Stability analysis of bank slope under conditions of reservoir impounding and rapid drawdown

Stability of an ancient landslide in a reservoir area is analyzed by using centrifugal model tests, soil laboratory tests and numerical analysis. Special attention is paid to variation in water level, simulation of large-scale heterogeneous prototype slope, and strength reduction of sliding zone soils after slope sliding. The results of centrifugal model test show that reservoir impounding can reduce sliding resistance at the slope toe, followed by toe collapsing and front cracking of slope. Rapid drawdown can produce hydrodynamic pressure towards reservoir at the front of slope. Deformation is observed in the middle and upper slope, which reduces the slope stability further and forms the pull-typed landslide trend. Reinforcement of slope toe is effective for preventing the progressive failure. The results of laboratory test show that slope toe sliding will lead to the redistribution of soil density and moisture content, which will reduce the shear strength of soil in sliding zone, and the cohesion of immersed soil is reduced gradually and finally vanishes with time. The numerical results show that the strength reduction method used in finite element method (FEM) is very effective in capturing the progressive failure induced by reservoir water level fluctuations, and the evolution of failure surface derived from numerical simulation is very similar to that observed in centrifugal model test.

Evaluation of behaviors of earth and rockfill dams during construction and initial impounding using instrumentation data and numerical modeling

In this study,the behavior of Gavoshan dam was evaluated during construction and the first impounding.A two-dimensional（2D） numerical analysis was conducted based on a finite difference method on the largest cross-section of the dam using the results of instrument measurements and back analysis.These evaluations will be completed in the case that back analysis is carried out in order to control the degree of the accuracy and the level of confidence of the measured behavior since each of the measurements could be controlled by comparing it to the result obtained from the numerical model.Following that,by comparing the results of the numerical analysis with the measured values,it is indicated that there is a proper consistency between these two values.Moreover,it was observed that the dam performance was suitable regarding the induced pore water pressure,the pore water pressure ratio r;,settlement,induced stresses,arching degree,and hydraulic fracturing probability during the construction and initial impounding periods.The results demonstrated that the maximum settlement of the core was 238 cm at the end of construction.In the following 6 years after construction（initial impounding and exploitation period）,the accumulative settlement of the dam was 270 cm.It is clear that 88% of the total settlement of the dam took place during dam construction.The reason is that the clay core was smashed in the wet side,i.e.the optimum moisture content.Whereas the average curving ratio was 0.64 during dam construction; at the end of the initial impounding,the maximum amount of curving ratio in the upstream was 0.81,and the minimum（critical） amount in the downstream was 0.52.It was also concluded that this dam is safe in comparison with the behaviors of other similar dams in the world.

Socioeconomic Impact of Small Water Impounding Projects in Quirino Province, Philippines

The study was conducted to determine the socioeconomic impact two (2) Small Water Impounding Projects (SWIPs) namely Burgos SWIP in Cabarroguis and Divisoria Norte SWIP in Maddela, both in the Province of Quirino, Philippines. The study made use of structured questionnaire to gather the socio-demographic profile and determined the socioeconomic impact of the SWIPs to the farmer-beneficiaries. Most of the farmer-beneficiaries of Burgos and Divisoria Norte SWIPs are male, 40 - 59 years old, married, have below four household members, Roman Catholic, high school graduates and some are members of cooperatives. They are rice farmers, and have attended one to two training courses related to farming and own a 1.00 to 1.99 hectares farm. “Utilization of water in the reservoir for other purposes like wallowing pool for carabaos” and “integration of fish production and livestock raising” were the identified benefits at the farmer’s level while “introduction of new techniques in farming by the Local Government Units,” “availability of cheap fish and additional income as a result of integration of fish production” and “construction of better roads” were the identified benefits at the community level. The SWIPs have positive impact as confirmed by the increase in the annual mean yield and in the annual gross income of the farmer-beneficiaries. It contributed a significant increase of 2.14 tons/year in the mean yield generating an additional income of PhP 32,113.64 for Burgos SWIP. On the other hand, it contributed a significant increase of 2.08 tons/year giving an additional income of PhP 31,313.60 for Divisoria Norte farmer-beneficiaries. The result of this study will provide strategic information to policy makers of agricultural and irrigation agencies on the existing weaknesses of irrigation systems in the country and determine in a more quantifiable terms level of potential improvement and investment targets.

