The two-dam coexistence (TDC) is a special measure adopted by the Chinese Fengman Dam for risk management of old and dangerous dams, which acts as selective withdrawal by demolishing a partial elevation of the original dam. This study combined measured water temperature data for many years with the characteristics of the TDC to reveal the influence of the TDC on the thermal stratification in front of the dam and the withdrawal water temperature (WWT) process. The results showed that the TDC accompanying the operation and outflow daily regulation of the two power stations caused a general decline and periodic fluctuation in temperature contours. The metalimnion moved upward, and stratification strengthened with the temperature gradient from 0 to 10.5°C/m. The vertical mixture was weakened with the difference between the surface and bottom water temperatures ranging from 6.8°C to 26.8°C. The stratification stability of the water column was 1.6 times that with the one-dam condition (ODC), and there were interannual differences. The buoyancy frequency decreased correspondingly. Additionally, the TDC had a higher WWT in the warming period (3.2°C on average from May to Jun.) and a lower WWT in the freeze-up period (0.2°C on average from Jan. to Feb.) than the ODC. The maximum temperature difference between the front of the two dams occurred at the bottom of the reservoir (3.5°C to 12.2°C), verifying that the main WWT difference was caused by releasing water above the demolition elevation. With the TDC, the withdrawal layer raised 19.6 m from near the intake to near the demolition elevation. Thus, more stable stratification and various changes in the WWT played a vital role in several sensitive elements and reservoir management. This study can provide practical experience for the effect of similar disease reservoirs treatment and can provide a reference for optimizing water quality management strategies.
Research Article
Regime Impact of Two-dam Coexistence: A Special Withdrawal Measure in Fengman Reservoir, China
https://doi.org/10.21203/rs.3.rs-1434935/v1
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two-dam coexistence
thermal stratification
selective withdrawal
withdrawal water temperature
equivalent withdrawal elevation
Modern artificial hydrological construction designed to impound water and meet living and production needs (Ignatius and Stallins, 2011; Smith et al., 2002) provides important ecosystem services for humans. A corresponding healthy ecosystem is stable and sustainable and can possess resilience against stress to maintain its organizational structure (Costanza et al., 1992) and maintain economic activity and human health (Rapport et al., 1998). However, these ecosystems and services are being altered by human activity in ways that we do not understand and cannot predict (Dobiesz et al., 2010). In some cases, within a few years, homeostasis indicators may change radically (Kong et al., 2016; Aleksandr et al., 2021). The construction, operation or reconstruction of dams is an example of structural change in reservoir ecosystems (Vitousek et al., 1997), whose impact includes ecosystem and even water quality (Bardach et al., 1973; Yang et al., 2013).
Thermal stratification is a basic physical feature of lakes and reservoirs (Magee and Wu, 2017; Yang et al., 2020), determining the vertical convection and mixing process (Rogora et al., 2018, North et al., 2014). It affects the material exchange inside the water, such as nutrients and dissolved oxygen (Zhu et al., 2018). Due to seasonal stratification, the epilimnion and hypolimnion are separated by the metalimnion (or thermocline) (James et al., 2017), and the strengthening process of thermal stratification has a negative impact on the aquatic river ecosystem (Yang et al., 2021), thereby affecting the utilization of reservoir water bodies (Weber et al., 2017). The stratification responses of reservoirs are dependent not only on the reservoir’s characteristics but also on the operation of hydraulic facilities that particularly control the outflows and the intrusion depth of inflow (Çalişkan and Elçi, 2009). For example, many northern temperate countries mostly used bottom water intakes to strengthen the vertical mixing of dissolved oxygen in the 1960s, such as in Shasta Lake (Hanna et al., 1999), Lake Powell (Richard marzolf et al., 2000) and Lake Dillon (Lewis et al., 2019). Later, widespread use of selective withdrawal facilities alleviated the severe quality impact of low intake (Nurnberg, 1987). Studies have shown that deeper withdrawal could facilitate heat transfer in the water column, increase the thickness of the metalimnion and weaken stratification stability (Casamitjana et al., 2003; Ma et al., 2008). When the intake moves upward, the thermocline deepens, the withdrawal height controls the thermocline depth in summer, and the inflow depth and hydrodynamics are affected simultaneously (Mauric et al., 2021; Ren et al., 2020). However, reservoir reconstruction-induced thermal regime alteration has rarely been assessed through field observations, especially the variation among one-dam and two-dam conditions.
