the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Dec 2021
Regularity of transportation for cohesive bank-collapsed materials
Abstract. The transportation of bank-collapsed materials is a key issue among river evolution processes. In this study, a series of flume experiments were conducted to monitor riverbank collapse processes and to explore the regularity of transportation for cohesive collapsed materials. The collapsed materials, both the bed and suspended loads, that transformed from collapsed materials were intensively evaluated under experimental conditions. The results showed that the collapsed materials contributed to 12～20 % sedimentation in situ, 8～14 % suspended loads and 70～80 % bed loads. In addition, the bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. This research provides theoretical and practical benefits for predicting channel evolution processes.
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Interactive discussion
Status: closed
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RC1: 'Comment on esurf-2021-97', Anonymous Referee #1, 12 Jan 2022
Comments to the editor:
The authors dedicated to explore the regularity of transportation for cohesive collapsed materials. The regularity contains two parts: quantity and energy. Physical experiments were conducted to monitor riverbank collapse processes, and the percent of collapsed materials were calculated by critical particle method. The bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation materials in terms of energy dissipation. The research contents and technique proposed in the study are interesting and useful in understanding river evolution process. The manuscript is basically well organized. Therefore, I would suggest the manuscript be accepted after moderate revisions.
Comments to the authors:
The authors dedicated to explore the regularity of transportation for cohesive bank collapsed materials. The regularity contains two parts: quantity and energy. Physical experiments were conducted to monitor riverbank collapse processes, and the percent of collapsed materials were calculated by critical particle method. The bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. The research contents and technique proposed in the study are interesting and useful in understanding river evolution process. The manuscript is basically well organized. However, the manuscript still needs to be revised. Some problems are given as follows:
- Line 37, “Langendoen & Simon,2008”, suggest adding blank before 2008.
- Line 65, “Rijin and Leo”, suggest changing and to &.
- Line 100, “(45°, 60°, 75°, 90°)”, ensure consistent formatting of commas.
- Line 110, suggest changing “propeller” into “propeller-type current-meter”, “shows” to “showed”.
- Line 131, in Table 1, for each group of the experiments (No.1, 2, 3, 4), there were two “Flux” and “Water discharge time”, why?
- Line 131, please explain how you design the bank morphology and water discharge time.
- Line 137, 153, suggest changing “toe of the bank” to “bank toe”.
- Line 152, in Table 3, for each group of the experiments, there is only one “Collapse amount”, does the amount represent the whole collapse amount?
- Lines 160, 161, …, suggest changing the units “Nm-3”, “ms-2” to “N•m-3”, “m•s-2”, …
- Lines 171-175, please add literatures to support the sentences.
- Lines 192-193, Lines 209-213, the units such as “ms-1”, “ms-2” should be changed to “m•s-1”, “m•s-2”.
- Lines 194-198, it’s better to add literature to explain why you selected the sediment carrying capacity factor (U3g-1R-1ω-1). Please add literatures.
- Line 216, Line 242, in Figure 7 and Figure 8, whether the curves were the show of the equations (2) and (7)? If not, it is better to provide trend lines.
- Line 237, “κ=0.4, p=0.3551, N=0.72”, line 240, “ks=2D”, suggest adding blanks before and after the equal sign.
- Line 238, change “m/s” to “ms-1”.
- Lines 250-251, “sediment suspension energy decreased because of the drag reduction of suspended sediments provided by Zhang (1963)”, what does it mean? Please explain the drag reduction of suspended sediments.
- Lines 266-275, “There are still limitations that need to be addressed within future research …”. More specific research work should be addressed within future research based on this study, please rearrange this part.
-
AC1: 'Reply on RC1', Guosheng Duan, 23 Jan 2022
Dear Editor,
We would like to thank editor and reviewers for carefully reviewing the manuscript entitled “Regularity of transportation for cohesive bank-collapsed materials” (esurf-2021-97).
We very much appreciate editor and reviewers for their positive and constructive comments and suggestions. Below is our response to all individual comment/suggestions. We hope that the quality of the manuscript is greatly improved. If you have any questions about this paper, please do not hesitate to let us know.
Thank you and best regards.
