Impoundment Stage Research Articles

A hybrid approach combining UD and GA-CV-SVM to evaluate shear performance in high asphalt concrete core

The shear effect on high asphalt concrete core is significant. However, studies on the reliability of 100-meter-scale cores against shear damage remain limited. A key challenge in this research field is establishing the control criteria for the core and improving the computational efficiency of implicit limit state function (LSF). Additionally, the impact of material parameter uncertainty on the shear failure reliability of the core during the dam construction and impoundment stages remains unclear. To address this, a safety evaluation method based on time discretization was proposed, combining uniform design (UD), K-fold cross-validation (K-CV), and genetic algorithm (GA) to optimize the support vector machines (SVM). The core parameters of 52 asphalt concrete-core rockfill dams (ACCRDs) were analyzed, with the statistical values of the basic variables considered in determining the reliability index. The theoretical derivation of the critical shear failure safety index established a stability formula to assess the safety state of the dam core. The significance parameters were identified, and the sample points were generated at each stage using UD, and the Support Vector Regression (SVR) was applied to reconstruct the LSF, and reliability was calculated through the checking point method.

Identification and Deformation Characteristics of Active Landslides at Large Hydropower Stations at the Early Impoundment Stage: A Case Study of the Lianghekou Reservoir Area in Sichuan Province, Southwest China

Reservoir impoundment imposes a significant triggering effect on bank landslides. Studying the early identification of landslides and their stability concerning reservoir water levels and rainfall is vital for guaranteeing the safety of residents and infrastructure in reservoir regions. This study proposed a method for establishing a dynamic inventory of active landslides at large hydropower stations using integrated remote sensing techniques, demonstrated at Lianghekou Reservoir. We employed interferometric stacking synthetic aperture radar (stacking-InSAR) technology, small baseline subset interferometric synthetic aperture radar (SBAS-InSAR) technology, and optical satellite images to identify and catalogue active landslides. Moreover, we conducted field investigations to examine the deformation characteristics of landslides. Finally, Pearson’s correlation analysis was employed to evaluate the response between deformation values, reservoir water levels, and rainfall. The results revealed 75 active landslides, including 12 long-term active landslides before impoundment and 63 new landslides after impoundment, which were primarily concentrated in the Waduo and Yazho–Zatou regions. The correlation coefficient between landslide deformation values and the reservoir level was high (0.93), while the correlation coefficient with rainfall was low (0.57). The results of this research offer a crucial foundation for preventing and mitigating landslides in reservoir areas.

Analysis of stress field in the head area of the Three Gorges Reservoir based on coupled fluid-solid theory

The Three Gorges Reservoir, one of the largest water conservation system in the world, has been of keen interest to scientists globally since its impoundment. After construction of the dam, there has been a significant increase in seismic activity in the head area of the reservoir. It is generally accepted that earthquakes in this region are predominantly caused by the Jiuwanxi and Xiannvshan faults. This study focused on the stress changes occurring in the research area. A three-dimensional finite element model of the reservoir area was constructed using the geological structure and digital ground elevation data of the reservoir area. The fluid-solid coupling theory was applied to calculate the dynamic spatial changes in pore pressure and Coulomb stress in the faults and surrounding rocks during reservoir impoundment. The findings indicated that the added head pore water pressure at the bottom of the reservoir had a maximum impact range of approximately −2800 m on the surrounding rock, whereas the Xiannvshan and Jiuwanxi faults had a maximum diffusion range of approximately −4300 m. Rock permeability also played a significant role in the water storage process. During the 1 56 m water impoundment stage, owing to rapid water storage activity, stress could not be transmitted to both sides in a timely manner, resulting in the formation of an extreme stress change zone at −4000 m inside the fault. This may have been the reason for the frequent earthquakes during this stage. The 17 5 m cycle water storage stage also exhibited a significant degree of seismicity, potentially attributable to the long-term infiltration of reservoir water and accumulation of stress in the previous stage. The stress in the study area at the four stages are in a process of accumulation-release-accumulation-release.