According to the 2011 database of the ICOLD (International Commission on Large Dams), there are 37,626 large dams all around the world, mainly built in the 1960s and 1980s, some of which are increasingly coming to their end of life. The solution is to maintain or demolish the dams, while the second is more adaptable to small construction due to the contradiction of considerable economic benefits and production demand (Michal et al., 2020). Dam demolition always arouses controversy (Coleen A., 2016). Thus, severe risk management of large reservoirs is lacking, which is a great challenge to the world. In Jilin Province, China, for the first time, a two-dam coexistence (TDC) and the original dam is partially removed to deal with the Fengman Dam, which was defined as a sick dam, and that could solve the safety problem of a dangerous reservoir (Lu et al., 2020). The study of the TDC can provide practical experience and a theoretical basis for dams with serious safety risks all over the world.
The Fengman Reservoir, located in northwestern China, presented a vertical stratification in summer and an inverse stratification in winter. In winter, affected by the severe cold climate, the surface water is frozen. The water body of the reservoir overturned twice a year, which was a dimictic reservoir. Additionally, a rime scenery stood at 70 km downstream of Fengman Dam formed by special water vapor conditions in winter, which was affected by natural conditions (outflow temperature and meteorological and freezing conditions) and human activities (such as dam reconstruction). The four spawning areas downstream were also sensitive to the thermal regimes of the reservoir, which together emphasizes variations in the thermal state of the reservoir among studies on the water environment of the TDC.
Giving attention to the study of the thermal response of the TDC, many researchers chose numerical simulations to simulate the stratification of Fengman Reservoir. For the effect of partial demolition on the released temperature and rime scenery, Tuo et al. and Lu et al. established a 2D water temperature model coupled with an ice cover model and found that the total heat storage of the reservoir throughout the year decreased with the TDC, and the outflow temperature decreased in winter with a small magnitude and increased in summer. The above studies focused on the impact of the TDC on the outflow temperature, rarely involving the thermal state response of the reservoir area. Moreover, the discharged water temperature was complex due to the mixed generation of new and old power station units. The prediction accuracy of the models was subject to the parameter value accuracy that needs to be calibrated based on the prototype observation results. Prototype observation was the most intuitive means to reflect the variation in the water temperature situation caused by the operation of the two dams.
To explore the influence of the TDC on the water temperature characteristics of the reservoir and the process of released temperature, this study conducted water temperature observations from Sep. 2019 to Oct. 2021, combined with historical water temperature data, compared the vertical water temperature and discharge water temperature in front of the dam with the one-dam condition (ODC) and two-dam coexistence condition (TDC), and mastered the thermal effect of the old dam as the front retaining wall. This study can not only provide a research basis for this special water discharge mode but can also provide new ideas for the treatment of dangerous reservoirs.
Study area and background
Fengman Reservoir (43°43′10″ N, 126°41′13″ E) is located in Jilin Province, China, on the Second Songhua River (Fig. 1a), with a watershed area of 7.34×104 km2, and is a large (1) type reservoir. Fengman Reservoir underwent three distinct periods of management and operation (Fig. 1). Since Fengman Dam was defined as a "dangerous dam" in 2007, comprehensive treatment was carried out, and its objectives were 1) to rebuild a new RCC gravity dam 120 m downstream of the old dam and 2) to demolish the old dam above an elevation of 240 m (Fig. 1b). The three periods were 1) Period A with the ODC through deep penstock withdrawal in 1943–2014, and 2) Period B for a reconstruction period from 2014 to 2019 and 3) Period C with the TDC from 2019 to 2021 (Fig. 1b). This study thus highlights Periods A and C of operation based on the reservoir's data history.
After reconstruction, Fengman Reservoir retained its characteristic water levels (such as normal water level and dead water level) and engineering functions. The engineering characteristic parameters of Periods A and C are shown in Table 1.