Yours sincerely
Haifei Liu
haifei.liu@bnu.edu.cn.
Detailed responses to reviewer 1’s comments
We appreciate the reviewer for the constructive comments to our manuscript entitled “Regularity of transportation for cohesive bank-collapsed materials” (esurf-2021-97). We have made the revisions as suggested. Please see our point-to-point responses below (our responses are in blue color).
- Line 37, “Langendoen & Simon,2008”, suggest adding blank before 2008.
RE: We have added “blank” before 2008.
- Line 65, “Rijin and Leo”, suggest changing and to &.
RE: We have changed “and” to “&”.
- Line 100, “(45°, 60°, 75°, 90°)”, ensure consistent formatting of commas.
RE: We have changed “(45°, 60°, 75°, 90°)” to “(45°, 60°, 75°, 90°)”.
- Line 110, suggest changing “propeller” into “propeller-type current-meter”, “shows” to “showed”.
RE: We have changed the writing.
- Line 131, in Table 1, for each group of the experiments (No.1, 2, 3, 4), there were two “Flux” and “Water discharge time”, why?
RE: For each group of the experiments, the experiment lasted 1 hour. In the first 30 minutes, one flux was selected, in the following 30 minutes, another flux was selected. And the two fluxes represented dry and flood seasons respectively.
- Line 131, please explain how you design the bank morphology and water discharge time.
RE: Experiments were performed in a 25 m long rectangular flume with a width and depth of 0.8 m (Figure 1). First, bank slopes were designed based on the in-site investigation along the Ningxia-Inner Mongolia reach of Yellow River where the most common bank slopes were “45°, 60°, 75°, 90°”; second, bank toe width of each side was 0.2m which was the half of river bed width, so that bank collapse process could be detailed monitored; third, bank top width was design based on bank toe width and bank slopes.
Water discharge time was designed based on two conditions. First, bank collapse could occur among the water discharge time, so the water discharge time should not to be too short. Second, there is rarely bank collapse at the end of water discharge time, and then it is convenient to calculate the quantity of bank collapse. In addition, several preliminary experiments were performed before the actual experiments to observe bank collapse, then water discharge time was decided.
- Line 137, 153, suggest changing “toe of the bank” to “bank toe”.
RE: We have made the change.
- Line 152, in Table 3, for each group of the experiments, there is only one “Collapse amount”, does the amount represent the whole collapse amount?
RE: Yes, the collapsed amount represented the whole collapsed amount for each experiment.
- Lines 160, 161, …, suggest changing the units “Nm-3”, “ms-2” to “N•m-3”, “m•s-2”, …
RE: According to your suggestion, we have made the changes.
- Lines 171-175, please add literatures to support the sentences.
RE: We have added the literatures and the sentences have been changed into “In sediment-laden flow, coarse particles are usually transported as bed loads, while fine particles are transported as suspended loads. Although there were mutual transformations between these two in the transport processes, the quantities of bed and suspended loads transported by the water flow remained roughly the same under certain flow conditions (Qian et al., 1983; Shu et al., 2019).”
- Lines 192-193, Lines 209-213, the units such as “ms-1”, “ms-2” should be changed to “m•s-1”, “m•s-2”.
RE: Thank you for your suggestion, we have made the changes.
- Lines 194-198, it’s better to add literature to explain why you selected the sediment carrying capacity factor (U3g-1R-1ω-1). Please add literatures.
RE: Thank you for your suggestion, the literatures have been added.
Li, J., Zhao Z., and Wu, Y. (Eds): Hydraulics, Hohai University Press, Nanjing, ISBN: 9787563015580, 2001.
Qian, N., Wan, Z., and Yang, J. (Eds): Sediment motion mechanics, Science Press, Beijing, ISBN: 9787030112606, 1983.
- Line 216, Line 242, in Figure 7 and Figure 8, whether the curves were the show of the equations (2) and (7)? If not, it is better to provide trend lines.
RE: Thank you for your careful review. Yes, the curves were the show of the equations (2) and (7) based on the experiment data.
- Line 237, “κ=0.4, p=0.3551, N=0.72”, line 240, “ks=2D”, suggest adding blanks before and after the equal sign.
