Operation Of Three Gorges Reservoir Research Articles

A Novel Time-Varying P-III Distribution Curve Fitting Model to Estimate Design Floods in Three Gorges Reservoir Operation Period

Design floods are traditionally estimated based on the at-site annual maximum flood series, including historical information of hydraulic structures. Nevertheless, the construction and operation of upstream reservoirs undermine the assumption of stationarity in the downstream flood data series. This paper investigates non-stationary design flood estimation considering historical information from the Three Gorges Reservoir (TGR) in the Yangtze River. Based on the property that the distribution function of a continuous random variable increases monotonically, we proposed a novel time-varying P-III distribution coupled with the curve fitting method (referred to as the Tv-P3/CF model) to estimate design floods in the TGR operation period, and we comparatively studied the reservoir indices and parameter estimation methods. The results indicate that: (1) The modified reservoir index used as a covariate can effectively capture the non-stationary characteristics of the flood series; (2) The Tv-P3/CF model emphasizes the fitness of historical information, yielding superior results compared to time-varying P-III distribution estimated by the maximum likelihood method; (3) Compared to the original design values, the 1000-year design peak discharge Qm and 3-day and 7-day flood volumes in the TGR operation period are reduced by approximately 20%, while the 15-day and 30-day flood volumes are reduced by about 16%; (4) The flood-limited water level of the TGR can be raised from 145 m to 154 m, which can annually generate 0.32 billion kW h more hydropower (or increase by 6.8%) during flood season without increasing flood prevention risks.

Quantifying the Regulation Capacity of the Three Gorges Reservoir on Extreme Hydrological Events and Its Impact on Flow Regime in a Changing Climate

AbstractThe Three Gorges Reservoir (TGR) is one of the world's largest hydropower projects and plays an important role in water resources management in the Yangtze River. For the sake of disaster prevention and catchment management, it is crucial to understand the regulation capacity of the TGR on extreme hydrological events and its impact on flow regime in a changing climate. This study obtains historical inflows of the TGR from 1961 to 2019 and uses a distributed hydrological model to simulate the future inflows from 2021 to 2070. These data are adopted to drive a machine learning‐based TGR operation model to obtain the simulated outflow with TGR operation, which are then compared with the natural flow without TGR operation to assess the impact of TGR. The results indicate that the average flood peaks and total flooding days in the historical period could have been reduced by 29.2% and 53.4% with the operation of TGR. The relative declines in drought indicators including duration and intensity were generally less than 10%. Faced with more severe extreme hydrological events in the future, the TGR is still expected to alleviate floods and droughts, but cannot bring them down to historical levels. The impact of TGR operation on flow regime will also evolve in a changing climate, potentially altering the habitats of river ecosystems. This study proposes feasible methods for simulating the operation of large reservoirs and quantifying the impact on flow regime, and provides insights for integrated watershed management in the upper Yangtze River basin.

Simulated climate effect of the Three Gorges Reservoir after its completion:Within surface and local scope instead of regional

针对以往三峡水库气候效应数值模拟研究中,水库参数化方案简约及模拟时段未涉及成库以来高水位运行阶段等不足,在中尺度气象模式中,通过扩宽水体面积和抬升水位高度的方式对三峡水库引起的陆面参数变化进行描述,进而采用敏感性数值试验及统计分析等手段,评估了三峡水库成库以来高温干旱(2013年)和低温洪涝(2020年)年景下,关键气象要素对水库运行的响应特征。结果表明:两种典型年景下水库运行均会造成近地层气温降低(0.98~1.27℃)、相对湿度增加(3.9%~5.5%)和风速增大(0.43~0.68 m/s),同时响应强度的日变化导致近地层气温和相对湿度的日较差减小、平均风速的日较差增大;尽管上述变量的变化幅度与该地区气候的自然变率相当,但水库运行对气温和相对湿度的影响范围基本限制于水库周边约2 km,垂直方向则大多低于200 m,对风速的影响范围可扩展至水库周边约12 km,垂直方向延伸至200 m左右,且响应强度均随水平距离和垂直高度的增加而显著减小。尽管数值试验放大了三峡水库的气候效应,但作为典型的河道型水库,三峡水库成库以来的不同气候年景下,水库运行产生的气候效应基本限制在近地层、局地范围内,未对区域气候产生明显影响。;In view of the shortcomings of the previous numerical simulation research on the climate effect of the Three Gorges Reservoir (TGR), such as the simplicity of the reservoir parameterization and the fact that the simulation period did not involve the reservoir impoundment period, the changes of land surface parameters induced by the TGR were numerically described by increasing the river width and raising the water level in the mesoscale meteorological model. The variation characteristics of meteorological elements after completion of the TGR were analyzed based on two parallel numerical simulations, control run (no TGR) and sensitive run (TGR), during the drought year with high temperature (2013) and the flood year with low temperature (2020). The results show that the operation of TGR under these two typical years both reduce the surface temperature (0.98-1.27℃), increase the relative humidity (3.9%-5.5%) and the wind speed (0.43-0.68 m/s), the diurnal range of near-surface temperature/relative humidity decreases and the diurnal range of wind speed increases with the diurnal variation of the response; Although the aforementioned responses are equivalent to the natural climate variability, the variations of temperature and relative humidity are basically limited to about 2 km around the reservoir, most of which are lower than 200 m in the vertical direction, and the variation of wind speed change extend to about 12 km near the reservoir with up to 200 m in the vertical direction. All of these responses decrease significantly with the increase of horizontal distance and vertical height. Therefore, although the climate effect of the TGR is amplified in our simulation experiment, as a typical river-type reservoir, the climate effect induced by the operation of TGR is basically limited to the surface and local area, and has no obvious influence on the regional climate.

