Effect of Mean Velocity-to-Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

Round 1

Reviewer 1 Report

see attached file.

Comments for author File: Comments.pdf

Author Response

Reviewer #1

 

 

Dear Reviewer,

 

The authors would like to appreciate your kindness and thoughtful comments. Thank you for focusing on different parts of the article and specifying both its positive and negative attributes. We have revised the manuscript based on your suggestions. All recommendations have been addressed and highlighted in blue in the revised manuscript. Also, the responses to your comments/suggestions are shown below. We would like to take this opportunity to thank you once again for your thorough consideration that has improved the quality of the manuscript.

 

 

1. A) General comments

 

 

 

Comment 1:

 

Please explicitly state the research objectives in the introduction and why was the research undertaken.

Please state the hypotheses that were tested for this research.

 

 

Reply to Comment 1:

 

As seen in the revised manuscript, the research objectives and the reason why it was carried out have been explicitly stated in the Introduction section. Moreover, the hypotheses considered for conducting these tests have been added to Materials and Methods section as follows:

 

 

Research objectives:

Since there have been no comprehensive experimental studies carried out on the incipient motion condition in meanders so far, the main objective of this research is to experimentally find the incipient motion of sediments in a meandering channel. To this end, a meandering channel with two consecutive 180-degree mild bends in-between straight upstream and downstream reaches has been designed and constructed by the authors. Investigations of the effect of different mean velocity to critical velocity ratios at the upstream straight reach on bed topography variations along the meander, the effect of the upstream bend’s geometry on bed topography variations in the downstream bend and the effect of the downstream straight end on the upstream bend are amongst the other objectives of this study.

 

 

Research hypotheses:

• Channel walls are rigid.
• Bed sediments are uniform.
• The mean velocity to critical velocity ratios in each test refers to its value at the upstream straight reach of the first bend.
• The test duration was considered 4 hours.

 

 

 

Comment 2:

 

Statements presented in the conclusions are a summary of the laboratory model results. What conclusions can be made that extend the results of this research to other laboratory model experiments, numerical model studies, or the study of natural channels?

 

Reply to Comment 2:

 

Given that the whole results presented in this study have been reported as dimensionless, the results of this study can be compared with the results of other researchers in experimental, numerical, or field models as these comparisons have been made in different parts in the article’s Results section with similar numerical and experimental studies. Moreover, as the measured range of the Froude and the Reynolds numbers in this channel are compatible with most rivers found in nature, the maximum sedimentation and scouring locations, bed topography variations, the bend’s behavior and the qualitative velocity pattern can be attributed to the natural rivers under similar conditions as well as to other physical models with an appropriate approximation.

Comment 3:

 

In the physical model, a straight channel reach leads to the entrance of the upstream channel bend. Meandering flow conditions are established by the end of the upstream bend which provides the important meandering flow and sediment conditions at the beginning of the downstream bend. Consequently, model results from the upstream bend are not nearly as interesting as results from the downstream bend. I suggest there be much less discussion in the manuscript narrative about results from the upstream bend. Randle [Reference 1] demonstrated why multiple consecutive channel bends should be used in physical and numerical model studies and that results from upstream bends should not be used when characterizing the hydraulics of meander bends.

 

 

Reply to Comment 3:

 

Following your recommendation and also the studies conducted by Randle [Reference 1], in explanations on Figs. 2 and 3 in the revised manuscript, this has been taken into consideration and the descriptions of the upstream bend have been summarized as much as possible. The initial version of the manuscript had completely explained the results of this section because the aim of this study has been to investigate meandering bends’ behavior under the effect of different u/uc values, which included the results of both bends. The less effect of the upstream bend and its fewer topography variations compared to the downstream bend are among the results of this study, which have been mentioned in the article.

 

 

 

Comment 4:

 

The abrupt change in curvature between the upstream and downstream bends is not normally found in natural channels and may create zone of flow separation at the beginning of the downstream bend. This may explain the sedimentation along the left channel edge at the beginning of the downstream curve.

 

Reply to Comment 4:

 

Yes, this is completely true. These variations have been observed owing to the change in the maximum velocity from the first to the second bend, the shift of the inner bank to the outer bank at the junction between the two channels and generation of a flow separation zone in this area.

 

Comment 5:

 

The maximum scour and sedimentation depths are made dimensionless by use of the median sediment grain size, which is appropriate. However, the median grain size is 1.85 mm (equivalent to 0.185 cm) while the scour and sedimentation depths appear to be measured in centimeters. Consistent units must be used to create dimensionless values. There are numerous lines in the manuscript where multiples of d50 need to be increased by a factor of 10. For example, 3.2d50 rather than 0.32d50 [line 128], 4.8d50 rather than 0.48d50 [line 141].

 

Reply to Comment 5:

 

This problem has been accordingly solved in the revised manuscript and the necessary modifications have been made for making the units consistent.

 

Comment 6:

 

There are several paragraphs that are too long. Different ideas or descriptions of different results should be divided into separate, shorter paragraphs.

Reply to Comment 6:

 

In accordance to your comment, after reviewing and proofreading the article, particularly in the Results section, the long paragraphs were converted into shorter paragraphs.

 

Comment 7:

 

When using degrees to identify model locations, use the term degrees of channel curvature (at least the first time).

 

Reply to Comment 7:

 

For clarification and better understanding of the degrees along the channel, a new figure has been used in the revised manuscript. As will also be noted in the following comment and in the revised manuscript, the gradation of the meandering channel plan, which consists of two consecutive 180-degree bends, has been precisely illustrated in Figure 1-b.

 

1. B) Materials and Methods

 

Comment 8:

 

Please note the basic physical model dimensions in Figure 1 or provide a separate figure with basic model dimensions (i.e., longitudinal segment lengths, channel width, wall height, and flow depth).

