Xiangxi River Research Articles

Effects of the Three Gorges Reservoir Operation on Vertical Distribution of Chlorophyll a and Environmental Factors in Tributaries

The hydrodynamics and environmental factors in the Xiangxi River (XXR) and Shennong River (SNR), which are tributaries of the Three Gorges Reservoir (TGR), were monitored from July to August (the low water level period) and in October (the impoundment period) in 2018. The vertical distribution characteristics of chlorophyll a and other indicators of the two tributaries were analyzed during the different operation periods, and the factors that affected the vertical distribution in each period were discussed. The results showed that the vertical distribution of dissolved oxygen, water temperature, pH value, and chlorophyll a of the XXR and SNR during the low water level period was relatively consistent. The indexes 0-10 m (0-5 m for chlorophyll a) from the surface of the XXR and SNR, respectively, showed significant stratification and decreased with increasing water depth; the stability index of thermal stratification (RWCS/H) was 13.71-29.07 m-1, which was stable. After the water depth reached 10 m (5 m for chlorophyll a), the indexes tended to be stable along the water depth. During the impoundment period, there was no obvious stratification for each index; the stability index of thermal stratification was 0-0.5 m-1, the stability of the water body was weak, and the vertical variation of each index was relatively stable. The comprehensive trophic state index (TLI) of the XXR and SNR were 55 and 53 during the low water level period, respectively, indicating that they were in a slightly eutrophic state, and 39 and 46 during the impoundment period, respectively, indicating a mesotrophic state. Linear regression analysis showed that chlorophyll a, dissolved oxygen, water temperature, and pH in the two tributaries were significantly correlated in the vertical direction in the low water level period, indicating that dissolved oxygen, water temperature stratification, and pH were important factors affecting the vertical distribution of chlorophyll a. During the impoundment period, a large amount of backflow from the Yangtze River, a large fluctuation in tributary water level, and the decrease in RWCS/H were the important factors that affected the small vertical change in the water body. The enhancement of vertical mixing and the decrease in Zeu/Zmix were the key factors affecting the nutritional status of the water.

Identifying key drivers of harmful algal blooms in a tributary of the Three Gorges Reservoir between different seasons: Causality based on data-driven methods

Intense harmful algal blooms (HABs) can occur in the backwaters of tributaries supplying large-scale reservoirs. Due to the characteristics of process-based models and difficulties in modelling complex nonlinear processes, traditional models have difficulties disentangling the driving factors of HABs. In this study, we used data-driven methods (i.e., correlation analysis and machine-learning models) to identify the most important drivers of HABs in the Xiangxi River, a tributary of the Three Gorges Reservoir, China (2017–2018), for the dry season (from October to mid-April) and wet season (from April to September). We utilized the maximal information coefficient (MIC) combined with a time lag strategy and prior knowledge to quantitatively identify the driving variables of HABs. An extra trees regression (ETR) model was developed to assess the relative importance of causal variables driving algal blooms for the different periods. The results showed that water temperature was the most important driver for the duration of the study, followed by total nitrogen. Nitrogen had a stronger effect on algal blooms than phosphorus during both the wet and dry seasons. HABs were mainly affected by ammonia nitrogen in the wet season and by other forms of nitrogen in the dry season. In contrast, rather than the water temperature and nutrients, the operation of the Three Gorges Dam (difference between inflow and outflow discharge rate) was the most significant factor for algal blooms during the dry season, but its influence sharply declined during the wet season. This study showed that the key drivers of HABs can differ between seasons and suggests that HAB management should take seasonality into account.