Investigation of the microfracture and damage characteristics of dam during impoundment at Sanhekou hydropower station

Dam stability is one of the most important issues in hydraulic engineering. Microfractures and damage commonly occur during impoundment, which might lead to serious dam problems. In this study, based on the engineering background of the Sanhekou hydropower station, microseismic monitoring and numerical simulation were employed to systematically investigate the microfracture and damage characteristics of the dam body. First, the microseismic monitoring system was established to capture the microfractures inside the dam. The results indicated that the rise in water level elevation has a significant effect on the microfracture and damage characteristics of the dam body, especially during the early stage of impoundment. This can be reflected by the variation in the derived source parameters, i.e., the b value, daily energy release, daily apparent stress and daily apparent volume. In addition, the failure mode of the microfractures could be determined by using the ES/EP value of microseismic events and the moment tensor inversion method. The cracking orientation of the failure surfaces could also be determined by the moment tensor inversion method. Subsequently, numerical simulation was conducted where the initial damage of the dam was considered by integrating the microseismic monitoring data. The simulation results suggested that dam deformation under impoundment considering microseismic feedback agrees well with the real field measured results. The stress level of the dam toe was larger than that of the dam heel, and both the dam toe and dam heel were under compression before impoundment. However, with increasing water level elevation, the stress status of the dam heel area changes from compression to tension. The findings in this study will provide a better understanding of the damage and failure mechanism of dams during impoundment, which might be helpful for the design and support of dams in hydropower stations.

Variable analysis for supporting reservoir impounding modeling

The impounding stage of the reservoir is a stage after the implementation of dam construction is finished. The process of reservoir impounding is started by closing the tunnel diversion gate and the elevation of the water level will be moving up slowly. The speed of the increasing reservoir water level elevation is affected by the dry season or rainy season, so it is arranged in such a way for obtaining the daily increase is not more than 1.0 m/day, due to dam safety. This research intends to build this modeling, at first by studying which variables are affecting in order to get the safe period of impounding. The methodology consists of a correlation analysis of inflow, outflow, and the ratio between storage volume and inflow volume to the optimal impounding equation. The statistical hydrology approach is used in this modeling that will assess the 7 observed reservoir impounding which is started from base elevation until Full Supply Level (FSL). This modeling will help in decision-making about the selection of the safe and optimal period of reservoir impounding. At the end of impounding, the reservoir will be full and it starts to be allocated regarding the utilization that has been designed.

Study of 3-D velocity structure characteristics in Dadu river Houziyan reservoir area at different impoundment stages

High-resolution three-dimensional VP, VS and VP/VS images in the Houziyan Reservoir Area were obtained by using Fast Marching Tomography Package (FMTOMO) with the travel time data from 6330 seismic events monitored by the Houziyan Reservoir Seismic Network. This analysis yielded the 3-D velocity structure, including longitudinal wave velocity (VP), shear wave velocity (VS), and the ratio of longitudinal and shear wave velocity (VP/VS) at different impoundment stages. The data changes at various impoundment times, depths of sections, and directions of profiles were analyzed to obtain these results. The final findings demonstrate the following results: 1) Through tomographic analysis, it was determined that the underground velocity structure in the Houziyan reservoir area was anisotropic before impoundment. 2) The area of high wave velocity increases in stage 1, stage 3, and stage 4. The area of low wave velocity increases in stage 2, especially in depth, indicating significant changes in the underground velocity structure at different impoundment stages. 3) Compared to the changes in underground velocity structures in other reservoirs after impoundment, the Houziyan reservoir exhibited a unique pattern. 4) In general, the underground velocity structure displayed an overall increasing trend after impoundment. However, it also exhibited instances of decreasing velocity, reflecting continuous dynamic adjustments to the underground velocity structure after impoundment. These conclusions highlight the impact of impoundment in the reservoir area on the underground velocity structure and provide scientific theoretical support for seismic risk assessment following impoundment in the reservoir area.

