Knickpoints and fixed points: the evolution of fluvial  morphology under the combined effect of fault uplift  and dam obstruction on a soft bedrock river

Chen, Hung-En; Chiu, Yen-Yu; Cheng, Chih-Yuan; Chen, Su-Chin

doi:https://doi.org/10.5194/esurf-12-1329-2024

Articles | Volume 12, issue 6

https://doi.org/10.5194/esurf-12-1329-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-12-1329-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 6

Research article

|

03 Dec 2024

Research article |

| 03 Dec 2024

Knickpoints and fixed points: the evolution of fluvial morphology under the combined effect of fault uplift and dam obstruction on a soft bedrock river

Hung-En Chen, Yen-Yu Chiu, Chih-Yuan Cheng, and Su-Chin Chen

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Final revised paper (published on 03 Dec 2024)
Preprint (discussion started on 30 Mar 2023)

Interactive discussion

Status: closed

RC1:
'Comment on esurf-2023-8', Anonymous Referee #1, 22 May 2023
I’ve reviewed the manuscript by Chen et al, examining fluvial geomorphology evolution caused by the factors like earthquake and dam construction. This manuscript presents interesting study and results, and it is well-written. I only have a few comments as follows:
What is the resolution of the DSM and the data from WRA that you use and how the resolutions might affect the interpretation of knickpoint locations and migrations?

Since the study is done by investigating temporal datasets, is there any significant changes in the amount of rainfall or precipitation rate over time that might have also influence/control in fluvial erosion and deposition processes. How can the variation in precipitation (year over year) impact the fluvial geomorphology evolution?

Moreover, do the amount of water flowing out of the dam is consistent year over year? If not, it might have affected the fluvial geomorphology in the downstream portion, and this should also be considered.

Another factor that seems to be important is geology or lithology variation. It might be good to discuss the lithology variation along the rivers and how it can affect your interpretation in knickpoint locations and migration.
Citation: https://doi.org/10.5194/esurf-2023-8-RC1
- AC1: 'Reply on RC1', Su-Chin Chen, 03 Aug 2023
  
  Thank you for your comments. We have taken your question seriously and carefully processed it. After collecting and analyzing relevant data, we are providing the following response in the hope that it addresses your concerns. Our detailed response is attached. The comments of the reviewers are shown in black text while our responses are provided in blue.
  
  Citation: https://doi.org/10.5194/esurf-2023-8-AC1
RC2:
'Comment on esurf-2023-8', Anonymous Referee #2, 27 Jun 2023

Chen et al. discuss the effects of co-seismic fault displacements and a dam on the evolution of longitudinal and cross-sectional forms of three bedrock rivers in Taiwan. The multi-temporal topographic data, used in this study are unique and valuable to understand the evolution of the bedrock rivers. The content of this study matches the scope of Earth Surface Dynamics; however, I believe there are some major issues to be addressed before publication. I provide the key comments below and the minor comments in the attached PDF.

Key comments:

1) Further descriptions and discussions on the combined effects of fault displacement and dam obstruction are required. Because the co-seismic knickpoint in the Dajia River disappeared due to river training in 2008, the upstream newer knickpoint and the evolution of the river following the migration of the newer knickpoint are not necessarily related to the fault activity. However, the current discussion implicitly assumes the newer knickpoint is related to fault activity. I think further explanations on how the river training in 2008 changed the channel morphology and the origin of the newer knickpoint. In addition, you have to explain Figure 13 in more detail. Although it must be the most important outcome of this study and you clearly state proposing the evolution model is the aim of this study, I do not see any descriptions on this figure.

2) It is not clear how you evaluated results of the 2-D river profile evolution. Although I am not familiar with the mathematical model in this study, the modeled river profiles do not seem to be consistent with actual river profiles. In the Daan river (fig 5), while the model predicts the river profile downstream of the knickpoint keeps its original shape during knickpoint retreat, the actual river profile changes its shape during knickpoint retreat (the knickzone stretches horizontally, and there is much less incision downstream of the knickpoint). In the Dajia River (fig 11), while the model predicts the channel slope upstream of the knickpoint is essentially the same except at the dam, the actual channel steepening occurred more extensively between the knickpoint and the dam.

3) Since the observations in the Daan river are similar to those presented in Cook et al. (2013) and Cook et al. (2014), you may want to clarify the difference between this study and previous studies or add newer implications. Although the data in 2017 is not included in Cook et al. (2014), it seems the current explanations on the Daan river are essentially the same as those in Cook et al. (2014).

Citation: https://doi.org/10.5194/esurf-2023-8-RC2
- AC2: 'Reply on RC2', Su-Chin Chen, 03 Aug 2023
  
  Thank you for your remarks and suggestions on our manuscript. Our detailed response is attached. The comments of the reviewers are shown in black text while our responses are provided in blue.
  
  Citation: https://doi.org/10.5194/esurf-2023-8-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Su-Chin Chen on behalf of the Authors (03 Aug 2023) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (21 Aug 2023) by Niels Hovius

Dear Chen Su-Chin and colleagues,

First, please accept my apology for the long delay of this editorial decision. Due to no fault of his own, our designated AE, Greg Hancock, has not been able to pursue his tasks. I have taken the unusual step of changing my role from senior editor to associate editor, and shall oversee further progress of your manuscript. I am aware of the fact that your study deals with examples on which I have worked earlier, and shall try to avoid any biass.