 

Reply to Comment 8:

 

Accordingly, in addition to the figure explained in the previous comment, where the gradation and the size of different parts of the channel have been specified, the channel’s cross section has also been added to Fig. 1-b in the revised manuscript in order to show the water depth and the channel height. As it can also be observed in the revised manuscript, the water depth in different experiments varies depending on the value of U/Uc, and there is no constant value mentioned for it in this figure (refer to the next comment and also to Table 1 in the revised manuscript, please).

 

Figure 1-b. Schematic illustration of plan and cross-sectional views of the meandering channel

 

Comment 9:

 

Please state the width-depth ratio of the physical model.

 

Reply to Comment 9:

 

Given that the water depth has been varied for determining different U/Uc ratios in different tests of the study, the width-depth ratio is also not constant. As it is also evident in Fig. 1, the channel is 1 meter wide. In different tests, the flow depth has been calculated considering the equation proposed by Neill (1968) and the following table has presented the water depth and the width-depth ratio in different cases.

Table 1- Flow depth characterization in different tests

Test 8	Test 7	Test 6	Test 5	Test 4	Test 3	Test 2	Test 1	Test No.
0.98	0.95	0.92	0.89	0.86	0.84	0.80	0.75	U/Uc
17.0	17.5	18.0	18.5	19.0	19.5	20.5	21.8	Water depth (cm)
0.17	0.18	0.18	0.19	0.19	0.20	0.21	0.22	Depth-width ratio

 

 

Comment 10:

 

A figure of the bed topography data collection mesh might be helpful.

 

Reply to Comment 10:

 

In accordance to your comment, a new figure illustrating the mesh used for bed topography data collection has been added to Materials and Methods section in the revised manuscript. ɵ in this figure represents different angles along the meander and x is the distance of the collection points at channel width. For data collection, the bed topography has been read and recorded at different sections and angles in 1-cm-long intervals using a laser meter in every test and its topography has been drawn using Tecplot. According to the following figures, which have been taken as an instance of the mesh for U/Uc=0.98, it can be observed that the collection interval is for every 10 degrees at the beginning of the upstream bend due to few bed topography variations and it is for every 5 degrees at the end of the bend. For every angle, the bed topography was collected at a distance of 1 cm. Bed topography variations at the downstream bend have been collected every 2 degrees along the bend and at 1-cm-long intervals for each angle considering the amount of variations for the sake of high accuracy of the results.

 

 

Figure 2. An instance of the mesh grid for data collection in U/Uc=0.98 test.

Comment 11:

 

What was the strategy for controlling the downstream flow depth? Was normal depth achieved and in which reaches? For a given model experiment, how did flow depths vary among the channel segments? Example water surface profile plots would be helpful.

 

Reply to Comment 11:

 

As previously explained in the manuscript, for adjusting the flow depth at the upstream straight reach, a butterfly gate has been used at the channel outlet. An instance of the water surface profile measured at distances of 2%, 50%, and 90% of the first bend’s outer wall (up to the 180-degree angle) and the second bend’s inner wall (from 180 to 360 degrees) has been drawn for U/Uc=0.95 in the following figure.

In this figure, B is the channel width and X is the distance of water surface profile collection from the first bend’s outer wall and the second bend’s inner wall per a fraction of the width.

 

Figure 1. An instance of variations in the measured water surface profile

 

Considering the addition of mesh figures (Figure 2) and the figures related to plan and cross section (Figure 1-b) as well as the explanations recommended by the respected reviewers to the manuscript in the revision stage, the volume of the article and the number of its pages have increased. Therefore, this figure has been placed here solely for presenting an instance of the data and gaining your kind approval and addition of it to the manuscript has been avoided.

 

1. C) Results

 

Comment 12:

 

Lines 124-125: “These 124 variations reach a maximum within the range of 200 to 300 degrees, and . . .” I suggest you reference Figure 2 to explain what the degrees of curvature mean.

 

 

 

Reply to Comment 12:

 

I appreciate your helpful suggestion. As also explained in Comment 7, for better understanding of the bend’s gradation, a new figure (Figure 1-b) has been added to the revised manuscript. In addition, the gradations are available in Figures 2 and 3, which make easier the understanding of the descriptions provided in this section.

 

 

Comment 13:

 

Figure 2: The four graphs within this figure are too small to easily to read, especially the color-scale magnitudes. Perhaps the figure could be expanded into four larger figures. I suggest that you identify the locations of maximum scour and sedimentation in each figure.

 

 

Reply to Comment 13:

 

 

Accordingly, the intended modifications have been applied to this figure in the revised manuscript. In addition to further magnification and clarification of the figures and the color-scale present in them, the maximum scour and sedimentation locations were added to Figures 2 and 3.

Please note that Figures 2 and 3 in this comment are in fact the same as Figures 2 and 3 in the initial version of the manuscript, while figure and table numbers have been altered in the revised manuscript due to addition of a figure and a table.

 

Comment 14:

 

Lines 137 to 161: The paragraph is too long. Please break the ideas into separate paragraphs.

Reply to Comment 14:

 

Following your kind recommendation, after reviewing and proofreading the article, this long paragraph has been changed into shorter paragraphs.

 

Comment 15:

 

Lines 137-138: “The maximum sedimentation height has also been developed insignificantly at the 200 degree angle.” What is meant by “developed insignificantly”?

 

 

Reply to Comment 15:

 

“developed insignificantly” in this part refers to little and non-significant maximum sedimentation height at the 200-degree angle. In fact, what is meant here is that the maximum sedimentation height at this angle is trivial and insignificant in comparison to other angles of the bend.