Susceptibility assessment method for reservoir landslides considering the effect of reservoir impoundment

Reservoir impoundment is the main factor inducing reservoir landslides, it is essential to obtain the dynamic landslide susceptibility by considering hydraulic factors. In this paper, we proposed the use of the original and revised logistic regression models (machine learning methods) to analyze the landslide susceptibility after the second stage of reservoir impoundment. Data of active unstable slopes before the first stage of reservoir impoundment were used to train the models. The results indicated that the two models both performed well with high accuracy. The revised logistic regression model performed better than the original logistic regression model in two aspects. Firstly, TPR and AUC of the revised model is 0.771 and 0.906 which is obviously higher than the original model of 0.743 and 0.896. Secondly, the revised model can predict the unstable slopes after reservoir impoundment which the original model can’t predict. The former could be used to obtain the dynamic landslide susceptibility in reservoir areas after different stages of reservoir impoundment and predict the potential reactivated unstable slopes more precisely due to hydraulic factors.

Earthquakes Induced by Rapid Loading of Faults During Pulichintala Reservoir Impoundment in the Stable Continental Region of India

AbstractThe first stage impoundment of the Pulichintala reservoir in the Proterozoic Cuddapah basin in the southeastern Indian shield began in August 2014. The second stage of impoundment started in 2019 when the height of the dam was increased and the filling of the reservoir was rapid. Soon after earthquakes started occurring which continued for a few months with a maximum magnitude of ML 4.6 on 25 January 2020. Two distinct clusters within the shallow crust were identified, which follow the faults/shear zones in the region. The estimated focal mechanisms with predominant strike slip motion on steep planes and the derived stress state are consistent with the regional stress and plate motion of the Indian plate. We simulated the influence of reservoir impoundment on the inferred seismogenic faults and found that the stress and pore pressure due to reservoir load indeed promoted failure, thus inducing earthquakes. We ascribe the induced earthquakes to rapid filling, longer and higher retention of high‐water levels during 2019–2020, which caused a stress change of 4–8 kPa. This stress change was at least two orders of magnitude higher than the typical tectonic loading rate in the stable continental region of the Indian Shield.

A collaborative numerical simulation-soft computing approach for earth dams first impoundment modeling

Uncertainty quantification plays a crucial role in the design, monitoring, and risk assessment of earth dams. To reduce the computational burden, we employ a combination of finite difference method and soft computing techniques to investigate material uncertainties in earth dams during the initial impoundment stage. The findings of sensitivity analysis with the Tornado diagram indicate that key material properties such as dry density, elasticity modulus, friction angle, and Poisson’s ratio significantly influence the displacements and stress analysis. In our study, we explore four variants of extreme learning machines (ELMs): the standalone ELM, hybridized versions with the improved grey wolf optimizer algorithm, ant colony optimization for continuous domains, and artificial bee colony. These methods are assessed across various training sizes to predict multiple parameters, including horizontal and vertical displacements, stresses, and the factor of safety (FoS). The hybridized ELM with the improved grey wolf optimizer algorithm emerges as the superior choice for most of the response variables. A minimum of 200 numerical simulations is required to establish a stable and accurate meta-model with an average prediction error of less than 3% for responses and the FoS.

Dam construction reshapes heavy metal pollution in soil/sediment in the three gorges reservoir, China, from 2008 to 2020

Dam construction interfered with the original environment of the river system and greatly affected the geochemical behaviors of trace metals. Thus, a set of toxic metals of Cr, Ni, Cu, Zn, As, Cd, Pb and Hg in soil/sediment of the Three Gorges Reservoir (TGR) during the period of 2008–2020 were analyzed and summarized. The results showed that levels of trace metals (except Cr) were apparently higher than the soil background in the TGR and China, in which Cu, Zn, As, Cd, Pb and Hg corresponded to the moderately to highly contaminated grade. As expected, most trace metals (except Ni and As) were observed an evident increase after the full impoundment stage of 2008–2014, suggesting the dam construction of the TGR that promoting the sediment adsorption effects for trace metals. For spatial patterns, metal levels largely depended on the sampling sites, that intensive anthropogenic activities might well be the primary contributors. Main stream with higher concentrations of trace metals in comparison with tributaries reflected the larger loads of metal pollution. In the water-level-fluctuating zone, hydrological regime induced by damming played a critical role on the redistribution of trace metals through eroding soil/sediment particles or bedrocks and altering the physiochemical characteristics and vegetation coverage of soil/sediment. Finally, submerged sediment seemed as a major sink of trace metals that had greater concentration than that in the water-level-fluctuating zone.

