Safety of small and medium dams in permafrost regions

Rudolf V.Zhang ,Sergey A.Velikin

1.Melnikov Permafrost Institute,Siberian Branch of the Russian Academy of Sciences,Yakutsk 677010,Russia

2.Vilyui Permafrost Research Station,Melnikov Permafrost Institute,Siberian Branch of the Russian Academy of Sciences,Chernyshevsky 678185,Russia

1 Introduction

Safe operation and performance of dams is one of the key issues in permafrost regions.Intensive geocryological processes (thermokarst,thermal erosion,frost heaving,suffosion,concentrated seepage along the voids left by melt ice and others) begin to develop at the early stages of construction.These processes are hazardous both in the upstream and the downstream areas.It should be noted that large Class I–II dams,50 m or more in height (primarily for hydropower production) have a lower catastrophic failure rate compared to medium (20 to 50 m in height)and small (20 m or lower) Class III and IV dams (At its 2000 meeting,the International Commission on Large Dams (ICOLD) has defined large dams as dams with a height of 15 m or more from the foundation,or dams between 5–15 m high with a reservoir volume of more than 3 million cubic meters).ICOLD statistics show that the failure rate for small embankment dams accounts for approximately 36% or higher of all dam failures.In Russia,half of the Class III and IV dams,90% of which are under the jurisdiction of the Federal Ministry of Natural Resources and Environment,are identified as unsafe (Zoteevet al.,2003).Dam failures can have disastrous consequences for populations and property in the downstream areas,especially where development occurs in the low floodplains in violation of all existing building norms and regulations.

In permafrost regions,the percentage of dam failures is higher due to the unique environmental and geotechnical conditions,as well as due to global climate change observed since the mid-twentieth century.In Central Yakutia,for example,about 90% of small embankment dams built for land reclamation fail within the first few years of operation (Zhang,1969).Over 40% of medium dams for domestic and industrial water supply are subject to failures,about 50% of which occur in the first 3 years of operation and 31%–35% occur between the third and fifth years(Pridorogin,1979).

At present,the surviving dams are 40–45 years old and they are reaching their design life limit.It is appropriate to mention here that not a single water project has been completed in Russia over the last half a century.The only projects currently under construction are the Boguchany and Bureya hydroelectric power stations initiated in 1974–1978 and to be completed in 2014,as well as the Svetlinskaya and Ust-Srednekanskaya hydroelectric power stations to be completed in 2016.In this context,rehabilitation becomes important to prolong the useful life of the dams that are adequately operating.This fact should be taken into account in dam safety declaration.It is known that ageing and degradation of the hydraulic structure as a whole and its materials begin from the moment of placement of the material (Mirtskhulava,2008).These processes are even more intensive in harsh permafrost environments due to the large thermal and moisture gradients and the resulting complex thermal stress-strain state in the structures(Grechishchev and Sheshin,1973;Zhang,2002).

2 Dam safety terms and definitions

The safety of dams and other hydraulic structures(HS) in Russia is regulated by the Federal Law of the Russian Federation of 21.07.97 No.117,On the Safety of Hydraulic Structures (State Duma of the Russian Federation,1997).This Law uses the following basic notions:

1) "operator of a hydraulic structure" is a public owned or municipal unitary enterprise or entity of any other organizational or legal form,on whose balance sheet the hydraulic structure is recorded;

2) "owner of a hydraulic structure" is the Russian Federation,federal unit,municipality,legal entity irrespective of its organizational or legal form,or a natural person,having the rights of ownership,usage and disposal of a hydraulic structure;

3) "area of a hydraulic structure" means a tract of land and/or water within the boundaries established according to existing land and water legislation;

4) "emergency" means a condition developed by HS failure which have or may result in loss of lives,damage to public health or the environment,or significant economic and social losses;

5) "safety of a hydraulic structure" refers to a HS quality to ensure the protection of the life,health and legitimate interests of persons,the environment,and property;

6) "safety assessment of a hydraulic structure"means the evaluation of compliance of the condition of a hydraulic structure and of the qualifications of operator’s employees with the standards and rules approved in accordance with the procedure established by the Federal Law;

7) "ensuring safety of a hydraulic structure" refers to the development and implementation of measures to prevent HS failure;

8) "declaration of safety of a hydraulic structure"is a document which gives the grounds for HS safety and defines measures to ensure HS safety;