Fig. 1. Maps of a) the Second Songhua River catchment and the reservoir, b) observation points and dam profile with its water control facilities indicated and c) on-site motorizing.
Table 1. Characteristic parameters of the Fengman Reservoir during Periods A and C.
Data collection
In this study, there were four monitored sections from Oct. 2019 to Sep. 2021, including the front of the old dam and new dam and withdrawal water temperature (WWT) observations of the old dam and new dam. The vertical temperature monitoring in front of the dam adopted the YSI6600 multiparameter water quality meter produced by YSI Company in the United States, which can automatically record the water depth and water temperature. The accuracy of the temperature sensor was within 0.15 °C, and measuring points were arranged every 1.0 m to 3.0 m in the water depth direction based on stratification. The WWT observation adopted the ZDR self-recording thermometer produced with a range from -40 °C to 100 °C and an accuracy of 0.1 °C.
Moreover, the collected data included discontinuous long series of data in Period A. Notably, among the data in front of the dams, 1952 to 1954 were selected with more monitoring frequency and different water levels to interpolate to obtain the continuous water temperature process. The different data uses and purposes are listed in Table 2.
Table 2. Data time series in different parts.
Index of thermal stratification and statistical analysis
To better evaluate the change in stratification stability, the SI reflecting the stability of deep water bodies was used to assess the stratification strength (Sahoo, 2016). The SI calculation expression is:
where Z is the depth of the water column from the surface, Z0, Zl and Z are the depths of the surface water, the lower end of the water column, and the centroid of the water column, respectively, and is the water density at depth Z. SI can be converted into energy by multiplying by the acceleration of gravity and the volume of each layer, and it represents the ideal energy estimate of the entire water column to achieve mixing in the depth range without an increase or decrease in heat.
Additionally, the thermocline strength index or TSI (Yu et al., 2010) was computed using the equation:
where ΔT and ΔH are the differences in water temperature (°C) and water depth (m) between intervals, respectively. The TSI represents only the steepness or the average gradient of the cline (Liu et al., 2019). Since many studies have provided different values where the thermocline begins to appear, this paper adopts a minimum gradient of 0.1 °C/m in the temperature profile. The temperature gradient for each depth interval was calculated using the actual temperature data, while all values equal to or higher than 0.1 °C/m after the depth interval were averaged and thus constituted the average TSI.
Equivalent elevation method of outflow temperature
The equivalent elevation method of outflow temperature (EEMOT), designed to identify the elevation of the mean plug flow corresponding to the WWT, employs the tail water temperature of each generator set as the released water temperature of the unit and finds the corresponding elevation as the equivalent elevation among the vertical water temperature in front of dams. It is worth pointing out that EEMOT is mainly established for stratified water bodies and not for stratospheric structures. The measurement accuracy of the released temperature and vertical water temperature will affect the determination of the equivalent elevation.
Vertical water temperature profile analysis in Periods A and C
Figure 2 shows a two-dimensional distribution map of the vertical water temperature structure, temperature gradient and buoyancy frequency distribution of the Fengman Reservoir during the high-temperature period in Period A (1952, 1953 and 1954) and Period C (2021). In Period A, there were interannual differences in water level and scheduling and thus the evolution of water temperature. In 1952, the average surface water temperature was 21.3 °C, and the temperature difference between the surface and bottom ranged from 4.9 to 22.8 °C. The temperature gradient ranged from 0 to 0.8 °C/m, and the buoyancy frequency ranged from 0 to 4.5×10-2 s-2. The thermocline mainly existed between elevations of 220 m and 245 m. The temperature gradient below 210 m was less than 0.2 °C/m, and the buoyancy frequency varied from 0 to 1.5×10-2 s-2. During the flood season (Jul.–Aug.) in 1953 and 1954, the inflow flow changed greatly, the maximum flow reached 7551 m3 s-1, and the vertical mixing was sufficient. The thermocline existed at an elevation above 240 m from Jul. to Aug. with a maximum temperature gradient of 0.74 °C/m. Another thin thermocline was located at an elevation of 210 m, which was close to the water intake where the maximum temperature gradient and buoyancy frequency were 1.2 ℃/m and 3.4×10-2 s-2, respectively. In Oct., the reservoir area basically tended to be vertically isothermal.
