What Controls the Flushing Efficiency and Particle Transport Pathways in a Tropical Estuary? Cochin Estuary, Southwest Coast of India

Round 1

Reviewer 1 Report

  See overview comments below. Detailed comments are provided on the appended marked up manuscript   * Paper is often hard to follow, with non standard terminology and some unusual sentence structure.
* FVCOM was used in the analysis but with a focus on 2 D flows. The model however was applied in 3D, as I understand it, but no results are presented for the vertical structure of the flow. No idea how important this is in the analysis. Also no sense of the importance of density induced flows (salinity structure in the vertical).
* The model application periods pre, during, and post monsoon are not clearly defined.
* Comparison of model predictions were made to observations but the reader is given no sense as to the structure of flows and salinity. All the comparisons are in terms of the correlation coefficients and agreement index. The reader is never given any sense of the underlying model predictions of water level, currents or salinity. Nothing on the vertical structure.
* The authors apply a particle transport model as a proxy for contaminant transport but the reader is given no sense how it works.  2 or 3 D? vertical-horizontal dispersion included or not? Not clear where particles that exit the model domain go.  Assume they disappear.
* A key problem with the paper is that there is no attempt to validate the particle transport model?  Significant application of it but no real validation against observations or anything else. Calls into question the results and how useful they are.
* Paper never answers the question posed in the title of the paper. Overview Comments: * Paper is often hard to follow, with non standard terminology and some unusual sentence structure.
* FVCOM was used in the analysis but with a focus on 2 D flows. The model however was applied in 3D, as I understand it, but no results are presented for the vertical structure of the flow. No idea how important this is in the analysis. Also no sense of the importance of density induced flows (salinity structure in the vertical).
* The model application periods pre, during, and post monsoon are not clearly defined.
* Comparison of model predictions were made to observations but the reader is given no sense as to the structure of flows and salinity. All the comparisons are in terms of the correlation coefficients and agreement index. The reader is never given any sense of the underlying model predictions of water level, currents or salinity. Nothing on the vertical structure.
* The authors apply a particle transport model as a proxy for contaminant transport but the reader is given no sense how it works.  2 or 3 D? vertical-horizontal dispersion included or not? Not clear where particles that exit the model domain go.  Assume they disappear.
* A key problem with the paper is that there is no attempt to validate the particle transport model?  Significant application of it but no real validation against observations or anything else. Calls into question the results and how useful they are.
* Paper never answers the question posed in the title of the paper.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In the manuscript "What controls the flushing efficiency and particle 2 transport pathways in a tropical estuary? Cochin 3 estuary, southwest coast of India" the authors use a numerical model to study the residence times and transport dynamics in a tropical estuary.

In my opinion the manuscript is of interest to the journal. However it needs some improvements prior to publication.

My main concerns are the following:

-  the description of the results regarding the model validation needs to be improved. The model is the base for the study, and the presented results for the model validation are not enough to allow a good assessment of the model. The authors just present the summarized results in terms of index/skill. However, it is important to have more model data-comparison (e.g. time series), which would allow a better understanding of the system and to assess the model behavior in more detail. In order to keep the manuscript with a reasonable length, part of this results could be presented as Supplementary Material;

- the English language, in particular the sentence construction, which makes the manuscript difficult to follow.

I am also attaching a commented version of the manuscript.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

My overview comments are as follows:

1. paper is still hard to read, combination of writing style and English.
2. Core objective of the paper as expressed in the title has not been met.
3. Still have no idea how the Lagrangian transport model handles dispersive transport. Appears as if it is ignored
4. Appears Lagrangian model simply assumes any particle that finds itself at one of the two inlet mouths of the system simply is held there. The concept of some of the material exiting on the ebb and coming back in on the next flood is not even mentioned.
5. More than a few problems with comparison of model predicted and observed currents. Normally strategy is to compare along channel vs cross channel values, while paper uses U and V with directions undefined.
6. Comparison to other estuarine systems focus on tidal range ( micro, meso, macro) but never mentions role of freshwater input to the system. both are important particularly for systems like those under study.
7. Paper still does not adequately answer the question posed in the title of the article.

Still think the paper needs more revision. Authors need to step back from paper and take a look at the overall quality of the presentation and key findings.

Comments for author File: Comments.zip

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The authors did a good effort reviewing the manuscript.

Author Response

Thanks to the reviewer for considering our effort in revising the manuscript according to the queries. 

Round 3

Reviewer 1 Report

 

I have reviewed version #3 of this paper and think the authors have done a good job of responding to my detailed comments on their manuscript from both prior versions that I reviewed. They have performed additional simulations looking at the relative role of tides and river flow on the flushing time of the system as I suggested. They have also performed comparison of model predicted to observed currents using along and cross channel direction convention. They have also updated their comparison to other estuarine systems to include not only the tidal range, but the freshwater input to the system. They have also clarified the fact that the model does not address the potential for a portion of the material that is discharged on the ebb tide at the inlets can reenter the system on the next flood tide. The particle trajectory model that they have selected does not include any effects from dispersive transport. They unfortunately have not attempted to understand its role in flushing of the system.

The model data comparisons for water level, currents, and salinity indicate that the model performance is reasonably good, particularly given the complexity of the system in terms of the geometry and substantial variations in freshwater input. They never show any sensitivity of model predictions to grid resolution, but the model data comparisons suggest that the grid system they have selected is adequate to resolve the key dynamics of the system. One serious problem with the paper is that there is no real validation of the particle trajectory model, but that is a result of the lack of field data so is understandable.

The paper remains difficult to read because of the sentence structure and the lack of careful editing. Editing by the authors have improved the structure somewhat but that has been partially offset by additional text added to address questions on the manuscript.

I recommend that the authors go through a final edit to address the sentence structure/wording problems and that the paper otherwise be accepted as is.