9) "failure of a hydraulic structure" means HS failure or damage caused by unforeseen conditions(outside the design assumptions or safety rules) and accompanied by the uncontrolled release of impounded water or mine tailings;

10) "failure risk level" is a HS safety characteristic which can be presented in the probabilistic form or in the form of a deterministic parameter (hydraulic-structure safety level) characterizing the deviation of HS performance or operating conditions from the requirements identified in the regulatory documents;

11) "permissible failure risk level" is a HS failure risk value established by the normative documents;

12) "controlled parameters" are quantitative parameters measured instrumentally or calculated from instrumented measurements,as well as qualitative parameters characterizing the condition of the hydraulic structure;

13) "diagnostic parameters" are the most significant controlled parameters which are used to assess the safety of the dam-foundation-reservoir system as a whole or its elements.

14) "safety criteria":

?K1 is the first (warning) level of diagnostic parameters,within which the stability and seepage strength of the dam and its foundation as well as the discharge capacity of outlet works and spillways are still in line with normal performance conditions;

?K2 is the second (threshold) level of diagnostic parameters,beyond which further operation of the hydraulic structure is inadmissible under design conditions.

15) "performance condition of a hydraulic structure":This term has no definition in the Law.It recognizes three different performance conditions("normal condition","potentially unsafe condition","pre-failure condition") which are defined further.

16) "normal condition" is the condition when the structure performs in accordance with the standards of practice and the design,and the diagnostic parameters do not exceed theK1 criterial values;

17) "potentially unsafe condition" means the condition when the value of any diagnostic parameter has become higher (lower) than its first,warning level(K1)or has gone beyond the range of values predicted for a given load combination.The condition of the structure does not meet the standards,but it is not in immediate danger of breaching and may be operated for a limited time;

18) "pre-failure condition" is the condition when the value of any diagnostic parameter has become higher (lower) than the second,threshold level (K2).Further operation of the structure is not allowed under design conditions without immediate measures taken to restore the required safety level and without special permission from the supervising agency.

3 Procedure for determining the dam safety criteria

To properly implement the Federal Law on the Safety of Hydraulic Structures,the Unified Energy System of Russia has developed a regulating document,RD 153-34.2-21.342-00,Methodology for Determining Safety Criteria for Hydraulic Structures (Unified Energy System of Russia,2001),as well as a guide to this regulation (Unified Energy System of Russia,2006).The regulating document applies to all agencies responsible for the development,approval,and employment of safety criteria for hydraulic structures of all classes with regard to projects in the power generation industry.It requires that criterial values for diagnostic parameters (safety criteria) should be established at two phases of a project:design phase and operation phase.Criterial values developed at the design phase should be corrected at the commissioning phase taking into account all the information obtained during construction,as well as possible extension of the scope of supervision.

The performance condition and safety of the hydraulic structure are evaluated by comparing the measured (or calculated from measurements) quantitative and qualitative diagnostic parameters with the respective criterial valuesK1 andK2,as well as with the predicted range of diagnostic parameters.

The criterial relationships have the following form:

(a) the hydraulic structure is in a normal (good)condition,if

(b) the structure is in a potentially unsafe condition,if

(c) the structure is in a pre-failure condition,if

whereFmeasis the measured or calculated value of the diagnostic parameter;K1 andK2 are the values which,if reached by any one diagnostic parameter,will indicate transition from one condition to another.

Apart from comparing the measured (calculated)parameters with their criterial values,diagnostic control includes comparison of the measured diagnostic parameter with its predicted value.In other words,the diagnostic parameter should also be checked against the confidence interval predicted for actual loads acting at the time of checkup:

whereFpredis the diagnostic parameter value predicted for actual loads and impacts by the deterministic or statistic model;δis the maximum permissible error of the prediction model.

Both checkups,comparisons with criteria(1)–(3)and with criterion(4)changing with actual loads and impacts,are mandatory.They are necessary and sufficient conditions of safety.

Determining safety criteria is a critical and difficult task in dam safety management.The existing procedures need to be continuously refined and improved depending on dam importance class.Some researchers (e.g.,Mirtskhulava,2008;Kayakin and Bobkov,2010) recommend introducing process development criteria (stability,destabilization,and extremality) for more objective assessment of dam safety,in addition to the existing two condition criteria.In other words,they call for a multi-factor dam–environment interaction system.As a rule,these multiple processes have a synergistic effect.

