Diversion Weir and Undersluice
An ogee shaped concrete weir is proposed across Jum Khola at about 1.5 km upstream of the confluence of Jum Khola and Lapche Khola. The length of the weir is 36 m and its crest level is fixed at an elevation of 2300.00 m amsl. The height of the concrete weir is sufficient to provide head for flushing of gravel and sediment from gravel trap and settling basin respectively. The diversion weir is designed for the safe passage of 1 in 100 years flood. A stilling basin of an average length 49 m is provided to control the hydraulic jump and consequent wear and scour of the basin floor.
Two numbers of undersluice are provisioned at the left bank of Jum Khola, adjacent to the intake arrangements. Each undersluice is 4 m wide and 3 m high with the sill elevation at 2290.53 m amsl. The undersluice consist of radial gates and stop logs.
An arrangement of 0.5 m thick hardstone lining on the undersluice floor and 16 mm thick steel lining on the walls of the undersluice are provided to control the abrasive wear.
The intake arrangements consist of side intakes along with trash passage in between intake and undersluice. The intake arrangement comprises of four orifices, having an opening of 2.8 m wide and 3 m high (each orifice). These openings are sufficient to withdraw design discharge of 25.45 m3/s (41.11% exceedance of flow). The intake also has coarse trash rack, in front of each orifice. The invert level of intake orifice is fixed at an elevation of 2296.00 m amsl.
The floating trash passage arrangement is 2.5 m wide and is gate controlled. The invert level of debris trap arrangement is fixed at 2299 amsl (1 m below the weir crest level) to attract the floating debris before entering into the intake.
A double chambered surface gravel trap is located immediately after the intake. The gravel deposited in the gravel trap is discharged back into the river through the flushing canal with conventional hydraulic flushing method. The size of each gravel trap chamber is designed to settle the particle size larger than 5 mm gravel.
Gravel trap is 10 m long, 6.6 m wide and 6.85 m high. The longitudinal slope of gravel trap is 1 in 40. Steel lining of 16 mm thickness is provided on the base of the gravel trap and at flushing canal that is exposed to continuous wear and tear. A coarse trash rack is provided at the end of gravel trap.
An approach tunnel conveys water from gravel trap to the settling basin. The tunnel is designed as pressurized tunnel. Two short approach culvert convey the flow from the gravel trap up to the inlet portals for the left and middle approach tunnels, where as a longer approach culvert at right (of length 37.6 m) conveys the flow to the shorter approach tunnel on right. The three approach tunnels are of length 86.63 m (left), 81.06 m (middle) and 62.31 m (right). The approach tunnel is of inverted D shape and its size is 4.2 m x 4.2 m.
Three chambered underground settling basin is provisioned to trap sand particles up to 0.15 mm diameter with efficiency greater than 91%. Each basin is 68 m long, 12 m wide and 7.5 m high. The length of inlet transition is 22.12 m and the horizontal transition of 110 is provided. The arrangement has longitudinal slope of 1:50 to facilitate the flushing of deposited sediments. From the outlet of the settling basin, the flow will be passed to headrace tunnel following a drop bend located just downstream of the conveyance tank.
Water is conveyed through three approach tunnel, with three inlet portals, into each of the three settling basin bays. An inspection/access tunnel of length 236 m will lead to the operation platform of the conveyance tank.
A conventional hydraulic flushing system is adopted for the flushing of deposited sediments. At the outlet of each basin chamber, outlet gate is provided for the operation during the flushing. Flushing culvert of size 1.5 m x 1.5 m will flush the deposited sediments. The inner face of the culvert will be provided with steel lining. The flushing culvert ends near the bifurcation of the flushing tunnel and the adit to the headrace tunnel, beyond which the flushing discharge shall be open flow. Another flushing culvert will be provided from the portal of the sediment flushing tunnel which will flush the sediment into the river.
Three inlet portals for the three approach tunnels of the settling basin bays are provided. Each of the three settling basin will convey the flow through three connecting tunnels which will join into the headrace tunnel. The headrace tunnel having a length of 1519 m passes along the left bank of the river and ends at the surge shaft. The cross section of the tunnel is inverted “D” shaped with 4.4 ᵡ 4.4 m dimension.
The geological condition along the tunnel alignment is good and based on preliminary study, 6 types of the tunnel supports have been considered.
An adit tunnel of length 390 m and diameter 4.4 m has been proposed which connects to the headrace tunnel at chainage 1+405 m. The adit tunnel has been proposed in a way that it shares about 100 m of its length with ventilation tunnel. A bifurcation has been planned to split adit tunnel and ventilation tunnel.
At the headworks area, adit tunnel has been proposed which will join the headrace tunnel just downstream of the junction of the three connecting tunnels. This adit tunnel will branch off as the sediment flushing tunnel, leading towards the settling basin.
