---
title: "RoR layout: pressure tunnel and free surface"
subject: "Hydropower Engineering"
chapter: 2
key: "semester_7/hydropower_engineering:ror-layout-pressure-tunnel-free-flow"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-layout-pressure-tunnel-free-flow
site: BCT Engineering
---

# RoR layout: pressure tunnel and free surface

Hydropower Engineering · Chapter 2: Planning of Hydropower Projects

- Asked in: 2078 Bhadra, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-layout-pressure-tunnel-free-flow
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-2

## Question

Draw a layout (plan and section) of a ROR hydro project for the following cases: (i) alignment with pressure tunnel, (ii) with free surface flow. Name the salient features also (draw with representative contours).

## Short answer

Pressure tunnel layout: weir/intake, desander, pressure tunnel, surge tank, penstock, powerhouse; safe from landslides, for steep valleys. Free surface flow layout: weir/intake, desander, open canal (slope ~1:1000), forebay, penstock, powerhouse; cheaper, needs gentle terrain and cross-drainage works.

## Answer

#### (i) Alignment with pressure tunnel
```text
Plan (contours 1000-1400 m)
        ____________
 ~~ River ~~ Weir/Intake/Desander
              \
               \== Headrace tunnel ===+ Surge tank
                                      |
                              Penstock (steep)
                                      |
                                  PH -> Tailrace -> River
Section
 FSL
  ~~|intake|=============== HGL =======(ST)
                      tunnel             \ penstock
                                          \__PH
```
**Salient features:** Weir/barrage, intake with trash rack and gate, gravel trap, settling basin, headrace pressure tunnel (circular, lined), surge tank, penstock (steel, surface or buried), powerhouse (surface or underground), switchyard, tailrace and transmission line. The tunnel flows full under pressure; HGL stays above the crown. Shorter than the canal, safe from slides, used in steep valleys.

#### (ii) Alignment with free-surface flow
```text
Plan
 ~~ River ~~ Weir/Intake/Desander
                 \_______ Open canal / free-flow tunnel
                         \______ Forebay
                                   \ Penstock
                                    PH -> River
Section
 ___weir__intake___ canal (S = 1:1000)______forebay
                                              \ penstock
                                               \_PH
```
**Salient features:** Weir, intake, desander, free flow canal (or tunnel flowing partly full with slope 1:1000), spillway at forebay, forebay with trash rack, penstock, powerhouse and tailrace. Cheaper, but the head is lost to the slope, and the canal needs bends and cross-drainage works; needs gentle terrain.

## Check yourself

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

1. What is true of the pressure tunnel?
   A. Has slope 1:1000
   B. Needs forebay spillway
   C. Flows full with HGL above crown
   D. Flows partly full
   Answer: C. It flows under pressure. (Key point: Pressure tunnel)

2. What is a disadvantage of free-surface canal?
   A. Expensive
   B. Head is lost to slope and needs cross-drainage
   C. Unsafe tunnels
   D. Needs pressure
   Answer: B. It needs gentle terrain. (Key point: Free-flow loss)

3. Which feature is in the free-flow layout but not pressure?
   A. Pressure tunnel
   B. Underground powerhouse
   C. Surge tank
   D. Forebay with spillway
   Answer: D. Pressure uses surge tank instead. (Key point: Salient features)

## Related in Hydropower Engineering

- [Development cycle; RoR layout with headrace canal](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adev-cycle-ror-layout-headrace-canal) — Hydropower Engineering, chapter 2
- [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
- [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3
- [Optimizing plant capacity of a RoR project](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-optimum-capacity-discussion) — Hydropower Engineering, chapter 3
- [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
- [Development cycle; storage plant layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adev-cycle-storage-layout) — Hydropower Engineering, chapter 2

## Related in other subjects

- [Tunnel advantages and pressure tunnel section](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Atunnel-advantages-pressure-tunnel) — Hydro Power, chapter 5
- [Tunnel alignment survey and ideal bridge location](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-alignment-survey-ideal-bridge-location) — Transportation Engineering II, chapter 5
- [Tunnel alignment from three boreholes](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-alignment-three-boreholes) — Engineering Geology II, chapter 6
- [Indirect sub-surface methods: tunnel and dam](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aindirect-subsurface-tunnel-dam) — Engineering Geology II, chapter 4