Higher-Order Velocity Moments, Turbulence Scales and Energy Dissipation Rate around a Boulder in a Rock-Ramp Fish Passage
Round 1
Reviewer 1 Report
Thank you for this interesting and well-written paper! Please find my general comments and recommendations, as well as some minor comments to specific lines below.
General comments and recommendations:
All Figures: It would be nice to increase the marker sizes in the legend to have the same size as in the plots
Instead of having all Tables and Figures at the end of each subsection, I would prefer to move them close to the position in the text where they are described. This would avoid lots of scrolling.
Use correct minus-symbols instead of dashes.
What is the Froude-scale of the model? Can you compare your model to a real rock-ramp fish pass? Was your model designed following any design guidelines?
Can you write something more about the accuracy and repeatability of the measurements? Did you conduct several measurements at the same location to quantify repeatability (e.g. on different days)? Ideally, confidence intervals should be included in the plots, such that it is clearer which differences are relevant.
I really appreciate the subsection “Implications for fish passages” as it tries to highlight that the study can be useful for designing rock-ramp fish passes. However, I still find it difficult to use the results for real applications, as I am missing clear key findings and recommendations. It would have been very beneficial if multiple dock-ramp fish pass designs were studied and compared with each other.
Comments to specific lines:
104/105: It is hard to know how these natural boulders exactly looked like. Can you please describe that in more detail (e.g. surface roughness) or include a picture in Figure 1?
125: Side-looking or down-looking Vectrino Plus probe?
152: missing space between u_rms and stands
249/250: dash (which is supposed to be a minus sign) is separated from its number
307: missing space
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
I have some comments and questions for the article:
- It is understandable that the stone chosen for measurements was selected in the central part of the profile. But why was the boulder chosen 409 cm from the beginning of the structure? Why not at a different distance (e.g. closer or farther to the beginning of the structure)?
- Why such spacing between boulders, and not larger or smaller?
- Why such a diameter of boulders and not another?
The article is not innovative but well done (experiences are done well, right). I would like the description of the experience itself "why was it done this way?" has been better described. It is a pity that no more variants of the experiment were made (with a different arrangement of boulders, with different boulder diameters, etc.).
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
This paper investigates turbulent velocity fluctuations around a submerged object in a flume. This type of research has been conducted, using the tools they report (acoustic Doppler velocimetry) for nearly 40 years. Because turbulence around submerged objects along a bed has been so extensively studied in the past, it's incumbent upon the authors to explain clearly how their work contributes to the field. The important contributions have not been clearly stated in this manuscript.
It appears that the contribution is the application to nature-like fishways. The primary purpose of nature-like fishways is to aid in the upstream migration of fish. If the novelty of this research is therefore the novel application of existing metrics on previously studied geometries, then the important contribution should be the connection of these metrics to the biological organisms effected by the turbulent metrics reported. Such connections are not made clear until the last two paragraphs prior to conclusion of this paper (paragraphs 2 and 3 of Section 3.4). These connections are not made for each metric reported, and the connections aren't substantiated by literature references. For this paper to be useful to the fish passage community, the authors need to connect their metrics and findings directly and clearly to migrating fish throughout the paper.
The following are more minor (detailed) comments for the improvement of the paper.
1) It's unclear how the work adds to the general body of knowledge about turbulence around submerged objects. What do they find that hasn't already been reported for turbulence in gravel bed rivers? An introduction which more clearly states what's known, what's not known, and what they add will clear things up significantly.
2) Section 2.3: The physical meaning of the skew in the u'w' plane isn't explained.
3) Line 176: Need to be careful about the use of the term "eddy" throughout the paper. Eddy in this case is a coherent bundle of fluid all moving at the same speed. That's in contrast to other places in the paper, and other citations, which use the term "eddy" to mean a rotating body of fluid. The physical meaning is different so the effects on fish swimming ability are likely to be different.
4) Lines 179 - 188: The energy dissipation rate metric and subsequently the Kolmogorov length scale depend on "assuming an isotropic and homogeneous turbulent flow". It's unlikely that this assumption's likely to be valid in all regions directly around a submerged object. The authors should either provide citations which support the use of this assumption in the regions it's being applied or find another way to calculate this metric. A third option would be to remove these two metrics from the paper to give more space for a thorough discussion of the remaining metrics.
5) Line 268: With r2 values ranging from 0.38 to 0.60, it's unclear that the best-fit lines are accounting for a significant trend. Please provide a statistical analysis which shows that the slopes of these lines are significantly different than zero. Perhaps more importantly, does the physics of this scenario infer that we should expect a linear change in length scales? It seems like it would be more likely to expect a threshold or non-linear pattern. Finally, these equations are reported to 2 sig-figs of accuracy, with such wide variation in the data a single sig-fig of accuracy seems more accurate.
6) Table 1: The energy dissipation rate is reported to 4 sig-figs of accuracy. Given the uncertainty of the assumption inherent in this calculation (see my 4th concern) it seems like 4 sig figs of accuracy is unlikely to be appropriate.
In general, I would rather see the authors report fewer metrics, but spend more time explaining why what they found is important to fish passage.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The authors have made substantial changes to the paper for the better. I would have still appreciated a more consistent integration of the impacts of their metrics to the upstream migration potential of fish (rather than leaving it to a final discussion section), but the changes made have significantly improved the paper. You've collected lots of data and begun to help the readers understand the importance of your work. Well done.