Other researchers (e.g.,Shchedrin and Kosichenko,2011) advocate that safety declaration should be simplified to an expert safety review and the use of complicated performance criteriaK1 andK2 (justified for large dams for power generation) must be cancelled for Class III–IV dams.It should be noted that RD 153-34.2-21.342-00 (Unified Energy System of Russia,2001) eases up on Class IV hydraulic structures,and if properly justified,on Class III hydraulic structures,permitting the use of criterionK2 only.The existing Federal law on hydraulic structure safety applies to all types of hydraulic structures.Apparently,this is not very correct,because its application to Class IV dams entails additional,unnecessary costs.

We would like to make a few remarks regarding the simplification of safety criteria assessment for medium and small dams with low storage capacity.The existing methodologies were developed for the safety analysis of large,primarily hydropower,projects.If applied to smaller water-retaining structures,they would lead to significant economic errors.For small and medium dams on permafrost,however,the following considerations must be kept in mind.

In permafrost regions with their complicated engineering-geological conditions,medium dams for industrial and public water supply are facilities of increased importance,because they are integrated into the mining and power generation complexes of high economic and social value for the region.We,therefore,think that the level of safety requirements for this category of dams should not be lowered.As for land reclamation dams,we are inclined to support the arguments in favor of simplifying the safety assessment procedures:the existing methodologies have been developed based on experience with large projects which,applied to small dams,will lead to significant errors;no reliable procedures for hydrologic characterization (streamflow volume,runoff generation,maximum discharge estimates,etc.) have been developed as yet;there are no building codes or other regulations for this type of dams;inadequate permafrost information is collected during dam site investigations due to funding constraints leading to serious flaws in the design,construction and operation of dams;and the lack of qualified and competent monitoring personnel.Given these circumstances,there appear to be too many unknowns to undertake analyses with the existing safety assessment methodologies.A simplified procedure of expert safety review is therefore thought to be economically justified for small dams.

4 Declaration of safety:discussion and a case study

As mentioned above,the existing law on hydraulic structure safety requires that the owner or operator of the hydraulic structure must file a declaration of safety with the relevant authorities.This declaration must be developed for each hydraulic structure supervised by the Mining Safety Inspectorate (Gosgortechnadzor)and the Technical Inspectorate (Gostechnadzor) following the current standards,guidelines and manuals in force in Russia.

The following case study is presented to illustrate the development of a dam safety declaration.

4.1 Project description

The Sytykan River Project in western Yakutia is a water scheme utilizing Sytykan River to meet the municipal,rural and industrial water needs for the town of Udachny,surrounding communities and diamond mines.This Class III project was commissioned in September 1976 (Biyanovet al.,1989;Aleshinet al.,2005).It is located within the zone of continuous permafrost.The permafrost at the project site ranges from 500 to 600 m in thickness.The depth of seasonal thaw varied from 0.7 to 3.5 m during the geotechnical investigation.The permafrost temperature varies from-7.2 °C to-5.3 °C.The permafrost is underlain by the highly mineralized Metiger-Ichera aquifer.A talik was present beneath the river bed extending to a depth of 17.0 m and to a width of 30.0 m.

The Sytykan is a semi-mountain river.The valley at the project site is U-shaped and the floodplain ranges in width from 120 to 200 m.The channel is 18.0 m wide at low water time and 36 m at high water time,with water depth varying from 2.3 to 6.5 m respectively.The river bed downstream the dam has a weighted mean slope of 0.009.The catchment area is 880 km2.The mean annual flow is 195 million m3,while the mean flood flow is 122 million m3.The 100-year flood flow in the river is 158 million m3,while the 200-year flood flow is 246 million m3.The period of spring high flow occurs,on average,from May 25 to June 10.Most of the annual flow,from 70% in a normal year to 98% in a dry year,occurs in spring.During the winter months,the river has no flow.

The bedrock in the dam and right abutment area consists,down to 50.8 m depth,of Lower Ordovician limestone with layers of marl.The limestone thickness varies from 10.0 to 40.8 m.The bedrock is overlain by 3.8 to 9.2 m of clayey silts with lenses of gravel to cobble-sized fragments of dolerite,limestone and marl.

The subsurface investigation in 1995 (The Declaration of Safety of the Sytykan River Project,2006)showed the dam embankment and foundation materials were in thawed and frozen states.The thawed material was water saturated.Borings revealed its table to be at depths of 5.0 to 11.0 m extending down to depths of 7.6 to 17.2 m.The frozen materials had massive,reticulate or fractured cryostructures.Ice contents were up to 5%.Groundwater was encountered while drilling with the steady level at 311.1 to 313.3 m a.s.l..

