Variations in Sedimentation Rate and Corresponding Adjustments of Longitudinal Gradient in the Cascade Reservoirs of the Lower Jinsha River
Aram Bayet-Goll
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe introduction must explicitly outline the study's aims. Clearly articulate the study topic and the significance of comprehending sedimentation rates and longitudinal gradient modifications in cascade reservoirs.
Additional background information regarding the Lower Jinsha River and its cascade reservoirs would improve comprehension for people unfamiliar with the region.
The discourse must connect sedimentation fluctuations to overarching hydrological or environmental influences (e.g., upstream development, climatic effects). Examine divergent results in relation to the current literature.
Discuss the practical ramifications of sedimentation and gradient alterations, including their impacts on dam functionality, water retention, and ecological interactions.
Summarise the principal findings concisely and clarify their importance.
Maintain uniformity in the application of technical terminology throughout the manuscript.
Streamline excessively intricate language to enhance clarity.
Conduct a comparative investigation among various reservoirs to uncover shared trends or distinctive anomalies.
Follow the below papers:
REVIEW OF RESERVOIR OPERATION
http://dx.doi.org/10.3808/jeil.202400129
Comments on the Quality of English LanguageSatisfactory
Author Response
Comments 1: The introduction must explicitly outline the study's aims. Clearly articulate the study topic and the significance of comprehending sedimentation rates and longitudinal gradient modifications in cascade reservoirs.
Response 1: Thank you for pointing this out. I agree with this comment. I have revised the introduction according to this comment. The revision can be found on page 2, lines 73-89 in the revised manuscript. The modified contents are as follows:
“It has been more than ten years since the completion of the Xiangjiaba and Xiluodu dams in the cascade reservoirs in the lower Jinsha River. The observation data on the water and sediment processes and river channel geomorphology in the river reach where the Xiangjiaba and Xiluodu reservoirs are located are relatively abundant, making them ideal research objects for understanding the sedimentary changes in the cascade reservoirs and the morphological adjustments of the river channels in the study area.”
“The main objectives of this study are: (1) To estimate the average sedimentation rate of different sections of the Xiangjiaba and Xiluodu reservoirs in different periods; (2) To analyze the response of the variation in thalweg mean gradient of the different sections to the sedimentation rate; (3) To reveal the influence mechanism of the change in reservoir operation mode on the sedimentation rate in the reservoir area. The significance of this research work lies in the fact that the research results can serve as a reference for understanding the variation patterns of the sedimentation rate in cascade reservoirs and the morphological adjustments of the river channels in similar areas, thus helping to formulate comprehensive management countermeasures for the balanced development and sustainable utilization of the siltation in cascade reservoirs.”
Comments 2: Additional background information regarding the Lower Jinsha River and its cascade reservoirs would improve comprehension for people unfamiliar with the region.
Response 2: Thank you for pointing this out. I agree with this comment. I have supplemented the information mentioned in this comment. The supplemented and revised contents are on the lines from line 92 on page 2 to line 103 on Page 3, and from lines 109 on page 3 to line 126 on Page 4 in the revised manuscript. The modified contents are as follows:
The Jinsha River is the river reach above Yibin City in the upper reach of the Yangtze River (specifically, at the confluence of the Minjiang River) (Figure 1). Its river course is 3,841 km long, and its drainage area is 458,800 km², accounting for 77.3% and 45.88% of the upper reach of the Yangtze River respectively, and 55.3% and 25.49% of the entire Yangtze River basin [9]. According to the statistics of the measured data from 1952 to 2004, the average annual runoff of the Jinsha River is 144.2 billion cubic meters, and the average annual sediment transport volume is 250 million tons [23]. It is an important sediment source area in the Yangtze River basin. The Jinsha River runs through the Qinghai-Tibet Plateau, the Hengduan Mountains, the Yunnan-Guizhou Plateau, and the southwestern part of the Sichuan Basin from northwest to southeast, with complex and changeable terrain and significant differences in landforms [24]. The terrain is high in the northwest and low in the southeast, and the surface elevation ranges from 216 m to 6,293 m.