Influence of the impounding process of the Three Gorges Reservoir up to water level 172.5m on water eutrophication in the Xiangxi Bay

The water level of the Three Gorges Reservoir(TGR) rose to 172.5 m in two stages from September to November,2008.The hydrodynamic parameters and water quality parameters(e.g.flow velocity,temperature,turbidity and nutrient content) had been continuously monitored to reveal the influence of the impounding process of the TGR on the water eutrophication in the Xiangxi Bay(XXB).The maximal daily rise of the water level of the TGR was about 2.38 m.The water exchange between the mainstream(the Changjiang River) and the XXB of the TGR was enhanced by density current,and its main characteristics were a density-stratified flow in different directions.The water exchange reduced the differences of the water quality parameters(for example,temperature,turbidity,nutrient level,pH,etc.) between the mainstream and the XXB during the impounding process.Meanwhile,the water stability index in the mainstream remained small but it was lowered in the XXB.The main causes included the dilution by water of lower concentration of the chlorophyll a(Chl.a) entering from the mainstream,the decline of the water stability,and the increase of suspended silt,which induced the Chl.a to decrease in the XXB during the impounding process.The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process.However,as available nutrients were imported into the XXB from the mainstream during the impounding process the risk of water eutrophication increases in the XXB in the future.

Assessment of the Impact of the Grand Ethiopian Renaissance Dam on the Performance of the High Aswan Dam

A Large scale hydropower dam known as Grand Ethiopian Renaissance Dam (GERD) is under construction on the Blue Nile River just upstream of the Ethiopian-Sudan border in Ethiopia. The GERD has an active storage capacity of more than 60,000 Mm3 and is anticipated to produce 6000 MW of hydropower energy. The aim of this study was to assess the potential impact of the dam on the performance of the High Aswan Dam (HAD) during filling and operation phases using a simulation model using Mike Basin river basin simulation model. The results indicate the planned 6 years filling period is sufficient to fill the reservoir with little impact on the current irrigation water demands from HAD in Egypt without additional management investment. There will be about 12% and 7% of reduction of annual energy output from High Aswan Dam during the filling and after filling stage of GERD respectively. Cumulative Energy production from the two dams will increase significantly Water loss at HAD due to evaporation will decreases by 22%. Overall performance of HAD during and after filling of the GERD remains at a reliability level of 96%. The study advises to utilize the results cautiously as it is based on only one historical realization of the many possible scenarios that may evolve in the future.

PROBLEMS OF RESERVOIR SEDIMENTATION IN CHINA

ＰＲＯＢＬＥＭＳＯＦＲＥＳＥＲＶＯＩＲＳＥＤＩＭＥＮＴＡＴＩＯＮＩＮＣＨＩＮＡＪｉａｎｇＮａｉｓｅｎ（姜乃森）ＦｕＬｉｎｇｙａｎ（傅玲燕）ＣｈｉｎａＩｎｓｔｉｔｕｔｅｏｆＷａｔｅｒＲｅｓｏｕｒｃｅｓａｎｄＨｙｄｒｏｐｏｗｅｒＲｅｓｅａｒｃｈ，Ｂｅｉｊ...