The surge shaft is proposed at the end of the headrace tunnel to accommodate the surge generated during plant operation without any spillage. The shaft is 45 m high and 10 m finished diameter. The surge shaft is connected to the tunnel by a 4.8 m diameter orifice opening. Based on preliminary geological study, two different support types have been considered. The upsurge level is determined at 2310 m amsl and the downsurge level at 2289.55 m amsl.
A 4.4 m ᵡ 4.6 m (width ᵡ height) ventilation tunnel of length 211 m has been proposed which connects to the surge shaft at level 2312.92 amsl. The ventilation tunnel and adit tunnel both share a common portal and 100 m stretch of tunnel.
The penstock shaft comprises of a vertical shaft (222 m) and a horizontal shaft (165 m). The vertical shaft is circular shaped with diameter 4.25 m whereas the horizontal shaft is an inverted D-shaped tunnel of size (W ᵡ H) 4.5 ᵡ 5 m. The penstock shaft is of 387 m length (steel lined with steel of ultimate tensile strength of 410 N/mm2) and 3.25 m internal diameter, with 0.45 m thick concrete lining, which conveys the flow from the headrace tunnel to the four turbine units inside the powerhouse. Before entering the powerhouse, the penstock pipe is divided at four branches maintaining a constant velocity. After branching, 15 m long and 1.5 m internal diameter pipe conveys water to each turbine.
Powerhouse and Tailrace
An underground powerhouse which accommodates four generating units of capacity 14.43 MW is proposed on the left bank of Tamakoshi Nadi, upstream from the confluence point of Kharane Kholsi. The design flow will be then discharged back into the Tamakoshi Nadi through a tailrace tunnel. The length and width of the powerhouse are 75 m and 15 m respectively. The underground powerhouse will comprise of main powerhouse cavern and the transformer gallery.
The powerhouse is accessed through an access tunnel. The access tunnel will bifurcate into two tunnels which lead to the main powerhouse and the transformer gallery.
After the power generation, water will be discharged to the tailrace tunnel via a Draft Tube. Tail water level will be 2046 m amsl during the normal operation period. Stop logs are provisioned at the end of the draft tube. The tailrace tunnel will be of length 703.64 m.
Twenty sets of hydraulic gates have been envisioned at different locations, such as; headworks, settling basin and tailrace for regulation of discharge, maintenance of the structures and efficient operation of the power plant. The gates shall consist of gate leaf, guide frame, guide rollers, and hoisting equipment. The gates will be made of structural steel and shall be all welded. Rubber seals will be provided for water tightness of the gates. The guide frame of gate will consist of a sill beam, side frames, and lintel beam. Hoisting equipment of suitable capacity will be provided for the operations of gates.
Trashracks have been provisioned at two locations: one at the intake and the other at the outlet of gravel trap, to prevent floating objects, gravels and sediment other than the desired size from entering. The trashracks shall be designed to withstand the hydro-static forces, static loads and vibration phenomena which are likely to occur due to the flow of water through rack bars.
Each trashracks shall be made of structural steel plates spaced parallel to each other by spacers. The trashracks will be supported by horizontal beams, built up type construction and fixed by the steel bolts, nuts and washers. The rack bar spacing shall be of specified size.
Seven sets of stoplogs have been proposed at different location of headworks (undersluice, gravel flushing and sediment flushing) in order to facilitate maintenance works of hydro-mechanical equipment such as gates, trashracks etc. This arrangements also facilitate the smooth operation of the project in case the main hydraulic gates are not operational.
Grooves are provided at each side of walls which shall be furnished with channels to facilitate the placing of stop logs. The stoplogs will be made of structural steel and shall be all welded. Mono rail hoist for handling stoplogs will be provided at undersluice and intake and desanding basin.
Trash passage gate
A trash passage has been provided at the intake to conveniently allow passage of trash, wooden logs, or other floating debris.
The penstock shaft conveys water from surge shaft to the powerhouse. The penstock shaft consists of a 3.25 m dia. penstock pipe encased with concrete. The penstock pipe bifurcates into four branches of diameter 1.5 m before entering the powerhouse. The total length of the penstock pipe is 387 m including branching for generating unit. The thickness of steel pipe varies from 14-55 mm (considering structural steel, E350: IS 2062 or equivalent)
In addition to the penstock shaft, steel pipe has also been used in gravel flushing culvert. 1 m dia. steel pipe of about 120 m length has been used for gravel flushing.
Four sets of vertical axis Francis turbines have been selected with respect to the gross topographical head of 254 m and design discharge of 25.45 m3/s.
Each turbine will have an output capacity of 14.43 MW. Each turbine is equipped with the digital governor which should be able to control speed under all conditions of load and head. The governor should be able to operate in both automatic and manual mode.
Indoor GIS (Gas Insulated Switchgear) type switch gear has been proposed. The switch gear will comprise of various components such as: Indoor switchgear, current and voltage transformer, circuit breakers, surge arrestors, disconnectors and others.
The power generated from the plant will be evacuated by an approximately 37 km long double circuit 132 kV transmission line to Garjyang Substation, Ramechap District, (NEA).