In period C, the water temperature in front of the dam fluctuated periodically, and the water temperature in each layer generally showed a downward trend with time. The average surface water temperature was 22.7 °C, and the temperature difference between the bottom and surface layers ranged from 6.8 to 26.8 °C. The temperature gradient ranged from 0 to 10.5 °C/m, and the buoyancy frequency ranged from 0 to 10.9×10-2 s-2. The surface water temperature variation ranged from 14.0 to 32.9℃, and the bottom water temperature varied from 6.0 to 7.3℃ with a maximum amplitude of 1.3℃. From Jul. to Sep., the thermocline existed at elevations between 240m and 250m and between 225m and 230m, with average temperature gradients of 0.5 and 0.7 °C/m, respectively. In Oct., the upper thermocline disappeared, and the lower thermocline persisted until the end of Oct.
Fig. 2 Two-dimensional contours of (a) temperature, (b) vertical temperature gradient and (c) buoyancy frequency labeled A1~A3 (1952, 1953, 1954) and C (2021), as representative of Periods A and C.
Withdrawal water temperature analysis in Periods A and C
Figure 3 shows the monthly average WWT process in Period A (Sep. 2010 to Dec. 2013) and Period C (Oct. 2019 to Sep. 2021). In Period C, the WWT was affected by the mixed power generation of the old and new power stations and the number of units operating, leading to a greatly fluctuating WWT. After excluding the water temperature when the new power station and old power station were not generating power at the same time, the average value of the outflow temperature of the new and old power stations was used as the WWT in this period. The average heating period (from Jun. to Sept.) was 8.3 °C, 13.7 °C, 15.5 °C, 16.8 °C, 11.6 °C, 14.7 °C, 16.9 °C, and 17.2 °C and the average heating rates were 0.071 °C/d and 0.047 °C/d, respectively.
During the freezing period (Jan. to Feb.), the discharge water temperatures of Periods A and C were 2.7 °C, 2.5 °C, 2.4 °C and 2.4 °C, respectively. The WWT of Period C was lower than that of Period A, which may have been caused by the participation of the old power station in operation. From May to Jun., the released mixed water temperature in Period C was significantly higher than that in Period A, with an average increase of 3.2 °C. During this period, the vertical stratification in front of the dam mainly formed on the surface and developed. The withdrawal water of the new power station was taken from the middle and upper layers of the reservoir, the front retaining wall of the old dam could stabilize the selective withdrawal, and the effect was obvious. In Jul. and Aug., stratification had developed to 20 m underwater and formed a stable state where the outflow power had difficulty overcoming the buoyancy formed by the flow of the main layer. The participation of the old power station caused an insignificant difference in WWT.
Fig. 3. Comparisons of monthly average WWT in different periods: Period A from 2010 to 2013 and Period C from 2019 to 2021.
Effects of TDC on thermal regimes and stratification stability
The box diagram in Figure 4A shows the distribution of water temperature at different elevations during the same period as in Figure 2. The elevation of 250 m was close to the surface layer of the reservoir, and the elevation of 220 m was close to the bottom layer. The average and median water temperatures at an elevation of 250 m in Period C were 22.3 °C and 22.8 °C, respectively, which were significantly higher than those in Period A, which were 19.7 °C and 20.4 °C (`p=0.008), respectively. Moreover, the water temperature in Period C had a larger fluctuation range from 14.1 °C to 29.8 °C, which was related to the use of upper water temperature and meteorological factors. As the elevation decreased, the mean and median water temperatures in Period C decreased and were 1.2 °C and 1.4 °C, 2.9 °C and 2.7 °C, and 7.0 °C and 7.1 °C lower than those in Period A, respectively, which was due to the strong buoyancy inhibition of the up moving thermocline in Period C and the more uniformly mixed water of the deep layer. Among them, the 240 m elevation difference between Periods A and C was insignificant according to the paired T test (p>0.05) because the driving force of the water temperature change was gradually transformed from the various dynamic effects of the water surface to thermal diffusion caused by the temperature difference and vertical mixing due to mid-level flow.