Numerical analysis and geophysical monitoring for stability assessment of the Northwest tailings dam at Westwood Mine

The Westwood Mine aims to reuse the tailings storage facility #1(TSF #1) for solid waste storage, but,downstream of the Northwest dike is considered critical in terms of stability. This paper uses numerical modeling along with geophysical monitoring for assessing the Northwest dike stability during the restoration phase. The impact of waste rock deposition in the upstream TSF #1 is considered. The geophysical monitoring is based on electrical resistivity methods and was used to investigate the internal structure of the dike embankment in different deposition stages. The numerical simulations were performed with SLOPE/W code. The results show a factor of safety well above the minimum recommended value of 1.5. Geophysical monitoring revealed a vertical variation in the electrical resistivity across the dike, which indicates a multilayer structure of the embankment. Without any current in situ data, the geophysical monitoring helped estimating the nature of the materials used and the internal structure of the embankment. These interpretations were validated by geological observation of geotechnical log of the embankment. Based on this study, it is recommended that the water polishing pond be partly filled before waste rock is deposited in TSF #1. In addition, to ensure the stability of the dike, the piezometric head monitoring prior to and during waste rock deposition is recommended.

Long-term monitoring revealed fish assemblage zonation in the Three Gorges Reservoir

Reservoirs are important artificial ecosystems that modify the hydrological and ecological characteristics of a river. Knowledge of the basic characteristics of fish assemblages in reservoirs is a first step toward the development of effective conservation policies. We used the information collected over a 10-year period (2006-2015) to assess the structure of the fish assemblages in the Three Gorge Reservoir (TGR) in a river-dam gradient. Three fish zones were detected in TGR. Species richness was the highest in the upper zone and lowest in the lower zone. The riverine zones were dominated by rheophilic species Coreius guichenoti and Pelteobagrus vachelli. The transitional zones were dominated by Coreius heterodon and Rhinogobio cylindricus. The lacustrine zones were dominated by eurytopic species Hypophthalmichthys molitrix, Aristichthys nobilis, Hemiculter bleekeri and Cyprinus carpio. For the functional characteristics, fish assemblages in riverine and transitional zones were dominated by insectivorous species, equilibrium strategists and rheophilic species (e.g., Coreius heterodon and Coreius guichenoti). In lacustrine zones, the fish assemblage was dominated by habitat generalists common to lakes and reservoirs (e.g., Hemiculter bleekeri, Hypophthalmichthys molitrix, Aristichthys nobilis). Moreover, 18 exotic species (e.g., Protosalanx hyalocranius, Ictalurus punctatus, Megalobrama amblycephala, Tilapia) were collected in TGR, most of which only existed in the lacustrine zone. The results highlight the importance of freely flowing riverine reaches for conserving native fish in the upper Changjiang River and adaptive management strategies for fisheries in TGR.

Analysis of working behavior of Jinping-I Arch Dam during initial impoundment

To study the stress, deformation, and seepage pressure during the initial impoundment of the Jinping-I Arch Dam, monitoring analysis and numerical calculation were used in a dam behavior analysis that focused on the working behavior of the dam during the late period of the initial impoundment up to the end of November 2014. The numerical calculation was performed based on feedback analysis of the deformation and stress of the arch dam through inversion of the elastic moduli(E) of the dam body and foundation, using a three-dimensional finite element model for the linear elastic material of the arch dam. The main monitoring indices presented insignificant changes in the late period of the initial impoundment, and the results of feedback analysis were consistent with monitoring results. Analysis results also show that the deformations of the dam body and dam foundation were within the design range; the dam stress distributions were normal, with values lower than the design control criteria; and the seepage flows through the dam body and dam foundation were lower than the design drainage capacity of the deep-well pump house, demonstrating that the Jinping-I Arch Dam was in good working condition, and the initial impoundment had been successfully completed. The results of the working behavior analysis of the Jinping-I hydropower project during the initial impoundment can provide references for safe operation of similar projects.

