Estimation of sedimentation in the Manwan and Jinghong reservoirs on the Lancang river

When it comes to hydraulic engineering, one of the most important problems is the trapping of sediment (Walling 2006). It is assumed that the global sediment flux that is trapped in reservoirs is over 30% (Vorosmarty et al. 2003), and 1% of the world's existing reservoir volume is lost each year due to sedimentation (WCD 2000). As a result, sediment flux measured from hydrological stations exhibits significant decreases. This circumstance also happens in the Lancang–Mekong River. With hydropower stations constructed one after another, the changes of water discharge and sediment flux in the Lancang River as well as their impact on the lower Mekong have become the focus of research (Räsänen et al. 2012; Liu et al. 2013; Zhai et al. 2016; Han et al. 2019; Binh et al. 2020). Studies have shown a decrease in the sediment flux of the Lancang river and it is generally agreed that in the Lower Mekong 50% of the total suspended sediment originates from China (Milliman & Meade 1983; Roberts 2001; ADB 2004). Sediment brings important nutrients to the downstream and has a significant impact on the production and life of downstream areas. At the same time, changes in the amount of sediment also play a key role in power generation, navigation, as well as river bank erosion, and may even affect the change of national boundaries, ecological security of river basins and sustainable development. Therefore, research on reservoir sedimentation to grasp detailed information on sediment changes is of great importance. In addition, sedimentation is also a problem that has to be paid great attention to in the operation and management of reservoirs.

As mentioned above, the changes of water and sediment have attracted much attention from scholars and some results have been achieved. However, there is little recent systematic research on the reservoir sedimentation in Lancang River. Kummu & Varis (2007) estimated the theoretical trapping efficiency of proposed dams as well as the amounts of sediments that trapped in reservoirs. Subsequently, Kummu et al. (2010) developed a protocol to estimate the trapping efficiency of reservoirs in the Mekong basin by means of the Brune method and made a prediction of the trapping efficiency if eight dams are all constructed. Fu & He (2007) estimated the trapping efficiency of the Manwan dam based on the sediment load data from the hydrological stations upstream and downstream of the Manwan reservoir and compared the result with that estimated by modified Brune and Siyam models. Furthermore, Fu et al. (2008) calculated the sedimentation in the Manwan reservoir and also revealed its impacts downstream. It can be seen that there is a paucity of recent research on reservoir sedimentation and most studies are about the Manwan reservoir. Detailed and up-to-date theoretical calculations of reservoir sedimentation are lacking. However, as an international river, the Lancang River occupies an important position in the production, life and economic development of the riparian countries (Hecht et al. 2019). Quantitative study on the sedimentation of the Lancang cascade is essential to assess the current status and further ensure its sustainability. Only by mastering the situation of reservoir siltation and exploring effective methods to reduce the sedimentation can the river and hydropower stations be better servers of the national economy as well as of people's livelihoods and thus make them sustainable. As a result, it is of great scientific value and practical significance to analyze the current state of reservoir siltation in the Lancang river.

In this paper, we first present several empirical methods to estimate trapping efficiency and give a result of estimation. Then, annual sedimentation is calculated theoretically using suspended sediment data from the upstream and downstream of reservoirs during the period 1964–2019. Finally, a simple comparison and analysis is performed between the measured and calculated values.

The Lancang-Mekong River is a large international river and flows through six countries, that is, China, Myanmar, Laos, Thailand, Cambodia and Vietnam. The upper Mekong, known as Lancang in China, contains a rich resource of hydropower (Liu et al. 1998; He et al. 2005) and eight dams have so far been completed with the Manwan dam first constructed in 1992 and the Nuozhadu dam the last finished in 2011 (Figure 1). Reservoir information for the Lancang cascade is shown in Table 1.

