---
title: "Hydraulic tunnels and design features"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:hydraulic-tunnel-design-features"
source: https://www.bctengineer.com/q/semester_6/hydro_power/hydraulic-tunnel-design-features
site: BCT Engineering
---

# Hydraulic tunnels and design features

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2075 Bhadra, 1+5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/hydraulic-tunnel-design-features
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

Explain hydraulic tunnels and its design features.

## Answer

A **hydraulic tunnel** is an underground conduit excavated through rock or soil to convey water, used in hydropower as headrace, tailrace, diversion or spillway tunnels. It may run **free-flow** (partly full, like an open channel) or **pressure flow** (full, under internal pressure).

#### Design features
1. **Alignment:** as straight and short as possible; through sound rock, avoiding faults, shear zones and weak strata; crossing weak zones at right angles; adequate rock cover (vertical and lateral) so that the rock weight resists the internal water pressure (Norwegian criterion: cover $C_{RM} \ge \dfrac{h_s \gamma_w F}{\gamma_r \cos\beta}$).
2. **Type of flow:** pressure tunnels need sound rock and lining for leakage control; free-flow tunnels need about 15–25% freeboard area above the water surface.
3. **Shape of section:**
   - **Circular** – best for high internal pressure, minimum perimeter, used for TBM tunnels.
   - **Horseshoe / D-shaped (inverted-D)** – easy drill-and-blast excavation and flat invert for traffic, common in Nepal.
   - **Modified horseshoe** – for external rock pressure.
4. **Size (diameter):** fixed by hydraulic and economic considerations—the **economic diameter** minimises the sum of construction cost and value of energy lost in friction; minimum size about 2.0–2.5 m for construction access. Usual velocities: 1–2.5 m/s in unlined, 2–4 m/s in lined tunnels.
5. **Hydraulic losses:** head loss by Manning, $h_f = \dfrac{n^2 V^2 L}{R^{4/3}}$; smooth lining reduces losses ($n \approx 0.012$–0.014 concrete, 0.03–0.04 unlined).
6. **Lining and support:** unlined in good rock; shotcrete and rock bolts in fair rock; concrete or steel lining in weak rock, low-cover zones and near the powerhouse. Lining reduces friction and leakage and resists water pressure.
7. **Grouting and drainage:** contact and consolidation grouting; drainage to relieve external water pressure when the tunnel is empty.
8. **Slope:** gentle slope (about 1:300–1:1000) towards the outlet for drainage and flushing; pressure tunnels kept below the minimum hydraulic grade line everywhere.
9. **Transient (water hammer/surge) design:** surge tank at the downstream end to limit pressure rise and allow quick load changes.
10. **Rock trap and access:** a rock trap near the downstream end and adits for construction, inspection and maintenance.

```text
   Circular        Horseshoe        D-shaped
    .--.            .--.             .--.
   /    \          /    \           /    \
  |      |        |      |          |    |
   \    /          \    /           |    |
    '--'            \__/            |____|
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which alignment practice is recommended for a hydraulic tunnel?
   A. Straight and short, through sound rock, crossing weak zones at right angles
   B. Curved to follow the river, crossing faults along their strike
   C. Through the weakest strata so excavation is easy
   D. As deep as possible regardless of length
   Answer: A. The tunnel should be straight, short, in sound rock and cross faults/shear zones at right angles with adequate rock cover. (Key point: Tunnel alignment)

2. Why is adequate rock cover needed over a pressure tunnel?
   A. To reduce the friction factor of the lining
   B. The rock weight must resist the internal water pressure
   C. To provide freeboard above the water surface
   D. To make the tunnel slope gentler
   Answer: B. Cover (Norwegian criterion) ensures the rock weight resists the internal pressure and prevents uplift or hydraulic fracturing. (Key point: Rock cover)

3. How is the economic diameter of a tunnel fixed?
   A. It is the smallest diameter that can be excavated
   B. It gives the maximum velocity allowed
   C. It minimises construction cost plus the value of energy lost in friction
   D. It equals the diameter of the penstock
   Answer: C. Economic diameter balances construction cost against the value of friction energy loss. (Key point: Economic diameter)

4. What is the purpose of a surge tank at the downstream end of a pressure tunnel?
   A. To trap rocks entering the tunnel
   B. To reduce the friction in the tunnel lining
   C. To provide the freeboard for free-flow
   D. To limit pressure rise from water hammer and allow quick load changes
   Answer: D. The surge tank handles transient (water hammer/surge) effects; a rock trap is a separate feature. (Key point: Surge tank and slope)

## Related in Hydro Power

- [Best hydraulic rectangular canal design](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Abest-rectangular-canal-design) — Hydro Power, chapter 1
- [Criteria for designing and laying out tunnels](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Ahydraulic-tunnel-design-layout) — Hydro Power, chapter 5, asked 2×

## Related in other subjects

- [Hydraulic design of tunnel; alignment criteria](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-hydraulic-design-alignment-selection) — Hydropower Engineering, chapter 6
- [Hydraulic design of surge tank and pressure tunnel](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-tank-hydraulic-design-pressurised-tunnel) — Hydropower Engineering, chapter 6
- [Hydraulic and energy grade lines](https://www.bctengineer.com/question.md?key=semester_4%2Ffluid_mechanics_mechanical%3Ahgl-egl-reservoir-pump-turbine) — Fluid Mechanics, chapter 8
- [Hydraulic jump stilling basin at spillway toe](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astilling-basin-hydraulic-jump-80cumec-2070) — Hydropower Engineering, chapter 4