Meeting

1. TURGA PUMPED STORAGE PROJECT
1000 MW, WEST BENGAL
Meeting on Fixation of Lower Dam
Complex Parameters
Day 2
Date: 16th May 2023

2. 1. Lower Dam Design Parameters
a) Area Capacity
b) Freeboard Calculations based on shifting of dam axis
c) Different alternates for spillway arrangement
d) Recommended spillway type
Agenda of Mission

3. DPR Salient Features
S. No. Particulars Values
Lower Reservoir
1. Maximum Water Level (MWL) EL 318.53 m
2. Full Reservoir Level (FRL) EL 316.50 m
3.
Maximum Drawdown Level
(MDDL)
EL 280.40 m
Pondage at FRL 18 MCM
4. Pondage at MDDL (444.40m) 3.8 MCM
Lower Dam
5. Type of Dam Concrete Gravity
6. Top of Dam EL 320 m
7. Length of Dam at top 872 m
8. Maximum Height of Dam 64 m
9. Top width of Dam 10.00 m
S. No. Particulars Values
Lower Dam Spillway Arrangement
10 Type Overflow Ogee Type
11 Crest Elevation EL 316.50 m at FRL
12 Design Flood 428 m3/s
13 No. of Bays 5 bays, 15m wide each
14 No. of Piers 4 Piers, 3m wide each
15 Waterway 87m

4. Comparison of Design Parameters for Lower Dam
S. No. Description DPR Stage Post DPR Stage Remarks
1.
Area Capacity Curve
i. Gross Storage
ii. Live Storage
18 MCM
14.2 MCM
18.55 MCM
14.56 MCM
Due to shifting of lower dam axis 30m
upstream and updated topographical
survey, area-capacity of lower dam
has been updated.
2.
Spillway
5 bays of 15m
width was
proposed to pass
the Design flood of
428 m3/s
9 bays of 7.75m width to
pass the Design flood of
428 m3/s
The width of bay has been modified
from 15m to 7.5m because the
construction of 15m solid slab shall be
difficult.

5. Post DPR Studies
• In DPR Stage, topographical survey carried out on 2m contour interval.
• In post DPR stage, detailed survey work taken up and topographical map on
1m contour interval developed.
• The investigations works for computing shear design parameters were also
carried out at project site. The details are provided in Geological Assessment
Report.
• Location for Lower Dam axis modified due to the recent development at the site
in form of strong opposition of the locals due to religious sentiments attached to
the following.
a) Shri Ram Mandir
b) Sacred Banyan Tree
• During the 8th mission, it was decided to shift the lower dam axis 30m upstream
in order to avoid the above mentioned sacred structures.

6. Shifted Dam Axis of Lower Dam

7. Area Capacity
S. No. Parameter DPR Stage Post DPR Stage
1. FRL 316.50m 316.50m
2. MDDL 280.40m 280.40m
3. Gross Storage (at FRL) 18 MCM 18.55 MCM
4. Dead Storage (at MDDL) 3.8 MCM 3.99 MCM
5. Live Storage 14.2 MCM 14.56 MCM
Area Capacity Curve (DPR Stage) Area Capacity Curve (Post DPR Stage)
250
260
270
280
290
300
310
320
330
0.00
10.00
20.00
30.00
40.00
50.00
60.00
250
260
270
280
290
300
310
320
330
0.00 5.00 10.00 15.00 20.00 25.00
Contour
Value
in
(m)
Area in (Ha)
Contour
Value
in
(m)
Capacity in (MCM)
AREA_CAPACITY CURVE

8. Dam Design Parameters
 FRL
FRL has been kept same as in the DPR i.e. EL. 316.50 m
 MWL
MWL has been kept same as in the DPR i.e. EL. 318.53 m

9. Dam Design Parameters
 Height of Dam
• The geotechnical investigation test to calculate shear design parameters conducted at site in Post
DPR stage.
• Due to the results of the above test and revised topographical survey work, the rockline has revised
resulting in the lowering of the Lower Dam foundation level from EL. 256m in DPR to EL.246m in
Post DPR Stage.
• Height of the Lower Dam increased by 10m.
• Maximum height of dam in DPR 64m.
• Maximum height of dam in Post DPR stage 74m.

10. Dam Design Parameters
 Freeboard
• Freeboard for Lower Dam revised due to shifting of dam axis 30m upstream.
• Calculated free board is 1.463 m and 0.474 m above FRL and MWL for lower dam.
• As per IS 10635:2014, minimum freeboard at MWL shall be 1.0m in case of concrete dam.
• Therefore, top of dam calculated as per freeboard at FRL and MWL for Lower Concrete Dam is EL.
317.963m and EL. 319.53m respectively.
• The Top of Lower Dam has been fixed as EL. 320.00m.