RE: According to your suggestion, we have added blanks before and after the equal sign. And the sentence has been changed into “κ = 0.4, p = 0.3551, N = 0.72”.
- Line 238, change “m/s” to “m•s-1”.
RE: we have changed “m/s” to “m•s-1”.
- Lines 250-251, “sediment suspension energy decreased because of the drag reduction of suspended sediments provided by Zhang (1963)”, what does it mean? Please explain the drag reduction of suspended sediments.
RE: Based on the literature (Zhang, 1963), it is pointed out that in sediment laden flow, the increase of suspended sediment content can reduce the energy dissipated by water flow. Then the theory was considered as the drag reduction of suspended sediments in several literatures (Miu et al., 1986; Shu, 2010). The literatures were listed as following:
Miu, H. Criteria for drag reduction of suspended sediment, J. Sediment Res., 2, 73-78, https://doi.org/CNKI:SUN:NSYJ.0.1986-02-009, 1986.
Shu, A. Study on efficiency coefficient of suspended load motion in sediment-laden flows, J. Basic Sci. Eng. J., 18, 209–216, https://doi.org/10.3969/j.issn.1005-0930.2010.02.0003, 2010.
- Lines 266-275, “There are still limitations that need to be addressed within future research …”. More specific research work should be addressed within future research based on this study, please rearrange this part.
RE: Thank you for your suggestion, we have rearranged this part as following:
There are still limitations that need to be addressed within future research. First, the quantity of the collapsed materials, bed and suspended loads in this study were obtained under specific flow conditions. For the complicacy of natural rivers, more bank shapes, angles and series of flow processes including both dry and flood seasons should be added. Second, although the law of energy dissipation is a promising approach to describe the transportation of collapsed materials, studies of sediment transportation in terms of energy dissipation are usually qualitative. More accurate measurement tools need to be explored and applied to obtain the energy consumed by the bed and suspended loads. Finally, the relationship between quantities and energy dissipation should be studied further to analyze the transportation of collapsed materials and benefit channel evolution prediction.
-
RC2: 'Comment on esurf-2021-97', Anonymous Referee #2, 30 Jan 2022
The transport of cohesive sediments and riverbank collapse processes are not well understood, though they are of vital importance for most (if not all) natural rivers. This study aims to improve our understanding of riverbank erosion using an experimental flume. The authors run experiments with different types of cohesive materials to examine sediment transport styles. They use sediment transport formulations to determine the amounts of sediment that were transported in bedload and suspended load based on grain size and flow velocity.
I found the paper to be very well-written and easy to read. While the ideas presented are at first intriguing—running large flume experiments to understand the role of materials in modulating riverbank erosion and their subsequent transport down channel—I have very significant concerns with the design and execution of this study. The biggest problems are 1) experimental design and reporting of results 2) use of obscure old sediment transport formulations and references that essentially ignore over half a century of extensive work. These problems make the paper currently unfit for publication.
Experimental Design and Results
The experiments are designed such that two different natural materials are placed on either side of a flume. Figures 1 and 3 show the experimental setup, which from the description and images looks generally acceptable to me. However, the figures and text mention various measurements that were taken that are not reported anywhere in the paper. For example, pore pressure gauge measurements, flow velocity measurements, and sediment concentration measurements at the outlet. Though the authors mention these, I find it bizarre that the only actual results from these experiments- which I know took a lot of hard work to conduct- are calculations of the weight and cross sectional area of collapsed sediment as shown in figure 6. In reality, the experiments as presented give no information about sediment flux, bedload vs. suspended sediment transport, or the role of different bank materials. The decision to use two different bank materials in a single experimental run is strange- on one hand, I can guess that the authors were hoping to compare the role of materials properties in consistent flow conditions, which is understandable. But without the ability to measure independently any of the subsequent sediment transport- for example by measuring sediment flux at the outlet- the difference between the two materials used in each experiment cannot be determined. If any of the measurement results exist and were not included in this paper, I highly recommend that the authors massively overhaul their presentation of the experiments. At present, they don’t really serve any purpose in the manuscript except to illustrate the sediment transport calculations that follow.