Small run-of-river hydropower dams and associated water regulation filter benthic diatom traits and affect functional diversity

Knowledge of benthic diatom traits can help understand ecosystem function and guide biodiversity conservation. This is particularly important in rivers on which there are small run-of-river dams, which currently receive less attention. These dams generate power by drawing water from upstream and discharging it downstream after a large drop in penstock. We examine 15 functional diatom traits in habitats upstream, surrounding, and downstream of 23 small run-of-river dams in Xiangxi River, China. We compare the effects of these small dams on benthic diatom species traits, and taxonomic and functional diversity, from 90 sites. Dams change local environmental (e.g., channel width, flow velocity, depth) and physicochemical (e.g., dissolved oxygen, water temperature) variables, and a shift in diatom life forms and guilds is apparent, from taxa with strong attachment and low profile in high velocity waters (i.e., H1, H2 and H4) to those with weak attachment or that are planktonic below dams and outlets (i.e., H3 and H5), and towards high profile taxa below dams. Significant differences in biodiversity, particularly in functional richness, redundancy, and evenness, are apparent. Species and functional diversity indices are influenced by physical and chemical environmental factors (especially flow velocity and water depth). We found that diatom functional traits reflect longitudinal changes in flow and ecological condition, and suggest that monitoring such traits could be useful in adjusting flows to minimize ecosystem impacts. To maintain ecological flow and reasonable water depth within rivers we advocate for improved connectivity, carrying capacity and resilience of water ecosystems via a long-term, trait-based understanding of the impacts of small run-of-river dams.

An Environmental Flow Framework for Riverine Macroinvertebrates During Dry and Wet Seasons Through Non-linear Ecological Modeling

A suitable environmental flow is critical for the functional maintenance of riverine ecosystems. Hydropower plants alter the flow regime by decreasing or even drying up the streamflow downstream of the dams, thereby affecting ecosystem sustainability. In this study, we aimed to develop a robust environmental flow framework that can provide scientific evidence for sustainable water resource management. Using ecological niche modeling based on non-linear responses of species to habitat factors, we assessed the environmental flow in the Xiangxi River Basin of Central China during dry and wet seasons from a multi-year perspective. The most abundant macroinvertebrate taxon (i.e.,Baetis) was selected for model testing. The results showed seasonal differences in the minimum ecological water requirements and optimal environmental flow. These two hydrological metrics were higher during the wet season than during the dry season. During the dry season, the minimum ecological water requirement ofBaetiswas 1.3 m3·s−1, and the optimal environmental flow was 1.6 m3·s−1. During the wet season, the minimum ecological water requirement ofBaetiswas 2.5 m3·s−1, and the optimal environmental flow was 2.6 m3·s−1. This study provides a theoretical basis for the robust management of water resources in river basins.

Distribution, sources and risk assessment of PAHs in soil from the water level fluctuation zone of Xiangxi Bay, Three Gorges Reservoir.

Information on PAH distribution in the water level fluctuation zone (WLFZ) of Three Gorges Reservoir is limited. In this study, we investigated PAH distribution and sources and assessed PAH risks, over one annual water level fluctuation cycle (June 2017-June 2018) at four elevations spanning the WLFZ (145m, 155m, 165m and 175m) at seven locations in the water level fluctuation zone along Xiangxi River. The mean total PAH concentration in June 2018 (953ngg-1) was significantly higher than in June 2017 (494ngg-1), and the horizontal and vertical distributions of PAHs changed significantly. The changes in distribution patterns provided evidence for the cause of increased PAH levels, which were attributed to construction of the Xiangxi River Bridge. Thus, this study of PAH dynamics in the WLFZ soils of Xiangxi Bay also provided valuable information on the impact of bridge construction on WLFZ soils. The change in PAH levels among stations implicated sediment disturbance resulting from bridge construction as the major contributor to the increased PAH levels. Source characterization, based on the ratios of certain PAHs, indicates that PAHs are mainly from the combustion of petroleum fuels, biomass and coal. These ratios indicated that the proportion of PAHs from fuel combustion increased from 2017 to 2018, implicating the heavy equipment used during bridge construction as another source of the increased PAH levels. The incremental lifetime cancer risk (ILCR) model was used to assess the health risk of the PAHs and the range among all age groups (10-5-10-4) indicates a potential health risk. The mean effects range-median quotient (M-ERM-Q) was used to assess the ecological risk of PAHs and the range (0.1-0.5) indicates low to medium risk. The increase in PAH levels from 2017 to 2018 increased the risk to public health and the environment. The results of this investigation provide a reference for ecological restoration of the WLFZ and support development of effective policies for environmental and public health. Further, the results provide information on the impact of bridge construction on WLFZ soils and identify research needed to more fully understand PAH dynamics in WLFZ soils.

