Effects of Dam Construction in the Wang River on Sediment Regimes in the Chao Phraya River Basin

Round 1

Reviewer 1 Report

This work investigated the effects of three dams in the Wang river on the sediment. This paper includes river flow and sediment data in different locations along the river for 90 years. The systematic investigation highlights the construction of these three dams in the Wang river did not affect the sediment significantly. I think this work is rigorous and meaningful to the community. I don’t have further comments and suggestions. I recommend acceptance in the present form.

Author Response

Reviewer#1

This work investigated the effects of three dams in the Wang river on the sediment. This paper includes river flow and sediment data in different locations along the river for 90 years. The systematic investigation highlights the construction of these three dams in the Wang river did not affect the sediment significantly. I think this work is rigorous and meaningful to the community. I don’t have further comments and suggestions. I recommend acceptance in the present form.

Response to Reviewer #1:

Thank you very much for your positive comments.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript is prepared professionally with all required information for readers. The only concern I have is the ratio of bedload to the total sediment load. In many stations, this ratio seems very high compared to those found in the literature. The authors may want to cross-check them with other studies in neighboring watersheds. These ratios are spacially very high for dry season (Table 2)

Also, numbers reported in lines 231-234 don't match the values in Figure4b. Either these numbers have not been reported correctly or the unit of the y-axis of this figure is not right.

Author Response

1. The manuscript is prepared professionally with all required information for readers. The only concern I have is the ratio of bedload to the total sediment load. In many stations, this ratio seems very high compared to those found in the literature. The authors may want to cross-check them with other studies in neighboring watersheds. These ratios are spacially very high for dry season (Table 2)

Response to Reviewer #2 (1):

Thank you for your constructive comment. The high bed-to the total sediment load ratio (BL/TSL) in the upper and middle reaches of the Wang River was mainly found downstream of the Kiew Koh Ma Dam (XW.4 and XW.5) and Kiew Lom Dam (XW.8). The BL/TSL at these stations was high during the dry season because the water released from the dams contained less suspended sediment (clear water), while the high velocity of the released water caused significant bedload transport. Similarly, the high BL/TSL was also found in the Ping River system, the adjacent river basin, in which the BL/TSL of 67% was found downstream of the Bhumibol Dam. By contrast, the BL/TSL in the Yom and Nan Rivers were 4% and 12%, respectively [1][2]. Based on field observation data, bedload transporting along the Ping and Wang Rivers was relatively high compared to that of the Yom and Nan Rivers. Then, many sand mines along the lower Ping River reach resulted [3].

References:

[1] Bidorn, B.; Kish, S.A.; Donoghue, J.F.; Bidorn, K.; Mama, R. Change in sediment characteristics and sediment load of the Nan River due to large dam construction. Proceedings of the 37th IAHR World Congress 2017, Kuala Lumpur, Malaysia, 13–18 August 2017; IAHR: Kuala Lumpur, Malaysia, 2017, 403–412.

[2] Namsai, M.; Bidorn, B.; Chanyotha, S.; Mama, R.; Phanomphongphaisarn, N. Sediment dynamics and temporal variation of runoff in the Yom River, Thailand. International Journal of Sediment Research 2020, 35(4), 365–376.

[3] Namsai, M.; Charoenlerkthawin, W.; Sirapojanakul, S.; Burnett, W.C.; Bidorn, B. Did the construction of the Bhumibol Dam cause a dramatic reduction in sediment supply to the Chao Phraya River? Water 2021, 13(3), 386.

 

2. Also, numbers reported in lines 231-234 don't match the values in Figure4b. Either these numbers have not been reported correctly or the unit of the y-axis of this figure is not right.