The lower reach of the Jinsha River refer to the Jinsha River section with a river course length of 765 km below the Sanduizi Hydrological Station [9]. The catchment area within this section is 86,000 km² [25], accounting for 22.1% and 18.75% of the Jinsha River basin respectively. In the lower reach of the Jinsha River, the average terrain is high in the south, low in the central and northern parts, and high on the northeastern edge, and the surface elevation ranges from 216 m to 4,281 m. The average annual precipitation ranges from 597 mm to 1172 mm, and the average annual temperature ranges from 8℃ to 25℃ [25]. The average annual net runoff production and net sediment transport volume in the lower reach account for 19% and 63.2% of the Jinsha River basin respectively, making it the main sediment-producing area in the Jinsha River basin or the Yangtze River basin.
The four reservoirs, namely the Xiangjiaba, Xiluodu, Wudongde, and Baihetan reservoirs located in the lower reach of the Jinsha River, began to impound water in October 2012, May 2013, May 2020, and April 2021 respectively. The storage capacities of these reservoirs range from 5.16 billion m3 to 20.6 billion m3, and the total storage capacity is as high as 45.83 billion m3. Given that the large storage capacities of this cascade reservoir group can intercept a large amount of runoff and sediment, the entire lower reach of the Jinsha River has changed from a long-term erosive state in the past to a state of intense sediment interception and deposition.
Comments 3: The discourse must connect sedimentation fluctuations to overarching hydrological or environmental influences (e.g., upstream development, climatic effects). Examine divergent results in relation to the current literature.
Response 3: Thank you for pointing this out. I agree with this comment. According to this comment, I have added relevant content, which can be found in lines 584-611 on page 16 in the revised manuscript. It is listed as follows:
“Sedimentation in reservoirs is a severe global challenge affecting the efficiency of surface water storage systems [32]. The temporal variation of sedimentation in reservoirs reflects the continuous accumulation of sediment and the reduction of reservoir live storage capacity. For example, for the Kadana Dam in India, which was completed in 1989, sediment deposition has led to a 38.52% loss of its live storage capacity by 2020, with 22.79% of this loss occurring after 2000 [32]. As canyon-type reservoirs with a length of 150−200 km and a "V"-shaped cross-section, each cascade reservoir in the lower reach of the Jinsha River exhibited a high sedimentation rate in the initial stage after dam closure. However, for several reasons, the sedimentation rate of these cascade reservoirs will gradually decrease over time. First, as the thickness of sediment deposited in the reservoir area increases, the bottom of the "V"-shaped river valley becomes flatter, and the cross-section that receives a large amount of sediment deposition gradually widens. Assuming the sediment inflow remains constant, the amount of sediment deposited per unit length of the cross-section will decrease. Second, as cascade reservoirs are successively built and trap sediment, the sediment inflow and deposited sediment volume of a single reservoir will relatively decrease. Third, small- and medium-sized reservoirs are still being continuously built and start to trap sediment in the middle and upper reaches of the Jinsha River main stream and some tributaries of the basin, resulting in a continuous reduction of sediment inflow to the lower Jinsha River. Fourth, other soil and water conservation measures in the basin are being continuously promoted, thus continuously reducing the sediment inflow to the lower river reach. Although the temperature in the Jinsha River Basin is rising and the precipitation has slightly increased [33], the sediment transport volume is continuously decreasing. Therefore, climate change will not lead to an increase in sediment inflow to the lower reach. Thus, when the impact of climate on sediment inflow is not significant, dam construction and soil and water conservation measures in the basin will reduce the sediment transport to the lower Jinsha River, thereby gradually reducing the sedimentation rate of large-scale cascade reservoirs in the lower river reach.”
Comments 4: Discuss the practical ramifications of sedimentation and gradient alterations, including their impacts on dam functionality, water retention, and ecological interactions.