Displacement characteristics and prediction of Baishuihe landslide in the Three Gorges Reservoir

In order to reach the designated final water level of 175 m, there were three impoundment stages in the Three Gorges Reservoir, with water levels of 135 m, 156 m and 175 m. Baishuihe landslide in the Reservoir was chosen to analyze its displacement characteristics and displacement variability at the different stages. Based on monitoring data, the landslide displacement was mainly influenced by rainfall and drawdown of the reservoir water level. However, the magnitude of the rise and drawdown of the water level after the reservoir water level reached 175 m did not accelerate landslide displacement. The prediction of landslide displacement for active landslides is very important for landslide risk management. The time series of cumulative displacement was divided into a trend term and a periodic term using the Hodrick-Prescott(HP) filter method. The polynomial model was used to predict the trend term. The extreme learning machine(ELM) and least squares support vector machine(LS-SVM) were chosen to predict theperiodic term. In the prediction model for the periodic term, input variables based on the effects of rainfall and reservoir water level in landslide displacement were selected using grey relational analysis. Based on the results, the prediction precision of ELM is better than that of LS-SVM for predicting landslide displacement. The method for predicting landslide displacement could be applied by relevant authorities in making landslide emergency plans in the future.

Sedimentation retention basin utilization for best management practice

Approaches to the artificial impoundment and theoretical design of sedimentation retention basin are reviewed with particular attention to best management practice(BMP) to control agriculture and surface runoff. Sediments retention basins are the small version of farm pond used where a criteria of farm pond is not met. Such basin traps the pollutants and suspended solids prior to entry into streams and lakes. The study is focused with special reference to the assessment and control of non point source pollution(NPSP) from the sub basin area of Tai Lake in the Xishan County of Wuxi City of China. The author suggested two different approaches to conduct this study including theoretical design for sedimentation retention basin and computation of flow, sediment transport and deposition during the artificial impoundment of retention basin for BMP's utilization. Theoretical design will provide a useful function as a first line defense against the movement of sediments and transport of pollutants into the Tai Lake while the assessment of sediments deposition will help to make its proper use and periodic cleanup.

Interception, degradation and contributions of terrestrial organic carbon obtained from lignin analysis in Wujiang River, southwest China

Understanding the fate of terrestrial organic carbon(OC)in a cascade impoundment system is critical for recognizing the role of carbon sink for reservoirs.Surface sediments collected from eight cascade reservoirs across the Wujiang River,southwestern China,were analyzed for elemental and stable carbon isotopic(δ^13C)composition,and lignin phenols(∑8 andΛ8)to investigate the spatial distribution,contribution,origin and degradation of sedimentary terrestrial OC.The values of total organic carbon(TOC)and∑8 exhibited a remarkable reduction along the upstream-downstream transect suggesting the trapping effect of cascade-damming.A relatively broad range ofδ^13C(-26.61 to-25.54%,95%CI)and C/N(6.80-18.20)indicated mixed allochthonous/autochthonous OC sources in surface sediments.The quantitative simulation indicates that the OC of the sediments mainly was derived from terrestrial organic matter.Soilderived OC rather than C_(3)vascular plant-derived OC makes a major contribution to sedimentary terrestrial OC in reservoirs on karst terrain.As evidenced by lignin compositions andδ^13C,the predominant vascular plant origins of terrestrial OC along the Wujiang River are non-woody angiosperm C_(3)plants.The aged reservoirs showed a trend of increasing contribution of autochthonous OC,which potentially weaken the role of carbon sink for reservoirs.The relationship between runoff inputs,watershed area/water surface area ratios,and water residence time and Λ8 were explored,indicating the natural and anthropogenic influences on terrestrial OC remains very complex in a cascade-damming river.

