HEAD RACE TUNNEL (HRT)

Head Race tunnel (HRT) takes water from connecting channels and conveys it to the forebay or directly to the penstock furnished with a Surge shaft relying upon the project and design requirements and is on occasion additionally called a Power tunnel. It is the shape that consists of water from the Intake to the Power House for Power technology.

LAYOUT OF HEAD RACE TUNNEL (HRT)

The layout of a Head Race Tunnel (HRT) in a hydropower project depends on various factors, including the topography. The head Race tunnel of the (Singoli-Bhatwari Hydroelectric project) in Uttarakhand is 11.256 km long. The tunnel has a finished dia. of 4.9 m and is concrete-covered and metal-covered with various thicknesses depending on the geological condition of the rock. The excavated phase of the top Race tunnel has been kept as D-formed with a backside width of 5 m. The excavated diameter of the tunnel is 6m. The excavation of the tunnel has been performed thru five construction ADITs on the right bank, every D-shaped with lengths varying from 163m to 546m.

The length of the tunnel among the ADITs varies from 680 m to 5635 m. The geological features of the site, the separation between the water source (intake) and the power plant (penstock entry), the available head, and the area. Here is a general outline of an HRT’s design.

Intake Structure: The HRT starts at the intake structure, which is often found where the water is switched from its normal flow pattern (for example, a river or reservoir). The intake structure, which is intended to manage water flow into the tunnel, frequently has gates or valves to control water flow.
Head Race Tunnel: The HRT begins as a sizable underground or partially concealed tunnel from the intake building. The water is transported through this tunnel from the intake to the power plant’s turbines.
Tunnel Alignment: From the intake to the power plant, the alignment or path of the HRT is planned to take the shortest and most practical route possible, taking into account aspects like avoiding steep hills, limiting excavation, and abiding by safety and engineering regulations.
Tunnel Diameter and Shape: The alignment or path of the HRT is planned to travel the shortest and most practicable route possible from the intake to the power plant, taking into consideration factors like avoiding steep slopes, limiting excavation, and adhering to safety and engineering requirements.
Support Structures: In order to reinforce the tunnel walls and maintain stability, support structures like rock bolts, shotcrete, or steel ribs may be utilized, depending on the local geology.
Ventilation and Drainage: To ensure a secure working environment and avoid water buildup inside the tunnel, sufficient ventilation and drainage systems are included in the HRT design.
Access and Maintenance Shafts: Access shafts that are periodically spaced apart are constructed along the length of the HRT to provide entry and exit sites for maintenance, inspection, and emergency scenarios. These shafts may also serve as ventilation holes.
Gratings and Trash Racks: At the intake structure and other entry locations, gratings and trash racks may be added to stop debris from entering the HRT and harming the turbines or other components.
Gate Chambers and Valves: Along the HRT, gate chambers and valves can be installed to manage pressure and control water flow, enabling modifications as necessary.
Transition to Penstock: The penstock, a pipe or conduit that transports water from the HRT to the turbines at the power plant, is where the water is routed after exiting the HRT.

TYPES OF HEAD RACE TUNNEL (HRT)

On the basis of flow:
i. Pressure flow tunnel
ii. Gravity flow tunnel
On the basis of shapes:
i. Semi circular
ii. Circular
iii. Elliptical
iv. Horseshoe
v. Square with an arched ceiling

On The Basis Of Flow

Pressure flow tunnel– When there is a substantial elevation difference between the water source (intake) and the power station (turbines), a Pressure Flow Tunnel (PFT) is one particular type of Head Race Tunnel (HRT) that is employed in hydropower projects. The pressure flow tunnel generates the required pressure for water transportation without the use of extra pumping by making use of the natural slope or head of the terrain.

Here are the features considerations for a Pressure Flow Tunnel (PFT)

Slope Utilization
Pressure Pipe or Conduit
Inlet Structure
Tunnel Diameter and Shape
Pressure Control Devices
Ventilation and Drainage
Support Structures
Outlet Structure

Gravity flow tunnel– Another type of Head Race Tunnel (HRT) utilized in hydropower projects to transport water from the intake to the power plant housing the turbines is a gravity flow tunnel. Gravity Flow Tunnels (GFTs) rely just on gravity to flow water through the tunnel, in contrast to Pressure Flow Tunnels (PFTs), which also use the natural slope of the terrain to create water pressure.