Response 4: Thank you for pointing this out. I agree with this comment. According to this comment, I have added relevant content, which can be found in lines 612-633 (on page 16-17) in the revised manuscript. It is listed as follows:
A high sedimentation rate raises concerns about the lifespan of reservoirs. The average annual suspended sediment inflow to the Xiluodu Reservoir in the study area from 1999–2019 was 105.1 million tons [9,18]. Assuming an average dry density of 1.5 tons per cubic meter [34], the total annual sediment transport volume is 0.7006 billion m³. If all this sediment is intercepted by the Xiluodu and Xiangjiaba Reservoirs, it would take 251.88 years to fill their total storage capacity (17.647 billion m³). Considering that the Wudongde Reservoir (total storage capacity of 7.408 billion m³) and the Baihetan Reservoir (total storage capacity of 20.627 billion m³) were completed in 2020 and 2021 respectively upstream of the Xiluodu Reservoir, the total storage capacity of these four cascade reservoirs is as high as 45.682 billion m³. If the average annual sediment transport volume remains unchanged, it would take 652 years to fill these four reservoirs. In fact, as mentioned above, the sediment inflow to the lower Jinsha River is gradually decreasing under various human interventions. As a result, the effective lifespan of the cascade reservoirs in the lower Jinsha River will be longer than the years mentioned above. Obviously, cascade reservoirs with large reservoir capacities can share the task of intercepting the sediment transported from the basin. Coupled with the implementation of optimized regulation measures, the effective lifespan of each reservoir can be maintained for a long time. Thus, the utilization of water resources and clean electricity production can be ensured in the long term.
In any case, understanding the spatiotemporal changes of sedimentation rates in cascade reservoirs is of great significance for understanding the distribution areas of intense sedimentation and the response characteristics of river channel morphology. Meanwhile, the results can provide guidance for the formulation of effective reservoir management measures.
Comments 5: Summarise the principal findings concisely and clarify their importance.
Response 5: Thank you for pointing this out. I have added relevant content, which can be found in lines 636-651 on page 17 in the revised manuscript. It is listed as follows:
In this study, we have focused on the sedimentation and gradient adjustment of cascade reservoirs in the lower Jinsha River, and the following conclusions are drawn:
(1) The average sedimentation rate in the Xiangjiaba reservoir decreased along the course, with a maximum of 20.75 m yr−1 in the first half year after dam closure. It then dropped significantly, reaching a maximum of only 0.35 m yr−1. In the Xiluodu Reservoir, the rate ranged from 0.65 m yr−1 to 3.97 m yr−1, decreasing in both upstream and downstream directions from the upper part of the middle region.
(2) The thalweg gradient in the Xiangjiaba Reservoir increased significantly in the middle part during the first half year, and then adjusted relatively little. In the Xiluodu Reservoir, the gradient increased along the course.
(3) In the cascade reservoirs, the gradient is controlled by differences in sedimentation rates, and the sedimentation rate is controlled by the amount of sediment input. The upstream dam has a large sediment interception capacity and can significantly reduce sediment inflow to downstream reservoirs.
(4) The findings help understand sedimentation trends in similar cascade reservoirs and guide the formulation of sediment regulation measures for the reservoirs.
Comments 6: Maintain uniformity in the application of technical terminology throughout the manuscript.
Response 6: Thank you for pointing this out. I checked the inconsistent terminologies and corrected them to ensure they remain consistent throughout the text.
Comments 7: Streamline excessively intricate language to enhance clarity.
Response 7: Thank you for pointing this out. I have tried to simplify those sentences that were expressed in a very cumbersome way as much as possible.
Response 8: Conduct a comparative investigation among various reservoirs to uncover shared trends or distinctive anomalies.
Response 8: Thank you for pointing this out. I compared the reservoir sedimentation in the study area with the literature suggested here and analyzed the differences in loss rates of storage capacity of different reservoirs. The relevant modifications in this regard can be found in section of “3.3. Changes in sedimentation rate and capacity of reservoirs”.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsJournal: Water
Manuscript ID: water-3411081
Title: Variations in Sedimentation Rate and Corresponding Adjustments of Longitudinal Gradient in the Cascade Reservoirs of the lower Jinsha River
Authors: Suiji Wang
Dear Editor,
Thank you for inviting me to participate in the review process of this manuscript. I have read the manuscript thoroughly. The manuscript, entitled “Variations in Sedimentation Rate and Corresponding Adjustments of Longitudinal Gradient in the Cascade Reservoirs of the lower Jinsha River” investigates the impact of sedimentation changes in cascade reservoirs on sedimentation patterns and river channel morphology. The study focuses on the Xiangjiaba and Xiluodu reservoirs, analyzing temporal and spatial changes in sedimentation rates and the response of the longitudinal gradient before and after dam closure. Key findings include significant variability in sedimentation rates, with the Xiangjiaba reservoir experiencing a maximum rate of 20.75 m/yr shortly after dam closure, and notable gradient adjustments controlled by sedimentation changes. The research highlights the importance of understanding sedimentation dynamics for effective reservoir management and offers practical applications for optimizing reservoir operations and maintaining river channel stability.