 

 

 

Comment 16:

 

Lines 149-150: Does the maximum sedimentation at 200 degrees correspond to a flow separation (eddy) caused by the abrupt change in channel curvature?

 

 

Reply to Comment 16:

 

Yes, according to explanations available in Comment 4, and as it has also been explained in the manuscript, experimental observations have shown that scouring has occurred at the outer bank of the upstream bend and sediment particles have left it. Therefore, at the location of the shift in the bend’s center of curvature, these sediment particles move towards the inner bank and the flow orientation in the first half of the bend is towards the inner bank.

 

 

 

Comment 17:

 

Lines 152-153: “The flow velocity has grown at the downstream bend given the decline in the water height under the downstream straight path's effect.” Please explain why there is a decline in flow depth through the downstream bend.

 

 

 

 

Reply to Comment 17:

 

As found in experimental results of other researchers, the bend has a complicated and variable behavior due to the generated vortex flows. The water surface difference in the inner and outer bends of single bends has also been observed, which is due to presence of helical flows in the bend and the flow orientation shift at different points of the bend (Vaghefi et al. 2017). Due to the flow attack towards the inner bank in the first half of the bend, the water height is reduced at the inner bank, and these variations are more complicated in a meandering channel and have more effects. The water surface profile has been collected for different cases. For every case, the water depth difference at upstream and downstream bends was observed even at the onset point of the experiment. Water depth variations along the meandering channel and its smaller values in the downstream bend are also evident in the Figure 1 provided for Comment 11.

In addition to the reasons mentioned above, this can be attributed to the flow shift from the first outer bend to the second inner bend, which results in an increase in the flow velocity and a decrease in the water depth. Further, the sediment motion towards the inner bend has led to channel constriction in this area, which is itself a cause of water height reduction over sedimentary stacks and velocity increase.

 

 

Comment 18:

 

Lines 170 to 214: the paragraph is too long. Please break the ideas into separate paragraphs.

 

 

Reply to Comment 18:

 

Following your suggestion, after reviewing the article in the revision stage, this long paragraph has been changed into shorter paragraphs.

 

 

Comment 19:

 

Lines 215-218: The long sentence is difficult to understand. I believe your point is that results from the downstream bend for this study were compared with results from another physical model with a single bend and with numerical model results for this same single bend.

 

 

Reply to Comment 19:

 

I appreciate your sincere and kind attention. In the revision stage, the article has been reviewed and proofread several times and all the ambiguities found in the article like this one have been solved. As it can be seen in the revised manuscript, this part has been edited, modified and made fluent.

 

Comment 20:

 

Lines 235-237: Results from the recent article by Randle [Reference 1] show that the maximum shear stress occurs and the beginning of the next downstream bend, which agrees with the statement in the review manuscript.

 

 

Reply to Comment 20:

 

These descriptions on Randle [Reference 1] have also been added to the revised manuscript in order to complete the verification of these results and its correspondence with this part of the article.

 

 

Comment 21:

 

Lines 246-248: The sentence begins by saying that maximum bed topography variations occur between 200 and 300 degrees. However, the sentence continues by saying that maximum scour depth occurs at 195 degrees. I assume the second sentence means that the maximum scour occurs at a location without much sedimentation. Please be clear on this point.

 

 

Reply to Comment 21:

 

What is meant by the first sentence has been explicitly specified in the sentence itself; it has been stated that the maximum bed topography variations (including sedimentation and scour) have been observed within the angles of 200 to 300 degrees, while the precise location of the maximum scour is the 195-degree angle, which has occurred near the 200-degree angle, thus the same 200 to 300-degree range.  

 

 

Comment 22:

 

Lines 257-258: “. . . the downstream bend functioned as an obstacle against the flow present at the upstream bend.” Does this mean there is additional flow resistance along the downstream bend because of point bars which creates additional backwater through the upstream bend? Could the decreased water depth through the downstream straight section be a result of the downstream boundary water surface elevation being somewhat below normal depth?

 

 

Reply to Comment 22:

 

This sentence means that the second bend’s geometry and curve have created an obstacle against the outlet flow of the first bend. In fact, this has been stated in comparison to the case where a straight reach exists downstream of the bend. In other words, when there is another bend, instead of a straight reach, downstream of a bend, the flow is in a slower motion than that in the case of a straight path downstream of the bend; the second bend works like an obstacle against the flow. However, in the second half of the downstream bend, the flow is under the influence of the downstream straight reach, has a higher velocity and moves more easily.

 

 

Comment 23:

 

Lines 273 to 296: The paragraph is too long. Please break the ideas into separate paragraphs.

 

 

Reply to Comment 23:

 

Following your suggestion, this long paragraph has been changed into shorter paragraphs in the revised manuscript.

 

 

Comment 24:

 

Line 327: Find a better transition between these two sentences, rather than using the same phrase (“in Figure 5-d”) to end and then begin the consecutive sentences.

 

 

Reply to Comment 24:

 

As per your comments, this part has been modified in the revised manuscript and consecutive mention of “in Figure 5-d” has been avoided.

 

 

Comment 25:

 

Table 1. Comparison of maximum scour and sedimentation results.

• Why do the reported results vary from two to four digits of precision? I suggest you consistently report three digits of precision.
• Present the results as factors of increase, rather than percent increase, to be consistent with narrative of the manuscript. The numbers in table 1 must agree with numbers stated in the narrative of the manuscript. I did not find that to be the case.
• Why are the table results from the upstream bend so inconsistent? Are they worth reporting?
• Why are the table results from the downstream bend so inconsistent?