Sediment transport formulations and background literature
Because the authors were not able to experimentally measure the percentage of sediment that travels as bedload and suspended load, they use the grain size distribution from the experimental sediments to calculate expected values for sediment transport. Again, this illustrates the trivial nature of the experiments as presented- the authors could have chosen theoretical grain size distributions at random without running any experiments and obtained the same results. As it stands, this study is more of an exercise in doing sediment transport calculations than gaining any understanding in riverbank collapse. For example figures 7 and 8, while well presented, are just demonstrations of the already well-known dynamics of the equations used- they are not novel results because they do not compare with any measurements of bedload or suspended sediment flux from the experiments.
To make these calculations, the authors choose to use very old, obscure sediment transport formulations that ignore a very large body of work in fluvial geomorphology and engineering. For example, Eqn. 1 is used to calculate the size of sediment that would be transported under certain flow velocities. It is a bizarre formulation that includes the ratio of the diameter of a water molecule to sediment grain diameters (an approach I have never seen before and seems extremely suspicious). Decades of work has provided the background needed for predicting incipient particle motions in fluids, where the most canonical formulation is the shields criterion This concept can be found in many textbooks and hundreds of studies on sediment transport. An updated (but still over 30 years old!) formulation for the onset of sediment motion can be found in Wiberg and Smith, 1987. To be clear, there is certainly much room for improvement for shields stress formulations for sediment transport, and I would have no problem with the use of alternative formulations if they are adequately defended in the text. But to completely disregard the concept of shields stress entirely in a paper about incipient sediment motions causes me to worry that the authors are entirely unfamiliar with the entire field of sediment transport. The authors go on to use a Bagnold-like formulation that predicts the efficiency of the flow in moving sediment through bedload and suspended load. Again, though the general idea is ok, they use the original, very old Bagnold formulation from the 60’s that has since been modified and updated extensively (for an example see Martin and Church, 2000).
Suggestions for the authors
Reading the literature: It is possible that the authors have limited access to scientific studies, many of which are behind a paywall- if this is the case, then that is no fault of the authors. However, the classic sediment transport formulations that should be used in this study (or at least be mentioned, before the authors explain why they choose to use uncommon formulations), can be found in any modern geomorphology textbook even without the need for accessing dozens of scientific papers (e.g., Anderson and Anderson 2010). I would encourage the authors to dig into one of these textbooks to understand the current state of science in the study of sediment transport thresholds for motion. In the end, it’s possible that using a shields stress formulation may not change their calculations very much at all- but would help link their future work to the broader study of sediment transport, making it both more scientifically defensible and useful to the community.
Scientific study design: I think a more carefully designed set of experiments similar to what was described in the paper could result in an interesting study. The most interesting aspect of the paper to me is the idea of better understanding how different bank materials modulate bank collapse and sediment transport in rivers. Experiments could be run using only one bank material at a time. Carefully controlling the flow and conducting repeat experiments- say, 3-5 for each type of bank material- will ensure that results are robust. By using one material, the sediment flux at the outlet can be measured and reported for each different material, giving information about sediment transport styles and rates. Measurements within the flume could give more information about bedload vs suspended sediment fluxes that can be compared with theoretical formulations (like an updated Bagnold transport formulation). This would make the study much more robust, and would illustrate the utility of combining experiments and theoretical analysis.
References
Anderson, R. S., & Anderson, S. P. (2010). Geomorphology: the mechanics and chemistry of landscapes. Cambridge University Press.
Wiberg, P. L., & Smith, J. D. (1987). Calculations of the critical shear stress for motion of uniform and heterogeneous sediments. Water resources research, 23(8), 1471-1480.
Martin, Y., & Church, M. (2000). Reâexamination of Bagnold's empirical bedload formulae. Earth Surface Processes and Landforms: The Journal of the British Geomorphological Research Group, 25(9), 1011-1024.
Citation: https://doi.org/10.5194/esurf-2021-97-RC2 -
EC1: 'AE comment on esurf-2021-97', Jens Turowski, 02 Feb 2022
Dear authors,
祝你们虎年快乐!
We have now received two reviews for your paper. Reviewer #2, in particular, raises some serious concerns, related to the presentation and use of the experimental data and to the modelling appraoch. I generally agree with this assessment and ask you to provide appropriate revisions. Both points seem substantial to me and could require major revisions of the paper.