The most part the work seems to be solid and conclusions supported by evidence. The authors have worked hard and many data are provided. I believe that it is worth considering as a publication; nevertheless, before publication, it needs the following modifications/amendments.
Before it can be published it needs a minor revision for reasons outlined in the annotated manuscript.
I have included an annotated word document that contains all the thoughts I had while reading through the manuscript, as well as suggested revisions that I believe would help the readability and completeness of the work. The Discussion and Conclusion section is not the place for details about your methodology or results.
Comments for author File: Comments.pdf
Author Response
Reply to Reviewer 2
Comments 1: The second sentence (“However, the research on the impact of the coordinated changes of sedimentation in cascade reservoirs on the sedimentation pattern in the reservoirs and the adjustment of river channel morphology is still insufficient, and thus it is a scientific problem worthy of exploration.”) is a bit complex and could be clearer.
Response 1: Thank you for pointing these out. The abstract (including this sentence) has been completely modified. The specific reply is included in the reply to Comment 2.
Comments 2: The abstract of the recent manuscript is quite descriptive. It would benefit from highlighting the key findings and the significance of the work more explicitly. Additionally, mentioning the practical applications of the methods used in the study would be helpful. By emphasizing these aspects, the abstract will better capture the reader's attention and convey the importance of the research.
Response 1 and 2: Thank you for pointing these out. I agree with the comments 1 and 2. I have revised the Abstract according to these comments. The revision can be found on page 1, lines 10-31 in the revised manuscript. The modified contents are as follows:
Abstract: The Xiangjiaba and Xiluodu reservoirs, as important components of the large cascade reservoirs in the lower Jinsha River, the interactive changes in sediment trapping amounts, the differences in sedimentation dynamics and the potential mutual influence mechanisms among them are scientific issues worthy of attention. Based on the multiple observed data of thalweg elevation before and after the completion of the dam construction, this study calculated the average sedimentation rates of all 20-km-long segments of the above-mentioned reservoirs in different periods. Meanwhile, the local mean gradients between adjacent segments and the regional mean gradients from the segments to the dam in the corresponding periods were calculated. The results show that the maximum and average sedimentation rates of the Xiangjiaba Reservoir, which was built earliest and is located downstream, were as high as 19.62 m yr−1 and 8.88 m yr−1 respectively in the first half year after the dam closure. After the completion of the Xiluodu Reservoir, an adjacent cascade reservoir upstream, the average sedimentation rate of the Xiangjiaba Reservoir in the following seven years dropped to 0.67 m yr−1. The maximum and average sedimentation rates of the Xiluodu Reservoir were 9.07 m yr−1 and 4.15 m yr−1 respectively within one year after the dam closure, and its average sedimentation rate in the following six years was 2.51 m yr−1. The spatial variations of sedimentation rates in these two reservoirs follow different changing patterns. There is an obvious correlation between the change in average gradient and the change in sedimentation rate. The sequence of dam construction, the relative positions of the reservoirs, the differences in sediment trapping amounts and operation modes are the key factors controlling the changes in sedimentation rate and gradient in the reservoir area. This study reveals the interactive changes in sedimentation rates among cascade reservoirs and the response mechanism of river channel morphology, and has a guiding role for the formulation of effective measures for the sustainable utilization of cascade reservoirs.
Comments 3: it is important to clarify the basis for the increase or decrease in sedimentation, or its maxima and minima. Please include the supporting evidence or data that has been examined to substantiate these claims.
Response 3: Thank you for pointing these out. I agree with the comments 3. According to this comment, the required sedimentation rates have been added to the corresponding parts of the Abstract. The revision can be found on page 1, lines 19-24 in the revised manuscript.
Comments 4: the term "sediment thickness" should be clearly defined, and the method used to determine it should be explained in the Materials and Methods section.