 

 

 

Reply to Comment 25:

 

• Following your comment, all the numbers in this table have been presented with three digits and there are no more inconsistencies regarding the decimals.
• As per your comment, we have reported the results in the revised manuscript as factors of increase rather than percent increase so as to be consistent with the descriptions provided in the article.
• The results on the upstream bend are worth reporting since the lower effect of the upstream bend and its fewer topography variations compared to those of the downstream bend are one of the results of this study, where expression of these values in this table makes it easier to state them more understandably. Moreover, the results provided on the upstream bend are not contradictory. For instance, the values of the maximum scour depth and the outlet sediment volume obtained in the test for U/Uc=0.89 have changed from a very small value close to zero to a larger value for U/Uc=0.92. Therefore, because the stated values are attributed to percent increase, this percent increase is significant.
• As it was explained above, the results provided on the downstream bend and those on the upstream bend are not contradictory. Rather, because for small U/Uc values, the variations are highly low and near zero, the number of these variations grows with velocity increase, and such an increase will become significant.

 

 

Comment 26:

 

Lines 338 to 364: The paragraph is too long. Please break the ideas into separate paragraphs.

 

Reply to Comment 26:

 

 

Following your comment, this long paragraph has been broken into shorter paragraphs in the revised manuscript like the other paragraphs you kindly specified.

 

 

Comment 27:

 

Figure 5. Cross section plots: Consider plotting all cross sections using the same vertical scale.

 

 

Reply to Comment 27:

 

Different scales had been used for the vertical axis in the revised manuscript because in some cases and some parts of the bend, the variations are very small, and if the scale of all plots are the same and drawn in a larger range, some figures may have unclear pictures. Nevertheless, following your recommendation, this has been modified in the revised manuscript and all the plots have now been presented with the same scale. In addition, at this stage, for the figures whose clarity has suffered due to using the same unit for them, magnified illustrations of their important variation ranges have been provided so that the trend of such variations can be observable and comparable with high quality.

 

 

Comment 28:

 

Figure 6. Cross section plots: Consider plotting all cross sections using the same vertical scale.

 

 

Reply to Comment 28:

 

As explained in the comment above, this has also been modified on Figure 6 in the revised manuscript.

 

 

Comment 29:

 

Lines 491-493: This sentence is a bit long and somewhat confusing. I assume the point of the sentence is that photographic results presented in Figure 8 agree with the photographic results presented in Figure 4.

 

 

Reply to Comment 29:

 

 

Following your comment, this part has been edited in the revised manuscript for solving ambiguities and making the text fluent. Yes, you are totally right. The trend illustrated in Figure 8 corresponds to not only the research conducted by previous researchers, but also the pictures of the laboratory about scour and sedimentation patterns provided in Figure 4.

 

 

 

1. D) Conclusions

 

Comment 30:

 

Lines 506-507: In general, there was little bed topography variation through the upstream bend. Why does this suggest the effectiveness of the downstream bend in altering the incipient motion? Perhaps the difference in responses along the upstream and downstream bends is explained by the differences in flow velocity patterns entering each bend.

 

 

 

Reply to Comment 30:

 

As stated before in Comment 22, because the downstream bend affects the upstream bend as an obstacle against the flow motion, the flow velocity in the upstream bend is smaller and this has led to variation in the incipient motion condition and further decreased in the mean velocity to critical velocity ratios in the upstream bend than the downstream bend. Therefore, fewer variations are observed under the same flow conditions in the upstream bend.

 

 

Comment 31:

 

Lines 512-515: More hydraulic explanation is needed to describe why water depths decreased through the upstream and downstream bends.

 

 

Reply to Comment 31:

 

As it has been explained in Lines 512-515, this change has taken place as a result of changing U/Uc from 0.89 to 0.98 because, as also shown in Table 1, the water depth for U/Uc=0.89 is 18.5 cm, while the depth has reached 17 cm for U/Uc=0.98. In fact, water depth reduction here means the intentional water depth reduction for adjusting U/Uc based on the formula proposed by Neill (1968) and there was no other reason for it.

 

 

 

References

 

Neill, C.R., 1968. Note on initial movement of coarse uniform bed-material. Journal of hydraulic research, 6(2), pp.173-176.

 

Vaghefi, M., Ghodsian, M. and Akbari, M., 2017. Experimental investigation on 3D flow around a single T-shaped spur dike in a bend. Periodica Polytechnica Civil Engineering, 61(3), pp.462-470.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors experimentally investigated the effect of flow velocity on channel bed topography in a meandering channel. Details of bed topography variations along the channel due to varied flow velocity were presented in the study. The paper is generally well written and organized. The research has novelty. Findings are interesting. I recommend publication after minor revisions. My specific comments are

(1) The authors need to give the definition of critical velocity.

(2)There are many curves indistinguishable for the velocity range in figures5-7. I suggest to remove some curves to make graphs look better.

(3)It's better if the authors can use some theoretical models to explain their measured data.

Author Response

Reviewer #2

 

The authors experimentally investigated the effect of flow velocity on channel bed topography in a meandering channel. Details of bed topography variations along the channel due to varied flow velocity were presented in the study. The paper is generally well written and organized. The research has novelty. Findings are interesting. I recommend publication after minor revisions.

 

 

 

Dear Reviewer,

 

Thank you very much for focusing on the different parts of the article and your positive view about this article. We have revised the manuscript, as suggested. All your recommendations have been addressed and highlighted in red in the revised manuscript. Moreover, the responses to your comments/suggestions are shown below. We would like to take this opportunity to thank you once again for your thorough consideration that has improved the quality of the article.

 

 

 

 

Comment 1:

 

The authors need to give the definition of critical velocity.