I am looking forward to seeing your revised manuscript. Please include a rebuttal, including replies to all of the comments and detailing the changes you made.
With all the best wishes,
Jens Turowski
Citation: https://doi.org/10.5194/esurf-2021-97-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on esurf-2021-97', Anonymous Referee #1, 12 Jan 2022
Comments to the editor:
The authors dedicated to explore the regularity of transportation for cohesive collapsed materials. The regularity contains two parts: quantity and energy. Physical experiments were conducted to monitor riverbank collapse processes, and the percent of collapsed materials were calculated by critical particle method. The bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation materials in terms of energy dissipation. The research contents and technique proposed in the study are interesting and useful in understanding river evolution process. The manuscript is basically well organized. Therefore, I would suggest the manuscript be accepted after moderate revisions.
Comments to the authors:
The authors dedicated to explore the regularity of transportation for cohesive bank collapsed materials. The regularity contains two parts: quantity and energy. Physical experiments were conducted to monitor riverbank collapse processes, and the percent of collapsed materials were calculated by critical particle method. The bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. The research contents and technique proposed in the study are interesting and useful in understanding river evolution process. The manuscript is basically well organized. However, the manuscript still needs to be revised. Some problems are given as follows:
- Line 37, “Langendoen & Simon,2008”, suggest adding blank before 2008.
- Line 65, “Rijin and Leo”, suggest changing and to &.
- Line 100, “(45°, 60°, 75°, 90°)”, ensure consistent formatting of commas.
- Line 110, suggest changing “propeller” into “propeller-type current-meter”, “shows” to “showed”.
- Line 131, in Table 1, for each group of the experiments (No.1, 2, 3, 4), there were two “Flux” and “Water discharge time”, why?
- Line 131, please explain how you design the bank morphology and water discharge time.
- Line 137, 153, suggest changing “toe of the bank” to “bank toe”.
- Line 152, in Table 3, for each group of the experiments, there is only one “Collapse amount”, does the amount represent the whole collapse amount?
- Lines 160, 161, …, suggest changing the units “Nm-3”, “ms-2” to “N•m-3”, “m•s-2”, …
- Lines 171-175, please add literatures to support the sentences.
- Lines 192-193, Lines 209-213, the units such as “ms-1”, “ms-2” should be changed to “m•s-1”, “m•s-2”.
- Lines 194-198, it’s better to add literature to explain why you selected the sediment carrying capacity factor (U3g-1R-1ω-1). Please add literatures.
- Line 216, Line 242, in Figure 7 and Figure 8, whether the curves were the show of the equations (2) and (7)? If not, it is better to provide trend lines.
- Line 237, “κ=0.4, p=0.3551, N=0.72”, line 240, “ks=2D”, suggest adding blanks before and after the equal sign.
- Line 238, change “m/s” to “ms-1”.
- Lines 250-251, “sediment suspension energy decreased because of the drag reduction of suspended sediments provided by Zhang (1963)”, what does it mean? Please explain the drag reduction of suspended sediments.
- Lines 266-275, “There are still limitations that need to be addressed within future research …”. More specific research work should be addressed within future research based on this study, please rearrange this part.
-
AC1: 'Reply on RC1', Guosheng Duan, 23 Jan 2022
Dear Editor,
We would like to thank editor and reviewers for carefully reviewing the manuscript entitled “Regularity of transportation for cohesive bank-collapsed materials” (esurf-2021-97).
We very much appreciate editor and reviewers for their positive and constructive comments and suggestions. Below is our response to all individual comment/suggestions. We hope that the quality of the manuscript is greatly improved. If you have any questions about this paper, please do not hesitate to let us know.
Thank you and best regards.
Yours sincerely
Haifei Liu
haifei.liu@bnu.edu.cn.
Detailed responses to reviewer 1’s comments
We appreciate the reviewer for the constructive comments to our manuscript entitled “Regularity of transportation for cohesive bank-collapsed materials” (esurf-2021-97). We have made the revisions as suggested. Please see our point-to-point responses below (our responses are in blue color).