The Sytykan River Project consists of a reservoir,a retaining dam,a side channel spillway,a refrigeration system,and outlet works.Figure 1 shows a general view of the project.

The reservoiris of mountain-valley type in topography.It has a full storage capacity of 34.1 million m3,and its active capacity is 27.72 million m3.

The damis a rockfill dam with a central silty clay core and a frozen membrane.The crest is 650 m long and,after adding material on the upstream and downstream faces,is 16 to 76 m wide.The maximum embankment height is 23.2 m.The upstream and downstream slopes are 3H:1V and 2H:1V respectively.The dam slopes are protected by a layer of dolerite riprap.The frozen impervious barrier is maintained by two refrigeration systems:a cold air refrigerant system consisting of 431 freeze pipes and a liquid refrigerant system of 162 freeze pipes.The maximum embedment depth is 37 m for cold air pipes and 55 m for liquid refrigerant pipes.

Two 500-mm-diameter steel pipes for seepage collection cross the dam at Sta.2+80 (temperature borehole #52).A siphon steel pipe 500 mm in diameter is located here to provide the release of water from the reservoir to the downstream area.Figure 2 shows the typical cross section of the dam.

Figure 1 The Sytykan River Project,view from upstream,2006 (The Declaration of Safety of the Sytykan River Project,2006)

Figure 2 The Sytykan dam,typical cross section

The dam rehabilitation plan provides for the creation of a combined grouting and freezing system,which will be completed in several phases.

The side channel spillwayis cut through bedrock in the right abutment of the dam and is used for channel diversion and emergency water release.The spillway is an open reinforced concrete trough with a trapezoidal cross section,changing to rectangular at the end.With the size of 10.0m×5.0m,length of 500 m and slope of 0.04,the spillway has a capacity of 293 m3/s.At its head portion,the spillway has a 3-m-high gallery with a 3-m-deep concrete cutoff.A cold air refrigerant system was installed from the gallery embedded to a depth of 7 m.Later,an additional refrigeration system was installed before the spillway crest,which consisted of 30 vertical freeze pipes embedded to a depth of 30 to 35 m.

The outlet works located on the right bank of the Sytykan River 500 m downstream of the dam provide a municipal and industrial water supply to the town of Udachny.The outlet works consist of a pumping station,two steel pipelines 802 mm to 273 mm in diameter,each 7,408 m long,and a 6-m-diameter well in the center of the pumping plant partitioned into two sections.The sections are connected by an overflow pipe 800 mm in diameter during normal operation.

4.2 Brief characterization of the performance of the Sytykan River Project facilities

Figure 3 shows the location of temperature boreholes (red line).Figure 4 shows the combined geological and geothermal profile along the Sytykan dam.During the first thirteen years of operation (1976 to 1989),no serious stability problems with any structures were experienced on the Sytykan River Project.The first signs of ground warming in the dam embankment,foundation and abutments,as well as along the spillway channel were detected in 1990.This warming caused by refrigeration system failures in 1989 triggered the development of seepage,icings,thermal suffosion,frost cracking and sinking,and slope slides.

Between 1995 and 1998 the thaw zone significantly increased both horizontally,extending 250 m into the left abutment,and vertically,deepening from 17.0 to 35.0 m.Seepage problems developed in and below the dam,especially near its right abutment,creating a serious risk of breach.A contingency plan was developed to prevent dam failure.

In early December 1999,Aqua-Eco Co.divers inspected the reservoir between the dam and the intake structure.To reduce leakage from the reservoir,Aqua-Eco Co.placed geomembrane liners covering a total area of 9,700 m2near the intake and approach channel area.During the period from 2003 to 2006,a project was undertaken,designed by Geostroyproekt Co.,to create a frozen grouting curtain in the right dam abutment and the right reservoir slope.As a result of these measures,the upward expansion of the talik into the dam embankment was arrested,but the talik continued to move to the right bedrock bank as seen in figure 4.In addition,a clay blanket was laid on the upstream face to reduce the seepage.Rockfill consisting of dolerite was added on the upstream and downstream slopes which enhanced the freezing of the dam slopes and foundation,thereby improving the structural stability of the dam.Surveys of the slopes,berms,crest and abutments showed no movement or sliding.No seepage exits were observed on the downstream slope of the dam embankment.However,seepage below the spillway continued,resulting in no reduction in total seepage discharge.The foundation below the right dam abutment thawed to a depth of 40.0 m (to an elevation of 282.00 m),i.e.,21 m lower than the minimum reservoir-bottom elevation.Permafrost degradation was also observed beneath the spillway channel.New seepage areas appeared on the right bank about 1 km downstream of the damsite.

