Investigating Deformation Mechanism of Earth-Rock Dams with InSaR and Numerical Simulation: Application to Liuduzhai Reservoir Dam, China

Round 1

Reviewer 1 Report

Summary: In this paper, InSAR technology and numerical simulation are used to investigate the deformation mechanism in Liuduzhai Dam, China. For earth-rock dams, dam displacement is the direct manifestation of hazard potential, which can be observed by InSAR technology. However, the deformation of earth-rock dams can be induced by a multitude of complex dynamic factors. The investigation into the deformation mechanism of earth-rock dams has always been a topic of great concern. In this paper, the authors acquired the deformation and dynamic seepage fields both pre- and post-reinforcement of the dam. Based on the findings, the authors have investigated the deformation mechanism of earth-rock dams. The present study is intriguing and the inferences drawn are sound. The main innovation is to reveal the deformation mechanism of earth-rock dams from a new perspective of InSAR time series deformation under a multitude of seepage conditions. However, there are some flaws in this paper.

Comments:

1.     My main questions and criticisms: It is well known that the SBAS-InSAR method can obtain more dense monitoring points than the PS-InSAR technology. The reasons for using two Mt-InSAR methods need to be explained carefully in this manuscript.

2.     Title, Page 1, line 1: In this paper, the numerical simulation method is utilized for seepage analysis. However, the numerical algorithm has not been improved. Therefore, We recommend replacing numerical modeling with numerical simulation or seepage analysis in the title.

3.     Abstract, line 21 and 22: The InSAR results indicated that the maximum deformation rate is -24.5 mm/yr before reinforcement, and the cumulative displacement exceeds 100 mm. However, the temporal coverage of ALOS-1 images spanning from 13 January, 2007 to 24 October, 2010 is less than four years. The accuracy of the maximum deformation rate needs to be carefully verified.

4.     Fig3b, Page 5, line 178: There is a discrepancy in the width of the water level line between the reservoir and the measuring weir.

5.     Title 3.1, Page 6, line 202: The correct terminology should be changed from SMT-InSAR to MT-InSAR.

The English language is readable and there are some minor language errors that need to be adjusted.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

This paper reads well and will be of interest to the audience. The application of InSAR monitoring to dams is of high potential value for society. We need to be clear about any uncertainty and limitations in the application.

A few uncertainties on this case study from my perspective:

Line 52 GNSS has high spatial resolution perhaps the advantage is in term of remote access where ground access may be difficult and also data collection time?

Line 74 do you mean temperature or change in temperature?

Line 137 Fig 2 shows stratigraphy not lithology and legend is not in stratigraphical order.

Line 164 what is the classification that designates third-class dam?

Line 360 appear to be 4 variables but only 2 figures - presumably SBAS for ALOS-1 and PS-InSAR for Sentinel 1? Whilst the different processing methods are described in section 3 the implications for data resolution and uncertainty of comparing the two different techniques do not appear to be considered. 

Line 357 The gap in any data between the two data sets, i.e. between 2011 and 2016 limits the interpretation that can be achieved. 

Line 391 Profiles aligned to touch, but not pass through zones of maximum deformation.

Line 414 would the negative pore pressure developing in d-f also lead to shrinkage.

Line 455 how has the high correlation of deformation and reservoir level been verified? It is not obvious in the time-series data. The gap in the time series makes it difficult to understand the rise in ground level 2016-2017. How does the lower resolution of the 2007 - 2011 InSAR data impact the interpretation, e.g. during guide wall construction?

Line 473 is this referenced to Fig 12b?

Re. subrosion is there any description of the core/ shell material does the grading (e.g. uniformity coefficient) suggest that this is likely as a process.

2018-2019 the reservoir was relatively low all year during this period, comparison with the InSAR data suggests that there might be a time lag to deformation that you could consider.  

Good. A few points to consider:

Line 43 what is meant by simple?

Line 62 assist could be deleted

Line 65 and elsewhere et al. full stop needed as this is an abbreviated word.

Line 81 does this mean compromised either because of poor regulation or damage?

Line 88 reusability = transferability?

Line 97 Furthermore could be deleted.

Line 107 deformation pattern in the context of dam related processes?

Line 140 faults that range in size.

Line 141 should it be Fault?

Line 144, 145 English spelling grey

Line 152 built between 1978 and 1986...

Line 159, 160 not sure if qualified in this sense relates to specification or quality assurance

Line 328 should this be in situ?

Line 359 does LOS need to be defined?

Line 590 should this be decrease?

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript:

"Investigating deformation mechanism of earth-rock dams with InSAR and numerical modeling: Application to Liuduzhai reservoir dam, China", by G. Liu, J. Hu, L. Liu, Q. Sun and W. Wu (Ref. No.: remotesensing-2477746-peer-review-v1.pdf),

contains interesting results and may potentially be suitable for publication. However, it is not well-organized and requires a significant elaboration. In particular, the objective and the uniqueness of this work are vague and should be stronger emphasized. Furthermore, this work does not provided data for the average deformation velocities per year before and after reinforcement. It would be also desirable to extrapolation the data to predict deformation velocities per year in the nearest future. The model also does not provide recommendation on how often and where (at which location) the reinforcement should be made in future.

English is acceptable. However, the manuscript requires some citations.

Apart from this, the following should be taken into consideration:

Abstract

1) Abbreviation for the InSAR should also be shown in the Abstract.

2) The sentence: “It is recommended that InSAR deformation monitoring should be incorporated into future dam safety programs to provide detailed deformation signals to assist in seepage safety assessments of dams”, looks too obvious. More specific details should be briefly provided.

3) What are average deformation velocities per year before and after reinforcement?

4) Does your model predict how often the reinforcement should be made?

