Estimation of Bed Shear Stress in Shallow Transitional Flows under Condition of Incipient Motion of Sand Particles Using Turbulence Characteristics

Round 1

Reviewer 1 Report

This study proposed an experimental study on estimation of bed shear stress by high frequency velocity measurement using ADV. Different methods on estimation of bed shear stress has been comprehensively compared. Over a sandy bed, different suggestions such as the measuring elevation of one-point-measurement by the ADV, were proposed by the authors. They also compared theirs results with existing studies. The methodology is appropriate. However, I have some concerns on the measurement itself, which the authors should clarify before this manuscript is accepted for publication.

General comments:

1.      Were all velocity data measured in case of incipient motion of sandy bed. As the manuscript mentioned, the movement of a larger number of sand partials was used as the judgement of sediment initiation, it is expected the sediment bed is in erosion or in bed load transport mode, i.e., the bed level would be keeping changed, during velocity measurement. So, to what extent this affects your results?

2.      How repeatable of the present experiments? Can the authors add a small discussion on this?

3.      The authors would like to address some issues induced by misalignment of ADV in the field conditions. But they conducted experiment with uniform and unidirectional flow, I am not sure if the vector addition of shear stress in different direction (i.e., u’v’ and v’w’) is necessary. Assuming a uniform and unidirectional flow, it is expected that the shear stresses in the main flow direction is the dominant components compared with others.

Other comments

1.      Line 17: “The necessity of vector addition of -u’w’ with other Reynolds shear stress components were examined.” As I mentioned before, again I suggest to not include this in the present manuscript. Otherwise, the additional experiment should be carried out, such as: misalignment of the ADV with the main flow direction and compare the results with the present study.

2.      Line 31: “The rate of sediment transport increases nonlinearly with the increase in bed shear stress.” I think this statement is only true when the bed sediment is sufficient for erosion. Please clarify.

3.      Line 53 and 56: the symbol of ρ is different in these two places and somewhere in the main text.

4.      Line 99-101: again, if the authors would like to study the misalignment of the ADV on the estimation of the bed shear stress, additional experiment should be designed. Otherwise, I suggest not list this as the major research content.

5.      Line 154: what is “Reynolds normal shear stress”?

6.      Line 176: Figure 1 can the authors add the sediment section in this figure? And in figure caption, I think the label (c) and (d) are reversed.

7.      Line 204: please include some formulations on how turbulence velocities are extracted, for the general reader.

8.      Line:205: Are these sand samples used in the experiment uniform in size? Can the authors provide grain size distribution figure in support information?

9.      Figure 3: are these velocity measurements all measured during sediment initiation of the sandy bed? Does the bed elevation change or does the bed load transport happen? Since only one ADV was used to measure velocity in different elevations vertically and horizontally, how the bed level change affects your results?

10.   Table 1: I suggest using ‘Re*’ but not “R*” to represent the shear Reynolds number.

11.    Line 314-318: The authors divided the vertical profile into three regions. Can the authors add a small discussion on how to divide these areas? Because when carrying our linear regression analysis, it is tricky to select data point in vertical. I suggest to add a discussion on how this arbitrary region separation affects the results?

12.   Line 320: So, the u’w’0 is used as the true value of bed stress to evaluate other metrics? If so, please explicitly mention this in the manuscript.

13.   Table 2: the symbols in the table and also the follow tables are not in good manners. For example, (1.22*-u’w’_d)/-u’w’_0 would result in confusion for general readers, and should be re-wright in a correct way.

14.   Section 3.3, 3.4, 3.5 and 3.6 can be combined and concise. Because the present experimental flow is unidirectional and uniform. The shear stress in other direction is for sure small compared to the u’w’. Again, if the authors would like to address the effects of misalignment of ADV in field conditions, additional experiments should be designed and carried out.

15.   Section 3.6-3.8: the coefficient using u’^2, v’^2 and w’^2 on calculation of bed shear stress is highly depend on your experimental results. Whether the coefficient can be applied in other situations should be discussed.

16.   The conclusion is a bit long and please only lists the major findings.

Author Response

Please see attached discussion letter. 

Thank you!

Jueyi

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper, the authors presented experiments of three different shallow water flows under hydraulically transitional flow condition to estimate bed shear stress under incipient motion of four groups of sand particles. The purpose of the research is clear, the writing is standardized, and it has an engineering value. However, the manuscript should be modified as necessary before publication, because it has some limitations, both technically and presentation style.

1. Figure 1 is not drawn to equal scale, which needs annotation. In addition, the actual photos of the experiment should be added to the manuscript.

2. ADV is used for velocity measurement. Why not use Acoustic Doppler Current Profilers (ADCP)?

3. This assumption of homogeneous sand is different from the real situation, and the authors should give some comments on these differences in the paper. The parameters of marine surface sediments may have some reference value, for examples: Evaluation of undrained shear strength of surficial marine clays using ball penetration-based CFD modelling. Acta Geotechnica. and Centrifuge experiment on the penetration test for evaluating undrained strength of deep-sea surface soils. International Journal of Mining Science and Technology.

4. The similar accuracy before the determination of the experimental scheme is missing, and it is recommended to add it to the revised version.

5. The symbols in Figure 3 should be distinguished by different colors.

6. What is the thickness of the boundary layer?

7. Chapter 3 shows many experimental laws with detailed data. However, the mechanism behind these laws should also be analyzed in more detail.

8. This paper lacks some typical experimental process diagrams.

9. This paper lacks photos of samples. What is the density (weight) of sand?

10. In the conclusion part, it is recommended to clearly show the research conclusions.

Author Response

Please see attached discussion letter.

Thank you!

Jueyi

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The new version has generally been revised based on my previous comments.