Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on a Horizontal Seabed

Round 1

Reviewer 1 Report

The authors carried out comprehensive flume experiments on Bragg reflection characteristics of porous submerged breakwaters. As the authors claim themselves, their work will supplement the understanding about the Bragg reflection performances of submerged breakwaters worked out mostly with theoretical and numerical investigations, and thus contributes to the further understanding of the characteristics of such breakwaters.

For the reasons mentioned above, I judge the presented manuscript is suitable for the publication, but before accepting for publication, I have several comments and questions as can be found in the followings, to which I recommend the authors pay due considerations and revise the manuscript accordingly.

 

1. The authors focus on the reflection performances of the breakwaters, but I understand transmission performances matter rather than reflection performances as the performances of breakwaters. If no energy dissipations take place in and around the corresponding breakwaters, large reflections imply small transmission and vice versa, but, since non-negligible amount of energy seems to be dissipated in and around the corresponding breakwater because of its permeable nature and of its geometry possessing sharp edges, large reflections do not necessarily imply small transmissions. Therefore, it will be more informative if transmission coefficients are presented in the manuscript together with the reflection coefficients. (Since the transmitted waves were measured with the wave gage WG.6 according to Figure 2, I guess this can be done without carrying out further experiments.)

 

2. It is mentioned in the ‘Introduction’ that the main objective of the authors’ work is to conduct experimental study on the effects of permeability of Bragg submerged breakwaters on their reflection performances, but what is the motivation to focus on the effects of permeability on the reflection performances? Did the authors expect that permeable breakwaters show better performances than non-permeable breakwaters? I myself expected permeable breakwaters will result in smaller transmissions than solid breakwaters. Although, according to Figure 8, larger permeability results in smaller reflections, this fact does not necessarily mean larger permeability results in larger transmissions because of the reasons mentioned in my comment 1.

 

3. Line 212.

It is mentioned that ‘When there is no bar on the bottom bed, there is still a small reflection coefficient, which may be due to uneven topography’, but I don’t believe that the bottom surface of the wave flume is so uneven that such meaningful reflections of incident waves take place. Instead, I suspect that it may be due to the errors resulted in the process of separating measured waves into incident and reflected waves.

 

4. The term ‘slowing trend’ appears in several places in the manuscript, but what does that mean?

 

5. Since I understand the breakwater proposed in the manuscript exploits certain kind of resonant phenomenon, reflection performances of the corresponding breakwater should be susceptible to energy dissipations that may take place in and around the breakwater. (It is well known that the resonant response is susceptible to the amount of damping.)

It is also that resonant phenomena usually occur at a particular single frequency whereas, in real seas, waves are irregular, that is, they are not composed of waves of single frequency. Then how should the breakwaters exploiting their resonant characteristic that can maximize their ability at a particular single frequency designed?

 

6. What is the definition of ‘Bragg resonant reflection coefficient’? Is that the ratio of the amplitude of the reflected wave to that of the incident wave?

 

7. Line 226

It is mentioned that ‘more waves pass through the pores’, but it is not an appropriate expression from the viewpoint of physics, because what pass through the pores are not waves but fluids.

Author Response

Dear reviewer,

 

Re: Manuscript ID: 1872124 and Title: Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed.

 

Thank you for your letter and the reviewers’ comments concerning our manuscript entitled “Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed” (1872124). Those comments are valuable and very helpful. We have read through comments carefully and have made corrections. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Revisions in the text are shown using red highlight for additions, and strikethrough font for deletions. The responses to the reviewer's comments are marked in red and presented following.

We would love to thank you for allowing us to resubmit a revised copy of the manuscript and we highly appreciate your time and consideration.

 

Sincerely,

Chun Chen.

Author Response File: Author Response.pdf

Reviewer 2 Report

This article touches an interesting topic as seen from the physics point of view. However, its practical applicability cannot be presently foreseen. I have three main comments on the manuscript:

1. The Bragg type of reflection can give increased values of the reflection coefficient only under narrow-range specific conditions relating the geometry of the bars layout with the wavelength of the impinging regular waves. This is a severe limitation as regards the applicability of the method, since in reality coasts are attacked by a wide range of wavelengths or, to be more accurate, wavelength is a notion that has no substantial meaning in irregular wave fields present in real life. Thus, the opening sentence of the Abstract sounds too far-fetching and should be replaced by a pragmatic statement acknowledging the above constraints.