1. Introduction

1) The sentence: “Xiao et al analyzed the InSAR time series deformation characteristics prior to the failure of the Uzbekistan Sardoba dam …”. Is there a common nature leading to deformation between Liuduzhai and Sardoba reservoir dams? If yes, the specific facts should be pointed out.

2) The sentence: “To further investigate the deformation mechanism of earth-rock dams, this study focuses on the Liuduzhai dam of a large (II) reservoir in Hunan Province, China”. The objective and originality of this work is vague. Therefore, objective and originality of this study should be stronger emphasized at the end of the section Introduction.

2. Study site and dataset

1) The sentence: “The geological strata exposed in the dam, ranging from ancient to modern, consist of the Jiangkou Formation of the Lower Devonian System (Zaj), Qiziqiao Formation of the Middle-Upper Devonian System (D2q), residual slope deposit of the Quaternary System (Qel+dl), alluvial deposit (Qal), and artificial filling soil (Qml)”, should be cited.

2) The sentence: “In 2000, the Hunan Province Water Resources and Hydropower Scientific Research Institute conducted a safety assessment of the dam, concluding that the seepage safety performance was unsatisfactory and classified as a third-class dam”, should be cited.

3. Methodology

1) The sentence: “However, this method is not effective in detecting small deformations in terms of phase decorrelation or low coherence”. It is not clear why this method is not effective in detecting small deformations. This should be described in more detail.

2) The sentence: “However, considering the characteristics of the SAR dataset before and after the reinforcement of Liuduzhai dam, the combination of the two methods could complement each other's limitations”. It is not clear how the combination of the two methods could complement each other's limitations. More descriptions are needed.

3) Equation (7) should be cited.

4. Results

1) Perhaps the sections ‘Results’ and ‘Discussion’ should be merged into one section named ‘Results and discussion’.

2) The sentence: "Comparing Fig 8a and b, it can be concluded that the reinforcement of the dam significantly reduced the deformation magnitude and range of the dam, thus suppressing dam deformation effectively". The maximum deformation velocity -1.2 mm/yr is still significant and potentially dangerous. What should be done to reduce the deformation further?

3) What is the average deformation velocity before and after reinforcement?

4) Is it possible to extrapolate data to predict the maximum and average deformation velocity over the next, say, five to ten years?

5. Discussion

1) The sentence: “Dam leakage is a primary triggering factor for the deformation and failure of earth-rock dams”. It is reasonable to assume that the water leakage is one of the main factors for the deformation. But how to prove that the leakage is indeed a primary triggering factor for the deformation and failure of earth-rock dams?

2) What are the disadvantages of your numerical simulation techniques?

6. Conclusions

1) The average deformations per year before and after reinforcement should be provided.

2) How often the reinforcement should be made?

The manuscript requires a major mandatory revision.

Quality of English is acceptable.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 4 Report

I suggest to move lines 111 to 120 to the introduction. It is currently under "geological site setting", but there is nothing concerning geology in this paragraph.

In line 152 I suggest to change "It was built in 1978", to "Construction started in 1978".

I noticed that dam design is not a straight line between the two mountain slopes but it is angular and protruding toward the downstream side. In my understanding this means the observed movements of some parts of the dam are indications that the dam material is stretched and may reduce mechanical stability and facilitate more seepage. I may have overlooked it but I think this aspect should be discussed. 

In line 359 it is stated that the data used are Line Of Sight (LOS) data. LOS data indicate movements in one dimension only, while in reality movements may happen in all three dimensions. I guess the dominant direction (x, y) would be horizontal while there is probably relatively litte vertical movement. I do not understand how you converted LOS data to (horizontal?) movement data. In many parts of the article you describe "deformation". Is this LOS deformation? If yes, it should be pointed out that this is not corrected horizontal movement. I suspect that actual horizontal movements are higher than the LOS data indicate. From my point of view it is necessary to explain the limitations of LOS data, or how LOS data were converted and segregated into the x, y, z components of the movement.

Were any in-situ measurements of the dam deformation conducted? This could indicate how accurate the InSAR-based data are.  

English is fine as far as I can see, but I am not an English native speaker.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The manuscript:

“Investigating deformation mechanism of earth-rock dams with InSAR and numerical simulation: Application to Liuduzhai reservoir dam, China”, by G. Liu, J. Hu, L. Liu, Q. Sun and W. Wu (Ref. No.: remotesensing-2477746_v2),

has been improved after major revision. In particular, the authors included more material, numerical results and required citations. Moreover, they provided clarifications and explanations. Therefore, the manuscript looks more complete. However, the manuscript requires some minor elaborations and the following should be taken into consideration:

1) The sentence: "It is recommended that InSAR deformation monitoring should be incorporated into future dam safety programs to provide detailed deformation signals". It is not clear how the InSAR deformation monitoring can be incorporated into future dam safety programs.

2) The sentence: “The dynamic seepage caused by water level fluctuations in the reservoir can easily lead to leakage [30,31]”. The sentence looks incomplete. It should be something like: “The dynamic seepage caused by water level fluctuations in the reservoir can easily lead to leakage due to soil erosion”.

3) The sentence: “However, the deformation development and its mechanism of the dam are still unclear”. The sentence should be rewritten something like: “However, the complete understanding of the deformation development and its mechanism of the dams are still unclear”.

4) The sentence: “The maximum deformation velocity of -1.2 mm/yr remains notably significant and potentially hazardous for dams”. Why the rate -1.2 mm/yr is potentially hazardous for dams? What are criteria to quantify potential hazardous for dams?

5) The sentence: “While the numerical simulation method employed in this study does not account for the calculation of the stress-strain field and dam stability …”. What are the possible side of ignoring the calculation of the stress-strain field and dam stability?

 The manuscript requires a minor revision.

Author Response

Please see the attachment 

Author Response File: Author Response.docx