Other structures and systems on the Sytykan River Project also suffered problems,requiring repair and modification.For example,the freezing system which is the key component in seepage and stability control was reconstructed several times.For better freezing of the ground,cold air and liquid refrigeration systems were used in a year-round operation mode.At present,the refrigeration systems,as well as the instrumentation system,are operating satisfactorily.

The spillway channel allows the safe passage of 100-year flood flow,but the concrete wall of the chute requires repair.Significant efforts are being undertaken to control seepage beneath the spillway.

The outlet works with the pumping station,as well as the pressure water pipelines are in a good condition and are able to fully perform their water-supply functions.

A special monitoring program was developed for permafrost control at the Sytykan River Project.The analysis of temperature observations indicates that the thermal regime is very complicated and strongly variable.In some parts of the Project area the ground temperatures hold steadily below 0 °C preventing seepage through the dam and its foundation,while in others ground warming continues,calling for sophisticated seepage control measures.

Figure 3 The Sytykan dam,location of temperature boreholes

Figure 4 The Sytykan dam,the longitudinal profile showing geology and temperature distribution

4.3 Safety declaration

As shown above,the Sytykan River Project has had a recurrent history of seepage over its 38-year operation period.The operator has to take costly remedial measures to maintain the project’s function of supplying municipal and industrial water to the town.The declaration of safety is intended to promote effective monitoring of the performance of all critical components of the project.Six declarations have been developed for the Sytykan River Project and the seventh is now under development.The declaration contains all the information on the condition of the dam,reservoir and appurtenant structures and details the measures to ensure their safe performance.

The first step in developing a safety declaration is identifying the subjects of monitoring and the diagnostic parameters.

Based on the analysis of potential risks and operation conditions,the following subjects of monitoring have been identified for the monitoring program in accordance with the guidelines (Gosgortechnadzor of Russia,2001;Unified Energy System of Russia,2001):(a) structures and systems—the dam,including all its elements (foundation,embankment,core,frozen barrier,refrigeration system,instrumentation system);the reservoir;the side channel spillway;the siphon spillways;the pumping stations;and the water pipelines;(b) the environment;and (c) project documentation.

The main measures used in evaluating the project safety are the safety criteria.The performance parameters to be monitored include qualitative and quantitative parameters for the dam,spillway,and reservoir,and qualitative parameters for the spillway,pumping stations,outlet works,instrumentation system,environment,project documentation,and operations and maintenance and monitoring programs.

The qualitative parameters are determined by visual inspections and expert assessment,while the quantitative parameters are monitored instrumentally or calculated from measurements.The quantitative diagnostic parameters provide the best indicators of the condition of the hydraulic structure and its impact on the environment.Omitting the calculation procedures which are performed in accordance with the current regulations and guidelines (Gosgortechnadzor of Russia,1998,2001,2002a,b;Unified Energy System of Russia,2004,2006),we will list the parameters and their criterial and actual values for the structures and systems of the Sytykan River Project.

1) List of quantitative and qualitative performance parameters

Qualitative parameters:reservoir water level;normal freeboard;temperatures in the dam embankment and foundation;dam geometry;settlements in the dam embankment and foundation;seepage line in the unfrozen dam portion;seepage rate through the dam embankment;changes in total seepage through the dam embankment,foundation and abutment;changes in drainage discharge from the pumping stations;seepage gradient in the core of the dam unfrozen zone;spillway channel geometry;spillway crest elevation;liquid level in the liquid refrigerant system.