Response to Reviewer #2 (2):

Thank you for pointing this out. Yes. The unit of the y-axis of Figure 4b is not accurate. Figure 4b with a corrected unit of the y-axis has been replaced as Figure 5b in the revised manuscript. (page 11)

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript entitled: “Effects of Dam Construction in the Wang River on Sediment Regimes in the Chao Phraya River Basin” analyze the sediment discharge from a 91-years of sediment record to discuss the impact of the construction of the dam on the sediment regime of the Chao Phraya River Basin. The dataset used in the manuscript is comprehensive. The organization and writing of the manuscript are well. I have a few suggestions as follow:

1. The result indicates that the average total sediment load in the upper basin increased from 40,000 t/y upstream of the dam to 70,000 t/y downstream of the dam, with a slight increase to 100,000 t/y in the middle basin. Therefore, the construction of the dams in the Wang River did not greatly impact sediment supply. Because the variation in total sediment load reflects the total results of the changes in rainfall change, land-cover change, and dam construction, it is interesting to clarify major sources of sediment supply in the Wang River basin to better explain the increasing or decreasing of the total sediment load.  
2. The results show that the annual sediment load Wang River and the capacity of the three dams are both considerable. Generally, there will be more or less sediment trapped behind the dams and changed the sediment regime. Is there any data associated with the sedimentation of the major dam to support the conclusion?

Author Response

The manuscript entitled: “Effects of Dam Construction in the Wang River on Sediment Regimes in the Chao Phraya River Basin” analyze the sediment discharge from a 91-years of sediment record to discuss the impact of the construction of the dam on the sediment regime of the Chao Phraya River Basin. The dataset used in the manuscript is comprehensive. The organization and writing of the manuscript are well.

Response to Reviewer #3:

Thank you for your positive comments.

I have a few suggestions as follow:

1. The result indicates that the average total sediment load in the upper basin increased from 40,000 t/y upstream of the dam to 70,000 t/y downstream of the dam, with a slight increase to 100,000 t/y in the middle basin. Therefore, the construction of the dams in the Wang River did not greatly impact sediment supply. Because the variation in total sediment load reflects the total results of the changes in rainfall change, land-cover change, and dam construction, it is interesting to clarify major sources of sediment supply in the Wang River basin to better explain the increasing or decreasing of the total sediment load.  

Response to Reviewer #3 (1):

Thank you for raising this issue. We agree that the sources of sediment should be addressed in the Conclusion as reviewer suggested. We added the possible major source of additional sediment to the lower Wang River reach that has mentioned in the Discussion (lines 471-472) and the Conclusion as follows:

“The sediment load noticeably rose to more than 200,000 t/y in the lower basin. The maximum sediment load of 450,000 t/y occurred in the middle portion of the lower basin mainly due to more sediment supplied from Mae Tam and Mae Chang tributaries.” (lines 579-582)

 

2. The results show that the annual sediment load Wang River and the capacity of the three dams are both considerable. Generally, there will be more or less sediment trapped behind the dams and changed the sediment regime. Is there any data associated with the sedimentation of the major dam to support the conclusion?

Response to Reviewer#3 (2):

Thank you for pointing this out. We agree that the data on sedimentation in the reservoirs will strongly support the conclusion of the article. Unfortunately, reservoir sedimentation study in the Wang River and most of Thailand’s rivers has not been done yet. Since sedimentation in a reservoir is one of the crucial information for policing sustainable water management, it will be added to our future research works.

Author Response File: Author Response.docx

Reviewer 4 Report

The paper analyses the influence of dams on the sediment regime of the Wang river (Thailand). The analysis is based on historical datasets and additional complementary recent dataset.

Major issue:

Looking to the previous publications of the team (Namsai et al 2020 &2021), similar analysis has been performed on the Ping and Yom rivers. Even the structure of the papers is very similar. So the authors should better highlight what is the contribution or novelty of this new paper compared to both previous ones.  

 

Minor issues:

Section 2.2 More details/definitions should be provided about the p-value and the trend (used in table 1 and later).

Line 149: what is a wadable flow?

The section 3 is a bit long and difficult to read. It could be better if 3.2 (the new dataset) is presented first and then 3.1 and 3.3. It looks like 3.3 is similar to 3.1 but using rather the relationships (eqs. 1-8). It would be thus better to merge 3.1 and 3.3.