The TDC caused changes in water temperature at various elevations (Fig. 4B) and changed the reservoir stability characteristics accordingly. The SI represents the work performed to achieve further mixing over the depth range of the entire water column without adding or reducing heat. Generally, the larger the SI is, the greater the amount of work the reservoir can do to resist buoyancy, and the more stable the water column. With the TDC, the increase in the intake layer reduced the thermal mixing of the bottom water temperature, and the water column was more stable.
Fig. 4. Variation in WT at different elevations (A). (a) 250 m, (b) 240 m, (c) 230 m, and (d) 220 m. The box symbol denotes the mean value of the dataset, with the horizontal lines in the box denoting the 25th, 50th, and 75th percentile values. SI changes (B) in front of the new dam (scatter points) and old dam (columns, red for Period A and blue for Period C). Temperature profile (C) in front of the new dam and old dam in Period C for eight different years. The red dotted line represents the dismantling elevation of the old dam. Accumulated thickness (D) of all layers including the epilimnion, metalimnion and hypolimnion in front of the two dams and the TSI comparisons during period C.
The TDC formed a pool with a storage capacity of approximately 1.5 million m3 between the two dams that was fully connected with the reservoir area in front of the old dam above the demolition elevation of 240.2 m and was blocked with the water bodies beneath 240.2 m. The difference in front of the two dams confirmed the water temperature effect of new dam released water (Fig. 3). From the eight prototype observation data, it can be seen that with the demolition elevation of 240.2 m as the boundary, the water temperature difference between the two vertical lines was small above 240.2 m but significant below it. The largest difference occurred at the bottom of the reservoir, with an average maximum difference of 8.4 °C. The obstruction of the old dam directly caused the water temperature to rise below the elevation before the removal of the new dam. It is worth noting that the difference in water temperature in Sep. was greater than that from Jun. to Aug., only because Fengman Reservoir was a dimictic reservoir where water was turned over twice a year. There was a cumulative effect on temperature stagnation in front of the old dam. The temperature difference continued from the stratification to the back and had a cumulative impact on the temperature hysteresis in front of the old dam.
Based on the analysis of temperature profiles in front of the two dams, the vertical water column was divided into the epilimnion, metalimnion and hypolimnion (FIG. 4D). It can be seen that the average thickness of thermocline layer in front of the old dam was 25.8m, larger than that before the new dam (8.9m) due to insufficient mixing between the bottom and the surface. The thickness of the epilimnion and hypolimnion was opposite. However, the TSI in front of the new dam (0.71 °C/m on average) was larger than that before the old dam (0.67 °C/m), which wasn’t consistent with the rule of SI. The reason was that SI in Figure 4B was the stability of water column 50m underwater, while TSI was the average strength of metalimnion on water column monitored, and the water depth and metalimnion thickness in front of the new dam and the old dam were different. The increase of metalimnion thickness may lead to weak average TSI. Therefore, combined with TSI and SI analysis, it can be seen that the water temperature stability in front of the new dam was significantly weaker than that in front of the old dam after the coexistence of two dams.
Effects of TDC on vertical mixing
During the high-temperature period, heat transfer and vertical mixing affected the downward transfer of heat to the deeper water layer (Prats & Danis, 2019). TDC mainly regulated the thermal state by releasing the epilimnion and metalimnion water bodies, which had a significant impact on the vertical water temperature structure in front of the dam. Figure 5A shows the distribution comparison of thermocline parameters between the TDC and ODC. Selecting a buoyancy frequency equal to 1.5×10-2 s-2 as the threshold, the upper and lower bounds of the thermocline were greater than 1.5×10-2 s-2 and less than 1.5×10-2 s-2 in the beginning, and the difference was the thickness of the thermocline. From Jul. to Aug., when stratification was strongest, the heat exchange between the epilimnion and hypolimnion was restrained, and the vertical mixing was weakened after the epilimnion and metalimnion water bodies were taken. The thermocline was concentrated between 4 m and 30 m underwater, with an average thickness of 26 m. With the ODC, the use of cold water led to a deeper thermocline lower boundary (37 m) and a thicker thermocline (35 m). From 1952 to 1954, the depth and thickness of the thermocline were 37 m and 35 m, 39 m and 39 m, and 47 m and 47 m, respectively.