Quantitative parameters:presence of established seepage pathways (seepage exits on the downstream face or in the tailwater);signs of seepage appearing as wet areas,icings in winter,puddles,muds,springs,boils,or streams;settlement or heaving on the dam crest;localized slides (slope failures);caverns and sinkholes in the embankment and foundation;cracking along the boundaries between the soil/bedrock zones with different mechanical and hydraulic properties or between the embankment and concrete structures;internal erosion and seepage along the bottom of the concrete portion of the spillway channel;piping and formation of deposition cones;scours,erosion gullies and runnels;cracking in the embankment(transverse or longitudinal,surficial or deep-seated frost cracks);squeezing of the soil on the dam face or foot;solifluction of the downstream face,abutments and banks;icings and frost blisters on the downstream face and adjacent area;reservoir ice conditions;ice formation in the voids in the downstream shell;frost-related slope processes,thermokarst,frost heaving and frost weathering in the dam embankment,reservoir bed,and downstream area;settlement and deformation along the water pipelines;condition of the wells;condition and performance of the pumping stations;condition of the side spillway,accumulation of snow,ice,sediment or debris obstructing flood flows;condition of the siphon spillways;condition of the geomembrane;performance of the cold air and liquid refrigeration systems.

A summary description of the compliance of the Sytykan River Project with the safety criteria as of 2005 is given below (The Declaration of Safety of the Sytykan River Project,2006).

2) Adequacy of quantitative parameters

Dam:

·crest elevation—adequate (criterial value ? 321.00 m,actual value ? 321.50 m);

·crest width—adequate (criterial ≥16 m,actual 16 to 76 m);

·physico-mechanical characteristics of the dam embankment and foundation—adequate;

·normal freeboard—adequate (criterial ≥1 m,actual 1.29 m);

·deformations—adequate (decreasing);

·ground temperature in the dam embankment and foundation—adequate in the embankment and inadequate in the right abutment (criterial ≥-2 °C,actual-7 to-14 °C in the embankment and 2 to 8 °C in the right abutment);

·seepage line position in the thawed zone of the dam—adequate (no seeps on the downstream face);

·turbidity of seepage water—adequate (same as in reservoir water);

·seepage rate through the unfrozen embankment and foundation zone—inadequate (actual 7×103–8×103m3/h).

Reservoir:

·normal water level—inadequate (criterial ?371.71 m,actual ? 317.60 m);

·dead storage level—adequate (? 311.00 m);

·water quality—adequate (complies with the drinking water standard).

Spillway channel:

·bottom width,depth,and crest elevation—adequate(10 m,5 m,and ? 317.71 m).Liquid refrigeration systems:

·kerosene level in the collector—adequate (20 cm below the expander lid).

3) Adequacy of qualitative parameters

·documentation—compliant with the regulations;

·operations and maintenance program and monitoring program—compliant with the regulations;

·performance condition of the Project components—not compliant with the regulations;

·surveillance—compliant with the regulations and the monitoring plan;

·location and elevation of instrumentation—deficient(6 surface plates not installed);

·cold air refrigeration system—compliant with the Temporary Operations and Maintenance Manual for the Sytykan River Project.

5 Conclusion

The declaration of safety has shown that the components of the Sytykan River Project are mostly in compliance with the safety criteria.The safety of the structures is largely assured by measures taken to freeze the right abutment of the dam and its foundation,the spillway foundation,and the right shoreline of the reservoir,as well as to reduce seepage from the reservoir.The structures of the project are performing in accordance with the design,except for the temperature and seepage parameters of the right abutment,spillway,and right reservoir shoreline.The dam and its foundation are in a thawed-frozen state.The measures undertaken so far to reduce seepage through the embankment,in the foundation and below the spillway channel have allowed the Sytykan River Project to supply the essential volume of water to the town of Udachny and mining facilities,however,it is still not capable of delivering a full design water supply.

Continual safety monitoring is implemented at the project.The maintenance and surveillance mostly conform to the requirements set in the PB 03-438-02,Safety Rules for Hydraulic Structures of Industrial Waste Liquid Storages (Gosgortechnadzor of Russia,2002a).The owner consistently seeks ways to improve the stability of the dam and reduce seepage losses from the reservoir by employing specialized research institutions.

Currently,the main task of the operational personnel is to strictly adhere to the recommendations presented in the safety declaration,namely to prevent talik development at the right abutment and along the spillway,as well as to maintain the frozen partial cutoff beneath the spillway crest.

It has to be admitted,however,that the seepage problem has not been satisfactorily resolved after 38 years of the dam operation.Seepage pathways are continuing to readjust in the right-bank slope.Recent geophysical investigations have detected new seepage areas along the right slope about 1 km downstream from the dam.In addition to the main talik,another large zone of increased flow has been detected 700 m from the reservoir shoreline east of the main talik(Shesternevet al.,2012).

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