Effect of thermal stratified flow on algal blooms in a tributary bay of the Three Gorges reservoir

After the impoundment of Three Gorges Reservoir (TGR), its tributaries have been frequented by algal blooms, especially the Xiangxi River (XXR). Stratified flow induced by temperature difference often occurs, which likely influences the water exchange and algae transport process between the tributary and TGR. To investigate the variation of flow field and algae concentration in XXR, a 2-dimensional hydrodynamic and water quality model is developed. The simulation results show good agreement with field observed data, reproducing the water exchange and algal bloom process. Notwithstanding the hydrodynamic environment of XXR is complicated, the thermal stratified flows are generalized into six typical patterns in this paper. Subsequently, algae transport processes under the influence of different thermal stratified flow patterns are examined. Thermal stratified flows from upstream and TGR intrude into the bay tributary through different layers with different combinations, resulting in water exchange of XXR is the major cause of algae transport difference. We show that when the mainstream intrusive flow and upstream inflow separately intrude into the XXR through the surface and bottom layer, hydrodynamic environments are favorable to algae transport, with the final chlorophyll-a (Chl-a) concentrations of entire XXR lower than algal bloom threshold value. The worst scenarios occur when the mainstream flow intrudes through middle or bottom layer while the upstream inflow enter through bottom layer, with the final algal bloom coverage rate (Ralg) of 82.3% and 78.1%, respectively. In terms of superimposed effect of different flow patterns, when the sequentially thermal stratified flows have different effect on algae transport direction, algae will concentrate in the reach rather than discharged which increase the risk of multiple algal blooms. These findings represent an important step towards understanding the multiple outbreaks of algal bloom in XXR during the spring and provide a new perspective for the research of algal bloom governance.

Study of uncertainty of satellite and reanalysis precipitation products and their impact on hydrological simulation.

Satellite and reanalysis precipitation products are potential alternatives in hydrological studies, and it is very important to evaluate their accuracy and potential use for reliable simulations. In this study, three precipitation products (Tropical Rainfall Measuring Mission 3B43 Version 7 (TRMM 3B43), spatial interpolation grid data based on 2472 national meteorological observation stations in China (GRID_0.5), and National Centers for Environmental Prediction-Climate Forecast System Reanalysis (NCEP-CFSR)) were evaluated against gauge observations in the Xiangxi River watershed of Hubei Province. The performance results indicated that the results of the three precipitation products were correlated with those of the rain gauges; however, there were differences among the three products. TRMM 3B43 tended to overestimate precipitation with the highest correlation coefficient, while NCEP-CFSR tended to underestimate precipitation with the least satisfactory performance, and the performance of GRID_0.5 ranked between them. However, the annual and monthly mean errors differed, as the errors of most of the results driven by NCEP-CFSR were lowest. The errors varied at different time scales. During years with high precipitation, the results were often underestimated, while the results are often overestimated during years with low precipitation. According to the average monthly results, the GRID_0.5 results were closest to the gauge observations for most months. During the wet season, TRMM 3B43 performed better, while NCEP-CFSR precipitation performed better during the dry season. The errors from precipitation to streamflow, NPS pollution, and water environmental capacity (WEC) driven by the three precipitation products increased gradually, ranging from 10% for precipitation to over 20% for NPS pollution and almost 100% for WEC. The error increase for NCEP-CFSR was lower than that of the other two products. Although the simulation error from precipitation to the WEC results driven by the three precipitation products gradually increased, the degree of overestimation and underestimation became smaller.