The Manwan reservoir, located in the middle reach of the Lancang River, was the first dam constructed in the mainstream of the Lancang as seen in Table 1. With Jiuzhou station upstream and Gajiu station downstream, the data of sediment inflow and outflow can be easily obtained. Jiuzhou station is located about 270 km upstream from the Manwan dam and remains in a natural state of fluvial sediment transport. Gajiu station is located downstream of the dam site by a distance of about 2 km. After the construction of Manwan reservoir in 1992, sediment discharge at Gajiu station was immediately changed. The difference of sediment discharge between Jiuzhou and Gajiu stations before and after the reservoir constructed can be used to assess the amount of reservoir sedimentation. Yunjinghong hydrological station, located about 3.5 km downstream of Jinghong reservoir, has plenty of measured data about water and sediment. However, there is no suitable data from the upstream hydrological station available for the reservoir. Thus, only the sediment data from Yunjinghong station were used in the calculation of sedimentation in Jinghong reservoir. Annual suspended sediment data at Jiuzhou, Gajiu and Yunjinghong stations were obtained from the Yunnan Hydrology and Water Resources Bureau for corresponding hydrological stations and the multi-year average values were then calculated and listed in Table 2. Normally, the single-point method is used to observe suspended sediment in the Lower Mekong (Hou et al. 2020). The acquired data has a large error and is often questioned by scholars. However, the sediment data in this paper is measured by hydrological stations using the depth integration method according to the Code for measurement of suspended load in open channels (2015). Thus, the data has high quality.

The Manwan and Jinghong reservoirs studied in this paper are located in the middle and lower reaches of the Lancang river. In order to better understand their potential to sedimentation, it is essential to explain the situation of land use in the upper and middle reaches. The upper reaches of Lancang river belong to the Qinghai-Tibet Plateau, which is the least populated area in the entire basin. Overall, the growth and decline of forests in the upper reaches is not obvious (Zhang & Xu 2003). The middle and upper reaches, from the territory of Yunnan Province to Nuozhadu hydropower station, bear the densest population as well as the highest degree of economic development due to the flat terrain and suitable climate. As a result, the human interference is stronger and impacts on land use are the most pronounced. The rate of forest coverage has decreased drastically since the late last century and the regional ecological environment has been greatly affected, which has led to a serious condition of soil erosion (Jiang et al. 2006). The population density in the lower reaches is lower and there is also less human interference. At the same time, the natural environment is better and the vegetation can recover at a faster rate. Therefore, the situation of land use in the lower reaches is relatively stable.

There are various ways to quantitatively assess reservoir sedimentation, empirically and theoretically. Most empirical methods are made with regard to reservoir capacity and the inflow sediment, while theoretical methods are made mainly about the sediment in and out of the reservoir, that is, sediment data from upstream and downstream hydrological stations. As Manwan reservoir is large with fixed gauging stations upstream and downstream, it is possible to measure the sediment inflow and outflow (Xu et al. 2006; Dai et al. 2008; Kummu et al. 2010). With the sediment inflow and outflow known, sediments trapped in the reservoir can be calculated theoretically. However, there are reservoirs for which inflow and outflow were not both measured. Under this condition, the storage capacity and the measured sediment inflow can be used to estimate the sedimentation. The sediment yield can also be estimated by comparing the reservoir drainage area with measured sediment outflow (Dendy 1974).

USDA-NRCS (1983) transformed the envelope curves developed by Brune into curves for three kinds of sediments. Correspondingly, there are three curves: the upper curve, median curve and lower curve. The three curves were further translated into an MS Excel thanks to the work of the Summit County Soil and Water Conservation District (Harbor et al. 1997). Different sediment textures correspond to different curves and the corresponding equations are shown in Table 3.

Reservoir sedimentation is generally estimated by the amount of sediment inflow and outflow. The theoretical calculation of sediment siltation is mainly based on the data of sediment transport from upstream and downstream hydrological stations. For reservoirs with only upstream or downstream sediment data, the trapping efficiency is combined with upstream or downstream data to calculate reservoir sedimentation.

In this paper, the Jiuzhou hydrological station upstream of Manwan reservoir and Gajiu hydrological station downstream of Manwan were selected as the sediment control stations for the inflow and outflow sediments to estimate the sedimentation in Manwan reservoir. Generally speaking, when the external environment is not very different, sediment transport in the upstream and downstream of a river is quite relevant. Therefore, we have taken 1993, the year when Manwan reservoir was built and operated, as the node. Sediment transport of Jiuzhou and Gajiu during 1964–1993 is regarded as that under the natural state, while sediment transport during 1993–2008 is that impacted by reservoir construction and operation. Reservoir sedimentation and storage capacity loss of Manwan could then be calculated separately in the two periods by linear regression analysis. Therefore, data stability and causality of the time series at Jiuzhou and Gajiu stations needs to be tested before linear analysis is performed, which is necessary to avoid pseudo-regression simulation.

