Morphodynamic model of the lower Yellow River: flux or entrainment form for sediment mass conservation?

An, Chenge; Moodie, Andrew J.; Ma, Hongbo; Fu, Xudong; Zhang, Yuanfeng; Naito, Kensuke; Parker, Gary

doi:https://doi.org/10.5194/esurf-6-989-2018

Articles | Volume 6, issue 4

https://doi.org/10.5194/esurf-6-989-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-6-989-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 6, issue 4

Research article

|

06 Nov 2018

Research article |

| 06 Nov 2018

Morphodynamic model of the lower Yellow River: flux or entrainment form for sediment mass conservation?

Chenge An, Andrew J. Moodie, Hongbo Ma, Xudong Fu, Yuanfeng Zhang, Kensuke Naito, and Gary Parker

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Final revised paper (published on 06 Nov 2018)
Supplement to the final revised paper
Preprint (discussion started on 12 Jun 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of manuscript Chenge An et al.', Anonymous Referee #1, 14 Jul 2018
RC2: 'Review: Chenge An et al', Anonymous Referee #2, 17 Jul 2018
AC1: 'Reply to referees' comments', Chenge An, 19 Aug 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Chenge An on behalf of the Authors (19 Aug 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (03 Sep 2018) by Jens Turowski

RR by Anonymous Referee #3 (06 Oct 2018)

Suggestions for revision or reasons for rejection

This paper presents one-dimensional morphodynamic modeling results in which the so-called “flux” and “entrainment” forms of the Exner equation of sediment mass conservation are compared with one another. The Lower Yellow River is used as an example because it has high equilibrium sediment supply which recently has been reduced due to dam operations, and because it has fine enough sediment that the entire system can be modeled with suspended load only. The paper focuses on short-duration (<1 year) response, and finds that when sediment mixtures are modeled, the sorting patterns are quite different, with the flux-based model predicting grain-size-specific kinematic waves, while the entrainment-based model predicts a more diffusive sorting response. The paper goes on to characterize adaptation length scales, and suggests that the entrainment-based formulations are more appropriate for fine-sediment systems with long adaptation length scales and small fall velocities.

Overall I think this is an interesting, well-written paper on a topic that merits attention, and it is likely to be of interest to anyone who works in the field of river morphodynamic modeling. I notice that the paper has undergone revisions already (I was not a reviewer on the previous version) and it shows, as I have relatively few comments or suggestions for improvement.

My primary suggestion concerns the presentation of the results comparing the two forms of the Exner equation – i.e., the “delta” metric. I think it is fine and appropriate to calculate the relative difference of the different variables predicted by the two models, but I have a concern that as framed, the delta metric may over- or (especially) under-represent the relative difference between the two results. For example, delta(zb) is shown to be within about 4% after 0.2 years for the uniform sediment simulations. This will be highly sensitive to the choice of elevation datum used in the model; i.e., if the initial upstream bed elevation is 1000 m instead of 20 m, then the 2.3 vs. 3 m of scour observed would lead to a delta(zb) of about 0.07%! I recommend calculating the delta values with the deviation from the initial condition, rather than the final value, for both the elevation and other variables (grain size, sediment transport rate). If that were done, then the delta(zb) for the uniform case would be (3-2.3)/3 = 23%, instead of 4%. Calculating results this way may change some of the story, but I suspect the overall message would remain the same.

A major motivating question for this paper is which version of the Exner equation is appropriate. The authors provide some guidance, suggesting that the adaptation length scale of the sediment in the system should be an important consideration, and I think their reasoning is sound. Some additional comparison with actual data from the Lower Yellow River would be helpful as well though – Does the LYR show evidence of sorting waves, etc., which could provide additional context or support for this argument?

Additional comments by line number:

20: “local function of bed shear stress” = although it is common to use bed shear stress to calculate sediment transport rates, it is not the only possibility (i.e., one could use velocity, stream power, etc.). Suggest being a bit more general, perhaps “sediment transport is a function of local hydraulic conditions” or something.

21: “equilibrium” – suggest clarifying that here you mean equilibrium with hydraulic conditions (as opposed to local sediment balance)

22: “identify” is a strange word here – maybe use “represent”?

27: should be noted in the abstract that you are using a one-dimensional morphodynamic model.

33: Related to the general comment above – it would benefit the abstract to have a statement about which version of the Exner equation is appropriate in what circumstances.

43: see comment above about local bed shear stress (line 20)

50: “pioneering work *on* bedload transport”

166: “summer” should be “summed”

195: “form” should be “from”

260: double period at the end of the sentence

263: Can you give some justification as to why a relation without a hiding function is not appropriate for the LYR?

292: The porosity used in the modeling was already given in line 268

377-379: I am not convinced you need to include these sentences – just state that the supplement looks at hydrographs.

390: 173.7 Mt/a – is this correct? Adding washload nearly doubles the sediment load? Also, this value is larger than the stated range of 89-126 Mt/a.

Figures 6, 7, and 8: The vertical axes on the grain size graphs say the values are in mm, not um.

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RR by Astrid Blom (07 Oct 2018)

ED: Publish subject to minor revisions (review by editor) (08 Oct 2018) by Jens Turowski

AR by Chenge An on behalf of the Authors (17 Oct 2018) Author's response Manuscript

ED: Publish as is (22 Oct 2018) by Jens Turowski

ED: Publish as is (22 Oct 2018) by Tom Coulthard (Editor)

AR by Chenge An on behalf of the Authors (25 Oct 2018)

Short summary

In most models of river morphodynamics, sediment mass conservation is described by the Exner equation, which may take either the flux form or the entrainment form. Here we compare the two forms of the Exner equation under conditions typical of the lower Yellow River. We find that when using a single sediment grain size, there is little difference between the two forms. But when considering sediment mixtures, the two forms will show very different patterns of grain sorting.