Ecological responses of spawning habitat suitability to changes in flow and thermal regimes influenced by hydropower operation

AbstractLarge reservoirs and dams can fundamentally alter downstream flow and thermal regimes by resetting the flow and water temperature release boundary conditions, resulting in significant impacts on fish habitat suitability. Therefore, it is critically important to understand how the volume and temperature of dam releases to influence downstream hydraulic and temperature dynamics and predict the ecological responses of fish habitat suitability to these changes. In this study, a physically based coupled model framework was developed to identify the temporal and spatial variations of hydraulic and water temperature regimes and the spawning habitat suitability of four major Chinese carps (FMCC) in the middle reach of the Yangtze River below Three Gorges Reservoir (TGR) responded to different upstream boundary conditions established by dam operation. The results showed that the dam discharge volume from TGR mainly affected the downstream hydraulic regime, while dam discharge temperature would have most impact on downstream water temperature. The spawning habitat suitability of FMCC obviously decreased due to TGR operation, and the declined water temperature suitability and the delayed and narrowed window of suitable water temperature were the main influencing factors. For the purpose of maximizing the spawning habitat suitability of FMCC in the downstream reaches below TGR, dam discharge volume ranged from 10,000 to 22,500 m3/s, and dam discharge temperature varied from 22 to 23.5°C would be recommended as the outflow conditions in the ecological operation of TGR.

Sedimentation and regulation technologies in the Three Gorges Reservoir

AbstractSedimentation is one of the key technical problems present throughout feasibility demonstration, design, construction, and operation of the Three Gorges Project. Since the Three Gorges Reservoir (TGR) began impoundment operation in 2003, important progress has been made in sedimentation and regulation technologies through systematic research and practices. This paper reveals the asynchronous propagation pattern of flood and sediment peak inflows, sediment movement in the backwater area during drawdown period, sediment flocculation, and sediment deposition that compromises the flood control reservoir storage, and puts forward new sediment regulation techniques, including sediment peak discharge regulation during flood season, siltation reduction in the backwater area during drawdown period, dynamic adjustment of the reservoir level during flood season, and joint sediment regulation in cascade reservoirs. A real‐time monitoring, forecasting and dynamic regulation platform for water and sediment inflow in the TGR was developed; a new mode of dynamic sediment regulation called “regulating sediment for greater benefits” is proposed, which addresses the constraints of sedimentation on the optimal operation of TGR. By enabling more flexible operation, this mode improves the process and pattern of sedimentation, ensures the safety of reservoir operation, and significantly augments the comprehensive benefits of the TGR.