Comparison of Mohr-Coulomb and hardening soil constitutive models for simulation of settlements in the Karkheh earth dam

This paper presents the settlement behaviour of Karkheh earth dam during its construction and operation stages. Karkheh is one of the largest earth dams in the world in terms of its reservoir capacity and body volume. The settlement of such a large body of soil can affect the performance of the dam elements and endanger downstream areas; should a breach or failure occur in the dam, more than two million people will be affected. It is crucial to know the settlement behaviour of this structure and use the existing results to predict its future settlements and calibrate the existing stress-strain models. For anticipation of dam settlement the measured displacement from the portable probe anchor magnets installed in the dam body are compared to the results of numerical simulations. The available data cover a period of 12 years including construction, and two material impounding and operation periods of the dam. The numerical analysis is performed in 2D plane-strain conditions and two material models are used, including Mohr-Coulomb (MC) and Hardening Soil (HS) models. The comparison between the calculation results and the measured vertical deformations in the dam site reveals that the accuracy of model for the deformations in the middle levels of dam is better than those of the crest for both applied material models in construction and impounding stages. The maximum settlement differences between computed and observed values are 0.05 m for MC model and 0.01 m for HS model. For the operation stage, the error of calculated settlements for the MC model is smaller; hence the results of this model might be more reliable for prediction of future dam settlements. The similar trends, obtained from both material models, exhibit the suitability of the model parameters used in the simulations.

Application of 210Pbex in elucidating contemporary sedimentary dynamics in the water-level fluctuation zone of the Three Gorges Reservoir, China

Excess 210Pb (210Pbex) has been widely applied as a proxy indicator for establishing the chronology of sediments in lakes where the sedimentation processes comply with the assumptions for using 210Pbex dating models. However, little is known regarding the applicability of 210Pbex to elucidate contemporary seasonal sedimentary dynamics in reservoir water-level fluctuation zones (WLFZs). In the present study, sedimentary core and bulk sediments were collected from the WLFZ of the Three Gorges Reservoir (TGR) in China, and the physical and chemical properties were determined. Results for 210Pbex activity, total organic carbon, and total nitrogen of bulk sediments indicate pronounced elevation-dependent spatial variation (p < 0.05), but 137Cs activity and particle composition show no systematic trends, allowing 210Pbex activity to be used to differentiate seasonal stratigraphy. For the sedimentary core, therefore, sediment layers with lower 210Pbex activity were inferred to represent sedimentation during wet seasons, when the 210Pbex concentration was diluted by a higher suspended sediment load, whereas sediment layers characterised by higher 210Pbex activity were ascribed to sedimentation during dry seasons, when the tranquil water favoured 210Pbex fallout to be absorbed by suspended sediments and then deposited in the WLFZ. Hereupon, temporal variation is observed in annual sedimentary thickness, with relatively low sedimentation rates during the initial stage (2006–2008) of the TGR impoundment, followed by relatively high sedimentation rates from 2009 to 2013. Over the period from 2014 to 2017, sedimentation rates were low because the suspended sediment was intercepted by upstream cascade reservoirs. Subsequently, sedimentation rates were higher during 2018–2019 as a consequence of an increase in extreme rainfall events. The findings herein highlight that 210Pbex-based seasonal stratification discrimination is effective approach to establish high-resolution sedimentary chronosequences and that net sedimentation rates in WLFZ are controlled by inflow suspended load of the TGR.