Fig. 5. Comparison of thermocline parameters between the TDC and ODC, blue represents the thermocline with no TDC, red represents the thermocline with TDC, and purple represents the overlap, which is realized by the transparency of the two colors.
In other reservoirs, selective withdrawal measures also reduced the thickness of the thermocline. After the front retaining wall was adopted in the Dongqing Reservoir, the thickness of the thermocline increased by 11.8 m (Yang, 2021). The change in thermocline thickness was caused by the difference in the thermal state of the reservoir. Due to the temperature dependence of the coefficient of thermal expansion, the temperature gradient was stronger, and the average temperature was higher, resulting in increased stratification (Schwefel et al., 2016).
Using EEMOT, the vertical equivalent elevation points in front of the dam corresponding to the WWT were counted based on 6 groups of data with the ODC (including Feb. and Jun. to Oct.) and 8 groups of data with the TDC (Fig. 6). With the ODC, the equivalent elevation ranged from 221 m to 226.5 m with an average elevation of 223.4 m, which was located 8.4 m above the bottom elevation and 0.4 m above the top elevation of the old water intake. With the TDC, the water was mainly taken from above the demolition gap of the old dam, with an average equivalent elevation of 243.0 m, 2.8 m above the demolition elevation, and the equivalent elevation varied from 241.4 m to 246.7 m. Comparatively, the average equivalent elevation of Period C rose by 19.6 m. This long-term action of taking surface water would further increase the thickness of the hypolimnion of the reservoir after the operation of the new power station to further reduce the heat storage capacity of the reservoir.
Fig. 6. Comparison of the equivalent elevation and operating water level during period A and period C.
Engineering applications and further considerations
The withdrawal mode of TDC was especially proposed for the treatment of sick large dams in large reservoirs, in which there were few cases in the world. However, similar to many selective withdrawal facilities in principle, such as stop-log gates, temperature-control curtains and retaining walls, it raised the position of the mean plug flow and thermocline layer and inhibited vertical thermal mixing, simultaneously improving the withdrawal water temperature and mitigating water environmental hazards caused by deep withdrawal. The old dam acted as a front retaining wall in front of the new dam. Therefore, the water taken from the new dam was taken from the epilimnion, while the water taken from the old dam was taken from the hypolimnion (Fig. 7.), which made up for the defect that the front retaining wall can only use surface water (Yang, 2021). Under the engineering background of the Fengman Reservoir and the expiration of the service life of an increasing number of large dams, there are no available treatment methods. This study can provide a reference for the treatment of sick large dams, which can solve the problem of reservoir diseases and improve the adverse withdrawal water temperature caused by dam construction.
Fig. 7. Schematic diagram of the selective withdrawal effect. Red arrows represent the approximate position of the withdrawal layer.
From the perspective of sensitive objects, there was a rime landscape in winter and fish spawn in summer in the Fengman Reservoir. The maintenance of the rime landscape required long-distance water vapor exchange to ensure that the river channel was not frozen. The WWT in the freeze-up period was 0.2 °C lower than that in the ODC period on average (Fig. 3). In the process of cooling along the way, the time and freezing distance to 0 °C would lead to insufficient water vapor conditions in the river section and threaten the formation of rime. Therefore, how to improve the WWT and eliminate its negative impact on the rime landscape in winter should be considered. Corresponding to the inversion stratification in front of the dam in winter, the water intake of the old dam was low, and the higher temperature water was discharged, while the new dam released the lower temperature water of the epilimnion and metalimnion. Therefore, it was recommended to select the old power station and old dam for operation in winter to increase the WWT. In spring and summer, fish spawn was affected by the low-temperature water taken from the bottom of the old dam. The new dam should be used as much as possible in summer, and high-temperature water can be available.
Additionally, Fengman Reservoir started the construction of the main works in Oct. 2014, and the new power station was put into use in Jul. 2019. During a period of nearly five years, the reservoir ecosystem was in a transition stage from one steady state to another, including the special operation mode of the reservoir and the changes in the thermal structure and thermal state of the downstream river, which directly led to changes in the downstream spawning ground, rime landscape and other sensitive factors, such as water quality factors. The interpretation of this transitional stage also had a significant impact on the research on the treatment of the coexistence of the two dams. Therefore, it is necessary to integrate more prototype observation data to analyze the water temperature and water quality process in the transition stage.
