Spark Hydroelectric Company Limited is developing the Tamor-Mewa Hydroelectric Project (TMHEP) in Taplejung district in eastern Nepal. The survey License to carry out a Feasibility study and Environmental Impact Assessment of the Project has been granted to SHCL by government authority.

During early 1996, the Nepal Electricity Authority (NEA) engaged Canadian International Water and Energy Consultants (CIWEC) and carried out the feasibility study of some of the MHSP projects. NEA, under the Phase I of Medium Hydropower Study Projects (MHSP) work, involved in a screening and ranking exercise of all potential hydropower projects identified with capacity ranging from 10 to 300MW. Tamor-Mewa HEP was one of the identified potential projects. In March 1997 the Phase I process of MHSP confirmed the selection of Tamor-Mewa Hydroelectric project as one of the attractive project to proceed to full feasibility study. Accordingly, between the period of 1997 and 1998 the first feasibility study of the project was completed by CIWEC in October 1998.

During the developing sequence of the project, Department of Electricity Development (DoED) has confined the permission to construct only one power plant using one license and reduced project boundary. For evaluation and documenting changed scenario and hydropower policies, Professional Network for Engineering Services Pvt. Ltd. (PNet) has been employed since 27th April 2018. PNet has prepared updated feasibility study considering aforementioned conditions and constraints.

The proposed Tamor-Mewa Hydroelectric Project is located along the left bank of Mewa Khola and Tamor River. The project area lies in Taplejung Municipality (previous Phungling VDC), Meringden, Mikwakhola and Athrai Tribeni Rural Municipalities (Previous Khokling, Khamlung, Santhakra, Chage, Hangpang and Phulbari VDCs) of Taplejung District, Province 1 of Federal Democratic Republic of Nepal. The geographical boundary of project originally was between latitude of 27° 20′ 00” N and 27° 25′ 00” N and longitude of 87° 35′ 00” E and 87° 40′ 00” E. The geographical boundary after reduction by DoED is between latitude of 27° 20′ 00” N and 27° 24′ 08” N and longitude of 87° 37′ 15” E and 87°40′ 00” E.

The Project, designed at an installed capacity of 128 MW, is a run-of-the-river type project with gross head of 124.5 m and design discharge of 126 m3/s. Net annual marketable energy generated from the Project is 719.11 GWh, out of which 203.87 GWh of energy is generated in dry season.

Tamor River is one of the major tributary of Saptakoshi River. The Tamor basin can be separated by three physiographic regions, namely, High Himalaya, High Mountain and Middle Mountains. Most of the catchment lies on high Himalaya and High Mountain. The north boundary of Tamor catchment lies in high Himalaya regions and delineates the border between Nepal and China. Similarly the eastern boundary lies in the high Himalayas and delinated the border between Nepal and India. The highest elevation of Tamor catchment is 8586 masl at Mount Kanchanjanga, the third highest peak of the world. Total catchment area of Tamor upto proposed Intake site is 2062 Km2.

Mewa Khola is tributary of Tamor River. Major part of Mewa catchment lies on High Mountain. The highest elevation of Mewa catchment is about 5700 masl. Catchment area of Mewa upto intake is 574 Km2.

Design flood at the proposed headworks at Mewa scheme is taken 1853 m3/s (Q100-year) and at Tamor scheme is 4620 m3/s (Q1000-year). As downstream riparian release for environmental requirement, 0.77 m3/s flow for Mewa Khola and 2.15 m3/s for Tamor River has been considered which corresponds to 10% of the driest month’s flow of the respective river.

The surrounding area of project falls on the complex geological structures; tectonic window, thrusting with some imbricated faults. The project lies in the lithostratigraphic succession from the Taplejung window, eastern Nepal that is equivalent to the Upper Nawakot Group of central Nepal. The project area covers about 10 km long stretch from the Mewa headworks to Tamor powerhouse which has four distinctive lithological characteristics. They belong to the Tamor Khola Granite, Taplejung Group, Gondwana Group and Quaternary deposits.