Geochemical characteristics and ecotoxicological risk of arsenic in water-level-fluctuation zone soils of the Three Gorges Reservoir, China

The Three Gorges Reservoir (TGR) has formed the water-level-fluctuation zone (WLFZ) due to reservoir regulation. However, as a sensitive zone in reservoir, little is known about the geochemical process and ecotoxicological risk of arsenic (As) in WLFZ soils under the anti-seasonal flow regulation. Hence, the anthropogenic contamination, mobility and ecotoxicological risks of As in WLFZ soils of the TGR were comprehensively assessed using the geochemical baseline concentration (GBC), chemical fractions, diffusive gradients in thin films (DGT) and toxicity data. The As concentrations in WLFZ soils showed a trend of increasing at the early stage of water impoundment and then stabilizing in recent years, which presented a low ecological risk of As according to the assessment by pollution indices. Based on GBC calculations, the average anthropogenic contribution of As was 13.95 %, indicating a slight influence of human activities. The distribution of labile As measured by DGT in WLFZ soils was mainly controlled by the Fe/Mn oxides, pH and organic matter. The DGT-induced fluxes in soils (DIFS) model further implied that resupply of As to soil solution was partially sustained by the soil solid phase, in which the resupply capacity was low and limited by the adsorption and desorption kinetics. In addition, the DGT was combined with toxicity data to obtain the risk quotient (RQ) and probabilistic risk assessment. The RQ value was lower than 1, indicating a low toxicity risk in WLFZ soils. Furthermore, the As in WLFZ soils had a low probability (5.97E-3 % and 7.77E-2 % in the mainstream and tributary, respectively) of toxic effects toward the aquatic biota. This study provides a comprehensive evaluation for the mobility and toxicity risk of As in WLFZ soils, which is beneficial to the prevention and control of heavy metals pollution in the riparian soils of lakes and reservoirs.

Hydroacoustic survey on fish spatial distribution in the early impoundment stage of Yuwanghe Reservoir in southwest China

Understanding the fish community structure and spatial distribution characteristics is essential for appropriate reservoir fishery management, especially in the early impoundment stage of the reservoir, which could help in obtaining basic data and performing artificial adjustments to biological communities. On the basis of this concept, we conducted a survey of the fish community and distribution at the newly constructed reservoir in southwest China by using a combination of methods, including hydroacoustic survey and fish sampling. Fish sampling showed a single fish community structure (six species belonging to four families) assembled in the reservoir, and the dominant species was Pseudorasbora parva which accounted for 98.79% of the fish population. In the hydroacoustic survey, the average fish density was 318.7 ± 256.1 individuals/1,000 m2. Irregular distribution of the fish was observed in the horizontal direction. The fish densities in the head area, middle area, and tail area of the reservoir were 168.5 ± 60.1, 306.8 ± 124.7, and 696.4 ± 288.9 individuals/1,000 m2, respectively, which showed a trend of increase in fish density with an increase in distance from the dam. More than 97.3% of the fish in the vertical direction were distributed in the water layer of depth 0 to 12 m. The average total length of fish was 75.4 mm, and the mean fish biomass in the reservoir was approximately 984.8 g/1,000 m2. According to Pearson correlation analysis, the main factors affecting the spatial distribution of fish were water depth, water temperature, dissolved oxygen, total nitrogen, and plankton density. Our results suggested that the fish community structure should be adjusted by releasing native carnivorous fish to control the abundance of small fish. Moreover, filter-feeding fish such as silver carp and bighead carp should be released in the reservoir to control the plankton community; this will enable the maintenance of the reservoir ecosystem in a healthy state while increasing the economic benefits to the local area.

Engineering Safety Evaluation of the High Rock Slope of a Hydropower Project: A Case Study of 684 m-High Slope Related to Lianghekou Hydropower Project at Yalong River

The stability of the slope is a very important topic in the construction of hydropower projects, especially the slope engineering in the dam area, as its stability will directly affect the safety of engineering. Taking the inlet slope of the flood discharge structure of Lianghekou Hydropower Project as the research object, based on the analysis and exploration of the geological condition of the slope and the field monitoring data, GA-LSSVM is used to establish the non-linear mapping relationship, and the BP neural network is used to establish the mechanical parameters back analysis of the slope at different water impoundment stages. A numerical simulation model is also established to set up different reservoir impoundments to study the stability and sensibility of the slope and provide guidance for slope operation. This case study shows that there is a hysteresis in the response of slope deformation to reservoir impoundment. At the same time, the mechanical parameters of the slope will be weakened by the seepage. In the process of water level changes, the stability of the slope decreases due to the decrease in mechanical parameters. The study will be practically useful for engineering applications.