 

 

Reply to Comment 1:

 

 

The authors would like to express their most sincere gratitude for your kind attention. Following your suggestion, a short definition of the critical velocity has been added and highlighted in the revised manuscript, a complete description of which is as follows:

 

In alluvial streams, hydrodynamic forces are exerted on the sediment particles at the bed surface. An increase in flow velocity induces an increased magnitude of hydrodynamic forces. Consequently, sediment particles begin to move if a condition is eventually reached when the hydrodynamic forces go beyond a critical value. The initial motion of sediment particles is commonly called incipient motion. The condition that is just adequate to initiate sediment motion is termed threshold or critical condition (Dey and Papanicolaou, 2008) where the hydraulic conditions of the particles’ incipient motion is defined by the critical shear stress or the critical velocity (i.e., the flow velocity of the incipient motion). Therefore, the critical velocity is in fact the flow velocity under the incipient motion condition of the bed sediment particles.

 

 

 

Comment 2:

 

There are many curves indistinguishable for the velocity range in figures 5-7. I suggest to remove some curves to make graphs look better.

 

 

Reply to Comment 2:

 

 

Figures 5 to 7 are in fact instances of variations in the bed’s longitudinal and lateral profiles and they have been presented to state the changes in the bend’s behavior for different U/Uc values. Hence, a great number of plots are found in these figures, whose aim was to illustrate variations for all U/Uc cases.

Following the suggestion by the respected reviewer 1, the same scale has been used for vertical scales in these figures in the revised manuscript, and for more clarity of the important variation ranges, their magnified illustrations have been used for some of them. Therefore, it is no longer needed to remove certain cases for better clarity of the figures and all the figures follow a similar pattern so that they can be easily compared.

Please note that Figures 5-7 in this comment are in fact the same as Figures 5-7 in the initial version of the manuscript, while figure and table numbers have been altered in the revised manuscript due to addition of a figure and a table.

 

 

Comment 3:

 

It's better if the authors can use some theoretical models to explain their measured data.

 

 

Reply to Comment 3:

 

Prior to presenting the data in the Results section, and as it was explained previously in Materials and Methods, different theoretical models and recommendations by different researchers, including Leschziner and Rodi (1979), Raudkivi and Ettema (1983), Chiew (1992), and Neill (1968) have been used for designing and preparing the laboratory setup, whose references are also provided in the article. Furthermore, given the nature of scour and sedimentation patterns data, the data and results provided in this article have also been compared with the results of different researchers in the Results section.

 

 

References

 

Chiew, Y.M., 1992. Scour protection at bridge piers. Journal of Hydraulic Engineering, 118(9), pp.1260-1269.

 

Dey, S. and Papanicolaou, A., 2008. Sediment threshold under stream flow: A state-of-the-art review. KSCE Journal of Civil Engineering, 12(1), pp.45-60.

 

Leschziner, M.A. and Rodi, W., 1979. Calculation of strongly curved open channel flow. Journal of the Hydraulics Division, 105(10), pp.1297-1314.

 

Neill, C.R., 1968. Note on initial movement of coarse uniform bed-material. Journal of hydraulic research, 6(2), pp.173-176.

 

Raudkivi, A.J. and Ettema, R., 1983. Clear-water scour at cylindrical piers. Journal of hydraulic engineering, 109(3), pp.338-350.

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

Reviewer #3

 

This study presents a meandering channel with two consecutive 180-degree mild bends designed and built-up at Islamic Azad University (at least I think) in Iran, to investigate bed topography variations. The channel is composed of an 8-meter-long path upstream and one downstream, connected by two consecutive 180-degree bends with inner and outer radii of 3 and 4 meters. The channel wall was 0.80 m high and 1.00 m wide, and the ratio, R/B, of the bend radius, R, to the channel width, B, was equal to 3.5 (i.e. mild bend). The channel bed was covered with a 0.30-m thick layer of nearly uniform silica sand with d50=1.85 mm and a sediment gradation of 1.2. Tests were considered to run for 4 hours. The ratio U/Uc of the mean velocity, U, to the critical velocity, Uc, at upstream straight path ranged from 0.75 to 0.98 and the Froude number from 0.22 to 0.31. Results indicated that increasing U/Uc from 0.80 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 the maximum scour depth at the downstream bend increased by factors of 1.5, 2.5, 5, 10, 12, and 26, respectively and the maximum sedimentary height by factors of 3, 10, 23, 48, 49, and 56. The MS is of interest to the WATER readership and is written sufficiently well. I have appreciated the experimental work - rather uncommon in literature - and the schemes and figures that describe it. I didn’t find evident drawbacks in this manuscript, but some issues would deserve more discussion. In conclusion, this MS could be worthy of publication in Water journal. Its quality is already quite acceptable. However, I would suggest the following specific comments in the hope that they might improve the quality of this paper. A re-review is recommended. 

 

 

Dear Reviewer,

 

Thank you very much for focusing on the different parts of the article and specifying its positive attributes. Thank you again for your positive opinion and your careful attention to this article. We have revised the manuscript, as suggested. All your recommendations have been addressed and highlighted in green in the revised manuscript. Moreover, the responses to your comments/suggestions are shown below. We would like to take this opportunity to thank you once again for your thorough consideration that has improved the quality of the article.

 

 

1. A) Introduction

 

Comment 1:

 

It would be opportune citing some literature works (if any) with experimental stands similar to that used in this study. 

 

 

Reply to Comment 1:

 

It should be noted that this had been addressed in the first version of the article, too. As this manuscript is focused on the topic of incipient motion in a meandering channel, Introduction has also provided research works on the same topic only. Therefore, the literature works seem limited in the first version of the manuscript, which is because there have been very few studies so far focusing specifically on sediment transport in meandering channels. However, following your valuable comment, more experimental studies appropriate for the article’s topic have been cited in the revised manuscript. Now, these changes are observable in Introduction and References sections in green.