The stability and causality of suspended load series are mainly achieved by the Augment Dickey-Fuller (ADF) test (Fuller 1976; Dickey & Fuller 1979) and Granger causality test. Test results are listed in Tables 4 and 5. The ADF test results show that the probability values of suspended sediment at Jiuzhou and Gajiu are both less than 0.05, which indicates that the data series of the two stations are both stable. The stability can also be judged by the fact that ADF test value is less than the critical value at a significance level of 1%. Moreover, the ADF test also provides a prerequisite for the Granger causality test of suspended sediment series from the two stations as the causality test first requires the data series to be stable. The Granger test in Table 5 gets a conclusion that the null hypothesis H₀ (Suspended load at Jiuzhou does not cause that at Gajiu) is rejected, which means there is indeed a causal relationship between the suspended sediment of Jiuzhou and Gajiu and this can provide theoretical support for the simulation of suspended load at Gajiu under natural state, using that at Jiuzhou.

Data series of suspended sediment load at Gajiu and Jiuzhou stations were from 1964 to 2008. The correlations of suspended sediment load for the two stations are quite significant before and after reservoir construction, that is pre-dam 1964–1992 and post-dam 1993–2008. The correlation results with a significance value of 0.01 are shown in Figure 2. As is seen, suspended load at the two stations correspond to two combined processes during the two periods and each shows a good correlation. This also demonstrates that the two stations can be regarded as sediment control stations for siltation estimation of Manwan reservoir.

R², the coefficient of determination, for the two equations, was 0.71 and 0.83, respectively, which shows that Equations (6) and (7) fit well for the two suspended load series. The changes in the two coefficients and the goodness of fit illustrate that the annual suspended load downstream was reduced and tended to be stable. Then, the differences between the estimated and measured values at Gajiu station were presumed to be the amount of sedimentation caused by the Manwan dam. The annual amount of sediment trapped in reservoir and the total amount are both shown in Table 1. As is seen, the maximum sedimentation occurred in 2000 with a value of 64.87 million t and the minimum was 24.92 million t in 1994. The amounts of sediment interception around 2000 were relatively high and this should be related to the construction of the Dali-Baoshan and Dali-Yangbi highways. The implementation of these key projects is often accompanied by an increase in soil erosion, which leads to an increase in sediment flux and reservoir sedimentation for the corresponding section of Lancang river. The average annual sediment load at Gajiu was estimated as 37.04 million t during 1993–2008 under the condition that Manwan operates and the total amount was 592.56 million t. If we estimate that 3% of the suspended sediment is bedload (He 2004), then the average annual sedimentation of Manwan was 38.15 million t and the amount of sediment intercepted in Manwan reservoir during the 16 years of operation was about 610.43 million t. It should be noted that the amount of siltation in the reservoir area caused by riverbank landslides and collapse was not included. Taking the average bulk density of sediment as 1.4 t/m³, the loss of reservoir capacity caused by sediment interception in Manwan hydropower station was about 436 million m³ to 2008, which accounts for 47.4% of the total reservoir capacity of 920 million m³.

Theoretically, the revised method can be used to estimate the trapping efficiency of reservoirs with similar size on the Lancang river, such as Jinghong reservoir. For reservoirs lacking the measured topography and sediment data, it is a relatively simple method to estimate the trapping efficiency. With the reservoir capacity and average annual runoff at the dam site known, the trapping efficiency can be obtained.

Regarding the multi-year average suspended load at Gajiu station as that of Manwan reservoir, combined with Equation (10), the multi-year average sedimentation of Manwan reservoir during this period can be obtained as 4.57 million t. According to the estimation that bedload accounts for 3% of suspended load, the annual amount of sedimentation in Manwan reservoir is 4.71 million t. Taking the average bulk density of sediment to be 1.4 t/m³, the annual storage loss of Manwan reservoir is about 3.36 million m³ due to sediment siltation, accounting for 0.37% of the total storage capacity. From 2009 to 2019, the total sedimentation of Manwan reservoir was 51.70 million t, resulting in a capacity loss of 36.93 million m³, which accounts for 4.0% of the total storage capacity.