The Impact of the Three Gorges Reservoir on Water Exchange Between the Yangtze River and Poyang Lake

The Three Gorges Reservoir (TGR) operation has profoundly altered the hydrological regimes and ecosystem in the middle and lower Yangtze River basin. Its impacts on the river flow regime, sedimentation, lake level decline, and biodiversity have been extensively studied. However, the quantitative influence of the TGR on the downstream water exchange between the Yangtze River and Poyang Lake is lacking attention. In this study, we developed a machine learning model to investigate the river–lake water exchange variations under the TGR operation in typical hydrological years, compared with the natural scenario (i.e., no TGR). The results show that the machine learning model could provide a fast and accurate approach to identify the long-distance impact of reservoirs. The operation of the TGR has significant effects on the water exchange between the Yangtze River and Poyang Lake in the impounding period, with a 10,000 m³/s decrease in reservoir discharge approximately causing a 4,000 m³/s decrease in river–lake water exchange in the dry year and a decrease of 6,000 m³/s and 8,000 m³/s in reservoir discharge causing an increase of 4,000 m³/s and 6,000 m³/s in the normal year and the wet year, respectively. The TGR effect varies with different hydrological conditions of the river and lake, showing longer time range (from May to October) and greater degree (1.5 times the change rate in other years) in the wet year. The TGR operation is beneficial to the water maintenance of the lake in the dry year and flood control in the lake area in the wet year. This study provides a constructive approach and valuable information for decision making in water resource management and ecosystem protection in large river–lake systems.

Response of CO2 and CH4 transport to damming: A case study of Yulin River in the Three Gorges Reservoir, China

The growing number of dams in the Three Gorges Reservoir (TGR) make tributaries of TGR into spatially complex and temporally dynamic systems. To assess the influence of damming on the carbon emission in the tributary of TGR, we investigated the spatial heterogeneity of CO2, CH4, organic carbon, inorganic carbon, and evaluated the transport mechanisms of CO2 and CH4 within water column during different TGR operation periods. We found that mean CO2 and CH4 concentrations in water downstream (44.04 and 0.44μmolL-1 for CO2 and CH4, respectively) were lower than upstream (48.36 and 1.63μmolL-1 for CO2 and CH4, respectively) in the impoundment period of TGR, which was consistent with the spatial variations of organic carbon. In the drainage period of TGR, the mean CO2 concentration of upstream (58.71μmolL-1) was significantly lower than that of downstream (88.92μmolL-1). The higher CO2 concentration downstream was attributed to terrestrial input and higher microbial diversity of the water column, while the lower CO2 concentration upstream was due to the photosynthesis of phytoplankton. Furthermore, low CH4 concentrations (less than 0.1μmolL-1) of both upstream and downstream were detected in the drainage period. Based on results of 16s rRNA sequencing, quantitative PCR, and functional prediction, it was indicated that aerobic CH4 oxidation predominantly in the bottom water layer reduced CH4 of the water column in drainage period. Our results expand the theory of CO2 and CH4 transport within the water column in complex river systems and provide theoretical references for the distribution of carbon in the dammed tributaries of TGR.

Hydrological drought evolution and its response to meteorological drought in the middle-lower reach of the Yangtze River triggered by Three Gorges Reservoir