 

 

1. B) Materials and Methods

 

Comment 2:

 

At line 95 it reads “Given the criterion suggested by Chiew [23], the tests were considered to run for 4 hours”. However, the paper by Chiew refers to scour protection at bridge piers. Therefore, could the Authors clarify why runs lasted (only) 4 hours? Was this duration enough to achieve dynamic equilibrium conditions?

 

 

Reply to Comment 2:

 

The authors wish to express their gratitude for the respected reviewer’s careful attention. About line 95 and the citation attributed to Chiew, it should be noted that in the first experiments conducted after construction of this channel, the equilibrium time test was perforemed with the presence of a bridge pier so that it could be used as comparison and citation to Chiew’s studies. To this end, Chiew's work [reference 23] helped us understand the definition and the process of measuring this duration and it was observed that with the presence of the pier, after 4 hours, the bed topography variations were insignificant and a relative equilibrium had been maintained. Since we wish to compare the results of this study with those of the scouring in the tests for cases with the presence of a bridge pier as well, the selected duration for these tests has been 4 hours, too. In addition, these tests have been performed in this study with the aim of examining U/Uc variations, not the effect of the equilibrium time.

 

 

Comment 3:

 

At lines 102 and 103 it reads that the ratio U/Uc was determined considering the formula proposed in Neill [9]. However, I’m wondering whether the Authors observed conditions of quasi-incipient conditions according to the Neill’s formula. A comment in this sense would be suitable;

 

 

Reply to Comment 3:

 

It should be explained that, as also described in the manuscript, U/Uc considered for different cases is that of the upstream straight path, where the conditions considered for it are similar to those of the test performed by Neill (1968) in a straight channel. In addition, laboratory observations in the upstream straight path confirmed the value obtained from Neill’s formula, and the particles in incipient motion (U/Uc=0.98) evidently began to slide.

 

 

Comment 4:

 

At lines 108 and 109 it reads, “in these tests, the Leica laser bathometer (DISTO-D510) with a precision of 1 mm in 200 meters has been used for bed topography data collection”. I guess measurements were performed at the end of each run (but this should be specified in the manuscript). If so, I also guess that the channel was drained before measurements. But, did the bed morphology remain unchanged in empting the channel? In short, the Authors should better specify and discuss the conditions under which the measurements were collected;

 

 

Reply to Comment 4:

 

For better clarification of this part, thorough explanations and appropriate pictures have been used, showing the process of bed topography data collection better. In order to prevent too many pages and too much content in the manuscript, these explanations have been presented only here to you.

Prior to the onset of the tests, the bed of the meandering channel had been filled with 30 cm of uniform grain materials. As shown in the figure below, at the beginning of each test, the surface throughout the channel path was made even and uniform using a bed flattener instrument.

 

 

Figure 1. Bed flattening prior to each test

 

In order to assure that the bed level would not undergo change during the initial water refilling, this stage lasted for about 1.5 hours with a discharge of 1.5 lit/s. The following figure illustrates an instance of the initial water refilling in the channel.

 

Figure 2. Initial water refilling of the channel

                                                   

A timer was used for controlling test durations. 4 hours was considered as the duration of each test. When the test ended and the pump was turned off, an average duration of nearly 5 hours was considered for drainage and full water discharge throughout the path. This duration varied within 2 to 5 hours depending on the amount of scour, bed variations and water pooling inside scour holes. After full water drainage and bed surface drying, topography variations for each case was read using a laser meter. The following figure illustrates an instance of the bed drainage stages and bed condition after the pump is turned off.

The picture depicts how bed topography collection is carried out using a laser meter. Using a Leica laser bathometer (DISTO-D510) in every test in different sections and angles at 1-cm-long intervals, bed topography was read and recorded. The data have been first inserted in Excel and then the plots have been drawn using Tecplot.

 

Figure 3. Bed drainage stages

 

 

Figure 4. Laser meter and bed topography variation

 

 

Comment 5:

 

Finally the Authors should discuss at length the negligibility of scale effects.

 

 

Reply to Comment 5:

 

We appreciate the question raised by the reviewer, which is essentially based on the viewpoint of the relationship between small-scale laboratory tests and actual prototype conditions related to scour. As a response to this comment, Ettema et al. (1998), which is one of the most comprehensive references used on scale effect, has been cited. While we fully understand the need to accurately translate research findings to actual practice, the reality is that results from such small-scale laboratory experiments often cannot do so, notwithstanding the ideas proposed by Ettema et al. (1998) about scale effect. The weakness of the infamous “scale-effects” related to laboratory experiments on sediment transport clearly applies to scour research. All laboratory studies worldwide on this topic are essentially subjected to this limitation because a full similitude is impossible. Frankly, the method proposed in Ettema et al. (1998) also fails if one were to use it to re-analyze all published pier-scour data.

Notwithstanding the above comment, we have carefully read Ettema et al. (1988) and several other papers (e.g., Heller 2011) on scale effects. All of them recognize this limitation with laboratory tests on scour and sediment transport. In the present study, flows of all the experimental runs were fully turbulent with Reynolds number, Re > 4,000 in subcritical critical flow condition (Fr = 0.22-0.31 < 1). Based on customary understandings, this is the condition in most natural rivers and most other published research conducted in this field.

From the research perspective, one can consider ours as a kind of small “full-scale” test and bypass the scale-effects argument. By having the same (relevant) dimensionless numbers as those in the prototype, one can assume their resemblance. Obviously, this approach, strictly speaking, cannot be correct, but it is how research on sediment transport and scour is often based on presently. Frankly, researchers worldwide, including ourselves, do not know how otherwise because published methods, such as Ettema et al. (1998), used to address this issue, are also not without their shortcomings.

