The influence of a vegetated bar on channel-bend flow dynamics

Bywater-Reyes, Sharon; Diehl, Rebecca M.; Wilcox, Andrew C.

doi:https://doi.org/10.5194/esurf-6-487-2018

Articles | Volume 6, issue 2

https://doi.org/10.5194/esurf-6-487-2018

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

https://doi.org/10.5194/esurf-6-487-2018

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

Articles | Volume 6, issue 2

Research article

|

14 Jun 2018

Research article |

| 14 Jun 2018

The influence of a vegetated bar on channel-bend flow dynamics

Sharon Bywater-Reyes, Rebecca M. Diehl, and Andrew C. Wilcox

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Final revised paper (published on 14 Jun 2018)
Supplement to the final revised paper
Preprint (discussion started on 25 Sep 2017)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of the influence of a vegetated bar on channel-bend flow dynamics', Anonymous Referee #1, 27 Oct 2017
- AC1: 'Author Response to Reviewer 1 Comments', Sharon Bywater-Reyes, 09 Dec 2017
RC2: 'Review of 'The influence of a vegetated bar on channel-bend flow dynamics'', Anonymous Referee #2, 06 Nov 2017
- AC2: 'Author Response to Reviewer 2 Comments', Sharon Bywater-Reyes, 09 Dec 2017
EC1: 'Brief editorial comment', Jens Turowski, 14 Nov 2017
- AC3: 'Author Response to Editor Comments', Sharon Bywater-Reyes, 09 Dec 2017

Peer-review completion

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

AR by Sharon Bywater-Reyes on behalf of the Authors (03 Jan 2018) Author's response Manuscript

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

RR by Anonymous Referee #2 (16 Jan 2018)

Suggestions for revision or reasons for rejection

The authors have responded clearly to my comments and have improved the manuscript. However, I still have a few concerns regarding the methods used.

1. Unfortunately I still cannot work out the precise meaning of the text in the supplementary material, describing the gridding process. This may seem like a minor point, and I apologise if I am missing something, but I think it is important as others may wish to reproduce the modelling methods. It appears from the text that a curvilinear grid was created from the topography (2.5m); the authors then ran the model in Cartesian co-ordinates (topography mapped from the previous grid?) and then mapped these Cartesian velocities back on to a different curvilinear grid with higher resolution (2m)? The text suggests Cartesian velocities (Pg 8,Ln 17. Figs 3 &4) but the text also mentions that FASTMECH solves in streamwise and cross-stream co-ordinates (Pg 7, Ln 4-5)? Therefore, could the authors clarify:
• Was the hydraulic model run in Cartesian or orthogonal curvilinear co-ordinates? Why were two grid conversions required?
• What is meant by the “average cell size” (Supp pg 1, Ln 13) if “the grid size was constant for the whole domain” (Supp pg 1, Ln 15)? Does it mean constant area but not constant length/width?
• What precisely is meant by “unable to maintain a curvilinear, channel-fitted grid (nodes overlapped)”

2. In response to my initial comments, the authors confirm that for the comparison case in the calibration, the un-vegetated model outperforms the vegetated model. This is not necessarily surprising if the un-vegetated model has been pre-calibrated using C_d (i.e. vegetation possibly already accounted for in bed C_d). However, it does question the accuracy and usefulness of the vegetation model component and the resulting analysis. I agree that even if the vegetation model shows a higher RMSE in WSE, it might perform better at predicting local velocities, but I cannot see any evidence of this presented here. The results show that the vegetation model produces different results, but not necessarily more accurate results. Is it possible to use the field data to show that the vegetation model enhances prediction of local velocity patterns (i.e. apply the vegetation model at a different discharge where velocity data are available for calibration/validation)? At present, although I follow the logic of model development, based on the evidence presented I find the case for the application of the vegetation model unconvincing

3. It would be helpful to include the drag equation in its area-averaged form rather than in the single stem form given, so that the stem density, which is listed as a key variable, appears explicitly, as it would have done in the actual model equation. I expect it would look something like that in Nepf (1999, as already cited in the text) although that paper states it per unit mass.

4. In Figure 5 what is the j co-ordinate unit, cells? and is j defined as the cross-stream co-ordinate in the text? Might units of metres be better for comparing with other figures?

5. Pg 20 Line 11: typo: “…to be most…”

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RR by Anonymous Referee #1 (25 Jan 2018)

Suggestions for revision or reasons for rejection

I have read both versions of the submitted manuscript and think that the authors have addressed all of my previous major concerns. I only have minor editorial comments to help with clarification of some points.

Page 3, lines 14-29: I am a little confused about the topic of this paragraph, it begins with discussing how simply adding vegetation resistance corrections can be problematic and that use of cylinder drag may be better. It is clear that Vargas-Luna used cylinders, and presumably Iwasaki et al. did with drag corrections? What about Marjoribanks et al.? Can you clarify?

Page 4, line 20: do you mean drainage area “is”
Page 8, line 8, extra period here.

Page 19, line 1: I still think there is some confusion about negative v meaning a decrease in velocity that is carried over from the previous version of the manuscript. In the Figure 7 v becomes more negative to the right of the patch and you state this in the text but in parenthesis, you state that v was reduced by 180%. I don’t think this is true, if it has become more negative, it must have increased? Or am I misunderstanding something? If this is true, can you please make sure to go through all of the cases in which v becomes more negative and make sure that you do not say that it decreased?

Page 19, line 9 “Laboratory studies by Blanckaert (2010), representing sharp meander bends, identified relationships between zones of inward versus outward mass transport, transverse bed profiles, and curvature variations” I feel like this statement is pretty general and doesn’t actually describe the relations between all of these parameters, which makes it difficult to understand what the main point is here as it relates to your paper and the surrounding sentences. Maybe you could clarify some?

Page 20, first paragraph: This seems to be more of a restatement of the results than a discussion of what the results mean, why the results occurred, or how they relate to the previous literature. I am not really clear about the main goal of this paragraph?

Page 21, line 7 “Our analysis, more comparable to the flood-dispersed case, shows the potential for development of vegetated islands, but also for prevention of chute cutoff through bar-head maintenance; chute cutoff may be more likely in the absence of vegetation (Constantine et al., 2010).” But your analyses also show that the velocities within a potential chute on the point bar are increased by the presence of vegetation. So the velocity may be lower at the bar head with vegetation, but if velocities are increasing downstream in the incipient chute, couldn’t you have upstream migration of erosion within the chute that counteracts any potential deposition at the bar head? Just a thought….

Page 21, line 22. Does HIPS abbreviate something? Please define.

Page 21, line 28 Can you please briefly explain this “from increasing bank erosion rates that would tend to increase sinuosity,” Why would higher bank erosion rates result in a higher sinuosity?

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ED: Reconsider after major revisions (26 Jan 2018) by Jens Turowski

AR by Sharon Bywater-Reyes on behalf of the Authors (06 Apr 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Apr 2018) by Jens Turowski

RR by Anonymous Referee #2 (26 Apr 2018)

ED: Publish as is (02 May 2018) by Jens Turowski

ED: Publish as is (02 May 2018) by Tom Coulthard (Editor)

AR by Sharon Bywater-Reyes on behalf of the Authors (09 May 2018) Manuscript

Short summary

Channel bends and bars make up an important physical and ecological unit within rivers. Many riparian vegetation species need river bars for their life cycle, colonizing bars after flooding events. Once vegetation is established on bars, they can alter the flow and patterns of erosion and deposition. We used a hydraulic model to quantify the impact such riparian vegetation of various sizes and densities has on flow dynamics and inferred the expected changes in river erosion and deposition.