According to the calculation of both theoretical and empirical methods, the total sedimentation of Manwan reservoir from the closing of gates in 1993 to 2019 is about 662.1 million t. Taking the average bulk density of sediment to be 1.4 t/m³, the total storage loss of Manwan reservoir is about 472.9 million m³ due to siltation, accounting for 51.4% of the total storage capacity. Of this, the annual sedimentation during 1993–2008 was 38.1 million t, and the loss of storage capacity was about 4.27% of the total capacity. After the completion of Xiaowan hydropower station, sedimentation in Manwan reservoir was greatly eased. The annual siltation in Manwan reservoir was 4.70 million t during 2009–2019, and the loss of storage capacity was reduced to about 0.37% per year.

Based on the above results, the cumulative storage loss of Manwan reservoir for a certain number of years can be calculated and Table 8 shows the comparison between the calculated cumulative storage loss and design values of Manwan reservoir. Due to the confidentiality requirements for the sedimentation results of measured topography, we will not make relevant comparisons here. As is shown in Table 8, the cumulative storage loss of Manwan reservoir for 5 years, 10 years and 15 years is 17.5%, 34.6% and 47.4%, respectively. The theoretical result for 15 years is consistent with the design value, indicating that the method used in this paper has high accuracy. The storage capacity loss of Manwan during 1993–2013 was 49.2%, which is far less than the design value 61.0%. That was because the operation of Xiaowan station greatly changed the amount of sediment in Manwan reservoir after 2008, thus the change in reservoir capacity was significantly smaller than before. Under the condition that Manwan and Xiaowan reservoirs normally operated and no major bank collapse occurs, it is expected that the Manwan reservoir will maintain an annual storage loss of about 0.37%.

The Yunjinghong hydrological station, about 3.5 km downstream of Jinghong reservoir, has multi-year measured sediment data, which can be used as the outlet hydrological station of Jinghong reservoir. However, there is no suitable hydrological station available upstream of the reservoir. Therefore, the trapping efficiency of Jinghong reservoir can be calculated by the modified Brune method. The annual suspended sediment at Yunjinghong is regarded as the annual amount of outflow sediment from Jinghong reservoir, combined with its trapping efficiency, the amount of sedimentation and storage capacity loss can be computed year by year.

Dachaoshan hydropower station is located upstream of Yunjinghong station and was completed at the end of 2001. After that, Jinghong hydropower station was constructed in 2008, and at the end of 2012, the Nuozhadu hydropower station between Dachaoshan and Jinghong station was finished. Obviously, the process of sedimentation in Jinghong reservoir is affected by the operation of Nuozhadu reservoir. Therefore, the sedimentation process of Jinghong reservoir during 2009–2019 was divided into two stages by taking the year 2012 as the boundary when Nuozhadu station was operated, that is, the time from 2009 to 2012 is recognized as the first period and the period from 2013 to 2019 is the second. Then the two stages of sedimentation in Jinghong reservoir can be calculated separately.

Based on the storage capacity and annual average runoff at the dam site, the trapping efficiency of Jinghong reservoir is calculated to be 66.0% using the revised Brune method. As the annual amount of outflow sediment from Jinghong reservoir is known, the annual amounts of inflow sediment and intercepted sediment of Jinghong reservoir during 2009–2019 can be obtained by Equation (10) and are listed in Table 9.

From 2009 to 2012, the annual average amount of sedimentation in Jinghong reservoir was 2.74 million t and the total amount was about 10.96 million t. According to the estimation of bedload accounting for 3% of suspended load, the annual amount of sedimentation in Jinghong reservoir was about 2.82 million t and the total amount from 2009 to 2012 was 11.29 million t. Taking the average bulk density of sediment to be 1.4 t/ m³, the total storage capacity loss of Jinghong reservoir during this period was about 8.1 million m³ due to siltation, which accounts for 0.7% of the total storage capacity in 1.14 billion m³.

After the completion of Nuozhadu station, the annual suspended sediment of Jinghong reservoir was reduced to 1.87 million t during 2013–2019 and a total amount of 13.09 million t sediment was blocked. According to the estimation that bedload accounts for 3% of suspended load, the annual amount of sedimentation in Jinghong reservoir is 1.93 million t and the total amount from 2013 to 2019 is 13.49 million t. Taking the average bulk density of sediment to be 1.4 t/m³, the total storage capacity loss of Jinghong reservoir during this period is about 9.6 million m³, accounting for 0.84% of the total storage capacity.

