Simplified Marsh Response Model (SMRM): A Methodological Approach to Quantify the Evolution of Salt Marshes in a Sea-Level Rise Context

Round 1

Reviewer 1 Report

In this work, the author use a simplified model (SMM) to assess salt marsh changes due to sea level rise. The marsh area is represented with a DTM computed from LIDAR observations. The tidal field modulates the exposition of marshes to waters and tide gauge measurements are used. Sea-level rise projection and acceleration rates are also used as input to assess the long-term impact. Two case-studies in the Portugal coastal region are discussed in the manuscript.

I am not expert in modelling so I am not the right person to comment this part. My review only focuses on the role of the sea level. The net sea level changes are the combination of both sea level and land changes. It seems the author only refer to the sea level changes mainly due to climatic effects. DTM does not provide a picture of vertical land motion over time. This means that results are not considering effects such as subsidence that in some places have to be taken into account, e.g. using GPS measurements over time. Some tide gauge installations have also co-located GPS to de-couple the two signals.

The analyses is also not considering the wave and wind climate that can contribute to SLR too (e.g., see Melet, A., Meyssignac, B., Almar, R. & Le Cozannet, G. Nat. Clim. Change 8, 234–239 (2018)).

In summary, the work is certainly of interest as there is a need to better understand the dynamics at land-sea interface, however, the phenomena contributing to changes at this interface have to be contextualized taking into account their temporal and spatial scales. In my opinion, the authors have to better describe the regions of investigation in terms of sea level variability, contributors to this variability and quantify the local sea level and land changes at long term scales that are key in the usage of their models.

Author Response

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Reviewer 2 Report

This is an interesting paper dealing on the evolution of saltmarsh, an important ecosystem, playing important functions in particular for biodiversity, coastal protection and climate change.

The introduction is well written and clearly presents the scope of the study. Regarding the description of the test sites, I suggest the authors to add the hydrodynamical characteristics of the pilot site tests (wind, wave, tide…)

In the methodology section, could you please explain why acceleration values of accretion rates must be introduced. Also it is not clear how the different environments (high marsh, low marsh...) were defined. Only by comparing the elevation characterizing each DTM cell with the elevation thresholds? Did you also perform some checks with in situ observations or ortophotos? Did you define young/mature salt marsh?

Could you also explain how the impacts of the vegetation on the accretion/sedimentation rate were considered?

From my point of view, using lidar data form 2011, and comparing them with 2016-2021 GPS measurements is hazardous, considering that the horizontal resolution of the LiDAR varies between 1 and 2 m and its vertical precision varies from 8 to 24 cm, therefore the authors should further justify their choice, indicating for instance if some severe events occur between 2011 and 2021? I also suggest the authors to mention first that the GPS measurements and the Lidar have been realized in different periods (l. 252).

Please rewrite section 5, which in its present form is not clear.

Could you please add information regarding the sensitivity analysis in the methodology section, and, as a consequence section 6 should also be rewritten.

In its present form, the discussion is limited to the test sites and should be improved (local). The authors should further discuss their findings considering other relevant studies. What are the “consequences/impacts” of your results in term of salt marsh evolution? What does your study reveal?

The first two paragraphs have already been mentioned before, so you may cancel these paragraphs or better highlight the added-value of a simpler model to assess the evolution of salt marsh. Further discussion on the use and importance of models could be added.

The authors could further discuss the importance of the SLR by comparing their finding with other studies related to salt marsh evolution. You may also discuss the other parameters comparing the results with other studies. For instance, regarding the importance of the topographic setting, the authors only mention “When the RMSE of the DTM is considered, it is possible to identify a proportional change in the evolution of the marsh.” And? Similarly, the importance of the accumulation rates may be further discussed. Did your results agree with the other studies?

Did you also observe some differences between young and mature salt marsh?

Since you mention the importance of the salt marsh for the blue carbon, what are the impacts of the salt marsh evolution in term of blue carbon?

1. 535-537 not clear, rewrite

Consequently the conclusion should also be improved.

Author Response

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Reviewer 3 Report

This work is very well presented and deals with an important topic related to the climate changes. In the manuscript it is reported the impact of future sea-level rise in the marsh areas of Portugal, applying a new model to predict the predicted submerged surfaces. The presented Simplified Marsh Response Model (SMRM) considers four input data: a high-resolution DTM, tide phases, projection of Sea-Level Rise (SLR), and accretion rate. All input data have been very well presented and discussed and the results have been compared also with other models (Sea-Level Affecting Marsh Model SLAMM). There are only two points that I think could be discussed:

1) - I strongly suggest discussing also the contribution of the Vertical Land Movement (VLM) for the future SLR in the Section 7; on the other hand, the VLM is a critical factor for the assessment of Relative Sea-Level Rise and can be assessed through different remote sensing techniques (see suggestions in the pdf file in attachment).

2) - Furthermore, it is important to show the difference of the sea-level projection of IPCC 2014 with the recent projections of IPCC SROCC 2019 and IPCC 2021.

Other few comments were inserted in the attached pdf file.

Kind regards.

Comments for author File: Comments.pdf

Author Response

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Reviewer 4 Report

This paper presents a reduced complexity model, SMRM, to quantify salt marshes' evolution in a SLR context through the combination of field and remote sensing data. This is an interesting paper with useful results of practical importance. However, in order to be published in the journal, major revisions are required, especially more detailed descriptions for the model as listed in the minor items below.

[Major items]
1.    The description of remote sensing application is rather limited in this paper. The reviewer suggests the authors to emphasize more applications of remote sensing technology, so that the present paper satisfies the scope of the journal.
2.    The reviewer strongly suggests the authors to put the results for C. Tróia in the main part, instead of placing it as an Appendix, to make more comprehensive comparative study between these study areas

[Minor items]
1.    P. 6 “Local tidal levels”: Astronomic tide from harmonic analysis is not sufficient for your analysis. Describe more how to consider meteorological tide in the study area, which may cause more sediment intrusion into the wetland area, resulting in marsh modification. 
2.    P.4 Figure 1: These two study areas located very close each other. What is the reason why there can be difference in the future projection of marsh response in these two areas?
3.    P.4 “two-dimensional rule-based model”: What kind of 2-D model is it? Vertically or horizontally? Make more detailed explanation for this assumption, hopefully with an additional schematical explanation in the paper. In addition, elaborate more why you can use the assumption of 2-D phenomena, although the phenomena must be basically three-dimensional in general.
4.    P.5 Figure 2 “Sedimentation rates”: There are so many sediment transport rate formulas proposed until now. More detailed explanation is required how to estimate the sedimentation rate in your model.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have worked to revise the manuscript by taking into account my comments. The authors replied promptly to my questions (with details) and made the related changes to the manuscript where possible.

I feel the paper is now much improved than the original one submitted.

Author Response

I would like to thank you for the revisions. I think your revisions contribute to improve the quality of the document. We also did some improvements in the English.

Best regards,

Miguel Inácio,

On behalf of the co-authors

Reviewer 4 Report

I found that the paper has been satisfactory revised in response to the reviewer's comments. Hence the paper can be now published in the present form. 

Author Response

I would like to thank you for the revisions. I think your revisions contribute to improve the quality of the document. We also did some improvements in the English.

Best regards,

Miguel Inácio,

On behalf of the co-authors