Annex 5 - Hydropower Model Vakhsh The Vakhsh … 5.pdfAnnex 5 - Hydropower Model – Vakhsh 1 ... Nurek 3000 Baipaza 600 Sangtuda-1 670 ... If sediment builds up against the dam the

Annex 5 - Hydropower Model – Vakhsh

1 annex 5 - hydropower vakhsh_13oct11.docx - 6/8/2012


1. The Vakhsh Cascade

The construction of dams on the Vakhsh River started in the late 1950s with the construction of the

Perepadnaya diversion and power station. Until 2011 seven power stations have since been

constructed, with the main purpose of generating hydropower. Some of the dams also have

secondary purposes, such as feeding irrigation schemes.

Power station Installed Capacity (MW)

Nurek 3000

Baipaza 600

Sangtuda-1 670

Sangtuda-2 (Under Construction) 220

Golovnaya 210

Perepadnaya 29.6

Central 18.0

Rogun (Feasibility Study Ongoing) 3600

Shurob (Planned) 600

Table 1: Power Stations on the Vakhsh

Further extensions of the cascade are planned. Construction work on the Rogun dam and power

station had already started in the late 1970’s and came to a halt with the end of the Soviet Union.

Recently another feasibility study for the completion of the scheme has been commissioned.

Further possibilities to expand the scheme into the Vakhsh’s main tributaries Surhob and Obihingou,

as can be seen in Figure 1.

The power stations assessed in this report include Nurek, Baipaza, Sangtuda-1, Sangtuda-2 and

Golovnaya. Not included in the assessment are the Perepadnaya and Central Power stations, which

are on relatively small diversion weirs feeding irrigation canals and the Rogun HPP, for which no

information could be obtained.

Presently only the Nurek Reservoir provides seasonal storage for the Vakhsh cascade. The

downstream power stations either operate as run-of-river plants or provide daily regulation only.



Figure 1: Vakhsh Cascade Schematic Diagram



2. Available Data

2.1. Power station data The power station data shown in the data sheets below was obtained from public sources.

Data Sheet Nurek

Installed Capacity MW 3000

Head Water Levels

Minimum Operating Level MOL m a s l 857.00

Full Supply Level FSL m a s l 910.00

Maximum Flood Level m a s l 917.00

Dam Crest Elevation m a s l 922.00

Flood Design Criteria

Extreme Design Flood (1:10000 years) Q0.01% m3/s 5400

Spillway Capacity m3/s 4040

Max Powerhouse discharge m3/s 1360

Total maximum discharge capacity m3/s 5400

Turbine Data

Year of Commissioning 1972

Turbine Type Vertical Francis

Number of Units 9.00

Rated Head Hr m 230.00

Max Head Hmax m 267.60

Min Head Hmin m 209.75

Rated Power Pr MW 341.0

Rated Discharge Qr m3/s 158.0

Rated Efficiency % 93.92%

Turbine Speed n RPM 200.00

Runner Diameter D3 m 4.75

Assumed water to Wire Efficiency % 87.22%

Generator Data

Generator Efficiency % 97.75%

Generator Rating MVA 390.00

Power Factor 0.85

Generator Power Limit MW 333.33

Frequency Hz 50.00

Generator Voltage kV 15.8

Spillway Data

Spillway Type

No of Bays

Bay Width m

Sill Elevation m a s l



TWL Rating Curve

Q (m3/s) TWL (m)

150 642.4

450 644.2

1400 647.25

5400 649.8

Mean TWL 650 644.842

Data Sheet Baipaza


Head Water Levels

Minimum Operating Level MOL m a s l


Maximum Flood Level m a s l 631.50






Diversion Tunnel m3/s 1164


Turbine Data

Year of Commissioning




Max Head Hmax m 57.50

Min Head Hmin m






Assumed water to Wire Efficiency % 86.00%

Generator Data

Generator Efficiency %


Power Factor 85.00%


Frequency Hz 50.00




TWL Rating Curve

Q (m3/s)

TWL (m)

309 570.4

1236 573

4470 579.5

5400 581.2

Storage Capacity

km3

Reservoir Volume 0.1246

Live Volume 0.087

Data Sheet Sangtuda-1


Head Water Levels

Minimum Operating Level MOL m a s l 569.90


Maximum Flood Level m a s l







Turbine Data

Year of Commissioning 1989




Max Head Hmax m

Min Head Hmin m






Assumed Water to Wire Efficiency % 88.86%

Generator Data



Power Factor 90.00%


Frequency Hz 50.00




TWL Rating Curve

Q (m3/s)