A C-Vine Copula-Based Quantile Regression Method for Streamflow Forecasting in Xiangxi River Basin, China

In this study, a C-vine copula-based quantile regression (CVQR) model is proposed for forecasting monthly streamflow. The CVQR model integrates techniques for vine copulas and quantile regression into a framework that can effectively establish relationships between the multidimensional response-independent variables as well as capture the upper tail or asymmetric dependence (i.e., upper extreme values). The CVQR model is applied to the Xiangxi River basin that is located in the Three Gorges Reservoir area in China for monthly streamflow forecasting. Multiple linear regression (MLR) and artificial neural network (ANN) are also compared to illustrate the applicability of CVQR. The results show that the CVQR model performs best in the calibration period for monthly streamflow prediction. The results also indicate that MLR has the worst effects in extreme quantile (flood events) and confidence interval predictions. Moreover, the performance of ANN tends to be overestimated in the process of peak prediction. Notably, CVQR is the most effective at capturing upper tail dependences among the hydrometeorological variables (i.e., floods). These findings are very helpful to decision-makers in hydrological process identification and water resource management practices.

Effectivity and Efficiency of Best Management Practices Based on a Survey and SWAPP Model of the Xiangxi River Basin

A questionnaire survey was conducted among farmers in the Xiangxi River Basin to investigate the local livestock situation and the farmers’ understanding of and attitude towards pollution. The results showed that local farmers lacked environmental awareness and few livestock and poultry pollution treatment measures had been implemented. However, once farmers understood that livestock pollution would greatly influence their lives and interests, they would act to prevent Agricultural non–point source (ANPS) pollution. The farmers’ education level and satisfaction with the environment were the main factors affecting their awareness regarding ANPS pollution. The “Comprehensive Environmental Optimization Tool SWAT–APEX Interface” model (SWAPP) was used to simulate the reduction of ANPS by different best management practices (BMPs) and the construction cost was calculated. The results showed that compound bedding and piping systems and ponds were the most effective and economic measures for reducing ANPS pollution. Spatially, implementing BMPs in the upstream region was better for improving water quality. The nitrate reduction rate in upstream sub–basins reached 90%, which is 30% larger than that in downstream sub–basins with combined bedding and piping systems. Combining the farmers’ awareness of and engagement in livestock pollution with cost–effective BMPs can improve the BMPs’ effectivity and efficiency.

Quantifying Climatic and Anthropogenic Influences on Water Discharge and Sediment Load in Xiangxi River Basin of the Three Gorges Reservoir Area

Quantifying the relative contributions of climate change and human activities to water and sediment variations is of great importance for regional resources and river basin management, which has become a hot topic in hydrology research. In this study, water discharge and sediment load in Xiangxi River basin of Three Gorges Reservoir area were investigated based on hydrometeorological time series since 1960s. The non-parametric Mann-Kendall (M-K) test was used to detect both the trends and abrupt changes in the time series. Double mass curve and sediment rating curve method were used to quantify and evaluate the climatic and anthropogenic effects on water discharge and sediment load. Results indicated that both water discharge and sediment load showed significant decreasing trends with annual decrement rate of –0.08 × 108 and ‒1.011 × 104 t/yr, respectively. The abrupt change points were observed in 1991 for water discharge and 1999 for sediment load by sequential M-K test and cumulative anomaly curve analysis. This study demonstrated that water discharge was mainly influenced by precipitation in 1990s while by human activities ever since 2000, and sediment load was predominantly influenced by human activities (relative contribution 86–98%, regardless of whether the effect is negative or positive). In this study, soil and water conservation as well as land use change were primarily responsible for water and sediment variations in Xiangxi river basin especially since 2000. Further studies need to be done to quantify the importance of a specific driving factor at multi-timescales.

