• Under India-Bhutan Co-operation, Govt of India allotted Sankosh HEP for updation of DPR and Bunakha HEP (180 MW) for implementation. The river Sankosh, called Mo-chu in Bhutan traverses 214 km in Bhutan and further 107km in India. The total drainage area is 12061 sqkm. The river flow in steep gradient and narrow gorges upto the Indo-Bhutan Border making it an attractive potential for hydro power potential. 
    The Project is located on Sankosh river, near village Kerabari under Kalikhola Dzongkhang (Sub Division) of Sarpang Dzongkhang (District) in Southern Bhutan. The project is approachable by road diverting from the National Highway 31-C at Baribasa in West Bengal. The nearest rail head is Jorai (near Baribasa) on broad gauge line between New Jalpaiguri and Guwahati. The nearest airport is in Bagdogra near Siliguri (India) which is about 300 Km away from the Project Site. The distance between Baribasa and Kerabari village is 35 Km. The nearest sea port is Kolkata.
    In 1995, CWC, New Delhi and RGoB in association with various other Government organizations, prepared the DPR of Sankosh Multipurpose Project (4060MW) with 265m high  Rock fill dam.
    In 2008, Govt. of India entrusted THDC with the work of updation of DPR of Sankosh Multi Purpose Project (4060 MW). Updation of DPR of Sankosh Project was completed in April’2009.
    Subsequently, Ministry of Power, GoI, in a meeting held on 26.05.2009 at New Delhi directed THDC to conduct the feasibility study for adopting concrete dam option at Sankosh project due to better sediment management through concrete dam. 
    THDC conducted intensive field investigations, laboratory studies and desktop studies in order to conclude on the concrete dam feasibility. Following studies were conducted by THDCIL.
    Ø Sedimentation Studies (submitted in October, 2010)
    Ø Feasibility Studies for adopting concrete dam at Sankosh Project (submitted in February, 2011)
    Ø Detailed Project Report for Sankosh HEP with concrete dam option (submitted in May, 2011)
    From the financial analysis projected in DPR of Sankosh HEP with 4060MW installed capacity, it was observed that cost of the project and computed power tariff comes out to be very high compared to general norms prevailing in the country, thus, making the project less attractive in terms of economic viability. In order to improve the financial viability of Sankosh HEP, it was considered to reduce the height of Sankosh. The feasibility report on Sankosh-Punatsangchhu HEP III integrated complex was prepared and submitted to GOI in November, 2011. As per this study, the height of Sankosh Dam was optimized at 220m and type of Dam was RCC Dam.
    This Feasibility Report was discussed and reviewed in CEA and CWC along with the representatives of RGoB in January, 2012. The power potential optimization has been done keeping in view of the water requirement for irrigation, minimal spillage from the dam, cost-benefit and other aspects. Project layout was also restructured. As per this discussion, the most economical height of RCC Dam was found to be 215m.
    DPR of optimized Sankosh HEP(2560MW), Bhutan with 215m high RCC Dam has been prepared and submitted to GOI in August  2012. The levelised and 1st year tarrif as per this scheme has been found to be Rs 3.89 per Kwh and Rs 4. 78 per Kwh.