- Line 37, “Langendoen & Simon,2008”, suggest adding blank before 2008.
RE: We have added “blank” before 2008.
- Line 65, “Rijin and Leo”, suggest changing and to &.
RE: We have changed “and” to “&”.
- Line 100, “(45°, 60°, 75°, 90°)”, ensure consistent formatting of commas.
RE: We have changed “(45°, 60°, 75°, 90°)” to “(45°, 60°, 75°, 90°)”.
- Line 110, suggest changing “propeller” into “propeller-type current-meter”, “shows” to “showed”.
RE: We have changed the writing.
- Line 131, in Table 1, for each group of the experiments (No.1, 2, 3, 4), there were two “Flux” and “Water discharge time”, why?
RE: For each group of the experiments, the experiment lasted 1 hour. In the first 30 minutes, one flux was selected, in the following 30 minutes, another flux was selected. And the two fluxes represented dry and flood seasons respectively.
- Line 131, please explain how you design the bank morphology and water discharge time.
RE: Experiments were performed in a 25 m long rectangular flume with a width and depth of 0.8 m (Figure 1). First, bank slopes were designed based on the in-site investigation along the Ningxia-Inner Mongolia reach of Yellow River where the most common bank slopes were “45°, 60°, 75°, 90°”; second, bank toe width of each side was 0.2m which was the half of river bed width, so that bank collapse process could be detailed monitored; third, bank top width was design based on bank toe width and bank slopes.
Water discharge time was designed based on two conditions. First, bank collapse could occur among the water discharge time, so the water discharge time should not to be too short. Second, there is rarely bank collapse at the end of water discharge time, and then it is convenient to calculate the quantity of bank collapse. In addition, several preliminary experiments were performed before the actual experiments to observe bank collapse, then water discharge time was decided.
- Line 137, 153, suggest changing “toe of the bank” to “bank toe”.
RE: We have made the change.
- Line 152, in Table 3, for each group of the experiments, there is only one “Collapse amount”, does the amount represent the whole collapse amount?
RE: Yes, the collapsed amount represented the whole collapsed amount for each experiment.
- Lines 160, 161, …, suggest changing the units “Nm-3”, “ms-2” to “N•m-3”, “m•s-2”, …
RE: According to your suggestion, we have made the changes.
- Lines 171-175, please add literatures to support the sentences.
RE: We have added the literatures and the sentences have been changed into “In sediment-laden flow, coarse particles are usually transported as bed loads, while fine particles are transported as suspended loads. Although there were mutual transformations between these two in the transport processes, the quantities of bed and suspended loads transported by the water flow remained roughly the same under certain flow conditions (Qian et al., 1983; Shu et al., 2019).”
- Lines 192-193, Lines 209-213, the units such as “ms-1”, “ms-2” should be changed to “m•s-1”, “m•s-2”.
RE: Thank you for your suggestion, we have made the changes.
- Lines 194-198, it’s better to add literature to explain why you selected the sediment carrying capacity factor (U3g-1R-1ω-1). Please add literatures.
RE: Thank you for your suggestion, the literatures have been added.
Li, J., Zhao Z., and Wu, Y. (Eds): Hydraulics, Hohai University Press, Nanjing, ISBN: 9787563015580, 2001.
Qian, N., Wan, Z., and Yang, J. (Eds): Sediment motion mechanics, Science Press, Beijing, ISBN: 9787030112606, 1983.
- Line 216, Line 242, in Figure 7 and Figure 8, whether the curves were the show of the equations (2) and (7)? If not, it is better to provide trend lines.
RE: Thank you for your careful review. Yes, the curves were the show of the equations (2) and (7) based on the experiment data.
- Line 237, “κ=0.4, p=0.3551, N=0.72”, line 240, “ks=2D”, suggest adding blanks before and after the equal sign.
RE: According to your suggestion, we have added blanks before and after the equal sign. And the sentence has been changed into “κ = 0.4, p = 0.3551, N = 0.72”.
- Line 238, change “m/s” to “m•s-1”.
RE: we have changed “m/s” to “m•s-1”.
- Lines 250-251, “sediment suspension energy decreased because of the drag reduction of suspended sediments provided by Zhang (1963)”, what does it mean? Please explain the drag reduction of suspended sediments.
