Phukot Karnali PROR HEP

Background

Phukot Karnali PRoR Hydro Electric Project (PKHEP) is located in Kalikot district, Karnali Province of Nepal. The project will use the flow from the Karnali River for power generation, and the generated power will be fed into integrated power system of Nepal at the nearest grid substation proposed at Regil.

The installed capacity of the project is 480 MW including 6 MW from the plant at toe of dam from the environmental release with a total energy of 2447 GWh. The gross head of the project is 168.62 m and the design discharge is 348m3/s.

Accessibility

The project area is accessible throughout the year as it lies in the close vicinity to the Humla -Karnali Highway. The project site can be reached from Kathmandu by road by following route:

·         Kathmandu to Mugling via the Prithvi Highway: 110.6 km, paved road

·         Muglin to Narayangadh via the Narayanghat-Mugling Highway: 35.7 km, paved road,

·         Narayangadh to Kohalpur via the East West Highway: 353.4 km, paved road

·         Kohalpur to Jitegada via the Ratna Rajmarg and Karnali Highway: 249 km, paved road

·         Jitegada to the Project Site via the Karnali Highway: 16 km, all weather road

The total distance between the Kathmandu and the project site is about 780 km. Alternatively, it is also possible to fly to Nepalgunj from Kathmandu (about 55 mins) and travel via road to the project site via Nepalgunj-Kohalpur-Jitegada- Project Site (about 260 km).

Details

Topographic Survey

The detailed topographical mapping of headworks site, construction adit and powerhouse site are completed. Mapping covered a total area of 120.2 ha. The Karnali Highway near the dam location has to be realigned due to impoundment. The 20 km realigned section of the road Sanigad and Ghurbagar has also been surveyed in a scale of 1:1,000 covering 15m Right of Way.

Hydrology

The headwater of the PKHEP catchment originates from the High Mountain of Tibetan Plateau and then drains through Western Nepal into the Ganges river system in India. The catchment area of PKHEP is 16,902 km2. The long term mean monthly flows in the project has been estimated to have an average discharge of 328 m³/sec, maximum discharge of 881.50 m³/sec and minimum discharge of 77 m³/sec. These data have been used for the calculation of power and energy of the selected dam options.

The Probable Maximum Floods (PMF) at headworks and powerhouse were estimated as 15,882m³/s and 15,944 m³/s respectively. The flood of various return period has been estimated to be 3,648 m³/s, 4,813 m³/s and 5,956 m³/s. for 100 years, 1000 years and 10,000 years respectively.

Geology

The main rock types in the project area are banded gneiss, calcareous gneiss and augen gneiss. The head works area and upper part of head race tunnel passes through augen gneiss, the middle part of the head race tunnel lies in banded gneiss and surge tunnel and powerhouse/ tailrace tunnel areas are situated in calcareous gneiss formations.

These are mainly thickly to very thickly foliated rocks with occasional thinly foliated layers and the rocks are strong. Attitude of the foliation ranges from 095˚/24˚ to 054˚/24˚ (dip direction/ dip amount) in the project area. Two set plus random and three set of joints are observed in the project area. As per Q-system, the rock mass quality in the project area is ranging from poor to good, mainly falls in good and fair quality. The underground powerhouse and transformer caverns are proposed in the calcareous gneiss which is fair to poor in the rock mass class.

In the headworks site, bedrock is estimated at the maximum depth of around 50 m from riverbed. Riverbed deposit is partially compacted. Right bank slope is dip slope, relatively gentle and left bank slope is anti-dip slope, relatively steep. At powerhouse area, the Karnali River valley is V-shaped with steep to very steep slope on both banks and some overhanging cliff to the right bank slope, dissected with several gullies.

Project Components

As mentioned earlier, PKHEP utilizes water from Karnali River to generate 480 MW of electricity in total including 6 MW generation from the plant at toe of dam from the environmental release. The proposed headworks of the Project is about 1.5 km downstream from the confluence of Karnali and Sanigad Rivers. The PKHEP reservoir will stretch 11 km along the river from the dam to the Lapha Bagar. The drop-in riverbed elevation on this stretch will be about 104 m varying from 910 masl to

806 masl. The main civil structures of the project consist of two diversion tunnels, one Sediment bypass tunnel, roller compacted concrete (RCC) dam, intake, upstream (U/S) and downstream (D/S) coffer dams, two headrace tunnels, two surge tunnels, two pressure shafts / tunnels, underground powerhouse cavern, underground transformer cavern, two tailrace tunnels and so on.

