the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Feb 2024
Exploring the Tidal Response to Bathymetry Evolution and Present-Day Sea Level Rise in a Channel-Shoal Environment
Abstract. Intertidal flats and salt marshes in channel-shoal environments are at severe risk from drowning under sea level rise (SLR) ultimately ceasing their function in coastal defense. Earlier studies indicated that these environments can be resilient against moderate SLR as their mean height is believed to correlate with tidal amplitude and mean sea level. Recent morphological analyses in the German Wadden Sea on the Northwestern European Shelf contradicted this assumption as mean tidal flat accretion surpassed relative SLR; indicating that nonlinear feedback between SLR, coastal morphodynamics, and tidal dynamics played a role. We explored this relationship in the German Wadden Sea’s channel-shoal environment by revisiting the sensitivity of tidal dynamics to observed SLR and coastal bathymetry evolution over one nodal cycle (1997 to 2015) with a numerical model. We found a proportional response of tidal high and low water to SLR when the bathymetry was kept constant. In contrast, coastal bathymetry evolution caused a spatially-varying hydrodynamic reaction with both increases and decreases of tidal characteristic patterns within few kilometers. An explorative assessment of potential mechanisms suggested that energy dissipation declined near the coast which we related to decreasing tidal prism and declining tidal energy import. Our study stresses the fact that an accurate representation of coastal morphology in hind- and nowcasts and ensembles for bathymetry evolution to assess the impact of SLR are needed when using numerical models.
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Status: closed
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RC1: 'Comment on egusphere-2024-283', Anonymous Referee #1, 03 Mar 2024
In this paper, the authors implement numerical modelling to investigate the effect on tidal characteristics from bathymetry evolution and sea level rise in the complex channel-shoal environment in the Wadden Sea and specifically in the German Bight. They attempt to disentangle the combined effect of these two parameters (bathymetry and sea level rise) by comparing results of scenarios that included 1) only bathymetric evolution, 2) only sea level rise and 3) both with a reference case and in a period of 18 years equal to a nodal cycle. Their work highlights the importance of coastal morphodynamics when modelling the SLR impact on intertidal flats.
The paper is well-written, well-arranged, and well-documented with clear objectives and conclusions. It has the merit to be published. However, there are a few obscure aspects which it would be good if they could be further elaborated or commented in the manuscript to strengthen better the validity of the methodology. These are summarized as follows:
- The basic assumption in lines 123-125 that SLR is the dominant driver because the external forcing parameters are constant neglects the non-linearities between them which, even though constant, might cause effects surpassing that of SLR and features that could be present even without the inclusion of SLR.
- The 10.6cm RMSE for high waters is higher than the prescribed SLR of 5.9cm. A comment is needed to justify that the extracted conclusions on the role of the SLR are not affected by this difference.
- It is not clear from the manuscript if the sea level rise is implemented as a uniform increase over the entire domain. If this is the case, then the model carries out the same calculations but over uniformly increased depths while the depth gradient which is directly involved in the flow calculations, remains unchanged. As a result, the HW and LW increase is on a similar level as it is depicted in Fig. 3(a) and 3(d) where minor differences can be observed between them. So, this means that the conclusion would be similar irrespective of the implemented SLR.
- The authors imply in line 163 that the 2015 bathymetry includes already changes due to SLR but then mention that these were negligible and included. Which of the two is right? Was this a design scenario decision to not include changes due to SLR or were these investigated and then found negligible?
Minor corrections
Line 38 It is either sea surface level or sea surface elevation and not height.
Line 83 Write “approximately” and not “approx.”
Line 103 Delete the second (a) in the line.
Line 126 It would be good to add the Helgoland sea level gauge location on Figure 1 map
Line 135 As above, sea surface elevation and not height.
Line 162 It is scenario (2) and not scenario (4)
Line 168 Better to use ‘note’ instead of ‘mark’
Line 200 I think you mean constant or amplified tidal low water instead of range.
Line 197 “on the contrary” instead of “contrary”
Line 199 “on” instead of “with”
Line 325, ‘by’ Jacob and Stanev (2021)
Line 384 a “high resolution model”
Figures
Figure 1 (a) occurs twice in the first sentence of the legend. Please delete one of the two. Please make thicker the depth contours or at least more visible and add the depth indication on the map.
Figure 2 ‘heights (m NHN)’ instead of ‘heights in mNHN’ on the horizontal axis. The distinction between the 1997 and 2015 pdf is not visible. Please use another colour.
Citation: https://doi.org/10.5194/egusphere-2024-283-RC1 - AC1: 'Reply on RC1', Robert Lepper, 19 Apr 2024
-
RC2: 'Comment on egusphere-2024-283', Anonymous Referee #2, 06 Apr 2024
The authors present a very well presented manuscript of an interesting local high-resolution study of how changes in bathymetry and SLR can influence local tides, and eventually sediment transport capacity. Overall, I think this is worthy of publications, if anything because the findings, whilst not novel from an oceanographic point, are very useful from a management perspective. I have some minor comments and would like an extended discussion on mechanisms before publication. Consequently, I recommend minor revisions.
“Major” commentI miss a mechanistic explanation for the results, at least within the results sections. There are some attempts in the discussion but this is mainly focussed on the evolution of the channels and sediment transport capacity. Section 4.3 points to some possibilities, but this is at the local scale. What drives the large scale changes in the tidal prism? Comments on this, or even a speculation about mechanisms is needed.
“Minor comments”L56-57: please use the same units for SLR and accretion – cm yr-1 would be preferred. Also, please refer to Figure 1 here.