Based on the calculation results of the two periods, it can be seen that the total amount of sedimentation in Jinghong reservoir from 2009 to 2019 is about 24.78 million t and the total storage capacity loss caused by sedimentation is about 17.7 million m³, accounting for 1.54% of the total storage capacity in 1.14 billion m³. Of this, the annual storage capacity loss from 2009 to 2012 was approximately 0.175% and it was reduced to about 0.12% per year during 2013–2019 after the completion of Nuozhadu hydropower station.

From the impoundment of Manwan reservoir in 1993 to 2019, a total of about 662.1 million t sediment was intercepted. The total loss of storage capacity due to sedimentation was about 472.9 million m³, accounting for 51.4% of the total capacity. Of this, the amount of annual siltation during 1993–2008 was about 38.1 million t, accounting for 4.27% of the total storage capacity. After the completion of the upstream Xiaowan hydropower station in 2008, the sedimentation in Manwan reservoir was greatly alleviated. From 2009 to 2019, the annual amount of sedimentation in Manwan reservoir was reduced to 44.7 million t, and the annual storage capacity loss was also reduced to 0.37%. Based on the measured suspended sediment data at Jiuzhou and Gajiu stations, the sediment interception rate of Manwan reservoir was found to be 69% using the theoretical method. The trapping efficiency estimated by empirical methods was between 56.4 and 67.5%, and it can be concluded that the result of the Brune method is the closest to the calculation result. Therefore, the Brune method was revised through correction coefficient and can be used in the calculation of sedimentation in Jinghong reservoir.

From the operation of Jinghong reservoir to 2019, a total of about 24.78 million t sediment was blocked, and the total volume loss because of siltation was about 17.7 million m³, accounting for 1.54% of the total volume. Of this, the annual sedimentation volume during 2009–2012 was about 2.0 million t, causing a loss of 0.175% to the storage capacity. After the upstream Nuozhadu hydropower station finished in 2012, the sedimentation of Jinghong reservoir was relieved. Thereafter, the annual sedimentation volume has dropped to about 1.4 million t, resulting in an annual loss of 0.12% of total storage capacity.

With the operation of the Lancang cascade reservoirs, it is essential to study the situation of reservoir siltation and get a good knowledge of the current status of the reservoirs. Only in this way can managers make better decisions when taking measures to ensure the sustainable development of reservoirs as well as the rational use of water resources in the river basin. Then the people and nation can greatly benefit from the Lancang river and reservoirs. The results of this study show that the siltation of Manwan and Jinghong reservoirs has been relatively serious. Although the construction of Xiaowan and Nuozhadu hydropower stations has relieved a certain amount of pressure, it can be seen that the overall siltation of the reservoirs still needs to be paid more attention to. Due to the limitation of hydrological and sediment data, this paper only uses the available information to carry out the empirical estimation and theoretical calculation, but it is sufficient to illustrate the problem and its severity. Future research can be carried out with more detailed calculations, but more importantly, it should focus on how to innovate measures for dredging reduction, and apply effective methods and devices for silt reduction in Lancang reservoirs. Ensuring the sustainability of Lancang reservoirs should be the focus of research, but little research has been done in this area.

Based on reservoir information and sediment data, this paper uses empirical and theoretical methods to illustrate the siltation status of the Manwan and Jinghong reservoirs in the Lancang river, and the siltation situations of Xiaowan and Nuozhadu reservoirs are also reflected from the side. Results show that since Manwan reservoir was put into use, the storage capacity has lost 51.4% in just two or three decades, and the trapping efficiency has reached 69%. The siltation of Manwan reservoir is quite serious. The capacity loss of Jinghong reservoir reached 1.54% as of 2019 and the situation is much better. With the completion of Xiaowan and Nuozhadu reservoirs, the pressure on Manwan and Jinghong reservoirs has been relieved to some extent. The quantitative information this paper provided can enable managers to accurately grasp the current state of Lancang cascade reservoirs and provide scientific guidance for them to make relevant decisions. For Manwan and other reservoirs, practical and effective measures should be taken in time to ensure their sustainable development and prevent the sedimentation problems of other reservoirs from being more serious.

This work was supported by the Natural Science Foundation of China (91647209). Sincere thanks are offered for the professional guidance given by the editor and the anonymous reviewers.

Data cannot be made publicly available; readers should contact the corresponding author for details.