Characterisation of the Water Renewal in a Macro-Tidal Marina Using Several Transport Timescales
Round 1
Reviewer 1 Report
The paper is devoted to an often streched topic of water quality modeling. The exchange time for the water in an semi-enclosed bay is of particular interest in many projects to judge about problems with aeration, temperature, turbidity, algae bloom and so on.
In the paper three different time scales are evaluated and discussed under different tidal and atmospheric conditions for a macro tidal marina. The plots show indeed different distributions of the values. This is of interest for environmentalists, modellers and planers.
Of course the determination of the different time scales is not as easy as just run a model once. So for modelers it could be interesting how to optimize the computational time. Is there a random walk type diffusion in the particle model?
Computing concentrations and paricle path's is numerically quite a different approach, while the physical result should be the same above the "Lagrangean Integral Time Scale". Regarding the different time scales a comparison of definitions to the time scales used in the literature is advantagous and has been made. The flushing time is similar to half-life time.
In fact the system should be sufficiently large so that the particles would not cross the ourflow boundary within a single tidal cycle during ebb tide if there is a chance to return back (except for the case that (tidal) currents along the coast displace the particles from the boundary for shure - as in many backbarrier systems). Otherwise the results would strongly depend on the artificial position of the model boundary. And this would not be a very sound way of computation. Monsen et al 2002 identify pitfalls of time scale computations.
How did you compute the Residence and the Exposure Times compared to each other? By artificially stop the particles at the entrance to the marina (page 6)? But you know, the particles return back. Please comment on the usefullness of the Residence Time (or LRT) as you computed it.
In the results (figure 5) the Local Residence Time gives quite different results. It seems that this time scale is not very usefull if it predicts almost the same short exchange time for neap tide without wind as for spring tide with wind! I should say that the paper is a bit confusing in this respect. Why did you compute thsi quantity?
Author Response
Dear reviewer, we would like to thank you warmly for the advice and comments you provided during this first round of review.Several remarks concerned the absence of elements or the lack of precision with regard to the implementation of the model, the domain studied, the parameterization of equations etc...All this information is available in reference [19]: Huguet et al. Influence of floating structures on the tide and wind-driven hydrodynamics of a highly populated marina, J. Waterw. Port Coast. Ocean Eng. We were aware that the fact that this article is not published would be a handicap in reading and understanding this article and we apologize for that. Until then, this paper was under revision, but we are pleased to inform you that it is now accepted and in the process of being published. We therefore provide you with the manuscript in its raw format, where you can find a lot of information about the model, as well as the implementation and effect of floating structures on hydrodynamics. For the rest of the review, reviewers remarks are colored in black and our responses are in blue.
1) Is there a random walk type diffusion in the particle model ?
- No. A stochastic diffusion model has been implemented recently in the model but we wanted to perform particles release without diffusion in this study to compare it to advection-diffusion modelling of the tracer concentration.
2) How did you compute Residence and Exposure Times compared to each other ?
- Residence and exposure times were computed from the same simulations. We post-treated the simulated positions of the particles with Matlab. We set a condition of arrival/departure of the particle inside the system (the marina) by detecting the passage of particles through the several entrance sections (InterX.m script). The residence time was obtained by searching the first time the particle passes through the section. The exposure time took into account the several passages of the particles through the section until the particle leaves the bay and reach areas with high flushing rates.
3) Please comment on the usefulness of the Residence Time (or LRT) as you computed it
- We originally computed residences times in the marina. Then, we computed other timescales (exposure and flushing times) in order to compare them. During the last process of the study, we used residence times to characterize the return coefficient. We wanted to add the information of the residence times in the modelling firstly to show the marked difference with the exposure and the flushing times in terms of amplitude and behaviour. From this comparison, the discussion was supposed to highlight the differences and explain the behavior of the three timescales in such a semi-enclosed macro-tidal environment. Furthermore, as discussed with the associate editor, the issue of which time scale is sufficient or appropriate may depend on the scientific question asked at the beginning of the study. We discussed this at lines 544 to 548. In our sense, Residence Times could be interesting only for short-term studies (at the scale of one tidal cycle) of the water masses in such macro-tidal port/marina. However, we do not feel qualified to justify the use of each timescales for any biological/ecological/geochemical application.
- The comparison of residence times and exposure times was also made with the return-flow coefficient relation that inferred the return-flow amplitude in the marina. In this sense, the computation of the residence times was necessary to understand the importance of the return-flow in this system.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Please, find my comments in the attached file
Comments for author File: 
Comments.docx
Author Response
Please see the attachement below.
Author Response File: Author Response.docx
Reviewer 3 Report
I found the paper to be very interesting. It is very well written and organized. In my opinion it should be published as it fits the scope of water mdpi journal and for sure will be cited in the future. However I found one major issue to be discussed in the paper and a few minor, which should be improved before the possible publication.
Minor comments, are mostly related to the conclusion section and are associated with the results presented:
Page 18. Line 598: The authors wrote that the “Both the tide and the wind have a significant impact…”. Significant should be supported by at least ANOVA test of the data.
Page 18. Line 605: The authors wrote that “physical processes… are much more sensitive…” I do not see any sensitive analysis of these results.
Page 18. Line 611. The authors wrote that “diffusion processes dominate”. How the diffusion was calculated. It should be disscuses.
Page 18. Line 617. Again the author wrote “significant trapping of the water masses”. Significance of the results should be proven.
Page 18. Line 619. Again the authors wrote “The return-flow can be very sensitive to the…” It is proven or there is speculation. In the scientific papers the speculations should be avoided.
Major problem is related to the scale used in the paper and the “domain” problem. Recently several papers were published about these issues. First of all, it was shown that two different time scales should be considered for parameterization of diffusive behavior within intermediate and global scale ranges and for defining the local–intermediate and intermediate–global boundaries. Moreover, recent results indicated that the interpretation of the experimental data and observation may be biased due to the “window/domain” effect leading to potentially false conclusions about the regime of the diffusion of particles. It should be better discussed in the paper.
I strongly recommend to study the following papers:
Diffusion of bedload particles in open-channel flows: distribution of travel times and second-order statistics of particle trajectories (2015). RJ Bialik, VI Nikora, M Karpiński, PM Rowiński Environmental Fluid Mechanics 15 (6), 1281-1292
On the effect of the window size on the assessment of particle diffusion (2018). RJ Bialik, M Karpiński, Journal of Hydraulic Research 56 (4), 560-566
Experimental Censorship of Bed Load Particle Motions and Bias Correction of the Associated Frequency Distributions (2019). F Ballio, A Radice, SL Fathel, DJ Furbish, Journal of Geophysical Research: Earth Surface 124 (1), 116-136
Author Response
Please see attachement below.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The paper has been improved since the first time was reviewed. The effort of the authors done to improve the paper is strongly appreciated. They answered to all my concerns, which I like very much. Based on the manuscript and the attached cover letter I recommend acceptance the paper in the current form.
Author Response
We thank you for the review and the interesting comments involved.