L100: how do these errors propagate through the solutions when you change bathymetry? They are larger than your SLR signals later on. This must be commented on.
L168: Mark -> Note. Also on L356.
L221-223: Distinguished -> separated may be a better word.
L326: Is this a popular assumption? In my mind it is the opposite and SLR leads to weaker dissipation. I guess this depends very much on local conditions, so maybe this could be reworded and you can state that you see an decreased dissipation, as do several other papers (including global studies) whereas others show increases. This leads to an important conclusion: regional responses must be modelled explicitly, and one region is a poor representation for general signal in another region.
Citation: https://doi.org/10.5194/egusphere-2024-283-RC2 - AC2: 'Reply on RC2', Robert Lepper, 19 Apr 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-283', Anonymous Referee #1, 03 Mar 2024
In this paper, the authors implement numerical modelling to investigate the effect on tidal characteristics from bathymetry evolution and sea level rise in the complex channel-shoal environment in the Wadden Sea and specifically in the German Bight. They attempt to disentangle the combined effect of these two parameters (bathymetry and sea level rise) by comparing results of scenarios that included 1) only bathymetric evolution, 2) only sea level rise and 3) both with a reference case and in a period of 18 years equal to a nodal cycle. Their work highlights the importance of coastal morphodynamics when modelling the SLR impact on intertidal flats.
The paper is well-written, well-arranged, and well-documented with clear objectives and conclusions. It has the merit to be published. However, there are a few obscure aspects which it would be good if they could be further elaborated or commented in the manuscript to strengthen better the validity of the methodology. These are summarized as follows:
- The basic assumption in lines 123-125 that SLR is the dominant driver because the external forcing parameters are constant neglects the non-linearities between them which, even though constant, might cause effects surpassing that of SLR and features that could be present even without the inclusion of SLR.
- The 10.6cm RMSE for high waters is higher than the prescribed SLR of 5.9cm. A comment is needed to justify that the extracted conclusions on the role of the SLR are not affected by this difference.
- It is not clear from the manuscript if the sea level rise is implemented as a uniform increase over the entire domain. If this is the case, then the model carries out the same calculations but over uniformly increased depths while the depth gradient which is directly involved in the flow calculations, remains unchanged. As a result, the HW and LW increase is on a similar level as it is depicted in Fig. 3(a) and 3(d) where minor differences can be observed between them. So, this means that the conclusion would be similar irrespective of the implemented SLR.
- The authors imply in line 163 that the 2015 bathymetry includes already changes due to SLR but then mention that these were negligible and included. Which of the two is right? Was this a design scenario decision to not include changes due to SLR or were these investigated and then found negligible?
Minor corrections
Line 38 It is either sea surface level or sea surface elevation and not height.
Line 83 Write “approximately” and not “approx.”
Line 103 Delete the second (a) in the line.
Line 126 It would be good to add the Helgoland sea level gauge location on Figure 1 map
Line 135 As above, sea surface elevation and not height.
Line 162 It is scenario (2) and not scenario (4)
Line 168 Better to use ‘note’ instead of ‘mark’
Line 200 I think you mean constant or amplified tidal low water instead of range.
Line 197 “on the contrary” instead of “contrary”
Line 199 “on” instead of “with”
Line 325, ‘by’ Jacob and Stanev (2021)
Line 384 a “high resolution model”
Figures
Figure 1 (a) occurs twice in the first sentence of the legend. Please delete one of the two. Please make thicker the depth contours or at least more visible and add the depth indication on the map.
Figure 2 ‘heights (m NHN)’ instead of ‘heights in mNHN’ on the horizontal axis. The distinction between the 1997 and 2015 pdf is not visible. Please use another colour.
Citation: https://doi.org/10.5194/egusphere-2024-283-RC1 - AC1: 'Reply on RC1', Robert Lepper, 19 Apr 2024
-
RC2: 'Comment on egusphere-2024-283', Anonymous Referee #2, 06 Apr 2024
The authors present a very well presented manuscript of an interesting local high-resolution study of how changes in bathymetry and SLR can influence local tides, and eventually sediment transport capacity. Overall, I think this is worthy of publications, if anything because the findings, whilst not novel from an oceanographic point, are very useful from a management perspective. I have some minor comments and would like an extended discussion on mechanisms before publication. Consequently, I recommend minor revisions.
“Major” commentI miss a mechanistic explanation for the results, at least within the results sections. There are some attempts in the discussion but this is mainly focussed on the evolution of the channels and sediment transport capacity. Section 4.3 points to some possibilities, but this is at the local scale. What drives the large scale changes in the tidal prism? Comments on this, or even a speculation about mechanisms is needed.
“Minor comments”L56-57: please use the same units for SLR and accretion – cm yr-1 would be preferred. Also, please refer to Figure 1 here.
L100: how do these errors propagate through the solutions when you change bathymetry? They are larger than your SLR signals later on. This must be commented on.
L168: Mark -> Note. Also on L356.
L221-223: Distinguished -> separated may be a better word.
L326: Is this a popular assumption? In my mind it is the opposite and SLR leads to weaker dissipation. I guess this depends very much on local conditions, so maybe this could be reworded and you can state that you see an decreased dissipation, as do several other papers (including global studies) whereas others show increases. This leads to an important conclusion: regional responses must be modelled explicitly, and one region is a poor representation for general signal in another region.
Citation: https://doi.org/10.5194/egusphere-2024-283-RC2 - AC2: 'Reply on RC2', Robert Lepper, 19 Apr 2024
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