the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Effects of Sea Level Rise and Tidal Flat Growth on Tidal Dynamics and Geometry of the Elbe Estuary
Rita Seiffert
Jessica Kelln
Peter Fröhle
Abstract. Future global mean sea level rise (SLR), will affect coastlines and estuaries in the North Sea and therefore coastal protection as well as local unique ecosystems and important waterways. SLR will not only raise water levels, but will also influence tidal- and morphodynamics, which is why the tidal flats of the Wadden Sea can potentially grow to a certain extent with SLR. Investigations on the effects of climate change induced SLR and corresponding potential bathymetric changes inside of estuaries form an important basis for the identification of vulnerabilities and the development of appropriate adaptation strategies. With the help of a highly resolved hydrodynamic-numerical model of the German Bight, we analyse the influence of potential SLR and tidal flat elevation scenarios on tidal dynamics in the Elbe estuary. The results show an increase of tidal range in the Elbe estuary due to SLR and further reveal, that tidal flat growth can have no effect, decrease or increase the tidal range relative to sole SLR, depending on the location and amount of tidal flat elevation. Further analyses demonstrate, how geometric parameters of the Elbe estuary are changing due to SLR and tidal flat elevation. We discuss how these changes in estuarine geometry can be an explanation for the changes in tidal range.
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Tara Mahavadi et al.
Status: open (until 01 Oct 2023)
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RC1: 'Comment on egusphere-2023-1288', Anonymous Referee #1, 22 Aug 2023
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In the presented study, the authors applied a hydronumerical model of the Elbe Estuary in order to investigate future regional changes in tidal range in response to a combination of SLR and morphological changes in intertidal areas. For this purpose, two scenarios of SLR and different scenarios for the growth of intertidal flats were used. As shown by the authors projections, future tidal range in the Elbe Estuary will strongly depend on where and by which amount the intertidal flats will grow. As also highlighted, the observed evolution of tidal range is governed by changes in geometrical features of the estuary (convergence length, hydraulic depth, and relative intertidal area).
Generally, the manuscript is written in a comprehensible manner and the results are well-visualized. My main issue with the manuscript is that the impact of the study should be highlighted in more detail. What is the main motivation of the study? Why exactly are the results relevant/impactful? This aspect is only very briefly addressed in the current version of the manuscript. A detailed list of my comments is listed below:
- There are a couple of issues with the language of the manuscript, e.g., the use (or lack thereof) of definite/indefinite articles, wrong prepositions, wrong punctuation etc. In order to improve the quality of the language, please give the manuscript to a native speaker or send it to a language editing service.
- LL13-15: “The results show an increase of tidal range in the Elbe estuary due to SLR and further reveal, that tidal flat growth can have no effect, decrease or increase the tidal range relative to sole SLR.”
The way this is written, this almost sounds like a non-result. Please describe in more detail in the abstract, which of your investigated scenarios leads to which results. - LL30-32: “However, facing the future acceleration of SLR, is difficult to quantify the amount to which tidal flat growth can keep pace with sea level rise, and it remains questionable, whether present hydromorphodynamic equilibrium will be maintained in the future.”
Please give a reference for the future acceleration of SLR. - LL48-49: “The Elbe estuary is the part of the Elbe river extending from the weir in Geesthacht to the North Sea (Figure 5).”
Normally, the figure that is referenced first in your manuscript should be Figure 1. In my opinion, a map of the study area also makes sense as Figure 1. - LL56-57: “Nowadays the Elbe estuary is an amplified estuary, where the tidal amplitude increases in upstream direction and reaches its maximum close to the port of Hamburg.”
Is “amplified estuary” a commonly used term? At least I haven’t stumbled upon this before. What you describe sounds like a “hypersynchronous estuary” to me (see Nichols and Biggs, 1985). - LL60-61: “The future of the Elbe estuary depends not only on anthropogenic measures implemented on site, but also in particular on sea level rise and its implications.”
Previously, you have already used the abbreviation SLR for sea level rise. Please use the abbreviation after defining it. - LL64-66: “Understanding the future evolution of tidal dynamics due to sea level rise in heavily utilised estuaries such as the Elbe estuary is important for the development of adaptation measures, e.g. in navigation, port infrastructure and water management.”