RE: Based on the literature (Zhang, 1963), it is pointed out that in sediment laden flow, the increase of suspended sediment content can reduce the energy dissipated by water flow. Then the theory was considered as the drag reduction of suspended sediments in several literatures (Miu et al., 1986; Shu, 2010). The literatures were listed as following:
Miu, H. Criteria for drag reduction of suspended sediment, J. Sediment Res., 2, 73-78, https://doi.org/CNKI:SUN:NSYJ.0.1986-02-009, 1986.
Shu, A. Study on efficiency coefficient of suspended load motion in sediment-laden flows, J. Basic Sci. Eng. J., 18, 209–216, https://doi.org/10.3969/j.issn.1005-0930.2010.02.0003, 2010.
- Lines 266-275, “There are still limitations that need to be addressed within future research …”. More specific research work should be addressed within future research based on this study, please rearrange this part.
RE: Thank you for your suggestion, we have rearranged this part as following:
There are still limitations that need to be addressed within future research. First, the quantity of the collapsed materials, bed and suspended loads in this study were obtained under specific flow conditions. For the complicacy of natural rivers, more bank shapes, angles and series of flow processes including both dry and flood seasons should be added. Second, although the law of energy dissipation is a promising approach to describe the transportation of collapsed materials, studies of sediment transportation in terms of energy dissipation are usually qualitative. More accurate measurement tools need to be explored and applied to obtain the energy consumed by the bed and suspended loads. Finally, the relationship between quantities and energy dissipation should be studied further to analyze the transportation of collapsed materials and benefit channel evolution prediction.
-
RC2: 'Comment on esurf-2021-97', Anonymous Referee #2, 30 Jan 2022
The transport of cohesive sediments and riverbank collapse processes are not well understood, though they are of vital importance for most (if not all) natural rivers. This study aims to improve our understanding of riverbank erosion using an experimental flume. The authors run experiments with different types of cohesive materials to examine sediment transport styles. They use sediment transport formulations to determine the amounts of sediment that were transported in bedload and suspended load based on grain size and flow velocity.
I found the paper to be very well-written and easy to read. While the ideas presented are at first intriguing—running large flume experiments to understand the role of materials in modulating riverbank erosion and their subsequent transport down channel—I have very significant concerns with the design and execution of this study. The biggest problems are 1) experimental design and reporting of results 2) use of obscure old sediment transport formulations and references that essentially ignore over half a century of extensive work. These problems make the paper currently unfit for publication.
Experimental Design and Results
The experiments are designed such that two different natural materials are placed on either side of a flume. Figures 1 and 3 show the experimental setup, which from the description and images looks generally acceptable to me. However, the figures and text mention various measurements that were taken that are not reported anywhere in the paper. For example, pore pressure gauge measurements, flow velocity measurements, and sediment concentration measurements at the outlet. Though the authors mention these, I find it bizarre that the only actual results from these experiments- which I know took a lot of hard work to conduct- are calculations of the weight and cross sectional area of collapsed sediment as shown in figure 6. In reality, the experiments as presented give no information about sediment flux, bedload vs. suspended sediment transport, or the role of different bank materials. The decision to use two different bank materials in a single experimental run is strange- on one hand, I can guess that the authors were hoping to compare the role of materials properties in consistent flow conditions, which is understandable. But without the ability to measure independently any of the subsequent sediment transport- for example by measuring sediment flux at the outlet- the difference between the two materials used in each experiment cannot be determined. If any of the measurement results exist and were not included in this paper, I highly recommend that the authors massively overhaul their presentation of the experiments. At present, they don’t really serve any purpose in the manuscript except to illustrate the sediment transport calculations that follow.
Sediment transport formulations and background literature
Because the authors were not able to experimentally measure the percentage of sediment that travels as bedload and suspended load, they use the grain size distribution from the experimental sediments to calculate expected values for sediment transport. Again, this illustrates the trivial nature of the experiments as presented- the authors could have chosen theoretical grain size distributions at random without running any experiments and obtained the same results. As it stands, this study is more of an exercise in doing sediment transport calculations than gaining any understanding in riverbank collapse. For example figures 7 and 8, while well presented, are just demonstrations of the already well-known dynamics of the equations used- they are not novel results because they do not compare with any measurements of bedload or suspended sediment flux from the experiments.