Temporal variations and trends prediction of water quality during 2010-2019 in the middle Yangtze River, China.

Water quality plays an important role in river habitats. This study revealed the annual and seasonal variations and trend prediction of water quality in the middle Yangtze River after the third impoundment stage of the Three Gorges Reservoir. Multivariate statistical methods including principal component analysis/factor analysis (PCA/FA), Mann-Kendall (M-K) tests, discriminant analysis (DA), rescaled range (R/S) analysis, and the Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) were used. Herein, eight water quality constituents including pH, electrical conductivity (EC), chloride (Cl), dissolved oxygen (DO), ammonia nitrogen (NH3N), total phosphorus (TP), water temperature (T), and permanganate index (CODmn) were monthly monitored in the Jiujiang hydrological transaction during 2010-2019. The information of eight water quality constituents, related to salinity, nutrient status, and oxidation reactions efficiency, was extracted. Water quality status remained as fair-good during 2010-2019 based on the results of CCME-WQI, with the seasonal significance ranked as T > DO > Cl > pH > EC > TP > NH3N > CODmn. In the future decade, annual average T was predicted to continue to increase although it might decrease in the wet season. EC was predicted to continue increasing annually especially in the wet season while Cl might decrease. NH3N and TP might maintain a significant decreasing trend in the future wet and dry seasons. DO maintained significantly increasing especially in the future dry seasons, whereas CODmn will continue to decrease annually and seasonally. The continued alkalization trend of waterbody was predicted, which is more significant in the wet season. The results provide helpful references for the ecological protection of the middle Yangtze River.

Different Toppling Bank Slope Failures under Hydrodynamic Action during Impoundment of the Miaowei Hydropower Station Reservoir

Toppling is a common deformation and failure phenomenon in the reservoir bank slopes of hydropower projects. This paper studies the genesis and evolution of different toppling bodies during water impoundment at the Miaowei Hydropower Station Reservoir on the Lancang River in southwest China. Toppling properties were determined and second failure characteristics analyzed in different reservoir impoundment stages. Different degrees of toppling deformation were primarily affected by the transverse bending stress, while the regional tectonic stress has been shown to have a significant effect on the transverse bending of the rock layers. Combined with the on-site investigation and monitoring results, the failure mechanisms of the different toppling deformation bodies were analyzed. The second failure of the toppling rock mass caused by the reservoir impoundment process is mainly the hydrodynamic splitting along fractures, wave impaction and softening on the slope foot. The transverse bending effect of gravity is transmitted upward through joint misalignment, rotation and slip, accelerating the speed of secondary toppling failure and forming a compression-shear failure along the toppling tension crack. A model to predict the scope and time of failure in the toppling deformation banks under the action of reservoir hydrodynamics was proposed.

Evaluation of the Effect of Varying the Angle of Asphaltic Concrete Core on the Behavior of the Meijaran Rockfill Dam

The use of asphaltic concrete cores for sealing embankments and rockfill dams is very important. The self-healing properties of bitumen, simple construction in cold and rainy conditions compared to clay cores, good flexibility and connection with embankment materials are the essential characteristics of asphaltic concrete. The main concern regarding the use of asphaltic concrete cores in Iran is mainly the performance of these dams under seismic loads. The evaluations of the performance of these types of dams in other countries show that asphaltic concrete cores perform satisfactorily in the static state, but in earthquake conditions, the situation may be different. In this paper, the static and seismic behavior of the Meijaran dam in Iran, Mazandaran, is evaluated for three core angles of 90°, 60° and 45°. This evaluation was conducted at the end of the impounding stage and after applying seismic loads using FLAC 2D software and Mohr–Coulomb consitutive models. The results were matched with the ICOLD recommendation to use angled cores in dams with asphaltic cores and showed that the dam performs better with angled cores. Finally, for the Meijaran dam, the results from the dynamic analysis are compared with the results from the centrifuge test.