Model-scale tests to examine water pressures acting on potentially buoyant underground structures in clay strata

Throughout the service life, underground structures are subjected to transient and sustained hydrostatic pressures. The reservoir impoundment results in an increase in water level, as well as hydraulic gradient,which can endanger the uplift performance of infrastructure. In uplift design, a reduction factor is often suggested for buoyant force acting on underground structures in clays due to the time lag effect.However, the mechanism of pore pressure generation in clays is not fully understood. This investigation presents a novel U-shaped test chamber to assess the pore pressure generation with time in the horizontal branch subjected to an increase in reservoir level in the left vertical branch. A mathematical model is developed to explain the time lag effect of pore pressure generation. The test program also involves the evaluation of uplift pressure acting on foundation model in the right vertical branch due to adjacent reservoir impoundment. It is found that the time lag effect of pore pressure generation in clays can be observed irrespective of hydraulic gradient, but a higher hydraulic gradient can lead to a faster response in pore pressure sensors. A reduction factor of 0.84-0.87 should be considered to reduce the conservatism of uplift design.

The effect of large reservoirs impoundment to the spatial and temporal variations of regional crustal deformation in Hubei Province, China

The total capacity of Three Gorges Reservoir（TGR） and Danjiangkou Reservoir（DJR） is large and has significant seasonal fluctuations, which give rise to crustal instability. In this research, we focus on studying the temporal and spatial variation of crustal deformation in Hubei Province caused by reservoir impoundment of TGR and DJR.The Digital Elevation Model, historical hydrological information, GPS monitoring data and load-induced deformation model are combined to monitor the crustal deformation. The modeled results indicate that in the trapezoidal area between the TGR and DJR, the average vertical deformations at different latitudes have different variation tendencies. The vertical deformation modulus and fluctuation amplitude are larger at the latitude of 33 N/32.5 N from 2003 to 2006 and at the latitude of 31 N/32.5 N from 2008 to 2014, while the latter are much larger than the former. Moreover, from2008 to 2014, the frequency and the intensity of seismic activities are all enhanced significantly in this region. The modeled results at the GPS sites are consistent with the vertical displacement of GPS monitoring results in trends and the waveform. It can be inferred that the seasonal deformation is elastic. The horizontal deformation components have the same variation trends with that at each GPS monitoring station,which demonstrates that the whole region is moving toward the southeast. The spatial variation of crustal deformation demonstrates that the impoundment of TGR in2003 causes significant vertical displacements, with the maximum modulus of 32 mm downward located in Xiangjiang River＇s estuary. When the water storage increases, the maximum value will become larger, and the location will move toward the upstream.Besides, the earthquakes occurred more frequently in the region with maximum deformation modulus.

Tectonic environment and cause of earthquakes in the Three Gorges reservoir area

Seismotectonics in the Three Gorges reservoir area is investigated by using the P-wave tomography with earthquakes that occurred before the impoundment of the reservoir. The result indicates that most of these events occurred in or around the velocity-gradient belts between high-velocity and low-velocity anomalies. These belts have similar characteristics to buried-fauh zones. Stresses generated by movement of partially molten upper-mantle materials and thermal stress may have jointly contributed to the seismic activities along the faults and such buried faults, and possibly activated new earthquake ruptures.

Gravity change and its mechanism after the first water impoundment in Three Gorges Project

In this paper we have analyzed precise gravity survey and gravity effects resulted from water loading, crustal deformation, ground water level change and precipitation before and after the water impoundment in the Three Gorges Reservoir. We found that： ① In dam area of the reservoir, gravity effect resulted from water load increase is the most significant, maximum gravity change is 200×10^-8 m/s^2, but this effect is limited in amplitude and range. Gravity change can be observed about 5 km offshore. The gravity change caused by ground water level change is regional; and the impact of precipitation on it should not be neglected. ② At head area of the reservoir, the maximum gravity change is near Xiangxi. Monitoring the variation of gravity field and further study should continue in the future.

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu(XLD)reservoir is the second largest reservoir in China and the largest in the Jinsha River basin.The occurrence of two M>5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators.We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment.Following impoundment,earthquake activity increased significantly.The occurrence of M≥3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level,though this correlation weakened after five years because earthquake activity was far from the reservoir area.Earthquakes in the XLD reservoir area are clustered;near the dam(Area A),small faults are intermittently distributed along the river,while Area B is composed of multiple NW-trending left-lateral strikeslip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EWtrending right-lateral strike-slip minor fault.The geometries of the deep and the shallow parts of the NW-trending fault differ.Under the action of the NW-trending background stress field,a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment.The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.

