the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Mar 2023
Knickpoints and Fixpoints: The Evolution of Fluvial Morphology under the Combined Effect of Fault Uplift and Dam Obstruction on a Soft Bedrock River
Abstract. Rapid changes in river geomorphology can occur after being disturbed by external factors like earthquakes or large dam obstructions. Studies documenting the evolution of river morphology under such conditions have advanced our understanding of fluvial geomorphology. The Dajia River in Taiwan presents a unique example of the combined effects of a coseismic fault (the 1999 Mw 7.6 Chi-Chi earthquake) and a dam. As a result of the steep terrain and abundant precipitation, rivers in Taiwan have exhibited characteristic post-disturbance evolution over 20 years. This study also considers two other comparative rivers with similar congenital conditions: the Daan River was affected by a thrust fault Chi-Chi earthquake, too; the Zhuoshui River was influenced by dam construction finished in 2001. The survey data and knickpoint migration model were used to analyze the evolution of the three rivers and propose hypothesis models. Results showed that the mobile knickpoint migrated upstream under the influence of flow, while the dam acted as a fixpoint, leading to an increased elevation gap and downstream channel incision. Thereby, the Dajia river narrowing and incision began at both ends and progressively spread to the whole reach under the combined effects.
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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.
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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).
- AC2: 'Reply on RC2', Su-Chin Chen, 03 Aug 2023
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