TWL (m)

2700 510.8

5400 512.26

Storage Capacity

km3


Live Volume 0.012

Data Sheet Sangtuda-2


Head Water Levels






Extreme Design Flood (1:10000 years) Q0.01% m3/s

Spillway Capacity m3/s

Max Powerhouse discharge m3/s

Total maximum discharge capacity m3/s

Turbine Data


Turbine Type Vert Kaplan



Max Head Hmax m

Min Head Hmin m




Turbine Speed n RPM

Runner Diameter D3 m


Generator Data


Generator Rating MVA

Power Factor

Generator Power Limit MW

Frequency Hz



Generator Voltage kV

TWL Rating Curve

Q (m3/s)

TWL (m)

3380 490.4

Storage Capacity

km3

Reservoir Volume

Live Volume

Data Sheet Golovnaya


Head Water Levels




Dam Crest Elevation m a s l


Extreme Design Flood (1:10000 years) Q0.01% m3/s

Spillway Capacity m3/s

Max Powerhouse discharge m3/s

Total maximum discharge capacity m3/s

Turbine Data


Turbine Type Kaplan



Max Head Hmax m

Min Head Hmin m 15.00




Turbine Speed n RPM

Runner Diameter D3 m


Generator Data


Generator Rating MVA

Power Factor

Generator Power Limit MW

Frequency Hz

Generator Voltage kV



TWL Rating Curve

Q (m3/s)

TWL (m)

Storage Capacity

km3


Live Volume 0

2.2. Sedimentation Sedimentation rates were estimated from bathymetric survey data from 1989, 1994 and 2001. The

2001 survey showed that the reservoir volume increased between 1994 and 2001, which does not

appear plausible. Either the 2001 or 1994 survey results are probably flawed.

Figure 2: Nurek Storage Curves

The average annual loss of storage from 1972 to 2001 was approximately 70 Mm3/a. If

sedimentation progressed at this rate, the reservoir capacity would theoretically be reduced to

1.8 km3 by the end of the century. If the 2001 survey results are disregarded, the annual

sedimentation rate would be approximately 115 Mm3/a and thus the siltation would progress even

faster.

650

700

750

800

850

900

950

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0

Ele

vati

on

(m

a s

l)

Revervoir Volume (km3)

1972

1989

1994

2001

2100

2100 with Rogun



However, siltation is unlikely to be allowed to progress unimpeded until the end of the century as

operations and possibly dam safety would be severely affected.

If sediment builds up against the dam the operation of spillway tunnel (invert at 810 masl) and

power intake (invert at 837 masl) would be compromised. Therefore sediment control measures

need to be implemented in the medium term. Although other measures, such as sediment flushing

tunnels or sediment retention dams are conceivable, the most obvious solution to the

sedimentation problem would be the construction of the Rogun Dam, which would prevent bed load

and most sediment from reaching Nurek.

For the purpose of this study it is therefore assumed that sediment control measures will be

implemented by 2020.

2.3. Evaporation An evaporation estimate was obtained from BT’s water balance for the Nurek reservoir. It was noted

that the factors are identical as those used in BT’s water balance for Kairakkum:

month 10-6

m/s mm/d

Jan 0.0037 0.3197

Feb 0.0066 0.5702

Mar 0.0157 1.3565

Apr 0.0314 2.7130

May 0.0513 4.4323

Jun 0.0736 6.3590

Jul 0.0813 7.0243

Aug 0.0782 6.7565

Sep 0.0554 4.7866

Oct 0.0283 2.4451

Nov 0.0143 1.2355

Dec 0.0066 0.5702

Table 2: Evaporation Data

The annual evaporation is roughly 1180 mm. The evaporation over the surface area of the lake

(approx 90 km2) is therefore in the order of 0.1 km3 or approximately 0.5 % of the average annual

inflow. The model is therefore not sensitive to changes in reservoir surface evaporation.

Nevertheless the 2080 predictions for Layhsh (see Annex 2) have been used for the model.

2.4. Historical Flow, Level and Energy Data Historical data was not provided, except for the 1994 water balance. Therefore the operating rules

are only based on one year of observed values. Further the model could not be calibrated (see par. 4

below)



3. Model Description

Only the Nurek Reservoir provides significant storage capacity for the Vakhsh Cascade.

A sequential stream flow model was selected as the appropriate methodology to estimate the effect

of changes in environmental parameters to energy production. The model is essentially a reservoir

water balance model, in which inflow, outflows and losses are accounted for.