Study of the Effect of Himreen Dam on the Phytoplankton Diversity in Dyala River, Iraq

In present study three stations are located on Diyala River has been investigated to study phytoplankton community, the first station was located in jalawlaa area, second station in Himreen impoundment, while the third station is located after the Himreen Dam in Al-Soddur area, A total 134 taxa of Phytoplankton were identified during the study period, 63, 82 and 84 were identified in stations 1, 2 and 3 respectively. The higher density of Phytoplankton observed in station 3, result ranged between 21.16 - 23.88 × 102 cell/l during September and October respectively, then station 2 which has density ranged between 18.44 - 66.188 × 102 cell/l during January and September respectively, while the station 1 has the lowest values during the study comparable to other stations, the result ranged between 5.495 - 70.62 × 102 cell/l during January and April respectively. Species Richness Index ranged from 2.9 - 7.46 in station 1, 4.08 - 7.85 in station 2 and 5.71 - 8.77 in station 3. The result revels that the station 3 show to increase in Richness index value in the most of the months comparable to other stations. The Similarity index also has been calculated among the common species of Phytoplankton through the three station by using the Hierarchical cluster analysis is the highest Similarity percentage in reach in 43.5% for the common species of Phytoplankton between stations 2 and 3.

Evaluation method of dam behavior during construction and reservoir filling and application to real dams

In this paper we analyzed a real fill-type dam,named the Minami-Aiki Dam,to verify a numerical consolidation method with an elastoplastic model for unsaturated soils.The validities were conducted by comparing the analyzed and measured values during construction and impounding.The settlements calculated showed good agreement to those observed.Though the changing patterns of the measured and calculated pore water pressures were similar,the amounts of pore water pressures induced by the self-weights of embankment and the dissipation speeds were different.The earth pressures measured at the bottom of the core zone showed good agreement with those calculated.This method is valid for estimating behavior of fill-type dams during construction and impounding.

Rainwater harvesting,its prospects and challenges in the uplands of Talugtog,Nueva Ecija,Philippines

The prospects and challenges facing eight small water impounding projects(SWIPs)in Talugtog,Nueva Ecija,an upland municipality located in Central Luzon,Philippines were evaluated using rapid appraisal and documentation of projects,interview of farmers and local officials,and a review of related studies undertaken on the same project sites.The challenges include the deterioration of structural facilities,inactive farmers associations,watershed degradation,and climate change.It also aims to evaluate improvement and innovation in the future implementation of SWIPs as rainwater harvesting facilities.The site was selected because it has the largest number of SWIPs established as one of the coping strategies during the 1997-1998 severe El Nino.Because of its location,it has no major irrigation systems and relies only on local rainwater storage facilities.The study involves 8 SWIPs established in two clusters(i.e.,5 and 3 SWIPs in a watershed)as rainwater conservation and management facilities.Results indicated these clusters of SWIPs offer multiple benefits in terms of supplemental irrigation,inland fish production,and water for domestic purposes and livestock production.They also serve as strategic small-scale upland structures that enhance recharging of groundwater,prevent flooding,and provide value-adding activities such as recreation,soil and water conservation,and environmental benefits.Previous studies also identified their benefits at the farm and community levels as conserved rainwater through storage in SWIPs is translated into more economic uses.However,some SWIPs are confronted with various challenges;deterioration of structural facilities,inactive farmer associations,unabated watershed degradation,and threats of climate change.These are seriously affecting the overall performance of SWIPs.Immediate actions should include the strengthening of small water impounding system associations(SWISA),repair and climate-proofing of structural facilities through the(SWISA)themselves,and watershed protection and management through the adoption of appropriate soil and water conservation measures.