Here are the features considerations for a Gravity Flow Tunnel (GFT)

Tunnel Alignment
Tunnel Diameter and Shape
No Pressure Pipe
Sufficient Slope
Inlet Structure
Ventilation and Drainage
Support Structures
Outlet Structure

On The Basis Of Shapes

Shapes Based on their forms or cross-sectional arrangements, Head Race Tunnels (HRTs) can be divided into many categories. The efficiency of the water flow, the complexity of the construction, and the stability can all be impacted by the HRT’s shape.

Based on their shapes, the following are some typical HRT types:

Semi-circular– The cross-section of semicircular tunnels resembles the bottom half of a circle. They are appropriate for a variety of geological settings due to the strong structural strength provided by their design. When the underlying rock or soil is stable and capable of supporting the tunnel structure, semicircular tunnels are frequently used.
Circular- One of the most typical shapes for tunnels has a circular cross-section, and this is a circular HRT. The circular form is relatively simple to create and offers structural stability. It is frequently employed in numerous hydropower projects and is appropriate for a wide variety of flow rates.
Elliptical- An elliptical HRT combines the benefits of circular and rectangular tubes with an oval cross-section. It is appropriate for a range of flow rates and soil conditions since it has strong flow characteristics and structural stability.
Horseshoe- The flat invert (bottom) and semi-circular roof of the horseshoe-shaped HRT are both present. When a higher clearance height is needed, this shape is frequently utilized in larger tunnels since it offers more structural stability than circular tunnels.
Square with an arched ceiling– These tunnels have an arched ceiling and a square cross-section. The arching ceiling strengthens the structure while the square shape makes good use of the available space. This kind of tunnel is frequently used when a higher clearance height is required or when the surrounding terrain calls for a square shape for greater stability.

USES OF HEAD RACE TUNNEL (HRT)

In particular, hydropower systems that produce energy by using dams and reservoirs must include headrace tunnels. A headrace tunnel’s main function is to transport water from a reservoir or intake structure to the power plant, where it powers turbines to generate electricity. Headrace tunnels are mostly used for the following purposes:

Water Conveyance to Power Station: An HRT’s main job is to move water from the intake structure to the power plant, which houses the turbines. The turbines are propelled by the potential energy of the water that is obtained from its height or head and used to produce power.
Utilizing Natural Head: The natural slope or elevation decrease between the water supply (such as a river, reservoir, or aquifer) and the power plant is intended to be utilized by HRTs. In tunnels that carry water by gravity or pressure, the natural head generates the pressure needed to move the water and power the turbines without the use of additional energy.
Flow Regulation and Control: To regulate and manage the flow of water, HRTs are outfitted with gates, valves, and control devices at various locations throughout the tunnel. As a result, operators can modify the water flow rate as necessary for system functioning and power generation.
Diversion of Water: HRTs offer a route for water diversion, directing it from the intake to the power plant without adversely affecting the surrounding ecosystem, which is necessary for hydropower projects that involve diverting water from its natural course.
Sediment Management: Before the water flow reaches the turbines, sediment, and debris are taken out of it using gravel or sluice tunnels. This ensures that electricity generation runs smoothly by reducing the risk of damage to the turbines and other machinery.
Safety and Environmental Benefits: HRTs reduce the visual and environmental impact on the surrounding area by transporting water through underground or partially hidden tunnels. They also offer defense against bad weather and other dangers, resulting in a safer and more dependable power-producing system.
Facilitating Remote Power Generation: HRTs allow hydropower plants to be constructed in remote areas with adequate water resources. This enables the generation of clean, renewable energy in regions that might otherwise have limited access to electricity.
Facilitating Multi-Stage Hydropower Projects: HRTs link many intake structures to the various power stations in multi-stage hydropower projects, where multiple power stations are used to collect electricity from the same water source at varied elevations.

Recommended Books on Head Race Tunnels & Hydropower Engineering

Hydropower Engineering Handbook:- https://amzn.to/4i23sG3
Tunneling in Rock by Drilling and Blasting:- https://amzn.to/3DrhEcF
Design of Hydraulic Structures – R. S. Varshney:- https://amzn.to/4imyLev