As far as I understand, this is the main motivation of your study. Accordingly, you should dedicate more than one sentence to this. Please explain in more detail, what impacts might be expected in estuaries in response to SLR. And what processes might be triggered by an amplification of tidal range (e.g., see Winterwerp and Zhang, 2013)? And please also describe in more detail, what this will mean for the future management of the estuary. It might also be useful to briefly address the recent deepening of the Elbe in order to showcase the perspective of different stakeholders on the estuary and how they might be impacted by further developments in the future. - LL136-137: “Sea level rise is added at the open boundary of the German Bight Model.”
Why haven’t you added the sea level rise at the boundary of the Dutch continental shelf model? Isn’t the boundary of your German Bight model in areas, where tidal constituents will already be significantly impacted by SLR? Please discuss the effect of your assumption/simplification. - Figure 1.
Please use a different colourbar for the left panel. When showing the European Continental Shelf with water depths of several thousand meters, it doesn’t make sense to limit your colourbar to 37.5 m. Furthermore, when using different panels, they should be labelled by using (a), (b), (c), etc. This also applies to most of your other figures. - Figure 2.
Why are only three days shown here? Why isn’t a whole spring-neap cycle shown? Is the performance of the model better/worse during different phases of the spring-neap cycle? Even though you mention that the validation of the model is presented in another paper, it also wouldn't hurt to briefly describe the model quality here in terms of selected parameters (e.g., what is the mean RMSE across all tide gauges in the model domain). - Figure 3.
Apparently, the dashed-lines indicate certain cross-sections along the estuary that are shown in Figure 5. This should be explained. For readers not being too familiar with the Elbe Estuary, it could also help to use labels that highlight the location of the different sections along the river (e.g., “outer section”, “mouth section”, etc.). - LL320-323: “The scenarios with SLR of 55 cm are not visualised and analysed in detail.”
Even if the scenarios with a SLR are not visualized in detail, they should nevertheless be described/discussed in a little more detail. Otherwise, one might ask the question, why you mention the scenarios with a SLR of 55 cm at all? - Figure 8.
I first had to take a closer look at Figure 5 to understand why there are so few markers in the "Hamburg section". In my opinion, it would help to explain that the whole city of Hamburg is considered as one control volume (thus being relatively large in comparison to neighbouring control volumes). - L367: “To access the rate at which cross-sectional-flow-area of an estuary decreases in upstream direction, the geometric parameter convergence length (La) is calculated by fitting an exponential function (Eq. (2)) to the data sets (see 2.3.3).”
Do you really mean “to access” or should it be “to assess”? - Table 3.
I don't quite understand, why you compare scenario “slr110t0” to your reference scenario and all other scenarios to “slr110t0”? Why not comparing all scenarios to the reference scenario? Otherwise one might ask, why you don't show all the possible scenario combinations (e.g., “slr110t110e” to “slr110t55e”, etc.)? Due to the presented changes in A0 and La, it should still be possible to see that the differences between certain scenarios are negligible (e.g., “slr110t110e” to “slr110t0”). - Figures 9 & 10.
It is not described, what the different lines represent. I presume it is the mean of all control volumes in a section, right? - Figures 11 & 12.
You use different types of lines and colours in these figures, which are not explained. In Figure 12, it also isn’t mentioned, what SINT’ and MW’ stand for.
References
Nichols, M. M. and Biggs, R. B. (1985). Estuaries. In Coastal Sedimentary Environments. Second Revised, Expanded Edition, ed. R. A. Davis Jr. (New York: Springer). 77–186, doi:10.1007/978-1-4612-5078-4_2
Winterwerp, J. C. and Wang, Z. B. (2013). Man-induced regime shifts in small estuaries—I: theory. Ocean Dynamics 63, 1279–1292, doi:10.1007/s10236-013-0662-9
Citation: https://doi.org/10.5194/egusphere-2023-1288-RC1
Tara Mahavadi et al.
Tara Mahavadi et al.
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