Diversions structures includes the upstream and downstream coffer dams, inlet and outlet portals with diversion tunnels, bypass tunnel, and inlet for bypass tunnel arrangement requirements Two diversion tunnels of finished diameter 11 m on the right bank of the Karnali River have been proposed. The inlet of the diversion tunnels are about 1100 m downstream from the confluence of the Karnali and Sanigad River. The diversion tunnel 1 has a length of about 1145m and the diversion tunnel 2 has a length of about 945 m. A sediment bypass tunnel with its inlet about 280 m upstream of the diversion inlets has also been proposed. The sediment by-pass arrangement is designed to divert sediment laden water and help increase the trapping efficiency of the head pond during operation of the power plant. The sediment Bypass inlet consists of four set of gates and stoplogs of size 9.5 m X 8 m (B X H) followed by a weir with a labyrinth spillways whose crest is designed at 904 masl after which there is a vertical drop shaft leading to a tunnel that is connected with the diversion tunnel 1. 36m high upstream cofferdam and 10m high downstream coffer dams have been proposed about 235 m upstream and 375 m downstream from dam axis, respectively. An RCC dam with a height of 109.0 m high (from riverbed) has been proposed. It is designed with three gated overflow spillways of size 13.5 m X 19 m ( B X H ) and three gated breast wall spillways of size 7 m X 13 m ( B X H) designed to safely spill the design flood and the PMF, an auxillary spillway of size 4 m X 3 M (B X H) to pass the floating debris and a bottom outlet of size 3 m X 3 m ( B X H) which is 3 m above the original river level at the dam axis the purpose of which is to control water evacuation from reservoir during initial reservoir filling phases . Two power intakes have been planned and designed to handle the flow for the power generation The power intake is situated on the right bank of the Karnali River and consists of eight openings of width 4.8 m each. Two numbers of vertical gates of size 8.4 m X 8.4 m are proposed at the end of Intake structure.

From the power Intake, there are circular twin headrace tunnels of approx. length 6,044 m and 5,878 m with finished diameter of 8.4 m up to the surge tunnels. The headrace tunnels are designed completely on the right bank of the Karnali River. Two surge tunnels, each having length of about 862 m and 848 m and finished diameter of 6.5 m, have been proposed at the end of headrace tunnels. After the surge tunnels, the water is conveyed through steel lined pressure shaft which is divided into three portions i.e upper horizontal portion, vertical shaft and lower horizontal portion. The pressure shaft consists of two horizontal 6.5 m steel lined horizontal pressure tunnels followed by two vertical pressure shafts of finished diameter 6.5 m leading to the lower horizontal portion of two 6.5 m steel lined penstock. Each lower horizontal  pressure shaft  is  then branched  into  3  penstock  branches  with  the  help  of two manifolds of varying sizes. The water is then conveyed to the turbines through six steel lined penstock branches of different sizes.

An underground power station of dimension 156.0 m X 20.0 m X 44.6 m (LX B X H) has been proposed. The powerhouse is designed for the generation of 474 MW The main power house of PKHEP is equipped with six (6) number of Francis turbines – generator – transformer sets each with generating capacity of 79 MW and auxiliary and ancillary facilities for control and protection. From the draft tube at powerhouse cavern, the water is conveyed to two downstream surge chambers of size 71 m X 10 m X 30 m (L X B X H) through six tailrace tunnels of varying cross sections. Finally, two tailrace tunnels with finished diameter of 9.5 m conveys the water from downstream surge chamber back to the Karnali River.

A 2.3 km long transmission line with a transmission voltage of 400 kV with double circuit twin Moose configuration is planned to evacuate the power to the nearest grid hub substation at Regil.

Additionally, to maximize the financial benefits of the project, the water from the environment release is proposed to be used to generate 6 MW additional RoR energy. The discharge is passed through an intake at the dam to penstock and then to power plant downstream of dam and plunge pool. The powerhouse is located about 420m downstream of the dam axis. Two Francis turbine each capable of generating 3 MW has been proposed at the Powerhouse.

Construction Planning and statement

The project area lies on the Karnali Highway. Hence, separate access road is not necessary to reach the project site. The transportation of construction materials is proposed via trucks through Karnali Highway. The materials which are not available in Nepal shall be imported from India or other country via. Kolkata Port. Plenty of local construction materials are available near the project area, the other construction materials like cement, fly ash, reinforcement bars, steel etc. is expected to be purchased from the nearest Indian border. The construction works are divided into two packages. The infrastructure development activities which can be carried out by local contractors like access road development, camp facilities construction and construction power are grouped as package 1. The second package (package 2) consist of Civil, Hydro-Mechanical works Electro-Mechanical work, Transmission Line and Sub-station work. Total construction power required is estimated to be around 6,300 kW. All the underground excavation is expected to be carried out by drilling and blasting procedure. The total duration for the construction of the project is estimated to be about 5 years. In this project, the activities like, Construction of Diversion Tunnel, U/S Coffer dam and Headwork Dam Spillway lies in the critical path of the project. Therefore, it is important to complete the critical activities on time for the timely completion of the entire project.

SALIENT FEATURES