Characteristics of Phosphorus Speciation and Genesis in Typical Tributaries of the Three Gorges Reservoir

A comparative analysis of phosphate occurrence and its genesis during different dispatching periods of representative level I tributaries of the Three Gorges Reservoir was conducted. For this, water quality, hydrodynamic force, and environmental data were examined for backwater areas of the Xiangxi River and Shennong River during the low-water-level period (June), storage period (September) and high-water-level period (December) in 2018. The results suggest that the mass concentration of total phosphorus (TP) in the water body of the coves of Xiangxi River and Shennong River range from 0.049 mg·L-1 to 0.168 mg·L-1 and 0.059 mg·L-1 to 0.152 mg·L-1, respectively, surpassing the 0.02 mg·L-1 threshold for algal blooms. The mass concentration of TP, DP (orthophosphate), and PP (particulate phosphorus) in the coves of the tributaries were ranked as follows:storage period > high-water-level period > low-water-level period for TP; storage period > low-water-level period > high-water-level period for DP; low-water-level period > high-water-level period > storage period for PP. Based on Pearson's correlation coefficients, TP and DP were significantly correlated as well as temperature and pH, which are the key factors influencing the phosphorus in soils and sediments in the water-level-fluctuation zone. During the low-water-level and high-water-level periods of the Three Gorges Reservoir, the TP in the coves of the tributaries mainly exists as PP, accounting for more than 75% and 60%, respectively. The flow in the coves of the tributaries slows during storage periods, meaning that settlement of PP is enhanced, changing the dominance of TP to dissolved total phosphorus (DTP).

Evolution characteristics and displacement forecasting model of landslides with stair-step sliding surface along the Xiangxi River, three Gorges Reservoir region, China

Five large and many small landslides are developed in Jurassic strata along the lower reaches of Xiangxi River, where interbedded weak and hard bedrock layers foster the development of landslides with a “stair-step” sliding surface. The paper investigates the evolution characteristics of these landslides and presents a novel forecasting model for their displacements. The distribution characteristics and behavior of landslides developed along Xiangxi River is revealed by the database of landslides in the larger Zigui basin, of which this area is part. Most landslides occur at rather low elevations of <300 m and in areas of moderate rainfall. The geological evolution of landslides in the Xiangxi River valley can be divided into four stages, beginning with anticline formation, followed by valley incision, then by weathering and erosion, and culminating in formation of the colluvial landslides. The accumulative displacement curves of landslides with a stair-step sliding surface in Xiangxi River region also present obvious, step-like characteristics. A novel GA-CEEMD-RF algorithm was developed to predict the displacement of these stair-step landslides, which helps to define the combination of induced factors and weak stableness of prediction results using a single displacement prediction model and the multi-field monitoring data.

Life history of Caenis lubrica Tong and Dudgeon, 2002 (Ephemeroptera: Caenidae) in a Three Gorges Reservoir feeder stream, subtropical Central China

Life history of Caenis lubrica Tong and Dudgeon, 2002 (Ephemeroptera: Caenidae) was studied for the first time. We investigated it in Pangxie Stream, which feeds into the Xiangxi River Bay, Three Gorges Reservoir in subtropical Central China. Monthly samples of larvae were taken from January to December, 2017. Caenis lubrica was determined to exhibit trivoltinism in this system, having an overwintering generation and two summer generations. The cohort production interval (CPI) of the overwintering generation was six months, from November to April, and the CPIs of the two summer generations were one month each, from June to July and from July to August. The Accumulated Degree Days of C. lubrica for larval development of the overwintering generation and the two summer generations were estimated to be 1829.24 °C, 904.84 °C, and 859.83 °C, respectively.

Parameter Uncertainty and Sensitivity Evaluation of Copula-Based Multivariate Hydroclimatic Risk Assessment