To make these calculations, the authors choose to use very old, obscure sediment transport formulations that ignore a very large body of work in fluvial geomorphology and engineering. For example, Eqn. 1 is used to calculate the size of sediment that would be transported under certain flow velocities. It is a bizarre formulation that includes the ratio of the diameter of a water molecule to sediment grain diameters (an approach I have never seen before and seems extremely suspicious). Decades of work has provided the background needed for predicting incipient particle motions in fluids, where the most canonical formulation is the shields criterion This concept can be found in many textbooks and hundreds of studies on sediment transport. An updated (but still over 30 years old!) formulation for the onset of sediment motion can be found in Wiberg and Smith, 1987. To be clear, there is certainly much room for improvement for shields stress formulations for sediment transport, and I would have no problem with the use of alternative formulations if they are adequately defended in the text. But to completely disregard the concept of shields stress entirely in a paper about incipient sediment motions causes me to worry that the authors are entirely unfamiliar with the entire field of sediment transport. The authors go on to use a Bagnold-like formulation that predicts the efficiency of the flow in moving sediment through bedload and suspended load. Again, though the general idea is ok, they use the original, very old Bagnold formulation from the 60’s that has since been modified and updated extensively (for an example see Martin and Church, 2000).
Suggestions for the authors
Reading the literature: It is possible that the authors have limited access to scientific studies, many of which are behind a paywall- if this is the case, then that is no fault of the authors. However, the classic sediment transport formulations that should be used in this study (or at least be mentioned, before the authors explain why they choose to use uncommon formulations), can be found in any modern geomorphology textbook even without the need for accessing dozens of scientific papers (e.g., Anderson and Anderson 2010). I would encourage the authors to dig into one of these textbooks to understand the current state of science in the study of sediment transport thresholds for motion. In the end, it’s possible that using a shields stress formulation may not change their calculations very much at all- but would help link their future work to the broader study of sediment transport, making it both more scientifically defensible and useful to the community.
Scientific study design: I think a more carefully designed set of experiments similar to what was described in the paper could result in an interesting study. The most interesting aspect of the paper to me is the idea of better understanding how different bank materials modulate bank collapse and sediment transport in rivers. Experiments could be run using only one bank material at a time. Carefully controlling the flow and conducting repeat experiments- say, 3-5 for each type of bank material- will ensure that results are robust. By using one material, the sediment flux at the outlet can be measured and reported for each different material, giving information about sediment transport styles and rates. Measurements within the flume could give more information about bedload vs suspended sediment fluxes that can be compared with theoretical formulations (like an updated Bagnold transport formulation). This would make the study much more robust, and would illustrate the utility of combining experiments and theoretical analysis.
References
Anderson, R. S., & Anderson, S. P. (2010). Geomorphology: the mechanics and chemistry of landscapes. Cambridge University Press.
Wiberg, P. L., & Smith, J. D. (1987). Calculations of the critical shear stress for motion of uniform and heterogeneous sediments. Water resources research, 23(8), 1471-1480.
Martin, Y., & Church, M. (2000). Reâexamination of Bagnold's empirical bedload formulae. Earth Surface Processes and Landforms: The Journal of the British Geomorphological Research Group, 25(9), 1011-1024.
Citation: https://doi.org/10.5194/esurf-2021-97-RC2 -
EC1: 'AE comment on esurf-2021-97', Jens Turowski, 02 Feb 2022
Dear authors,
祝你们虎年快乐!
We have now received two reviews for your paper. Reviewer #2, in particular, raises some serious concerns, related to the presentation and use of the experimental data and to the modelling appraoch. I generally agree with this assessment and ask you to provide appropriate revisions. Both points seem substantial to me and could require major revisions of the paper.
I am looking forward to seeing your revised manuscript. Please include a rebuttal, including replies to all of the comments and detailing the changes you made.
With all the best wishes,
Jens Turowski
Citation: https://doi.org/10.5194/esurf-2021-97-EC1
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