Section 3.1 For the riverflow, the historical dataset is formed of daily data. What about the solid discharge? Is it also daily or less? If it is less how many data/year do you have?

Section 3.2 Is the median diameter collected in the riverbed or from the water column (suspension)?  It may sound a bit surprizing to have such coarse sand (2 mm) and mainly suspension. Or is it finer bed material in suspension (silt or clay)?

 

Author Response

The paper analyses the influence of dams on the sediment regime of the Wang river (Thailand). The analysis is based on historical datasets and additional complementary recent dataset.

Major issue:

1. Looking to the previous publications of the team (Namsai et al 2020 &2021), similar analysis has been performed on the Ping and Yom rivers. Even the structure of the papers is very similar. So the authors should better highlight what is the contribution or novelty of this new paper compared to both previous ones.  

Response to Reviewer#4 (1):

Thank you for the constructive comment. We have addressed the new finding from the analysis of sediment data in the Wang River compared to the Ping and Yom Rivers in the Discussion to benefit the readers as the reviewer suggested. The additional text reads:

“Similar to the Ping River Basin, the sediment in the lower basin increased remarkedly downstream of the Bhumibol Dam. Although the drainage area of the dam accounted for 77% of the total drainage area of the Ping River Basin, it caused only 5% of sediment reduction in the CPR [5]. In contrast, the sediment load in the Yom River reduced dramatically at the basin outlet, although there is no large reservoir in the Yom river basin. This study reveals that damming may not be the primary human activity causing the sediment load reduction in a river basin. The impact of a dam likely depends on dam location and basin characteristics, especially variation of sediment load along a river. If a dam site is located in a basin with low sediment load upstream and high sediment load downstream, the effect of dam construction on sediment load would be lesser than that with a high sediment load upstream and a low sediment load downstream.

As environmental impacts due to damming have been widely mentioned during the recent years, construction of large reservoirs as a large-scale water resource became controversial in water management. Because of rapid increase of water demand for national development, water shortage is now one of the critical issues in many countries worldwide due to the insufficient water supply. A dam can still be an effective tool to provide water security with a proper site selection to meet the recent United Nations Sustainable Development Goals in a region where water resources are limited (http://sustainabledevelopment.un.org/focussdgs.html.).” (lines 541 – 560)

Minor issues:

2. Section 2.2 More details/definitions should be provided about the p-value and the trend (used in table 1 and later).

Response to Reviewer #4 (2):

The details/definitions of MK trend analysis and p-value and have been added into Section 2.2. (lines 127-139)

3. Line 149: what is a wadable flow?

Response to Reviewer #4 (3):

Thank you for raising this question. Wadable flow is a river flow condition that the investigator can wade from one end of the reach to the other. To make the sentence clearer, the authors revised the sentence as:

“To evaluate suspended sediment load along the river, a depth-integrated sediment sampler, US D-49, was used to assemble water samples at each observation site. In contrast, a US DH-48 was used to obtain samples only when the river section was shallow enough for the investigator to wade across.” (lines 163-166)

4. The section 3 is a bit long and difficult to read. It could be better if 3.2 (the new dataset) is presented first and then 3.1 and 3.3. It looks like 3.3 is similar to 3.1 but using rather the relationships (eqs. 1-8). It would be thus better to merge 3.1 and 3.3.

Response to Reviewer #4 (4):

Thank you for this constructive comment. Ragarding the reviewer’s suggestion, we have rearranged sub-section in Section 3 as follows:

3. Results (line 199)

3.1. River Flow and Sediment Characteristics along the Wang River (line 200)

3.2. Historical River Flow and Sediment Loads (line 249)

3.2.1. Historical River Flow along the Wang River (line 250)

3.2.2. Historical Suspended Sediment Loads along the Wang River (line 285)

3.2.3. Relationship between River Flow and Suspended Sediment Load (line 327)

3.2.4. Variability of Sediment Loads along the Wang River (line 337)

 3.3. Effect of large dam constructions on Sediment Loads in the Wang River (line 368)

5. Section 3.1 For the riverflow, the historical dataset is formed of daily data. What about the solid discharge? Is it also daily or less? If it is less how many data/year do you have?

