A Brief Description of Budhigandaki Hydropower Project: - by Dr. Laxmi Devkota

  Budhigandaki Hydropower Project: A Brief Description Dr. Laxmi Devkota Chairman of Budhigandaki Hydro-electric Project Development C...

 Budhigandaki Hydropower Project: A Brief Description
Dr. Laxmi Devkota
Chairman of Budhigandaki Hydro-electric Project Development Committee
and

Chairman, Nepalese Society of Agricultural Engineers 

E-mail: lpdevkota@ndri.org.np


1.      Background
Of the total installed capacity of 802 MW in the Integrated Nepal Power System (INPS), the Kulekhani Hydropower (92 MW) is the only storage hydropower plant capable of seasonal regulation. The existing peaking capacity is insufficient to meet the peaking demand in the system. This is shown by the severe capacity and energy shortage during the dry season. Most of the plants that are being implemented or planned are Run of the River (ROR) plants. With the addition of more ROR plants, there will be increasing surplus of energy during the wet season and deficit in the dry season. The seasonal fluctuation of water discharge in the rivers of Nepal is the main cause for this imbalance. The dry season flow becomes almost one tenth of flows in the wet season. As a result, INPS will continue to be as sub-optimal system in the absence of peaking plants, resulting in the spilling of water during the wet season at ROR plants and capacity deficit during the dry season. A storage project of sufficient capacity is the only remedy for such a situation. Budhigandaki Hydropower Project, being centrally located from the main load centers (Kathmandu, Pokhara and Chitwan) and having large storage capacity is capable to address the national peak electricity demand and energy deficit during the dry season especially from Mansir to Baishak. Government of Nepal has, thus, decided to build it as soon as possible as a "National Pride Project".
2.      Project location
The Budhi Gandaki Hydropower Project is located on the Budhi Gandaki River, approximately 2 km from its confluence with Trishuli River at Benighat that can be accessed by the Prithivi Highway linking Kathmandu and Pokhara, about 80 km from Kathmandu (Figure 1)

Figure 1: Project Location

3.      Hydrology and Climatology
Figure 2: Mean annual discharge at Dam site

With a total area of 5005 km2, the catchment area of the Budhi Gandaki HPP is characterized by a large range of elevation, from green hills in the south to snow covered Himalayan peaks in the north. The mean annual rainfall is 1,495 mm with a high spatial variability while the mean annual discharge at the dam site was estimated to be 222 m3/s. It implies that the mean annual flow of the river at dam site is about 7.0 billion m3 (Figure 2). The 10,000 years flood was estimated to be about 6,260m3/s and the Probable Maximum Flood (PMF) was about 9,800 m3/s (See Appendix 1: Salient Features of the Project for details). Glacial lake outburst flood (GLOF) and landslide dam outburst flood (LDOF) were estimated but the corresponding peak discharges at the dam site were significantly less than the PMF and are therefore not governing values for the design of the dam and spillway structures.