利用1960—2016年长江流域183个气象站逐月气温和降水数据以及干流3个水文站逐月径流资料, 采用标准化降水蒸散指数(SPEI)和标准化径流指数(SRI)分析三峡水库蓄水运行前后长江中下游宜昌、汉口和大通站水文干旱的多时间尺度演变以及对气象干旱的响应特征. 结果表明: (1) 三峡水库运行后下游各站冬春季旱情明显趋缓, 而秋季干旱状况略有加重; 水库蓄水后各站中旱和重旱发生频率均呈减少趋势, 其中中旱减幅明显, 而特旱发生频率则总体表现为增加趋势; (2) 三峡水库蓄水后3个站平均干旱历时的变幅相对较小, 而干旱烈度和烈度峰值的均值增幅较大; 同时, 各站短时间尺度(1和3个月) 干旱特征变量的变幅总体呈现沿程递增趋势, 而长时间尺度(6和12个月) 干旱特征变量的变幅整体表现为沿程递减趋势; (3) 水库蓄水后各站短时间尺度SRI与SPEI的相关性减小, 但相关性随时间尺度增加而迅速增强, 12个月时间尺度的相关系数达到最大并略高于蓄水前; 在年内相关性上, 蓄水后各站短时间尺度SRI与SPEI的相关系数明显减小, 冬季表现尤为突出, 而长时间尺度的相关系数则略有增加; (4) 水库影响下不同时间尺度宜昌站水文干旱滞后于气象干旱平均天数有增有减, 而汉口和大通站的滞后天数均呈显著增加趋势, 且总体表现为沿程递增趋势. 研究结果可为变化环境下水文干旱预警与水库优化调度提供参考.;Based on monthly temperature and precipitation data from 183 meteorological stations in the Yangtze River (YZR) Basin and monthly runoff data from three hydrological stations in the mainstream of the YZR during 1960—2016, this study, using the standardized precipitation evapotranspiration index (SPEI) and standardized runoff index (SRI), analyzed the multiple timescales evolution of hydrological drought and its response to meteorological drought at Yichang, Hankou, and Datong stations in the middle-lower reach of the YZR before and after the operation of the Three Gorges Reservoir (TGR). The results showed that with the operation of TGR, drought situations at the downstream hydrological stations in winter and spring were significantly reduced, while drought conditions were slightly aggravated in autumn. The occurrence frequency of moderate and severe drought at each hydrological station showed a declining trend in the post-TGR period, particularly for the moderate drought. The frequency of extreme drought demonstrated an increasing trend in general. With the TGR was impounded, the average drought duration of three hydrological stations showed relatively mild variations, whereas the average drought severity and peak exhibited a significant upward trend. Furthermore, the variation amplitude of drought characteristic variables at the short-term timescale (1 and 3 months) from three hydrological stations displayed an increasing trend along the middle-lower reach of the YZR, while decreasing trend happened at the long-term timescale (6 and 12 months). The correlation between SRI and SPEI series from three hydrological stations at short-term timescale decreased significantly, but the correlation increased rapidly with timescale increase. Correlation coefficient between the SRI and SPEI series reached the maximum at 12-month timescale and were slightly higher than that in the pre-TGR period. Meanwhile, the intra-annual correlation coefficients between the SRI and SPEI series at the short-term timescale declined evidently in the post-TGR period, particularly in winter. However, the correlation coefficients of the SRI and SPEI series at long-term timescale increased slightly. With the impacts of the TGR, average day number that hydrological drought lagged behind meteorological drought at Yichang station at different timescales increased or decreased, while the number of lagged days at Hankou and Datong stations increased significantly, and had an increasing trend along the middle-lower reach of the YZR. These results can provide reference for hydrological drought warning and optimal operation of reservoir under the changing environment.

Impact assessment of the three gorges reservoir on the contribution rate for diversion through the three outlets

The Dongting Lake distributes water and sediment of the Yangtze River through the three outlets. Influencing factors of the diversion capacity are complicated. In order to evaluate the influences of the operation of the Three Gorges Reservoir (TGR) on the diversion of the three outlets, this paper investigates systematically the variation process of the runoff at the three outlets based on substantially raw data and principles of variable control. By using comparative analysis and the calculation method of runoff reduction, the change magnitude of the runoff at the three outlets in different periods were quantified and the contribution rates of different influencing factors were estimated. Results show that (1) Inflow difference from the Yangtze River is the primary factor causing the runoff reduction of the three outlets. After the operation of TGR, the Yangtze River was in dry years. Therefore, over 60% of runoff reduction can be attributed to the variation of natural inflow discharge. (2) As the flow is retained by TGR, the runoff of the three outlets further reduced. From 2003-2008, the contribution rates of natural flow, sediment trapping and reservoir regulation to the runoff reduction at the three outlets were 76.19%, 13.49%, 10.32%, while their counterparts after optimal operation of TGR became 66.05%, 14.20% and 19.75% respectively. (3) Reservoir operation after the flood season showed the most significant impact on runoff of the three outlets, resulting in an increase in the number of shut-off days of the three-distributary channels from September to October, especially at Shadaoguan and Kangjiagang.

Understanding the River-Lake Relationship after the Operation of TGR based on SWAT Model

Wang, H.Z., Zhong, H.P., Lu, J.Z., Yan, Q.Q., Li, S., and Zhou, Y., 2020. Understanding the river-lake relationship after the operation of TGR based on SWAT model. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 593–600. Coconut Creek (Florida), ISSN 0749-0208.It is an important issue how the operation of Three Gorges Reservoir (TGR) has impacted upon the hydrological relationship between the Yangtze River and the Poyang Lake. A SWAT model was built to simulate the runoff of the River and the Lake, and the energy difference (Fe) index was selected to characterize the river-lake relationship. The results show: (1) The SWAT model was verified suitable to simulate the runoff of the study area. (2) The monthly trend of River-lake relationship after the operation of the TGR from 2010 to 2018 was studied. The successive monthly strong effect of the River (Fe 0.1) appeared in November and December of 2013, which corresponded to the period of severe draught of the Lake basin. (3) The yearly trend showed the constant increase of the role of the Lake and the constant decrease of effect of the Yangtze River main stream. (4) Under the operation of the TGR, the Yangtze River plays a stronger role during the drainage and flooding periods, leading to the increase of the flood control pressure in the Poyang Lake basin; the effect of the Poyang Lake is enhanced during the water storage and dry period, leading to the increase of the risk of drought disasters in the Poyang Lake basin. The study systematically reveals the change pattern of the river-lake relationship after the operation of the TGR at different time scales.

