Assessment and forecast of changes of reservoir volumes due to thermal settling in permafrost areas of Russia

Ilja S.Sobol,Stanislav V.Sobol

Nizhny Novgorod State University of Architecture and Civil Engineering,Nizhny Novgorod 603950,Russia

1 Introduction

According to published data,242 reservoirs out of 327 water storage basins of volume over 10×106m3are located in the European territory of Russia and 85 are in the Asian territory of Russia;of the 1,936 reservoirs of volume between 1×106and 10×106m3,371 are located in the Asian territory and more than 30 of those are in permafrost regions (Chernyaev,2001).The majority of Russian reservoirs are exploited according to designed morphometric characteristics that lose their initial relative reliability in the course of time (Gromov and Sobol,2012.).The expected changes of reservoir volume are of practical interest for the economy of Russia,the same as in other countries.

Changing of the volume of Russian reservoirs with age is a geographically diverse process.On the western and central plains,the volume of reservoirs diminishes due to sedimentation,whereas in the permafrost regions of the northeast,the volume increases due to thermal settling of the reservoir beds (sedimentation in this case is of secondary importance,for the water turbidity of northern rivers is rather low,such as the 3 mg/L at the site of the Kankunskaya hydroelectric power plant being designed on the Timpton River).

Well-directed researches of reservoir volume changes in permafrost areas are still to be conducted.Nevertheless,fragmentary data on this issue are available and they are presented and analyzed in this paper.

2 Materials and methods

Field investigations of morphometric parameters have now been completed for two water reservoirs located in the permafrost regions:the Ust-Khantaiskoe reservoir with a designed volume of 23.52×109m3,the filling of which was started in 1970,and the Anadyrskoe reservoir with a volume of 6.19 million m3,which has been exploited with the present impounded water level since 1986.

Assessment of the volume increase of the Ust-Khantaiskoe reservoir was performed by the Gidroproekt Institute in 1989–1990 to determine the reasons for its water balance discrepancy.Deformation of its bed was found on a standard reference 350-km2area by comparing a 1:25,000-scale topographic map of the bottom relief made at the beginning of filling-up with that plotted 20 years later.The latter was based on the depths measured from a boat by an echometer on profiles taken at every 500 m.Comparison of the 1970 and 1990 profiles confirmed that the reservoir bed had lowered by an average of 3 m during that period.The increase of the designed total volume of the reservoir (deformation of the shores being taken into account) was 2.81×109m3or 12.15%.However,the process of thawing and settling of loose ground with high content of ice in the reservoir foundation had stopped by 1983 (Onikienko,1995).

In 2008 the VNIIG (B.E.Vedeneev) Institute performed echosounding of the bottom relief of the Anadyrskoe reservoir in several cross sections.Then the Nizhny Novgorod State University of Architecture and Civil Engineering (NNGASU) carried out a mathematical simulation of temperature-cryogenic conditions of the reservoir foundation,taking into consideration the settling of icy Quaternary sediments for the periods of exploitation from 1986 to 2008 and up to 2030.According to the field investigations,during 22 years of exploitation the total volume of the reservoir body had enlarged from 6.19×106to 7.23×106m3(i.e.,by 16.8%),and by 2030 it will reach 7.55×106m3(i.e.,an increment that would constitute 21.9% of the designed volume) (Gnyotovet al.,2013).

In the absence of targeted field researches on other water reservoirs in the permafrost regions,their volume change over time was roughly calculated by Sobolet al.(2005).The initial data about natural conditions and the composition and properties of the valley soils were taken from publications,design documents,and reports of exploiting companies.

Thus,the water reservoir of the Evenkiyskaya hydropower plant being designed on the Nizhnyaya Tunguska River will have an impounded water level of 200.0 m BS,with total and conservation storage of 409.4×109m3and 101.0×109m3,respectively,and a surface area of 9,400 km2;the flood area will constitute 8,680 km2(Yurkevich and Tsvik,2008).Ten percent to 25% of its lower part and 60%–80% of its upper part have permafrost 50 to 300 m thick with temperatures within 0.1 °C to-3 °C.The Quaternary sediments are 15 to 18 m thick and are characterized by masses of underground ice.Estimates by the Melnikov Permafrost Institute SB RAS show that during the first 10 years after the reservoir fills up,the frozen cover of dispersed rock will thaw to the depth of 5.5 to 8.0 m,and over the next 50 years it will fill to the depth of 12 to 18 m (Kunitskyet al.,2011).Assuming that the area of thawed rock equals 50% of the flood territory,the ice content is 3%,and the average depth of thawing over 10 years is 6.75 m and over 50 years will be 15 m,the volume increase due to bed thermal settlement can reach 8.78×109m3over 10 years and 19.53×109m3over 50 years (2.14% and 4.77%,respectively,of the reservoir total storage).

Table 1 presents the results of calculations of the possible total volume changes of some large reservoirs (over 10×109m3) located in the permafrost regions;these are plotted in figure 1.Figure 2 presents the authors’ assessment of the possible total volume changes of 15 small reservoirs (under 50 million m3).These analyses indicate that during about 30 years of exploitation,the covering layer of soft soils in flood zones would completely thaw through,and the increase of total volumes would reach 12.1% in the large reservoirs (after only 22 years of exploitation of the Ust-Khantaiskoe reservoir).In the small reservoirs,the volume increase can reach 51.5% (in a reservoir under design on the Vacha River in the Irkutsk region,this would happen after 80 years).

Table 1 Assessments of the total volume increase of large reservoirs of hydroelectric power plants over time in permafrost regions in Russia

Figure 1 Possible total volume increase of large reservoirs in permafrost regions.1:Ust-Khantaiskoe;2:Kureyskoe;3:Kolymskoe;4:Viluyskoe;5:Evenkiyskoe;6:Amguemskoe

Figure 2 Possible total volume increase of small reservoirs in permafrost regions.1:On the Dolgaya River;2:Arkagalinskoe;3:Anadyrskoe;4:On the Kamenushka River;5:Irelyakhskoe;6:Kadykchanskoe;7:Pevekskoe;8:On the Oyuur-Yurege River;9:Bilibinskoe;10:Sytykanskoe;11:On the Markha River;12:On the Uesya-Liendokit River;13:On the Ebelekh River;14:On the Vacha River;15:On the Khanmey River

These assessments indicate that there are similarities in how the total storage volumes of large and small reservoirs in permafrost areas of Russia are expected to increase in the future:

The general pattern of total volume increase of the large reservoirs,depending on age,shows that the natural increase of the reservoir total storage over 20–40 years (on average) can reach 6%–8%,with subsequent stabilization;and

The same pattern exists for small reservoirs,indicating that the average increase of their total volume will be approximately 20% of their total volume.

3 Discussion and conclusions

The presented information and analyses,performed for the first time,give a quantitative idea of the features,long-term dynamics,and average parameters of possible increase of the total volumes of many large and small reservoirs in the permafrost regions of Russia.This will be of great use in their design and future exploitation.

Acknowledgment:

The research has been conducted in accordance with the state assignment of the Ministry of Education and Science of the Russian Federation on theme 7.4059.2011:"Experimental and theoretical researches of the behavior of reservoirs and power dams during the exploitation phase of life cycle," State Registration No.01201256972.

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