Consequently, results obtained from empirical laboratory tests on scour should be used in the following manner: (1) it provides a means to understand the mechanism of scour/sediment transport, thereby offering a fundamental understanding of the physics involved. Extrapolation of the proposed equations (if any) for use in the full-size prototype condition must be conducted in view of its underlying physics; (2) the experimental data provide a means for numerical researchers to conduct their investigations for data calibration. A well-calibrated numerical model can then be extended to tests with a full-size condition, whereby these recommendations may be used with more confidence; and (3) the results or fitted equations should be checked against field data. If corrections are needed, those equations may need to be amended, and their use will then have a higher level of confidence.

 

 

1. C) Results

 

Comment 6:

 

This section is predominantly a description of the experimental observations. Some comparisons with literature findings are given sporadically. I’m not sure this approach might be sufficient to warrant an article in Water journal. I have appreciated the accuracy of the descriptions as well as in writing the text and preparing the figures. However, I believe the Authors should make further efforts in making their manuscript more attractive. For instance the attempt to simulate the experimental observations by through an existing numerical code would be an excellent way to complete an already good manuscript. 

 

 

Reply to Comment 6:

 

Once again, I wish to express gratitude for your positive opinion about different sections of the article. The expression of the results might seem a bit scattered because the nature of scour and sedimentation data allows for different ways of presenting the results and describing and analyzing them from different aspects. However, with a little care, it can be realized that not only are these figures and descriptions not sporadic and scattered, but they are also fully interrelated and integrated in a way that has created a deep connection in meaning in the manuscript between the figures and different parts, from the experimental observations of scouring and sedimentation process in Figure 4 with schematic views of the qualitative flow pattern in Figures 8 and 9 to the connection between Figures 2 and 3 and the like in bed longitudinal and lateral profiles in Figures 5 to 7. Hence, the unity has been maintained from the beginning to the end of the work. As also mentioned in the title and the objectives of this article, the main aim of conducting this work has been experimental simulation and determination of the incipient motion in this meandering channel, which has been designed and constructed to serve this goal. Using a numerical model and comparing it with the experimental results is one of the future research objectives for studies that follow, which was practically impossible in the 10-day period that we had been allowed.

Please note that Figures 2, 3, 4, 5, and 7 in this comment are in fact the same as Figures 2, 3, 4, 5, and 7 in the initial version of the manuscript, while figure and table numbers have been altered in the revised manuscript due to addition of a figure and a table.

 

 

 

1. D) Keywords

 

 

Comment 7:

 

The keyword “Open-channels” appears too generic and ineffective in identifying the crucial issues of this manuscript. I would substitute it for a more suitable one (e.g. Meandering channel).

 

 

Reply to Comment 7:

 

The authors would like to appreciate your valuable recommendation for replacing the keyword “Open-channels”. As also seen in the revised manuscript, in accordance to your comment, “meandering channel” has replaced “open-channels”.

 

 

1. E) Other minor changes

 

Comment 8:

 

At line 97. It reads “m^3”, but I would write “m³”.

 

 

Reply to Comment 8:

 

This has been modified in the revised manuscript.

 

 

 

Comment 9:

 

Table 1. I would avoid breaking the word “Downstream”;

Reply to Comment 9:

 

 

This, which has occurred due to the format of the provided table, has been corrected in the revised manuscript.

 

 

Comment 10:

 

Are the numbers 325 and 3693 in correspondence to U/Uc from 0.89 to 0.92 correct? They would appear strange.

 

 

Reply to Comment 10:

 

 

At this stage, for further precision in calculations, these numbers have been checked again in the revised manuscript and their correctness is verified.

In order to explain why these values seem strange, it should be noted that the values on the maximum scour depth and the outlet sediment volume obtained in the test for U/Uc=0.89 has reached from a very small value near zero to a larger value for U/Uc=0.92. Therefore, as the presented values are related to the percent increase, this percent increase will be significant. According to the comment by the respected Reviewer 1, the expression of results has been changed from percent increase to factors of increase so that the results should seem less strange although they are correct.

 

 

 

 

References

 

Chiew, Y.M., 1992. Scour protection at bridge piers. Journal of Hydraulic Engineering, 118(9), pp.1260-1269.

 

Ettema, R., Melville, B.W. and Barkdoll, B., 1998. Scale effect in pier-scour experiments. Journal of Hydraulic Engineering, 124(6), pp.639-642.

 

Heller, V., 2011. Scale effects in physical hydraulic engineering models. Journal of Hydraulic Research, 49(3), pp.293-306.

 

Neill, C.R., 1968. Note on initial movement of coarse uniform bed-material. Journal of hydraulic research, 6(2), pp.173-176.

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

General Comment:

The latest version of the manuscript contains many significant improvements over the previous version. The additional figures, and modification to existing figures, greatly improve the manuscript.

 

Remaining Comments:

 

Lines 143-148: The list of channel characteristics presented hypotheses do not really seem to be hypotheses. By experimental design, the channel walls of the physical model are rigid, the bed sediments are uniform, the ratio of mean velocity to critical velocity is applied to the upstream reach, and the test duration is 4 hours. The hypotheses should describe process-based predictions of outcomes that was not known before conducting the model experiments. For example, a hypotheses might be that secondary flows through meander bends can increase the occurrence of incipient motion relative to a straight reach where incipient motion is not achieved. Given that a straight channel reach is upstream from the first channel bend, secondary flow circulation exiting the upstream bend produces even stronger secondary circulation through the downstream bend.