Lack of methane hotspot in the upstream dam: Case study in a tributary of the Three Gorges Reservoir, China

River damming has seen a growing trend in demand worldwide and the impounded reaches are considered hotspots of greenhouse gas emissions. However, it remains unclear how the spatial distribution of C-gas in sediments and methane (CH4) emissions of dammed tributary changes under different operation periods of the Three Gorges reservoir (TGR). We measured CH4 and carbon dioxide (CO2) concentrations in sediment and CH4 emissions from a dammed river of the TGR, and evaluated the effect of damming on the spatial variability of carbon in the sediment and on CH4 flux. It was found that damming led to a distinct spatial pattern of total organic carbon (TOC) in the sediment, which resulted in higher CH4 and CO2 in upstream sediment compared to the downstream. During the TGR impounding period, the upstream CH4 diffusive flux (0.253-0.427mgm-2h-1) across the water-air interface was higher than in the downstream (0.093mgm-2h-1), which was consistent with the spatial variation of CH4 in the sediments. However, the CH4 emission was predominantly by ebullition and the flux in the downstream (169.173mgm-2h-1) was significantly higher than upstream (12.23-123.05mgm-2h-1) in the discharging period. This can be attributed to a sharp increase in TOC in downstream sediment due to riparian zone soil erosion on both banks, which was caused by periodic large fluctuation in the water level, and a shallow water depth in the downstream. This study adds to our understanding of effects of the TGR's operation on CH4 emissions from a dammed tributary and suggests that the water level fluctuation of tributaries which has direct influence on ebullition and methane oxidation caused by manipulation of the TGR should not be overlooked.

Multi-objective model and decision-making method for coordinating the ecological benefits of the Three Gorger Reservoir

Abstract The operation of the Three Gorges Reservoir (TGR) has changed the original flow regimes in the reservoir area, as well as middle and lower reaches of the Yangtze River, which may lead to algal blooms in the tributaries of TGR, changing conditions in the downstream of TGR for fish spawning and breeding, and saltwater intrusion in water sources area of Yangtze estuary. In this study, a multi-objective model is developed to improve the environmental issues both upstream and downstream of TGR by trading off the contradiction between the ecological benefits and the power generation considering these environmental problems mentioned above caused by the operation of TGR. Solving the multi-objective model with the non-dominated sorting genetic algorithm III (NSGA-III), the non-inferior solutions are obtained. And then the concept of subjective trade-off rate (STOR) is introduced to measure the level of preference of the decision-makers for each objective. Combining the STOR method and ecological risk analysis, a decision-making method is proposed to select the most suitable alternative. As a case study, the operation process of TGR in 2015 on a daily scale is simulated with the multi-objective model, and the most suitable operation rule is selected by decision-making method within the interval ranges for decision-makers’ preference. Analyzing the effect of different trade-off rates combinations on the level of objectives in the suitable alternative, and the result indicates that when the trade-off rates in the combinations increase, the power generation increases but the ecological benefits reduce. Considering this relationship and the trade-off rate ranges both determined to be [1,1.5] with a step of 0.05, some schemes are established. And then the most suitable solution is selected from these schemes, whose power generation is 1.74% less than the maximum and its ecological risk probability is optimal. The most suitable operation rule derived by the multi-objective model and decision-making method provides an operational reference for decision-makers to coordinate the water conflict between ecological benefits and power generation.