Extensive uncertainties exist in hydroclimatic risk analysis. Especially in multivariate hydrologic risk inferences, uncertainties in individual hydroclimatic extremes such as floods and their dependence structure may lead to bias and uncertainty in future hydrologic risk predictions. In this study, a parameter uncertainty and sensitivity evaluation (PUSE) framework is proposed to quantify parameter uncertainties and then reveal their contributions to the multivariate hydroclimatic risk predictions. The predictive risks are finally generated by “integrating” the values over the posterior distributions of the parameters. The proposed approach was applied for bivariate risk analysis of compound floods at the Xiangxi River to characterize the concurrence probabilities of flood peaks and volumes. The results demonstrate that the proposed approach can quantify uncertainties in a copula-based multivariate risk analysis and characterize effects and contributions of parameters in marginal and dependence structures on the multivariate hydroclimatic risk predictions. In terms of the bivariate risk for flood peak and volume at the Xiangxi River, uncertainties in model parameters would lead to noticeable uncertainties even for moderate floods. The performances of the copula model for flood peak-volume at Xiangxi River are mainly affected by the uncertainties in location parameters of the two individual flood variables. Also, parameter uncertainty in the dependence structure (i.e., copula) would also poses explicit impacts on performance of the copula-based risk analyses model. These uncertainties would result into higher bivariate predictive risks than the values obtained by “optimal/deterministic” predictions. This indicates that uncertain- ties are required to be considered to provide reliable multivariate hydroclimatic risk predictions.

Research on Relationship of Total Nitrogen Concentration and Runoff in the Xiangxi River Basin

The response relationship between pollutant and flow rate is very important and is one of the key factors for the reasonable simulation of the model for a basin. The basin land use type, soil types, weather, vegetation distribution, soil and water loss situation are taken as the influence factors to explore its concentration change in migration of pollutants in relation with the response of the flow in Xiangxi River watershed. Emphasis was put on the effects of nitrogen pollutants based on the mass conservation equation combined with the total nitrogen concentration of pollutants. Relationship between total nitrogen pollutants and flow of three types of response relation is preliminary established. Measured data of total nitrogen from 2014 to 2017 are used for verification and error analysis. Results show that the power function is the best relationship, while the logarithm function is the second one, and the exponential function is the third one through trend coincidence, fluctuation range and error analysis. This study is helpful to understand the response relationship between pollutant concentration and discharge in small watershed, so it can also be used for pollutant prediction in small watershed without measured data.

Features of Oxygen and Hydrogen Isotopes in Waters from the Karst Mountains, Xiangxi River Basin

Targeting the carbonate-dominated catchment of Xiangxi River, the first tributary of Three Gorges Reservoir, this paper explores the spatiotemporal distribution of stable isotopes in the stream water, and distinguishes the water sources and recharge processes in the river basin. Multiple water samples were collected from the catchments of the main tributaries of Xiangxi River, namely, Nanyang River and Gaolan River. The results show that the two tributary catchments had obvious spatial and seasonable variability in δ18O and δD. This is attributable to the seasonal changes in meteoric precipitation recharge, which depends on ambient temperature, precipitation, evaporation, and secondary evaporation of raindrops. Besides, there is a strong elevation effect on stable isotopes in stream water. In the samples from Gaolan River, the mean elevation effect on the δ18O and δD was -0.35‰, and -1.57‰ per 100m change in elevation in summer, and -0.12‰, and -1.19‰ per 100m change in elevation in winter, respectively. In the samples from Nanyang River, the mean elevation effect on the δ18O and δD was -0.14‰ and -0.89‰ per 100m change in elevation in summer, and -0.08‰ and -0.66‰ per 100m change in elevation in winter, respectively. The δD, δ18O, and deuterium excess (d-value) are not only affected by the elevation effect, but also by the seasonal effect in the study area. The seasonal variations and spatial changes in the tributary catchments reveal the difference between the two catchments in control factors like latitude effect, evaporation, and the mixing of different water sources.

Annual Variation of CO2 Absorption and Release in Xiangxi River of Three Gorges Reservoir

To analyze the annual variation of CO2 absorption and release in Xiangxi River of Three Gorges Reservoir, this paper set up six monitoring points in the Yangtze River and Xiangxi River to monitor CO2 emission flux at the air-water interface from July 2017 to June 2018 using static chamber method. During the investigation, this paper found that the variation rate of CO2 in the Xiangxi River averaged 0.67-2.14 ppm/min while the number for the Yangtze River was 1.07 ppm/min and 2.82 ppm/min respectively. The variation rate of CO2 was lower during water storage and higher during water discharge or in dry periods. The status of CO2 changed from absorption to release during water storage and vice versa during water discharge.

