Ashkot Hydropower Menu

Hydraulic Design


The project layout has been proposed on the left bank with diversion weir to sandtrap as surface structures and headrace tunnel to tailrace outlet as underground structures.

With a 20m high diversion weir a discharge up-to 309 m³/sec would be diverted into about 225 m long connecting channel to take the design discharge up to sandtrap with three chambers, each with a length of 180m. Water from the sandtrap will be conveyed further through a 6870 m long low pressure tunnel having diameter of 10.6m connected with vertical surge shaft of 30 m diameter and 82 m height. The 72.6 m long vertical pressure shaft and 58 m high pressure steel lined tunnel having diameter from 10.6m to 7.6 m are proposed to connect with 5 Francis turbines in an underground powerhouse. From the powerhouse 267 m long low pressure tailrace tunnel will discharge water into Neelum River.


At the selected weir axis, the valley is a v-shaped narrow gorge. Major environmental consideration of population re-settlement in the head pond area restricts construction of a high weir which is required to settle the sediments in river before power intake. The weir would divert the design discharge into headrace channel and to flush the sediment downstream of weir.

To pass the flood through the spillway, a low level gated structure is provided over entire length of weir. The concrete weir structure is equipped with six (6) hydraulically operated low level radial gates for flood discharge and for the flushing of sediments entering the reservoir.

The weir structure has been designed to safely withstand more than 10,000 year Flood and 1:1000 design earthquake without damage.

The gate-controlled spillway is designed to pass flood of 10,000 year 5545 m³/sec flood with all gates open. It is also designed to pass the 1:1000 AEP flood of 4333 m³/sec flood with one gate inoperative. In this case, the reservoir level remains below maximum flood level of 1338 m.a.s.l.


Power Intake
The power intake is designed to be operated in such a manner as to ensure that the required turbine flow shall be conveyed within acceptable limit of head losses. The intake would be capable of taking the design discharge of 309 m³/sec, plus the flushing flows for single chamber of sandtrap i.e. about 15.5 m3/sec.

Power intake structure will have 5 bays for the design discharge. On each bay, a trash rack has been provided at the entrance to prevent entry of debris into the power waterway system plus the flushing flows for single chamber of sand trap i.e. about 15.5 m3/sec. Provision has also been made for installation of stop-logs upstream of the control gates for maintenance and inspection. A trash rack cleaning area has been provided for removal of debris from the trash racks. A gantry crane has been provided for placement and removal of the stop-logs.

4Power Channel
The power channel has been planned and designed from weir to sandtrap. The topographic and geological conditions allow to have open channel with open sand trap at downstream.
Sandtrap is planned as an open structure before low pressure tunnel starts. For a design discharge of 309 m³/sec, three chambers have been designed for settling of sediment greater than 0.25 mm. An alternative scheme to the sand trap has been proposed and is also under study.

Headrace Tunnel

Alignment of Headrace Tunnel:
The alignment of low pressure headrace tunnel is fixed with the following considerations:

  • The tunnel after sandtrap takes a turn at an angle of about 135 degrees. The turn is in a smooth curve.
  • After the turn, the alignment of the tunnel adopts a curved path to the surge shaft with due care of required minimum cover over the tunnel.
  • Suitable locations and lengths of construction adits.

Dimensioning of Headrace Tunnel:
The optimized diameter of low-pressure headrace tunnel is 10.6 m and it is 6,780 m long.


Surge Tank
At the junction of the headrace tunnel and surge tank, a throttle with a cross-section area of not less than 50 % of the pressure shaft is arranged in accordance with standard design practice to dampen the pressure fluctuations. In view of the expected high flow velocities at the throttle, it will be steel lined.

In accordance with standard design practice and the hydraulic design criteria, the cross sectional area of the cylindrical surge tank is selected with a factor of safety 1.5 times larger than the Thoma-Criterion to ensure adequate stability of plant operation.

8Vertical Shaft/Pressure Shaft
The arrangement of waterway after the surge shaft to powerhouse is vertical shaft and then horizontal high pressure tunnel. The pressure tunnel opens into a manifold, which subdivides the pressure tunnel into five (5) steel made penstocks, each 3.4 m in diameter. Total length of the five (5) branches of the penstocks will be 267 m.
9Access Tunnel to Powerhouse
From the Neelum river, a 130 m long access tunnel would be excavated to access the underground powerhouse. The access tunnel would be D type, 7 m wide and 7 m high. The tunnel would generally be shotcrete and maybe concrete lined at some portion where weak rock zone is encountered. The bottom width would be concrete lined or metaled road for easy transportation of the equipment and material. The access tunnel would be ventilated from outside to the powerhouse cavern.

Power Complex
Around the powerhouse site no plain or mildly sloping terrace with good geological conditions is available to place a surface power complex. Steeply sloping ridges of somewhat sound rocks are present, the cutting and stabilization of these slopes for a surface powerhouse are not cost effective hence an underground power complex has been selected.

The powerhouse complex accommodates the main power generating electromechanical plant and the ancillary plant for its operation and maintenance. It comprises two caverns: a powerhouse machine hall cavern and separately a transformer hall cavern. The caverns are interconnected by four bus duct passages and one connecting passage. The transformer hall cavern lies upstream of the machine hall cavern.

  • The power complex comprises the following structures:
  • Underground Powerhouse Cavern
  • Transformer Cavern
  • Main Access Tunnel
  • Cable and Ventilation Tunnel
  • Open Switchyard

Tailrace Tunnel and Outlet Structure
From the outfall of each turbine, the water will be carried through varying ‘D’ shaped free-flow concrete lined tailrace branch tunnels, which will be combined into one free-flow concrete lined main tailrace tunnel. The free-flow concrete lined tunnel will terminate into Neelum River.

A reinforced concrete outlet structure is provided at the downstream portal of the tailrace tunnel. The sill of the outlet structure is located at 1000 year flood level of Neelum River, hence the operation of the plant is not affected by the flood water. The outlet structure is accessible via an access road built to excavate the cable, access and tailrace tunnels.