Geological formations are low grade metamorphic rocks and distinguished to Augen Gneiss, Mica Schist, Schistose phyllite and Granitic Gneiss. The formations contain low grade gneiss, two mica schist, phyllites, metasandstones, and quartzite with few amphibolites. The Quaternary deposits refer to the colluvium and alluvium deposits having varying thickness. The secondary structures within rock strata are well developed. The Main Central Thrust (MCT) separated Higher Himalayan rock sequence from Lesser Himalayan rocks and is results the inverse metamorphism in the lesser Himalayan rock strata. The MCT is breached into many imbricated thrust and Tamor Khola Thrust (TKT). Folds in the area are marked by the variation of the attitudes of rock orientations and the axis line is trending to 70ᵒ ± 20ᵒ. The foliations and joints are commonly found structures in the rock mass. The predominant joint set is foliation parallel joint set with striking 30ᵒ ± 20ᵒ dipping by 10ᵒ to 40ᵒ due N. The RMR of rock mass ranging from 40-60 and Q value from 4-8 and rock mass description is Fair in common but it ranging from good to very poor rock quality in the project area. The support classes ranging from II to V for the head race tunnel. The permeability test resulted moderate to high permeability of ground soil.

The probabilistic seismic hazard analysis of the project area is carried out to assess seismic hazard and to estimate design ground motion parameters. Different seismic sources are identified on the basis of seismicity pattern, past destructive earthquakes, seismo-tectonics. The peak ground acceleration for components of structures are calculated and Uniform Seismic Response is constructed using probabilistic approach for 100, 200, 500 and 1000 years.

The main components of the project are Mewa diversion, headworks with gated barrage, intake, desander, headrace tunnel, surge tank, pressure, powerhouse and tailrace. All major components of the project have been proposed on the left bank of Tamor River except Mewa diversion which lies between Mewa Khola and Tamor River.

Mewa diversion is designed with overflow concrete weir of 45m length and 2 sluicing gates of 5m X 4m (W X H). Four intake gates of 4m X 2.5m are proposed to divert 34.18 m3/s discharge from Mewa Khola. 363 m long connecting pipe, 2566m long tunnel and 160 m long pressure pipe is designed to convey water to pressure release system which comprises of three energy dissipating valves (EDV) and opens at upstream of Tamor headworks area.

Gated diversion barrage consists of two sluicing gates and four spilling gates of 10m X 6m dimension with normal water level of 700 masl has been proposed. Six number of intake openings 6.0 m wide, 3.5 m high each has been proposed. Three bay desander of 30 m wide and 160 m long is provided to settle particle size up to 0.20 mm.

A 4760 m long concrete lined, modified horse shoe shaped headrace tunnel with 7.0 m finished internal diameter connects inlet portal and surge tank. At the end of the headrace tunnel, a restricted orifice type surge tank of 15 m internal diameter and 58 m high is connected by means of branched tunnel to dissipate pressure surge in waterway. The top of the non-overflow type surge shaft shall be ventilated with an aeration tunnel.

Steel lined pressure shaft of 3 m diameter conveys water from surge shaft towards the proposed powerhouse, the overall length of which is 255 m incorporating valve chamber. The pressure shaft itself comprised of horizontal, vertical and horizontal components. The thickness of the lined steel varies from 20 mm to 36 mm.

Underground powerhouse consisting of machine hall size 70 m length, 20 m width and 29 m height has been proposed to accommodate three sets of vertical shaft Francis turbines. The turbines of capacity 44 MW each are coupled with generators of 54 MVA capacity running at 300 rpm synchronous speed. A 290 m long modified horse shoe shaped shotcrete lined tunnel discharges water from the turbines back into the Tamor River. The tail water level is set as 572.5 masl as minimum level with NWL of 575.5 masl.

An outdoor switchyard has been proposed above the transformer cavern located near powerhouse cavern connected by approximately 155 m high cable shaft. The generated power shall be evacuated to proposed Hangpang sub-station in Koshi corridor via about 8 km long 220 kV double circuit transmission line with 2*Bison conductor.

The construction period of the Project has been taken to be 5 years. The total financial cost of the Project on the basis of rates as of June 2018 is estimated to be NRs. 26.12 Billion. The Project shall be financed from debt and equity ratio of 75:25 from financing institutions. Financial analysis has been undertaken with 10% interest rate on the long-term loan yielding 13.76% IRR, 17.46% ROE, and B/C ratio of 1.33.

Mewa diversion cost without revenue has serious impact on project return. Hence, it is wise to produce power from this diversion too. However, 128 MW scheme confirms the project is technically viable, financially feasible, and environmentally acceptable.