?
Table 2: The terms given in brackets at the end of each line refer to terms used in the equations defined in Section 2.4. It would be great, if the connection between the model simulations and these notations would be highlighted more. Maybe change order: “WL: Baseline total water level” instead “Baseline total water level (WL)”
Line 437: Check formulation “is a dominant control” Do you mean: “Overall, these results demonstrate that TSI dominates the timing and magnitude of surge-related water levels.”?
Line 541: Typing error: (Linear_0)

Citation: https://doi.org/10.5194/egusphere-2026-3871-RC2

Xin Liu, Lidia Gaslikova, and Ralf Weisse

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Short summary

Sea level rise does more than lift the ocean’s surface; it also changes how tides and storm surges behave. Our model simulations for the German Bight show that the nonlinear interaction of tide and surge contributes a lot to the uncertainties of water level simulation, while the nonlinear effects of sea level rise on the oceanic components are systematic but small. Even so, these subtle changes matter for precise estimation of extreme water levels and for improving coastal risk assessments.


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