Greg Hancock's earlier efforts yielded two reviews of your work. Both referees are convinced that your manuscript can make a valuable contribution to the literature on erosional river dynamics. However, they highlight several issues that will require attention prior to any decision about publication.
Referee #1 is concerned about the influence of rainfall variations and changes in discharge over time. They also request attention to the role of lithology. It is true that the lithology of your two examples is particular, and I agree that it would be helpful to have some discussion of its effects on the portability of your results and findings.
Referee #2 has raised some important points about the development of the Daan and Dajia knick zones. They draw attention to the manipulation of the Dajia river, and to discrepancies between the observed and modelled evolution of the Daan long profile. In addition, they ask about insights that reach beyond those that were published earlier (this is where I must refrain from comment, and simply act as go between).

I encourage you to address these points and any others raised by the referees in their reports, and look forward to receiving a revised manuscript. It is likely that this new version will be sent out to reviewers, but we shall endeavour to keep the process lean on our side.

Best wishes,
Niels Hovius

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AR by Su-Chin Chen on behalf of the Authors (14 Sep 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Jan 2024) by Niels Hovius

RR by Anonymous Referee #2 (25 Jan 2024)

RR by Anonymous Referee #3 (17 Feb 2024)

Suggestions for revision or reasons for rejection

This is the review report for the manuscript, entitled: “Knickpoints and Fixpoints: The Evolution of Fluvial Morphology under the Combined Effect of Fault Uplift and Dam Obstruction on a Soft Bedrock River” by Chen et al. This study utilizes high-resolution elevation survey data combined with remote sensing images to obtain nearly 20 years of river longitudinal profiles with a resolution of submeter since 1999. The 1-D knickpoints evolution model based on the diffusion model, along with the digitization of sandbar within the river channel, contributes to the 3-D of river network configuration. Overall, the author successfully constrains the elevation changes of river channels and the main flow paths in three rivers along the Chelungpu Fault in central Taiwan. The author claims that the evolution of these three rivers can represent the influences of faults, dams, or a combination of both on river topography. This is highly meaningful research that distinguishes human activities from natural driving forces in shaping the natural landscape, providing insights into the field of anthropogenic geomorphology.

However, I share some concerns that need to be addressed before publication. Firstly, the quantification of contributing factors is crucial for establishing a comparative platform. The role of the fault: the vertical displacement caused by the 921 earthquake along the Chelungpu Fault led to a north-high and south-low displacement, creating discontinuities in river longitudinal profiles. Afterward, does the fault lose its role, and does it no longer contribute to tectonic uplift? Regarding the role of dams: how far can the tool effect of the dam persist, and does the nature of sediment on downstream change? The title may lead readers to expect a discussion of the similarities and differences between dams and knickpoints, but the content lacks corresponding discussion. Secondly, the manuscript extensively presents speculative inferences without citations. The author should avoid subjective inferences and excessive assumptions without literature support. The discussion section on the evolution processes of the three rivers is currently weak, primarily offering descriptive phenomena. Thirdly, restructuring sentences and simplifying images could enhance the readability of the paper

Line-by-line comments:
L21 Are there significant differences in the impact of “natural” tectonic movements and “artificial” tectonic movements on river topography?
L21 please specify “river equilibrium”.
L25 delete “significant”
L25-L28 The formation of Knickpoints is repeated in these two sentences; a suggestion to rephrase is advisable.
L30 delete “sudden”
L30-31 The occurrence of knickpoints does not necessarily alter the slope of the river channel.
L32-37 please rephrase the text to better elucidate the theme of this paragraph.
L41-44 Information provided is overly fundamental.
L59-65Suggest removing the introduction to the research area.
L71-72 Lacks appropriate citation of references.
L82 -83 Overly assertive.
L87-119 While the author has introduced the lithological settings of the riverbed, it is important to note that the incision or migration of the riverbed is also influenced by factors such as sediment particle size, sediment concentration, sediment distribution, and the compressive strength of rocks. Moreover, the discussion should ideally focus on the tectonic uplift during interseismic periods and the runoff discharge. Therefore, it is recommended that the author supplement relevant data to address these aspects in the study.
L121-133 The author has not appropriately separated the materials from the methods. There is a lack of clear explanation regarding the purpose of analyzing the width and depth of the river.
L135-137 Reasons for the Need for Mathematical Models?
L184-185 move to method
Please maintain consistency in the name of the Jiji (Chichi) dam.
L324-331 Please provide information in the materials section regarding the source and processing of the flow data.
Fig.1 Lacks information.
The subfigure in Figure 5 is unclear and lacks sufficient information.
Fig. 12a If the numbers on the T-bar represent the ratio of W/D, some values appear to be quite peculiar, for instance, the W/D values in the upper reaches of the Dajia River. I think a simple table can replace this figure.

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ED: Publish subject to minor revisions (review by editor) (10 Sep 2024) by Tom Coulthard

AR by Su-Chin Chen on behalf of the Authors (19 Sep 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (07 Oct 2024) by Tom Coulthard

ED: Publish as is (14 Oct 2024) by Tom Coulthard (Editor)

AR by Su-Chin Chen on behalf of the Authors (16 Oct 2024)

Short summary

This study explores the fluvial morphology evolution in three rivers in Taiwan caused by natural tectonic movements (the 1999 M_w 7.6 Chi-Chi earthquake) and human-made structures (dams). Knickpoints resulting from riverbed uplift shift, leading to gradual evolution from instability to equilibrium. Dams, on the other hand, cause continuous degradation of the bed. When both effects exist on a reach, the impact of the knickpoint gradually fades away, but the effects of the dam on the river persist.