The general form of the water balance algorithm governing the operation can be expressed as:

Where: V2, V1: Storage volume at end and beginning of routing interval dt: Routing interval I: Inflow QP: Power station discharge QL: Leakage and other water requirements QS: Spill E: Net evaporation losses

During each time step, the reservoir level, surface area, average power and generated energy are

calculated. The routing interval dt was chosen to be 1 month.

Several assumptions and relations between the various parameters are introduced for the numerical

model:

1) It is assumed that leakage through the dam and spillway amounts to 2 m3/s. Abstractions for

irrigation were adopted from BT’s 1994 water balance:

month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

m3/s 3.5 3.5 3.5 3.5 6.2 10 10 10 10 3.5 3.5 4.6

Table 3: Irrigation Abstractions Nurek

2) Reservoir volume and level are related through the storage curve (Figure 2). Volume is

calculated as function of level and year so that the model takes gradual loss of storage through

siltation into account.

3) Reservoir surface area is calculated from the original design data.

4) Evaporative losses from the reservoir are estimated by multiplying the reservoir surface area

and the evaporation factor. The monthly evaporation factor (mm/day) was derived from BT’s

1994 water balance. It is assumed that this factor includes rainfall gain on the reservoir surface.

5) Maximum turbine discharge was estimated from the data presented in par. 2.1.

6) Net head is calculated as the difference between reservoir level and tailwater level. Friction loss

in the power tunnels is assumed to be in the order of 2 m.

7) Power station output is calculated from net head, turbine discharge, number of units available,

and an estimated water-to-wire efficiency.

8) The tailwater level is assumed to be approximately 645 masl.



9) Water discharged from Nurek (i.e. the sum of turbine and spillway discharges and leakage) is

assumed to pass through the downstream power stations in the same time step, i.e. live storage

of all downstream power stations is neglected. As no further information was available it had to

be assumed that lateral inflows and irrigation discharges and evaporation losses between Nurek

and Golovnaya are roughly equal.

10) For each power station, maximum turbine discharge, net head and efficiencies are estimated

from the data presented in par. 2.1.

The model as presented above is governed by a set of rules which can be modified to reflect current

and future reservoir operation. For this project the following rules are used:

1) Reservoir operation follows a rule curve, i.e. it is assumed that turbine operation aims at

controlling the reservoir level to a pre-set curve throughout the year. The level in 1994 is

assumed to be representative of the reservoir rule curve.

2) If the reservoir level exceeds the full supply level (FSL) the volume in excess of FSL is spilled and

the next time step of the simulation commences with the reservoir at FSL.

As the equations used in the model are interrelated and follow non-linear relationships. Iterations

are performed for each time step until the solution converges. As a check, the mass balance for the

reservoir is calculated for each time step.

4. Data Analysis and Model Calibration

As no historical level and energy generation data could be obtained, the model could not be

calibrated. Instead the base case flow series was fed into the un-calibrated model and the average

annual energy calculated. A comparison can then be made to published production figures (USSR

Energomachexport, 1990; SNC-Lavalin International Inc., 2010; ADB, 2008).

Nurek Baipaza Sangtuda-1 Sangtuda-2 Golovnaya

Model 10917 2529 2642 1015 992

Energomachexport (1990) 14900 3000 2700 1000 1100

SNC – Lavalin (2010) * 11850 2525 2970 840

ADB (2008) 11200

Table 4: Vakhsh Cascade - Annual Generation (GWh/a)

* The SNCL report assumes that Nurek is upgraded to 3200 MW.

The energy data estimated by the hydropower model falls short of the original design data but

matches the recently reported numbers reasonably well.



5. Climate Change/Hydrology Scenario Combinations

5.1. General Assumptions The following assumptions were made for the forecast scenarios:

- As outlined above, the model is not sensitive to changes in evaporation. Increase of

evaporation through the reservoir surface is therefore neglected.

- Sedimentation progresses at 70 Mm3/a until 2020, where after sedimentation control

measures are assumed to be effective.

- Future upstream dam construction is not taken into account.

- The monthly flow distribution follows the normalized 1977-1987 pattern (Figure 3):

Figure 3: Nurek - Assumed Inflow Distribution

The assumption about future dam construction is probably the most critical one as the Rogun dam is

not considered in the following scenarios. In order to assess the influence of Rogun on the energy

production of the existing power stations of the Vakhsh cascade, access to the currently ongoing

feasibility study would be required.



5.2. Scenarios

5.2.1 Baseline

The baseline scenario assumes a long-term average inflow of 607 m3/s.