Response 4: Thank you for pointing these out. I agree with the comments 4. According to this comment, the term "average sediment thickness" and the term “average sedimentation rate” have been clearly defined. These modifications can be found in lines 207-211 on page 5 and lines 218-219 on page 6 in the revised manuscript. The specific changes are as follows:
“The average sediment thickness of a certain river section in a certain period is defined as the average value of all elevations in the latter measurement time minus the average value of all elevations in the previous measurement time. The average sedimentation rate is defined as the sediment thickness divided by the number of years between two observation times (with the unit of m yr−1).”
“The calculation result in the brackets of Equation (1) is the sediment thickness.”
Comments 5: Please consider that the average sedimentation rate should account for the erosion rates and the erosional processes involved, especially considering the thalweg of the channel and the erosional events within it. While annual average sedimentation is considered, it is important to note that both sedimentation and erosion rates vary seasonally. Thes e rates can reach maxima in some seasons and minima in others. Evidence of erosion and seasonal changes in sedimentation is often indicated by changes in sediment size, particularly in stratification.
Response 5: Thank you for pointing these out. I agree with the comments 5. In some segments of the reservoir, especially in the backwater area at the tail of the reservoir, thalweg erosion is common, and thus the erosion rate will occur. In this paper, a negative sedimentation rate indicates erosion, and the absolute value of the negative value represents the erosion rate value. Therefore, the existing calculation method for sedimentation rate is actually also applicable to calculating the erosion rate. The content related to this aspect has been added in lines 219-223 on page 5 in the revised manuscript. The specific supplementary content is as follows:
It should be noted that in some sections of the reservoir, especially in the backwater area at the reservoir's tail, the thalweg may have an erosion rate due to erosion. If the sedimentation rate calculated according to Equation 1 is negative, it indicates that erosion has occurred, and the absolute value of the negative value represents the erosion rate. Therefore, Equation 1 is also suitable for calculating the erosion rate.
Comments 6: When referring to erosion being predominant in other areas, it is crucial to specify the evidence supporting this claim.
Response 6: Thank you for pointing these out. The period dominated by erosion was actually the period when the dams had not been built yet. Meanwhile, the sections with high erosion rates were related to human sand mining activities. The explanations regarding this aspect can be found in lines 257-261 on page 7 in the revised manuscript. The specific supplementary content is as follows:
“During that period, no dams had been built in the lower reach of the Jinsha River. This river reach was a natural mountain river reach and was naturally dominated by erosion. This was also the fundamental reason why erosion was dominant in this river reach before the dams were built. The sections with very high erosion rates were the result of local sand mining activities [20].”
Comments 7: it is essential to specify the basis for calculating erosion thickness. Please clarify whether it has been calculated based on scour depth.
Response 7: Thank you for pointing these out. According to this comment, I have clarified that the calculation of the erosion thickness is based on the scour depth. The modification can be found in lines 269-270 on page 7 in the revised manuscript. The specific changes are as follows:
but the erosion thickness calculated based on scour depth was less than 0.71 m, and the erosion rate was less than 1.70 m yr−1.
Comments 8: it is important to explain the relationship between the average gradient, sediment thickness, and erosion rate. An increase in gradient typically leads to increased erosion and a decrease in sediment thickness. This relationship should be clearly outlined and supported by relevant data.
Response 8: Thank you for pointing these out. I agree with the comments 8. In response to this comment, it has been added that the sharp increase in the gradient of the river section is a consequence of the substantial increases in the sediment thickness and sedimentation rate of the adjacent upstream river section. At the same time, Figure 3 is cited to clarify that the river section with intense sedimentation is exactly the adjacent upstream section of the river section with a significant increase in gradient. The supplementary content can be found in lines 349-352 on page 10 in the revised manuscript. The specific content added is as follows:
The significant increase in the gradient of these river sections was actually significantly affected by the larger increases in the sediment thickness and sedimentation rate of the adjacent upstream section (Figure 3), and it was an inevitable result of the local gradient responding to the intensity of sedimentation.
Comments 9: Why both?
Response 9: Thank you for pointing these out. The repeated words have been deleted. The modification can be found in line 485 on page 14. The original sentence has been modified as follows:
The dam that was closed first enables the reservoir to intercept the sediment from the upstream basin of the dam at the earliest time.
Author Response File: Author Response.pdf
