---
title: "Hydraulic design of surge tank and pressure tunnel"
subject: "Hydropower Engineering"
chapter: 6
key: "semester_7/hydropower_engineering:surge-tank-hydraulic-design-pressurised-tunnel"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/surge-tank-hydraulic-design-pressurised-tunnel
site: BCT Engineering
---

# Hydraulic design of surge tank and pressure tunnel

Hydropower Engineering · Chapter 6: Water Conveyance Structures

- Asked in: 2080 Bhadra, 3+3 marks
- Video: [Design of Surge Tank(Shaft) | Water Hammer Effect | Hydropower Engineering | IOE , TU , PU](https://www.youtube.com/watch?v=r1J4tINkknw) — Parash Joshi - Civil Construction and Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/surge-tank-hydraulic-design-pressurised-tunnel
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6

## Question

Briefly discuss the hydraulic design considerations of the surge tank and pressurised hydraulic tunnel.

## Short answer

Surge tank design considers location near powerhouse, type (simple, restricted, differential), cross-section larger than Thoma area, maximum upsurge/downsurge limits, oscillation period, and connection with overflow/air vents. Pressurized tunnel design includes economic diameter with velocity 2.5-4 m/s, friction and local losses, internal pressure including water hammer, adequate rock cover to prevent rupture, HGL above crown, concrete/steel lining, and grouting.

## Answer

#### Hydraulic design considerations of the surge tank
- **Location**: as close to the powerhouse as possible, on stable ground at a level high enough that the maximum surge level stays below the tank top, and the minimum level stays above the tunnel crown.
- **Type**: simple, restricted orifice, differential (Johnson), or with an upper/lower chamber, chosen by the head, topography and the required reduction of the surge.
- **Size**: the cross-section must be larger than the **Thoma area** for stability under small load changes:
$$
A_s\ge A_{th}=\frac{L\,A_t}{2g\,\alpha\,(H_0-h_f)}
$$
- **Surge levels**: maximum upsurge on full load rejection must not overflow, and minimum downsurge on full-load demand must not expose the tunnel crown (air entry) or the tank bottom; the oscillation period $T=2\pi\sqrt{LA_s/(gA_t)}$ should be checked with the governor.
- **Connection**: the area of the connecting orifice and the shaft should be sized to give small head loss, but enough damping.
- Provide an overflow, air vents, and access.

#### Hydraulic design considerations of a pressurised tunnel
- **Diameter and velocity**: from the economic diameter; velocity usually 2.5-4 m/s.
- **Friction and other losses**: Manning/Darcy, with $n$ of lining; plus entry, bend and transition losses.
- **Internal pressure**: the maximum internal pressure includes the static head and the surge/water-hammer rise.
- **Minimum rock cover**: the cover should resist the internal pressure (about $C_{rm}\ge\dfrac{h_s\gamma_w F}{\gamma_r\cos\beta}$); otherwise the section is steel-lined.
- **Hydraulic grade line** should stay above the crown at all times, in the transient too, so as to avoid vacuum/negative pressure.
- **Lining and grouting**: concrete lining for smoothness, steel lining where the rock is weak or the cover is low; contact and consolidation grouting; drainage.
- **Protection**: air vents and an emergency gate at the intake, and a surge tank at the downstream end to reduce the water hammer in the tunnel.

## Check yourself

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

1. What is the Thoma area criterion?
   A. Tank area equals tunnel area
   B. Tank height equals head
   C. Tank area must be larger than A_th = L A_t/(2g alpha (H0-hf)) for stability
   D. Tank area must be smaller than A_th
   Answer: C. Tanks smaller than the Thoma area give growing oscillations. (Key point: Thoma area)

2. What limits the minimum downsurge level?
   A. It must exceed max upsurge
   B. It must be at tailwater
   C. It must reach the powerhouse
   D. It must not expose the tunnel crown (air entry) or tank bottom
   Answer: D. Exposing the crown lets air enter and the tunnel loses pressure. (Key point: Surge levels)

3. Why must the HGL stay above the tunnel crown even in transients?
   A. To reduce discharge
   B. To allow free surface flow
   C. To increase friction
   D. To avoid vacuum or negative pressure
   Answer: D. Negative pressures can collapse the lining. (Key point: Hydraulic grade line)

4. What does maximum internal pressure comprise?
   A. Only static head
   B. Only friction loss
   C. Static head plus surge/water-hammer rise
   D. Tailwater level
   Answer: C. Transient rises add to the static head. (Key point: Internal pressure)

5. Why is a surge tank provided at the downstream end of the pressure tunnel?
   A. To store sediment
   B. To increase velocity
   C. To add head loss
   D. To reduce water hammer in the tunnel
   Answer: D. It reflects pressure waves and limits the length where water hammer acts. (Key point: Surge tank role)

## Related in Hydropower Engineering

- [Surge numerical: 13 m tank, 1.8 km tunnel](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-13m-tank-1-8km-tunnel) — Hydropower Engineering, chapter 6
- [Surge tank: tunnel 5 m dia, 28 cumec](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-tunnel-5m-28cumec) — Hydropower Engineering, chapter 6
- [Surge tank: 7.5 m³/s through 2.5 m tunnel](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-7-5cumec-2-5m) — Hydropower Engineering, chapter 6
- [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
- [Surge and water hammer: 25 m³/s, 15 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-25cumec-2-penstocks-15m-tank) — Hydropower Engineering, chapter 6, asked 3×
- [Surge and water hammer: 45 m³/s, 20 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-45cumec-3-penstocks) — Hydropower Engineering, chapter 6, asked 2×

## Related in other subjects

- [Time to discharge pressurised tank](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Apressurised-tank-emptying-orifice) — Fluid Mechanics, chapter 6
- [Surge tank functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-functions-design) — Hydro Power, chapter 5, asked 2×
- [Three reservoirs with pressurised closed tank](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Athree-reservoir-closed-tank-pressure) — Hydraulics, chapter 3
- [Structural components and the surge tank](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Astructural-components-surge-tank) — Power Plant Design, chapter 3