Variability in water chemistry of the Three Gorges Reservoir, China

The environmental influence of the Three Gorges Reservoir (TGR) on the Changjiang River has been widely studied since the Three Gorges Dam (TGD) began operation in 2003. However, the changes in water chemistry in the reservoir in response to damming effect variations are poorly documented in the area of this large reservoir. The results suggest that in comparison to the water chemistry before the TGR operation, the inflow concentrations of Mg2+, K+, Na+ and Cl− increased in the TGR, and the abundance of Ca2+ and HCO3- decreased in the inflow in the period after the TGR filling as a result of climate change and human activities in the Changjiang River basin. The ionic composition in the TGR is primarily controlled by contributions from the upstream region of the Changjiang River but was modified by the interaction between water and rocks within the TGR. The concentrations of most major ions as well as the equivalent ratios of the major ions increased in the TGR after the operation of TGD. This change yielded a 6% increase in the major ion loading downstream of the TGD. The Three Gorges area strongly contributes to the increase in ion loading in the TGR due to enhanced water and rock interactions in comparison with the period before TGD operation.

Dry cropland changes in China's Three Gorges Reservoir Region during the period 1990 to 2015

Monitoring and analyzing changes in the extent of cultivated land may inform strategic decisions on issues of environmental and food security. The dry cropland area of 12000 km2 in the Three Gorges Reservoir Region (TGRR) of China is essential for feeding the local population of ≈20 million, but is highly prone to soil erosion, leading to the delivery of excessive amounts of sediment and associated pollutants to the Three Gorges Reservoir (TGR), and causing serious eco-environmental consequences. Against this background, this paper used Landsat images and a digital elevation model to analyze the altitudinal distribution of, and dynamic changes in, the area of dry cropland during the period 1990 to 2015. The results suggest that dry cropland was mainly distributed in the elevation range of 200-600 m. The dry cropland area decreased from 12525.37 km2 to 11796.27 km2 during the 25-year study period, including a particularly significant decrease in the rate of decrease from 6.93 km2/yr to 43.99 km2/yr after 2000. The largest decline in the dry cropland area occurred in the elevation range of 600-900 m. The transformations between dry cropland and forest revealed the impact of the TGR operation on the extent of dry cropland. A total of 528.79 km2 of dry cropland with slopes >25° were converted to forest after 2000, whereas a total of 642 km2 of forest was converted to dry cropland during the study period, and these conversions mainly occurred between the elevation of 200–900 m. These spatiotemporal changes in the dry cropland area are likely to raise new issues concerning food security in the TGRR.

Impact of the three gorges reservoir operation on the hydrological situation of Poyang Lake

The operation of Three Gorges Reservoir (TGR) has changed the hydrological situation of Yangtze River and its connected lakes. Based on the hydrodynamic model, the influence of TGR operation from 2009 to 2014 on the Poyang Lake has been evaluated. It is found that TGR impoundment reduced the discharge of the Yangtze River and the water level of Poyang Lake, brought the decreasing of the backflow phenomenon. Especially in October, the discharge and water level of Hukou station (HKS) decreased by 9.6% and 1.09 m respectively, and this effect could be up to the Kangshan area. Meanwhile, TGR operation has also increased the water level of HKS by 0.1~0.56 m from December to May, which only affected the vicinity of Duchang. In addition, TGR operation also increased the maximum water level of HKS from April to June by 0.1~0.34 m, and that from July to August decreased by 0.22~0.4 m which is beneficial to reducing the flood control risk of Poyang Lake. But TGR impoundment changed the water exchange process between Yangtze River and Poyang Lake, and advanced the dry season, which had a certain impact on the water supply and ecological environment of the lake area.

Effects of water level fluctuation on thermal stratification in a typical tributary bay of Three Gorges Reservoir, China.

Xiangxi River is a typical tributary of Three Gorges Reservoir (TGR) in China. Based on field observations in 2010, thermal stratification was significant in most months of the year. Through field data analysis and numerical simulations, the seasonal and spatial variation of thermal stratification as related to the impact of the operation of TGR were investigated. Thermal stratification was most pronounced from April to September in the Xiangxi River tributary. Air temperature (AT) and water level (WL) were the two dominant variables impacting thermal stratification. AT affected the surface water temperature promoting the formation of thermal stratification, and high WLs in TGR deepened the thermocline depth and thermocline bottom depth. These results provide a preliminary description of the seasonal variation and spatial distribution of thermal stratification, which is important for better understanding how thermal stratification affects algae blooms in Xiangxi River.