Based on the historical and measured water temperature data of Fengman Reservoir, combined with the characteristics of the TDC, this study revealed the influence of the coexistence of two dams on the thermal stratification response in front of the dam and the withdrawal water temperature process. According to our study results, the TDC was accompanied by the mixed power generation of two power station units and a special daily regulation process, leading to the water temperature isoline in front of the dam showing an overall decline and periodic fluctuations with time. The thermocline moved upward with the temperature gradient from 0 to 10.5°C/m. Vertical mixing was weakened with the difference between the surface and bottom temperatures from 6.8 to 26.8°C. The water temperature of the hypolimnion was more stable with an average value of 6.5°C, while with the ODC, the water temperature of the hypolimnion ranged from 2.0 to 6.3°C, and the buoyancy frequency ranged from 0 to 1.5 × 10− 2 s− 2. The TDC also made the water temperature structure in front of the old dam more stable, and the SI was greater than that in front of the new dam and the ODC. From May to Jun., the WWT increased by 3.2°C on average compared with the ODC, and there was no significant increase from Jul. to Aug. The water temperature difference in front of the new dam and old dam also confirmed that the difference in WWT was mainly caused by the access of the water layer above the demolition elevation of the old dam. The equivalent elevation of the TDC increased by 19.6 m, which changed from near the water intake of the old power station to near the demolition elevation of the old dam.
The special water intake method of the TDC can provide a reference for the treatment of dangerous reservoirs and improve the low-temperature water in spring and summer, but the main construction period lasted longer, which represented a transition period with two steady-state ecosystems and deserves further attention.
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Author Contributions
Conceptualization：Youcai Tuo, Yun Deng.
Data Curation: Xiaoqian Yang, Yanjing Yang, Youcai Tuo.
Funding acquisition: Youcai Tuo, Yun Deng.
Investigation: Xiaoqian Yang, Yanjing Yang, Youcai Tuo, Yun Deng, Xingmin Wang.
Methodology: Xiaoqian Yang, Yanjing Yang, Youcai Tuo, Yun Deng.
Project administration: Youcai Tuo, Yun Deng, Min Chen
Supervision: Youcai Tuo, Yun Deng.
Writing-original draft: Xiaoqian Yang.
Writing-review & editing: Xiaoqian Yang, Yanjing Yang, Youcai Tuo, Xingmin Wang
Funding
This work was supported by the project of Fengman Dam Reconstruction Engineering Construction Bureau in Songhua River Hydropower Co., Ltd (Grant numbers 13H0180).
Competing Interests
The authors have no relevant financial or non-financial interests to disclose.
Availability of data and materials
All data generated or analyzed during this study are included in this published article.
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Table 1. Characteristic parameters of the Fengman Reservoir during period A and C.
|
Parameters |
Unit |
Period A |
Period C |
|
Normal water level |
m |
263.5 |
263.5 |
|
Dead level |
m |
242.0 |
240.2 |
|
Maximum dam height |
m |
91.7 |
94.9 |
|
Elevation of intake bottom |
m |
219.0 |
222.0 |
|
Total storage capacity |
m3 |
103.77×103 |
103.77×103 |
|
Capacity under normal water level |
m3 |
88.5×103 |
81.01×103 |
|
Installed capacity |
MW |
1002.5 |
1480 |
Table 2. Data time series in different parts.
|
Parts |
ODC |
TDC |
|
Part 3.1.1 |
Jul. to Oct. from 1952 to 1954 |
Jul. to Oct. in 2021 |
|
Part 3.1.2 |
Several years |
From 2019 to 2021 |
|
Part 3.1.3 |
- |
Several months |
|
Part 3.2.1 |
- |
From 2019 to 2021 |
|
Part 3.2.2 |
From 2010 to 2014 |
From 2019 to 2021 |
|
Part 3.2.3 |
- |
From 2019 to 2021 |
|
Part 3.3 |
From 2010 to 2014 |
From 2019 to 2021 |