11. Freeboard Computation at FRL for Lower Dam

12. Freeboard Computation at MWL for Lower Dam

13. Submergence Plan at FRL for Lower Dam

14. Submergence Plan at MWL for Lower Dam

15. Dam Design Parameters
 Crest Length
• Crest length revised due to change in contours
• Crest length for the Lower Dam - 872m (DPR stage)
• Crest length for the Lower Dam - 897m (Post DPR stage)

16. Lower Dam
Spillway

17. Comparison for Spillway Arrangement
S. No. Particulars DPR Stage Detailed Design Stage
1. Type of spillway Overflow Ogee Type Overflow Ogee Type
2. Crest Elevation EL 316.50 m at FRL EL 316.50 m at FRL
3. Design Flood 428 m3/s 428 m3/s
4. No. of Bays 5 bays, 15m wide each 9 bays, 7.75m wide each
5. No. of Piers 4 Piers, 3m wide each 8 Piers, 1.5m wide each
6. Waterway 87m 81.75m
7.
Type of Energy
Dissipator
Trajectory Type Bucket Stilling Basin

18. Detailed Design Studies
 The spillway arrangement for the Lower Dam has been restudied to avoid the Sacred Banyan Tree
at the project site and to check the effect in downstream portion of the Lower Dam in case of flood.
 During this phase, three (3) different types of changes have been proposed in the spillway
arrangement.
i. Change in size
ii. Change in type
iii. Change in location

19. Detailed Design Studies
i. Change in size
Under the detailed design phase, various types of spillways have been studied to ascertain the best suited spillway arrangement for the
Lower Dam due to the unavoidable circumstances at the project site. The calculations for the different alternatives studied are provided
in the table below.
Description DPR Stage Current Stage
(Ungated Spillway)
Current Stage
(Ungated Spillway)
Current Stage
(Gated Spillway
without MWL)
Current Stage
(Gated Spillway
with MWL)
Current Stage
(Gated Spillway
with MWL)
Unit
PMF 428 428 428 428 428 428 cumec
MWL/FRL 318.53 318.53 318.53 316.5 318.53 318.53 m
Crest
Elevation/FRL
316.5 316.5 316.5 312.5 314.5 312.73 m
Av. River bed level
for approach
velocity
280.0 280.0 280.0 280 280 m
No. of bay 5 4 9 2 2 2 No
Width of Bay 15 17 7.7 15 15 6 m
Discharging
Capacity of
Overflow Spillway
430.56 431.02 453.65 517.10 526.05 438 cumec
Size of gate Not Required 4.3m X 15.3m 4.33m X 15.3m 6.1m X 6.3m

20. Detailed Design Studies
i. Change in size
• 4 options are feasible i.e. the DPR option, ungated spillway option with 17m width, ungated spillway option with
7.7m width and gated spillway with MWL option with gate size 6.1m X 6.3m.
• Occurrence of flood is less in the project area as precedent from the Purulia Pumped Storage Project, provision of
gates is not an attractive option for the spillway arrangement.
• Periodic maintenance of the gates maybe an issue in the operation. Therefore, ungated spillway arrangement are
being preferred.
• Ungated spillway options with block width 7.7m preferred over the ungated spillway option with block width 17m
because the thickness of the deck slab and girder will be less in 7.7m spillway option and the need for prestressing
will be eliminated.
• Total width of the 7.7m spillway option is 81m i.e. 9 bays of 7.7m and 9 piers of 1.3m each.
• The width of this spillway option is less as compared to the width of the spillway option in DPR i.e. 87m.

21. Detailed Design Studies
ii. Change in type of spillway
• JPOWER suggested in its “Review Report of DPR, Relevant Part of Upper Reservoir, Lower Reservoir,
Spillways and Ancillary Structures (Part II)” to provide Stilling Basin in place of DPR proposed
Trajectory bucket type energy dissipation arrangement for the Lower Dam.
• In the Trajectory bucket type energy dissipation arrangement, the excavation process being carried out
twice i.e. once due the dam cutting and again because of the plunge pool arrangement.
• The advantage of providing a stilling basin type energy dissipation arrangement is that the excavation
of the stilling basin will be adjusted in the dam excavation and there will not arise a need to carry out
excavation twice.
• Therefore, the LC is of the same view as that of the FC that stilling basin type energy dissipation
arrangement shall be provided for the Lower Dam

22. Detailed Design Studies
iii. Change in location of spillway
• To safeguard Sacred Banyan Tree, the ungated spillway option with 7.7m bay width is recommended.
• The clear distance of the DPR spillway arrangement from the Sacred Banyan Tree is 10m which is very less because the roots of the
Banyan Tree might have a longer area of spread and this location may cause harm to the roots of the Tree. Therefore, it was decided
during the 8th mission to shift 1 block of the spillway portion towards the right bank.
• LC worked out 2 options for the shifting of the spillway arrangement.
 Option 1:
• The entire stilling basin with total width 81m to be shifted towards the right bank at the same level. The clear distance from the
diaphragm wall provided after the cutting of stilling basin is coming out to be 10.9m.
 Option 2:
• The other option is that the stilling basin may be constructed at 2 levels I order to optimize the cutting and filling quantities. The clear
distance from the diaphragm wall provided after the cutting of stilling basin is coming out to be 10.9m.

25. Cost Comparison for Stilling Basin Options
S. No. Options Classification
Extra Rock
Excavation for Stilling
Basin (in m3)
Concreting
(M25) (in m3)
Cost (in Cr.)
1 Option - 1 Previously Proposed 11060 6900 3.74
2 Option - 2
Stilling Basin spliting in
two parts (6 & 3 bays)
10360 6950 3.73
3 Option - 3
Stilling Basin in single
part (9 bays)
11515 6900 3.76

26. Conclusion
• Shifting of the stilling basin arrangement is inevitable to prevent the Sacred Banyan Tree from any
damage.
• The cost of the excavation and filling for the above mentioned options has been worked out and
found to be comparable.
• The most suited option among the above 2 options may be selected during the 11th mission based
on the merits and demerits of the options.