4.      Sedimentology
A sediment rating curve (sediment discharge vs. river discharge) based on a 3-year sediment sampling programme was applied to long term river discharge (1964-2012) and was used to estimate mean annual sediment inflow to the Budhi Gandaki reservoir. Almost 600 measurements of suspended sediment concentration and river discharge were used in the development of the relationships for the high flow and low flow seasons. Sediment inflow to the Budhi Gandaki reservoir was estimated to be 7.6±2.2 million m3/year. The volume of sediment due to rare catastrophic events such as LDOF or GLOF was estimated to be about 2 million m3 for an extreme event. The upper limit of estimated sediment inflow equal to 9.8 million m3/year is therefore considered as the annual sediment inflow to the Budhi Gandaki reservoir. Of this amount an estimated 9.6 million m3 of sediment per year (458 million m3 after 50 years) will be deposited in the reservoir without a sediment management strategy. The empirical USBR method, based on observations in existing reservoirs, was used to estimate the deposition patterns in the Budhi Gandaki reservoir. The sediment elevation estimated by this method (depth at which no reservoir capacity is available) after 50 years is 377 masl i.e. 85m below the power intake invert level which is at 462 masl. Sediment deposition will occur in both the dead and the active storages of the reservoir, however the results of the sedimentation study indicate that only about 7% of the active storage will be lost after 50 years and about 13% of the active storage will be lost after 100 years.
5.      Seismic Hazard
The Budhi Gandaki dam project falls in the region of possible future great earthquakes. A large amount of residual stress has to be released by an earthquake of magnitude, possibly not less than 8.5, with a rupture likely located along the Main Himalayan Thrust at ~16 km depth, ~20km to the north of the dam. Considering this fact deterministic Seismic Hazard Assessment was made and the following horizontal accelerations were taken into account in the design of the project structures: Operating Basis Earthquake (OBE): 0.6g and Safety Evaluation Earthquake (SEE): 1.2g
These values are equivalent to Reference Earthquake Magnitude of 8.7 Richter scale that occurs at 8km below the dam. It is noted here that TERI dam of India, which is 260.5 m high, was built for 8.5 Richter scale. The earthquake of 25th April 2015 of magnitude 7.8 Richter scale was of 0.16g. All these figure shows that the dam is quite safe from seismic hazard point of view.
6.      Reservoir
The salient features of the reservoir are illustrated in Figure 3 and Figure 4; and given in Appendix 1 too. The reservoir will store about 4.5 billion m3 water at full supply level (FSL) of 540 masl (meter above mean sea level). The minimum operating level (MOL) will be 496 masl. It means the active storage that lies between FSL and MOL will be 2.2 billion m3.  The surface area of the reservoir at FSL will be 63 km² which is about 15 times larger than the Phewa Lake of Pokhara.
Figure 3: Reservoir coverage
Figure 4: Reservoir area-volume curves and storage
7.      Dam
The dam of this project will be double curvature arch dam as shown in Figure 5. The maximum height of the dam will be 263 m with crest length of 760 m. The width of the dam at the base will be 80m while the width at crest will be of 15m. Total concrete required to construct this dam is about 5.75 million m3.
Figure 5a: Schematic diagram of the dam

8.      Power Generation and Annual Energy
The Budhi Gandaki project has been design and optimized to produce peak energy. The energy production, the firm power capacity and energy availability during the dry season has been maximised in order to compensate for the reduced capacity and/or energy outputs of the other run off river plants of Nepal. For this purpose the Budhi Gandaki project is associated with a large reservoir able to store the monsoon flows and to release a regulated discharge in the dry season. Since priority is given to peak power production, the waterways and the power plant have been sized to concentrate the production during the peak demand hours (Table 1) of the Nepal interconnected network. During low demand hours in the dry season, the Budhi Gandaki Hydropower Project will be on standby mode.
The turbine used in this project is Francis type. There will be 6 number of units with rated net head of 200 m. With rated discharge of 672 m3/s, the plant can generate 1200 MW of electricity. The mean annual energy generation by the plant will be 4250 GWh. It will generate 1623 GWh during the 5 months of dry season i.e. from Mansir to Baishak. It gives the plant factor of about 40% which is a quite good figure when we compare with other projects in the world.
Table 1: Capacity and energy output of the Budhigandaki Hydropower Project
9.      Affected Population 
Some parts of 27 village development committees (Gorkha: 14 and Dhading:13) fall under water (63 km2). It results in total displacement of 3,560 households (Physical and economic displacement) with a population of 20,260. The other 4557 household are economically displaced. It accounts the population of 25,351. Thus the total number of household affected and the number of people affected by the project will be respectively 8117 and 45,611.
10.  Cost and Duration of Construction
The estimated total cost of the project is Rs. 260 billion. The duration of construction is about six and half years.
The salient feature of the project is given in the Appendix 1.
11.  Other Development Possibilities
The project area is not only centrally located but also 1776 km Mid-Hill Highway linking 24 hilly districts and major emerging cities is passing through the site. The project is in close proximity to another highway joining China and India. Further the Budhigandaki reservoir is going to be about 15 times greater than the Pokhara’s Phewa Lake. These features have provided a lot of opportunities of development of different sectors. Tourism and modern settlement development around the proposed ring road, establishment of different trade centers: national and international and various shopping complexes with one complex for one product are some of them to name (please see the schematic diagram for the possible development given in Appendix 2).  Budhigandaki corridor is, thus, becoming an Economic Hub of Nepal after the completion of Budhigandaki Hydroelectric Project. 