2. The experimental layout (Fig. 2) cannot support a 2D (1DH) investigation, against the intention of the writers. This is due to the restricted length of the models that do not cover the full width of the flume. Such layout would induce wave scattering effects emanating from the end sections of the submerged models that could affect free surface displacement and contaminate measurements. This flaw can be properly fixed by redoing the experiments after enlarging the models as required; at the very least a convincing discussion is needed proving that the results obtained with the current layout can credibly support the comparisons and conclusions drawn in the paper.

3. Some of he comparisons (Figs 12, 14, 16) do not give the intended message. Fig.12 gives a confusing picture of results coming from various sources, as to the trend in Kr with respect to B/S, i.e. the bars geometry. In order that such a comparison would be meaningful, all other governing parameters (e.g. wave period) should remain constant in all sources. This is doubtful that holds here and, therefore, this comparison should be redesigned or deleted. Also, in Fig.14 the same approach is detected: you cannot compare Kr values based only on D/h, unless you keep constant all other important variables. Similar modifications as above are also required here. Figure 16 presents comparison of test and numerical results while the text states that "...test results...are consistent with numerical results...", whereas the peak Kr is 0.15 in the former and 0.50 in the latter method! This should be fixed; I suspect that the difference comes from the same source as above. The difference in the number of bars (3 against 4) may have an impact too, but this should be explained/discussed. The downshift behaviour due to the bars shape shown in Fig. 16 deserves also discussion; it appears in numerical results, which is reasonable, but not so in measurements. As a general rule, in order to do fair comparisons it is important to keep constant all parameters that are not exposed in the graph and control in a way or other the result described in there.

Additional minor points:

4. English, although acceptable at large, require a careful review throughout (see e.g. lines 285, 305, 318, ...). It is recommended to use "gauge" instead of "gage".

5. Improve configuration of panel captions in Figs 4, 5, 6.

6. Give more information for Fig.10 content: prediction is made by? Mase & prediction results assumed what porosity/permeability?

7. Check Fig. 15: max value 0.152 or 0.162 as in text?

Author Response

Dear reviewer,

 

Re: Manuscript ID: 1872124 and Title: Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed.

 

Thank you for your letter and the reviewers’ comments concerning our manuscript entitled “Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed” (1872124). Those comments are valuable and very helpful. We have read through comments carefully and have made corrections. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Revisions in the text are shown using red highlight for additions, and strikethrough font for deletions. The responses to the reviewer's comments are marked in red and presented following.

We would love to thank you for allowing us to resubmit a revised copy of the manuscript and we highly appreciate your time and consideration.

 

Sincerely,

Chun Chen.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper presented experimental results from wave flume tests of bottom-founded porous breakwater of various shapes, widths, heights and permeability. It is interesting to see from the experimental results that by adjusting the spacing between multiple bottom-founded porous breakwaters, we may increase significantly the reflection coefficient. The presented experimental results may be good references for practical designs as well as the on-going development in the field of porous breakwaters. such as for validation of theoretical/numerical models.

Below are some minor comments from the reviewer:

1. In the first paragraph of introduction, please discuss the importance of Bragg resonance.

2. In the final paragraph of introduction, please explain clearly 'Bragg reflection characteristics'.

3. Line 187, please give references for the statement that the Bragg resonance will occur when 2S/L is near an integer

 

 

 

 

Author Response

Dear reviewer,

 

Re: Manuscript ID: 1872124 and Title: Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed.

 

Thank you for your letter and the reviewers’ comments concerning our manuscript entitled “Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed” (1872124). Those comments are valuable and very helpful. We have read through comments carefully and have made corrections. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Revisions in the text are shown using red highlight for additions, and strikethrough font for deletions. The responses to the reviewer's comments are marked in red and presented following.

We would love to thank you for allowing us to resubmit a revised copy of the manuscript and we highly appreciate your time and consideration.

 

Sincerely,

Chun Chen.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Responses to Q2, Q3 cannot considered adequate. Technical discussion is required in the appropriate places of your text to address the relevant issues. The point is not to convince the reviewer but the reader, that the methodology applied serves its purpose.

Author Response

Dear reviewer,

 

Re: Manuscript ID: 1872124 and Title: Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed.

 

Thank you for your letter and the reviewers’ comments concerning our manuscript entitled “Experimental Investigation on Bragg Resonant Reflection of Waves by Porous Submerged Breakwaters on Horizontal Seabed” (1872124). Those comments are valuable and very helpful. We have read through comments carefully and have made corrections. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Revisions in the text are shown using red highlight for additions, and strikethrough font for deletions. The responses to the reviewer's comments are marked in red and presented following.

We would love to thank you for allowing us to resubmit a revised copy of the manuscript and we highly appreciate your time and consideration.

 

Sincerely,

Chun Chen.

Author Response File: Author Response.docx