Figure 4: Base Case - Inflow and Energy

Energy calculated for the cascade (Nurek, Baipaza, Sangtuda-1, Sangtuda-2 and Golovnaya) is

approximately 18000 GWh/a.

Firm capacity was calculated as shown in Figure 5 below. As firm capacity is calculated as the

minimum power over a 30 year interval the calculations are very sensitive to operating rules and

forecast monthly and annual flow factors. While In reality a different set of operating rules is likely to

be applied for exceptionally dry years, this is not considered in the model. It is therefore likely that

the model significantly underestimates the firm capacity. Nevertheless, the calculated trends are still

useful to compare the effects of different climate change scenarios.



Figure 5: Firm Capacity



5.2.2 Climate Change/Hydrology Scenario Combinations

Figure 6: WBM Hot- Dry

Figure 7: WGM Hot-Dry



Figure 8: WBM Central

Figure 9: WBM Warm Wet



Figure 10: REG Hot-Dry

Figure 11: REG Central



Figure 12: REG Warm-Wet

Figure 13: SRM Hot Dry



Figure 14: SRM Central

Figure 15: SRM Warm Wet



Figure 16: Climate Change / Hydrology Scenario Combinations – Energy Trend Summary

Figure 17: Climate Change / Hydrology Scenario Combinations - Firm Capacity Trend



6. Optimization

6.1. Effects of Rogun on the Vakhsh Cascade As demonstrated above, the main environmental factors affecting energy production will be

sedimentation and potentially reduced inflows.

The construction of the Rogun Scheme and/or other large dams will alleviate the sedimentation for

Nurek.

Further monthly peak flows would be attenuated by the Rogun reservoir, which in turn would

reduce spillage at Nurek and the other downstream power stations. As no information about the

planned operation of Rogun could be obtained, it was assumed that the monthly flow pattern will be

damped as shown in Figure 18.

As Rogun would do most of the annual regulation, operating rules for Nurek would be changed so

that the Nurek Reservoir would not be drawn down as it seems to be the case presently.

Figure 18: Assumed Nurek Inflow after Construction of Rogun



Figure 19: Baseline Inflow, Rogun & Nurek Operation at FSL

With these measures an increase in scheme output of approximately 5% appears feasible. This

conclusion would be valid for the near-baseline scenarios (all REG scenarios and WBM central).

Installing additional units or increasing installed capacity would not significantly increase energy

productions for the base case or any of the near baseline scenarios. Other benefits of increasing

capacity may still make up-rating worthwhile, if done as part of a major refurbishment.

Figure 20: Baseline scenarios



6.2. Other optimization opportunities Apart from the construction of the Rogun Dam, other measures could be considered to increase

energy production and firm capacity.

Installing additional generating capacity could be attractive for scenarios which predict a significant

increase in discharge. Further optimization would be possible by considering the effect of the

construction of the Rogun Dam and other dams upstream.

Figure 21: SRM Central - Rogun and Uprating

Figure 22: Effects of Climate Change/Hydrology Scenario Combinations, Rogun and Upgrading Nurek on Firm Capacity



Firm capacity would significantly benefit from the construction of the Rogun dam. Increasing

installed capacity would have no effect on firm capacity

Other measures could include changing the operating rules; however the current operating rules

would have to be made available for a further assessment.

6.3. Summary As most of the requested data was not provided, the above is based on assumptions and un-

calibrated models. Results must therefore be viewed with caution. Nevertheless some general

statements can be made:

The construction of the Rogun dam would be beneficial for annual energy and firm capacity.

Should the Rogun Dam not be built, then other sedimentation controls need to be investigated

and implement towards mid-century.

Increasing the installed capacity may be beneficial for annual energy production if one of the

scenarios that predict increased runoff materializes.

7. Bibliography

ADB. (2008). Proposed Asian Development Fund Grant Republic of Tajikistan: Nurek 500 kV

Switchyard Reconstruction Project. ADB.

SNC-Lavalin International Inc. (2010). CENTRAL ASIA - SOUTH ASIA ELECTRICITY TRANSMISSION AND

TRADE (CASA-1000) PROJECT FEASIBILITY STUDY UPDATE.

USSR Energomachexport. (1990). Nurekskaya Hydroelectric Power Station. Moscow: Vneshtorgizdat.

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Annex 5 - Hydropower Model Vakhsh The Vakhsh … 5.pdfAnnex 5 - Hydropower Model – Vakhsh 1 ... Nurek 3000 Baipaza 600 Sangtuda-1 670 ... If sediment builds up against the dam the