Response to Reviewer #4 (5):

Both historical river flow and suspended sediment (solid discharge) data provided by the Royal Irrigation Department (RID) were daily data. The type of sediment data obtained from the RID has been added in Section  2.2 as follows:

“To study the variability of river flow and sediment load along the Wang River, historical daily river discharge (Q_w) and daily suspended sediment (Q_s) data between 1929 and 2019 were obtained from the Royal Irrigation Department (RID).” (lines 107-109)

6. Section 3.2 Is the median diameter collected in the riverbed or from the water column (suspension)?  It may sound a bit surprizing to have such coarse sand (2 mm) and mainly suspension. Or is it finer bed material in suspension (silt or clay)?

Response to Reviewer #4 (6):

Thank you for raising this issue. We did not clearly explain the grain size analysis of bed load and bed materials in the Method section. As the median diameter (d₅₀) shown in Table 1 represents the mean grain size of the river bed, more details on bed material sampling have been added in Section 2, and the new text read:  

“In this study, bed materials were collected at left, middle, and right locations of a river cross-section.  At each sampling location, about 1-2 kg of surface bed material with a sampling depth of about 20 cm was taken using a Van Veen grab. Each sample was kept in a plastic bag and sealed. All samples were then delivered to a soil laboratory for analyzing grain size distribution using the same procedures as the bedload sample analysis.” (lines 194-198) 

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

The authors have correctly replied to the minor concerns.

Concerning the major concern, I am still not convinced that the contain of the paper is appropriate for a journal publication since the methodology has already been presented in previous papers. It looks to much as a copy/paste of the previous Water publication with mainly a change of studied site.

The authors have added a discussion in section 4.3.  It better highlights the new finding of this work but it would be better to also add more explanations in the introduction or to rise more clearly these issues in the introduction.

 A paper comparing the different tributaries and the impact of  their dams on the CPR system could be more interesting than separated studies of individual tributary.

Author Response

(1) Concerning the major concern,I am still not convinced that the contain of the paper is appropriate for a journal publication since the methodology has already been presented in previous papers. It looks to much as a copy/paste of the previous Water publication with mainly a change of studied site.

Response to reviewer #4 (1):

Thank you for pointing out this issue; we now realized the reviewer's concern. Since our study focused on systematic sediment study and evaluation of the dam effects on sediment loads, we then tried to detail the methods used in this study to assure readers of the sediment observation quality. We followed the writing style of our paper published in the WATER because the paper was improved regarding the WATER’s reviewers and editors suggestions, which has made the paper easy to read. However, we agree with the reviewer that some details published in a previous publication can be abridged from this manuscript to avoid repetition issues. Therefore, we have modified the Methods as the reviewer suggested in Sections 2.2-2.3. (lines 110-183)

(2) The authors have added a discussion in section 4.3.  It better highlights the new finding of this work but it would be better to also add more explanations in the introduction or to rise more clearly these issues in the introduction.

Response to reviewer #4 (2):

Thank you for the suggestion. We added more explanations on the issues relating to our new finding to the Introduction as the reviewer suggested in line xx-xx. The additional text reads:

Dams impound sediment behind the structure and alter river flow and sediment loads downstream; differences in geological setting, hydrological conditions, including dam location, may cause different impacts on sediment load in a river. Regarding the adverse effects of damming on sediment supply to coastal environments, it has been questioned whether a dam is still a proper tool for serving sustainable water management.  (lines 43-48)

 

(3) A paper comparing the different tributaries and the impact of their dams on the CPR system could be more interesting than separated studies of individual tributary.

Response to reviewer #4 (3):

Thank you for your constructive suggestion. We agree that comparing the different tributaries and the impact of their dams on the CPR system will definitely make our research paper more interesting.  Since systematic sediment study on other tributaries is an ongoing investigation by our research team, the comparison of the effect of damming in CPR’s tributaries will be presented in our subsequent publication.

Author Response File: Author Response.docx