Appendix 1

PROJECT SALIENT FEATURES

1. Reservoir
Full Supply Level (FSL):                                   540 masl
Maximum Flood Water Level:                          542 masl
Reservoir Upper Boundary:                              545 masl
Minimum Operating Level (MOL):                 496 masl
Gross Capacity at FSL:                                      4.467 billion m3
Active Storage between FSL and MOL:         2.226 billion m3
Surface Area at FSL:                                           63 km²

2. Hydrology
Catchment Area                                                  5005 km2
Long Term Average Flow:                                 222 m3/s
Construction Flood (20 years):                         3070 m3/s
Design Flood Discharge (10000 years):          6260 m3/s
GLOF                                                                     3000 m3/s
LDOF                                                                     5200 m3/s
Probable Maximum Flood:                               9800 m3/s

3. Sedimentology
Sediment Inflow:                                                 9.8 million m3/year
Active storage loss after 50 years:                   7%
Active storage loss after 100 years:                 13%

4. Diversion System
Design Flood:                                                       3070 m3/s
Number of tunnel:                                               2
Diameter:                                                              12 m
Crest Elevation of U/S Cofferdam:                 357 masl
Crest Elevation of D/S Cofferdam:                 332.50 masl

5. Dam
Type:                                                                     Double curvature arch
Maximum Height:                                               263 m
Crest Length:                                                       760 m
 Width at Base:                                                   80m
Width at Crest:                                                     15m
Total Concrete Volume:                                     5.75 million m3

6. Spillway
Type Orifice:                                                        Gated spillway with flip bucket
Number of bays:                                                 6
Bay dimensions:                                                  Width 5.6 m and Height 8.4 m
Design Flood (Routed):                                     6280 m3/s for PMF
Gate Type:                                                            Radial gate with hydraulic hoists
Gate size:                                                              5.6 m x 8.4m, Six gates

7. Waterway and Powerhouse
Intake structures:                                                 Bell mouth intake
Total Rated discharge:                                       6x112=672 m3/s
Number of intake:                                               6
Invert Level of Intake:                                       460 m
Number of Headrace Tunnel:                           6
Powerhouse and Transformer Building:         Outdoor in the left bank
Width, Length and Height:                                W 40 m x L 185 m x H 40 m

8. Generating Equipment
Turbine Type:                                                      Francis
Number of Units:                                                 6
Net Head at Rated Water Level:                       200 m
Rated Discharge:                                                 6 x 112 = 672 m3/s
Installed Capacity:                                              6 x 200 MW = 1200 MW
Mean Annual Energy:                                         4250 GWh
Winter Dry Season Energy:                               1623 GWh (Mansir to Baishak-5 months)
Normal Tailwater Level:                                    323.30 masl
Generator Type:                                                  Vertical shaft revolving
Generator Capacity:                                           6 x 235 MVA = 1410 MVA

9. Transmission Lines
Number:                                                                2
Voltage:                                                                 400 kV
Circuit:                                                                   Double circuit
Conductor:                                                           Quad Bundle MOOSE
Length BG HPP to Naubise:                             40.3 km
Length BG HPP to Hetauda:                            58.7 km
10. Substations
Number:                                                                2
Location                                                                1 in Naubise and 1 in Hetauda
11. Costs
Total Capital cost:                                               2593 MUSD


Note: Source of data and figures presented in this article is the Feasibility Report of Budhigandaki Hydroelectric Project

Appendix 2

Fig A:  Schematic Diagram Integrated Development of Budhigandaki Corridor
Name

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IOE Notes | Notices | Examination | Engineering in Nepal : A Brief Description of Budhigandaki Hydropower Project: - by Dr. Laxmi Devkota
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