  • A.                    LOCATION              
    1.         Country                                                                       Bhutan 
    2.         Dzong                                                                          Sarpang 
    3.         River                                                                            Sankosh 
    4.         Main dam                                                                    Near Kerabari  
    Longitude                                                                    89˚ 55’ 55” E  
    Latitude                                                                       26˚ 46’ 41” N
    5.         Regulating dam                                                            Near Kalikhola  
    Longitude                                                                    89˚ 51’ 42” E  
    Latitude                                                                       26˚ 44’ 09” N
    6.         Power House                                                 Two  Power House at the   toe of Main Dam (Left bank and Right bank)
    7.         Distance between Main Dam & Regulating                  13 km
    B.                     HYDROLOGY   
    1.                  Catchment Area                                                           9569 sq. km  
    2.                  Average annual rain fall                                              1619 mm 
    3.                  Average annual yield                                                   15220 M cum 
    4.                  90% dependable annual yield                                      14309 M cum 
    5.                  75% dependable annual yield                                      13803 M cum 
    6.                  Maximum observed flow                                             3939 Cumecs 
    7.                  Minimum observed flow                                             87 Cumecs 
    8.                  Probable Maximum Flood (PMF)                                 17455 Cumecs 
    9.                  Design flood for diversion                                           5520 Cumecs  
    (25 -year return period) 
    10.              Annual Sediment Load                                                0.6mm/year 
    C.                    MAIN DAM COMPLEX
    1.         Main Dam
    (i) Type                                                                        Roller-Compacted Concrete   (RCC Dam)
    (ii)River Bed Level                                                      EL135.0m
    (iii)Top elevation of Dam                                            EL350.0m
    (iv) Height   (Above river bed)                                                215.0m
    (v) Height of dam above deepest foundation level     235m
    (vi)     Height of u/s coffer dam                                   26 M
     (vii)    Height of d/s coffer dam                                  8M
                            (viii)     Length at Top of Dam                                     696.0m
                            (ix)  Upstream Slope below EL 195                             1 H: 0.625 V
                            (x) Downstream Slope                                     1.0 H: 0.9V
                            (xi) Top Width                                                             10.0m
    2.         Sluice Spillway
                (i) Design Flood                                                           12275 Cumecs
                (ii) Shape                                                                     Rectangular
                (iii) Number of sluices                                                  6
                (iv)  Size of sluices                                                      7mx8.3 m
                (v) Crest Level (elevation)                                          
                            Lower level                                                      EL 219.85 m
                            Upper level                                                       EL 228.15 m
    3.         Overflow Spill way                                                      
    (i) Design Flood                                                           6535 Cumecs
    (ii) Shape                                                                     Ogee type
                (iii) Waterway                                                              6 bays of 9.0 (W) x15.5 (H)
                (iv) Width                                                                    90 m
                (v)  Crest Level (elevation)                                         
                            Lower level                                                      EL329.50 m
                            Upper level                                                       EL345. m
    (vi) Dissipating arrangement                                        Flip bucket
    4.         Diversion Tunnel
                            (i) Number                                                                   1 
    (ii) Diameter                                                                12 m 
    (iii)  Shape                                                                   Circular
    (iv) Length                                                                   1050 m 
    (v) Inlet level                                                               137m 
    (vi) Outlet level                                                            131.75m 
    (vii) Bed slope                                                             1:200  
    (viii)  Diversion discharge                                          1600 m3/sec   
    5.         Construction sluices
    (i) Numbers                                                                 6
    (ii) Size                                                                        7mx7.7m
    (iii) Invert level                                                              EL145.0m
    6.         Reservoir
                (i) Full Reservoir Level (FRL)                                     EL 345.0 m
    (ii) Minimum draw down / Level                                 EL270.0 m
                                  Reservoir Area                                                      2321 ha
                                  Gross storage Capacity                                        1349.8 M cum 
     (iii) Full reservoir level                                                EL345.0 m
                                  Reservoir Area                                                      4626.0 ha   
                     Gross Capacity                                                      3919.20 M cum 
     (iv)  New Zero Elevation after 100 years                  190.5m                  
                             (v)  Live Storage                                                        2569.40 M cum
                             (vi) Length of reservoir                                              45 KM
                7.         Intake 
    (i) No. of gates                                                            8
                            (ii) Size                                                                        5.5 m x 7.0 m  
    (iii) Invert level                                                             EL 252.50 m 
    8.         Penstock
                            (i) Number                                                                   8 Nos. 
    (ii) Diameter                                                                7.00 m (finished) 
    (iii) Shape                                                                    Circular
    (iv) Thickness                                                             25 mm to 36mm
     (v) Length                                                                   245 m
    (vi)  Invert level                                                           252.50 m 
    (vii) Design discharge                                                 193.35 cumecs 
    9.         Power House (Dam Toe)
    (i) Type   & Number                                                    Surface - 2No (Both  
                                                                                     Side of river)
    (ii) Size of PH                                                               149m (L) x 87m (W) x69.5m (H)
    (iii) Installed Capacity                                                  2500 MW (8 X 312.5MW)
    (iv)  Type of Turbine                                                   Francis
    (v) Speed of turbine
                 Specific Speed                                  -           145.24 rpm
                Synchronous Speed                           -           142.86 rpm
    (vi)  Centre line of Units                                              135 m
    (vii) Net rated head                                                     177 m 
                            (viii) Over all turbine generator efficiency                   93.50 %
    10.       Tail Race Channel
                (i) Type                                                                        Open Channel 
    (ii) Av .Width                                                               94.76 m 
    (iii) Length                                                                   112.7 m
    (iv) Number                                                                 2
    (v) Design discharge                                                  774 m3/sec
    (vi) Tail race level                                                       140.0m (Min)  
                                                                                                                143.0 m (Max)        
    142 m (Avg)  
    11.       General
    (i) Type                                                                        Composite (Rockfill and  Roller-Compacted Concrete)
    (ii) River bed level                                                       101.0 m
    (iii) Top level                                                                146.50 m
    (iv) Length at top                                                         1672.0 m 
    12.       RCC dam
                            (i) Type                                                                        RCC Dam
    (ii) Length of dam at top                                              305.0 m
    (iii) Length of NOF (Non Overflow)                          138.0 m
    (iv)  Maximum height of dam                                     62.5 m 
        above deepest foundation level     
    (v) Acceptable foundation                                          84.0 m 
         level in river bed portion
    (vi) Top width                                                              8.5 m  
                13.       Spillway
                            (i) Type                                                                        Ogee type
    (ii) Length                                                                    167 m
    (iii) Crest Elevation                                                      131.0 m
     (iv) Maximum head over crest                                  12.30 m
    (v) Type of gates                                                        Radial
    (vi) Number of spans                                                  9 Nos.