 

4. Conclusions

 

The results of the hypothesis testing should be discussed in the conclusions. For example, the conclusions could say something about the straight flow entrance conditions at the upstream bend produce weaker secondary circulation, sedimentation, and scour through the upstream bend compared with stronger secondary circulation, sedimentation, and scour through the downstream bend.

 

I recommend that the conclusion state something like the following paragraph:

The dimensionless results of this study can be compared with the results of other experimental, numerical, or field studies that have a similar channel curvature, width-depth ratio, Froude number, and Reynolds. The measured conditions under this study (e.g., maximum sedimentation and scouring locations, bed topography variations, channel bend’s behavior and the qualitative velocity pattern) are expected to similar to other laboratory or natural channels with similar conditions.

 

Author Response

Reviewer #1

 

 

The latest version of the manuscript contains many significant improvements over the previous version. The additional figures, and modification to existing figures, greatly improve the manuscript.

 

Dear Reviewer,

 

The authors would like to appreciate your kindness and thoughtful suggestions. We have revised the manuscript based on your comments again. All recommendations have been addressed and highlighted in blue in the revised manuscript. Also, the responses to your comments/suggestions are shown below. Moreover, the paper's English language and style have been re-visited with great care, and the manuscript has improved accordingly. We would like to take this opportunity to thank you once again for your thorough consideration that has improved the quality of the manuscript for publication.

 

 

 

Comment 1:

 

Lines 143-148: The list of channel characteristics presented hypotheses do not really seem to be hypotheses. By experimental design, the physical model's channel walls are rigid, the bed sediments are uniform, the ratio of mean velocity to critical velocity is applied to the upstream reach, and the test duration is 4 hours. The hypotheses should describe process-based predictions of outcomes that were not known before conducting the model experiments. For example, a hypothesis might be that secondary flows through meander bends can increase the occurrence of incipient motion relative to a straight reach where incipient motion is not achieved. Given that a straight channel reach is upstream from the first channel bend, secondary flow circulation exiting the upstream bend produces even stronger secondary circulation through the downstream bend.

 

 

Reply to Comment 1:

 

We would like to greatly thank you for your thorough explanations and recommendations on the hypotheses. This section was generally edited in the revised manuscript, the previously added items were removed, and your intended hypotheses were added to this paper as follows:

• The downstream bend's presence influences bed topography variations and changes in incipient motion conditions along the upstream bend.
• In the downstream bend, in addition to the downstream straight path’s influencing the incipient motion conditions, the upstream bend geometry also affects bed topography variations in the downstream bend.
• There are more bed topography variations created in the downstream bend than in the upstream bend.
• The maximum sedimentation height occurs near the inner bank, and the maximum scour depth occurs near the outer bank.

Comment 2:

 

The results of the hypothesis testing should be discussed in the conclusions. For example, the conclusions could say something about the straight flow entrance conditions at the upstream bend produce weaker secondary circulation, sedimentation, and scour through the upstream bend compared with stronger secondary circulation, sedimentation, and scour through the downstream bend.

 

 

Reply to Comment 2:

 

Regardless of a lack of hypotheses in the paper in the first version of the manuscript, the obtained results had been discussed, and a summary of their most important notes had been presented. This has been addressed with greater care according to the hypotheses provided in the revised manuscript.

 

 

 

Comment 3:

 

I recommend that the conclusion state something like the following paragraph:

 

This study's dimensionless results can be compared with the results of other experimental, numerical, or field studies that have a similar channel curvature, width-depth ratio, Froude number, and Reynolds. The measured conditions under this study (e.g., maximum sedimentation and scouring locations, bed topography variations, channel bend’s behavior, and the qualitative velocity pattern) are expected to similar to other laboratory or natural channels with similar conditions.

 

 

Reply to Comment 3:

 

As you kindly suggested, major modifications have been applied to the Conclusion in the revised manuscript. As observed in the present revised manuscript, these explanations have been added to the Conclusion, and the previous explanations have also been edited.

 

Author Response File: Author Response.docx

Reviewer 3 Report

The Authors addressed all my concerns satisfactorily, though I would have liked that some comments in the Authors' Response file were reported in the manuscript. However, this paper mainly presents experimental findings and therefore it can be fine like that. I have appreciated the experimental work - rather uncommon in literature - and the schemes and figures that describe it, as already remarked in my previous review. In conclusion I would recommend the acceptance of this manuscript. Minor changes in terms of style and typos could be applied at the proofreading stage.

Author Response

Reviewer #3

 

The Authors addressed all my concerns satisfactorily, though I would have liked that some comments in the Authors' Response file were reported in the manuscript. However, this paper mainly presents experimental findings and therefore it can be fine like that. I have appreciated the experimental work - rather uncommon in literature - and the schemes and figures that describe it, as already remarked in my previous review. In conclusion I would recommend the acceptance of this manuscript. Minor changes in terms of style and typos could be applied at the proofreading stage.

 

 

Dear Reviewer,

 

Thank you very much for your positive view of this paper. While we are truly grateful for your helpful comments, it is necessary to note that the comments which did not have long explanations and could be feasibly added to the first revised version of the manuscript were included in the manuscript. As it was evident in the previous highlighted version, Comments 1, 7, 8, and 9 were applied according to your opinion. Moreover, the rest of the comments were not added to the manuscript because a number of them only provided an explanation for your consideration, and a few of them could not be added to the manuscript due to their explanatory nature and because they required presenting numerous figures in the manuscript.

Following your comment, in the present revised manuscript, the paper's English language and style have been re-visited with great care, and the manuscript has improved accordingly. We would like to take this opportunity to thank you once again for your thorough consideration that has improved the quality of the manuscript for publication.

 

 

Author Response File: Author Response.docx