Spatial variation in bacterial biomass, community composition and driving factors across a eutrophic river

Eutrophication is a global problem, and bacterial diversity and community composition are usually affected by eutrophication. However, limited information on the ecological significance of bacterial community during algae blooms of rivers has been given, more studies should be focused on the bacterial diversity and distribution characteristics in eutrophic rivers. In this study, we explored the spatial variations of bacterial biomass, community structure, and their relationship with environmental factors in the eutrophic Xiangxi River. The content of Chlorophyll (Chl) was about 16 mg/L in the midstream (S2, S3), which was in the range of light eutrophication. Significant spatial variation of bacterial community structure was found at different sites and depths (p < 0.05), and the driving environmental factor was found to be nitrogen, mainly detected as total nitrogen (TN), Kjeldahl nitrogen (KN), and ammonia nitrogen (NH4+) (p < 0.05). The midstream sites had some significantly different bacteria, including algicidal bacteria and dominant lineages during algal blooms. This result was consistent with the functional prediction, where significant higher abundance of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways was associated with algicidal substances in the midstream. At different water depths, some populations adapted to the surface layer, such as the class Flavobacteriia, and others preferred to inhabit in the bottom layer, such as Betaproteobacteria and Acidobacteria. The bacterial biomass was higher in the bottom layer than that in the surface and middle layer, and temperature and pH were found to be the major driving factors. The bacterial diversity increased with the increasing of depths in most sampling sites according to operational taxonomic units (OTUs), Chao1 and ACE indexes, and PO43− was demonstrated to be the most significant factor. In summary, this study offered the evidence for microbial distribution characteristics across different sites and depths in summer, and its relationship with environmental variables in a eutrophic river.

Xiakou Xiangxi River Bridge structure design

Xiakou Xiangxi River Bridge is located in the upstream of the Three Gorges reservoir area. It is a (90+238) m single tower, Leaning Tower central cable surface mixed beam cable-stayed bridge. Its main span adopts a cross the river to meet navigation and flood control needs. The main concrete beam and the steel box girder of the main span are of large-angle inclined web box section with cantilever flange. The steel and mixed surface is at 11.05m across the bridge tower. The full width of the bridge deck is 25.5m. This bridge adopts the oblique “pipa” shaped main tower, the height of the main tower is 126m, composed of the upper tower column, the lower tower column, diaphragm and so on. The tower body deviates from the vertical surface of the bridge by 10° and leans towards the bank side. The foundation of the main tower is an integral pile foundation of the pile bearing platform. 2×17×2 high-strength parallel steel wire stay cables are used to arrange the fan-shaped space cable surface.

Tributary oscillations generated by diurnal discharge regulation in Three Gorges Reservoir

Among the major consequences of dam construction and operation are the deterioration of water quality and the increasing frequency of occurrence of harmful algae blooms in reservoirs and their tributaries. Former studies at Three Gorges Reservoir demonstrated that the Yangtze River main stream is the main source of nutrients and pollutants to connected tributary bays. Eutrophication and other water quality problems reported for the tributaries along Three Gorges Reservoir are likely a consequence of density-driven exchange flows. Past work has focused mainly on the influence of seasonal and daily flow regulation on exchange flows, less attention has been paid to hydrodynamic processes resulting from sub-daily discharge dynamics. High-frequency measurements of flow velocity and water level in a eutrophic tributary (Xiangxi River) of Three Gorges Reservoir revealed the persistent nature of bidirectional density currents within the bay. Superimposed on this mean flow, we observed ubiquitous flow oscillations with a period of approximately 2 h. The flow variations were associated with periodic water level fluctuations with increasing amplitude for increasing distance from the river mouth (up to ±0.1 m at a distance of 27.4 km from the river mouth). They were caused by a standing wave in the tributary bay, which was generated by rapid increase or decrease in discharge following peak-shaving operation modes at Three Gorges Dam. The high-frequency wave made up the largest contribution to the temporal variance of flow velocity in the tributary bay and represents a so far overlooked hydrodynamic feature of tributaries bays in large reservoirs.