    (vii) Size of gates                                                        15.0 m x 12.0 m
    (viii) Energy dissipation                                               Slotted Roller Bucket Type
    14.       Rockfill dam
    (i) Length of dam                                                        1367 m
    (ii) Height of dam above                                             37.5 m 
         Stripped level
    (iii) Upstream slope                                                     2.5:1
    (iv)  Downstream slope                                               2.0 to 2.5:1
    (v) Top width                                                               12 m 
    15.       Regulating Dam Pond
                            (i) Catchment Area                                                     9758
    (ii) Maximum Water Level                                          143.30 m
    (iii) Full Reservoir Level                                              143.0 m
    (iv)  Minimum Draw down Level                                140.0 m
    (v) Gross storage    (at MWL, FRL, MDDL)               144 M cum
    (vi)  Live storage    (at FRL)                                        24.0 M cum
    (vii) Area at FRL                                                         821.0 ha
    (viii) Project affected villages                                     04 (Partial)
    (ix) Reservoir stretch              (Length)                                   13 km
                16.       Intake
                            (i) No. of Gates & size of gate opening                      3 of 5.0 m x 3.02 m
    (ii) Centre line of HRT                                                 132.0 m  
    17.       Penstock
                            (i) Number                                                                   3
    (ii) Thickness                                                               14 mm
    (iii) Diameter                                                               4.0 m
    (iv) Length                                                                   99.50 m each
    (v) Shape                                                                    Circular
    (vi)  Design discharge                                                 60 cumecs 
    18.       Power House
    (i) Type                                                                        Surface
    (ii) Size                                                                        71 m x 38 m
    (iii) Installed Capacity                                                  60 MW (3 x 20 MW)
    (iv)  Type of Turbine                                                   Francis
     (v) Centre line of Units                                               94.0 m
    (vi) Net rated head                                                      36.5 m
    (vii) Tail race level                                                       101.0 m (Min)
                                                                                                                115.40 m (Max)
    103.50 m (Nor)          
    19.       Tail Race Channel
                            (i) Type                                                                        Open channel
                            (ii) Number                                                                  01
    (iii) Length                                                                   43.0 m
    (iv)  Normal Water Level                                            103.50 m
    (v)  Minimum Water Level                                          101.0m
    (vi)  Maximum water Level                                         115.40 m   
    20.       Main Dam Power Plant
    i)        Firm Power (90% year)                                         410 MW
    ii)          Average Annual Load Factor                              27.10%
    iii)        Annual Generation (90% dependable yr.)            5848 GWh
    21.              Regulating Dam Power Plant
    I)        Firm Power (90% year)                                        17.44 MW
    II)       Annual generation (90% dependable yr.)                        367.35 GWh
    22.              Project as a Whole
    Annual generation (90% yr.)                                       6215.35GWh
    23.         Main Dam                                                                    Rs. 99344.85 Million
    24.         Regulating Dam                                                          Rs. 15374.96 Million
    25.         Total Cost                                                                  Rs 114719.81Million
    V.   COST OF GENERATION (PL-April, 2012)
    26.       Levelised Tariff                                                           Rs 3.89 per Kwh
    27.      First year Tariff                                                            Rs 4.78 per Kwh

Updated on : 18/02/2018