Sedimentation at estuary of 66 tributaries in the Three Gorges Reservoir

There are about 66 tributaries distributed in the Three Gorges Reservoir (TGR) area. As obvious deposition has been observed at estuary of most tributaries estuary since the water-impoundment of TGR, the possibility of sedimentation leading to form entrance bar at estuary of tributaries should be highly concerned. Based on data of topography and cross-sections, the DEM digital elevation model is established to calculate the aggradation volume at the estuary of tributaries in TGR, also the scouring and silting thickness at estuary will be demonstrated. Changes of cross-sections and thalweg caused by aggradation are also studied. The results show that: (1) The aggradation volume at estuary of 66 tributaries in TGR was 180 million m3 from March 2003 to November 2011; (2) hydrodynamic conditions at estuary of tributaries weakened after the operation of TGR, which is the root cause of sediment deposition. The extent of sediment silting was affected by sediment from the tributaries and backward water from the mainstream, meanwhile the dynamic state of river course at estuary had an important influence on the range of aggradation; (3) sediment deposited at the estuary mainly derived from backward water of mainstream, and less from the tributaries. As sediment concentrations both in mainstream and tributaries decreased obviously and will be continually maintain a low level, there is little risk for the formation of entrance bar of tributaries in TGR.

Research on Hydrodynamics with Water Temperature Characteristics and Spring Algal Blooms in a Typical Tributary Bay of Three Gorges Reservoir

After the impoundment of the Three Gorges Reservoir (TGR) in China, water environment problems induced by water temperature stratification in the Xiangxi Bay (XXB, a typical tributary bay of TGR) received wide attention. In this study, a 3-dimensional (3D) hydrodynamic and water temperature coupled model with the z-coordinate in the vertical direction was established with Delft3D software to simulate the continuous hydrodynamic and water temperature process of XXB in 2009, and the static stability and mixing depth were also analyzed. The results show that the upstream inflow is prevented from entering TGR with nutrients enriching and mixing in the upper and middle reach of XXB from winter to early spring, which is the primary cause of spring algal blooms in XXB. Therefore, measures such as improving the upstream hydrodynamic conditions and forcing the nutrients to flow into TGR in winter should be more effective in alleviating spring algal blooms than the artificial tide operation of TGR proposed by previous studies.

Impoundment Impact of the Three Gorge Reservoir on the Hydrological Regime in the Lower Han River, China

Reservoir regulation has been playing an increasingly important role in water resources development and its influence on the hydrological processes of downstream tributaries has attracted much attention. The lower Han River is selected as a case study to examine the hydrological and hydraulic influence of the upstream flow regulation of the Three Gorges Reservoir (TGR) in the middle Yangtze River, China. Based on a hydrodynamic model and the observed data, the hydrological processes in the lower Han River were simulated and their changes were analyzed under the impoundment influences of the TGR. The results indicated that there were obviously hydrological changes in the lower Han River after the TGR operation. The decreased stage downstream the TGR during the impounding periods of the TGR resulted in an increase in the stage difference, current speed, hydraulic gradient and the discharge ratio. In addition, the stage difference between the two rivers was decreased during the periods of water compensation from the TGR, which led to the outflow congestion in the lower Han River. The hydrological changes in the lower Han River were the response to the flow regulation of the TGR and the inflow of the two rivers. The variation in the rating curve in the lower Han River mainly resulted from the stage difference between the two rivers during the dispatching periods of the TGR. These results help to explain the hydrological variability under the impounding influence of the TGR for the lower Han River and they can be extended to other river tributaries downstream to the reservoirs.

Identification of flood seasonality using an entropy-based method

Identifying flood seasonality is critical in hydrologic applications as well as water resources management. We develop an entropy-based method (EBM) for identifying flood seasonality and partitioning the entire flood season into multiple sub-seasons. The performance of the proposed EBM is evaluated using a Monte Carlo simulation test and compared with current methods. The Three Gorges Reservoir (TGR) basin in the Yangtze River is selected as a case study to test the applicability of the proposed method. Results of Monte Carlo simulation test demonstrate that the EBM performs better than the probability change-point method and the improved relative frequency method with less bias and higher efficiency. The case study results illustrate that the EBM can appropriately divide the entire flood season of the TGR into pre-flood season (from June 1st to June 20th), main-flood season (from June 21th to September 10th) and post-flood season (from September 11th to September 30th). The flood limited water levels (FLWL) in these three sub-seasons can then be derived, which are 150 m, 145 m and 149 m, respectively. Compared with conventional operation rule, the seasonal FLWL scheme can generate more hydropower (0.93 billion KWh) annually with a reliability of 99.86%. Therefore, it is meaningful to divide the entire flood season into three sub-seasons and apply seasonal FLWL for TGR operation.