Static Parametric Stress-Strain Analysis for Asphaltic Concrete Core of Rockfill Dams

Abstract This study was conducted to investigate the effect of variations in the asphaltic concrete core Rockfill dam geometric parameters on the core behavior at different upper, middle, and lower points under two-dimensional static stress-strain analysis. The mentioned analysis was performed using SIGMA/W software. In the modeling and analysis process, the geometry of a large number of dams constructed around the world was studied and many numerical models with variations in eight classes of height, three dams upstream/downstream slope modes, and three different asphaltic core positions and thicknesses on a rigid foundation were created and analyzed. In addition, for each of the above models, four dam operation stages including “End of Construction”, “Full Reservoir”, “Half-Full Reservoir”, and “Rapid Drawdown” were considered. The results show, in four different stages of construction and impounding and three different slope modes, the maximum stress as well as horizontal and vertical displacements at the upper, middle, and lower points of the core increase with increasing height. In all models, increasing upstream and downstream slopes results in an increase in the amount of vertical displacement for all construction and impounding stages at the upper, middle, and lower points of the core. In the middle and lower points of the core, with increasing height, the amount of deviatoric stress increases for all construction and impounding conditions. In addition, for all operation cases, the maximum strain increases in all points of the vertical and tilted cores as the core thickness increases. Additional results are presented in the next sections.

The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China)

Using the test–retest data of the relative gravity field and earthquake monitoring catalog of the Three Gorges Reservoir (TGR) from October 2001 to October 2009, this paper systematically analyzes the changes in the gravity field in the head area of the reservoir and the temporal and spatial distribution characteristics of seismic activity during the impoundment process. It also employs the surrogate reshuffling tests to calculate the cross-correlation between the reservoir water level and the seismic activity sequence and discusses the influence of the rising reservoir water level on the gravity field and seismic activity in the reservoir region. Then, by constructing a three-dimensional finite-difference model based on the theory of fluid–solid coupling, the mechanism of reservoir-induced earthquakes is discussed from the aspects of direct reservoir water load and reservoir water infiltration. The results show that: (1) The rising reservoir water level has had a critical impact on the gravity field and seismic activity in the reservoir’s head area. The cumulative changes in the gravity field from October 2001 to November 2008 show that water impounding has led to a huge banded positive anomaly of gravity along the river near Xiangxi, which reached 450 × 10−8 ms−2. The seismicity activity dominated by micro-earthquakes after a 135 m water level rose rapidly, and the monthly average earthquake frequency increased from 2.00 before the impoundment to 92.60 after the 175 m stage. (2) From the beginning of the impoundment to the experimental impoundment stage of 175 m, the time series correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage to correlated when the time lag was 0 months at a 95% confidence threshold. This indicates that the seismic activity obviously has a direct relationship with the load pressure produced by the rapid rise of the reservoir water level, which causes the instability of the mines, karst caves, shallow rock strata, and faults within 10 km along the river and near the reservoir bank, and consequently induces earthquakes. (3) As the TGR enters the 175 m high-level operation stage, the cross-correlation test confirmed that the seismic activity and the reservoir water level show negative correlation characteristics under the time lag of 4 to 5 months, indicating that the seismic activity has a lagging response to the reservoir water level change. The continued infiltration of the reservoir water, followed by the softening of the faults and other actions, triggered the Xiangxi M4.1 earthquake at the center of the four quadrants of gravity anomalies near Xiangxi on 22 November 2008. The Xiangxi segment of the reservoir and its periphery, a triangular geological region where the Xiannvshan faults, the Jiuwanxi fault, and the Yangtze River meet, might be at risk of having reservoir-induced tectonic earthquakes.