---
title: "Forebay and surge tank comparison"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:forebay-vs-surge-tank"
source: https://www.bctengineer.com/q/semester_6/hydro_power/forebay-vs-surge-tank
site: BCT Engineering
---

# Forebay and surge tank comparison

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2080 Asoj, 3+3 marks; 2072 Asoj, 4 marks (asked 2 times)
- Video: [Forebay and Surge Tanks: Key Components and Hydraulic Design 🌊⚙️](https://www.youtube.com/watch?v=W3sEhaoyfQ0) — Hydropower
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/forebay-vs-surge-tank
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

Why are forebay and Surge Tanks provided in a hydropower plant? Mention the difference between these two.

## Short answer

Both handle sudden load changes in a hydro plant. A forebay is used at the end of an open canal to transition to pressure flow, store water, and spill excess. A surge tank is used at the end of a pressure tunnel to absorb water hammer and oscillations, with no spillway. Forebay water level is nearly constant; surge tank level oscillates.

## Answer

#### Why they are provided
Both are provided between the low-pressure conduit and the penstock to deal with **sudden changes of load** on the turbines.

- **Forebay** is provided at the end of an open **headrace canal**. It changes free-surface flow into pressure flow, stores water for load increases, spills water on load rejection, traps the last sediment and debris, and gives submergence to the penstock inlet.
- **Surge tank** is provided at the end of a **pressure tunnel or long pipeline** (as close to the powerhouse as possible). When the turbine gates close or open, it gives a free water surface that absorbs the water hammer, so only the short penstock below it is designed for high water hammer. It also supplies water when load increases and stores it when load decreases, and improves speed regulation.

```text
 Canal:  canal ==> [FOREBAY] ==> penstock ==> PH
 Tunnel: tunnel ==> [SURGE TANK] ==> penstock ==> PH
         (pressure)      free surface
```

#### Differences

| Point | Forebay | Surge tank |
|---|---|---|
| Used with | Open canal (free flow) | Pressure tunnel/pipe |
| Location | End of canal, head of penstock | End of pressure tunnel, near powerhouse |
| Main purpose | Transition and short-term storage | Relieve water hammer and surges |
| Water level | Nearly constant (canal level) | Oscillates up and down (mass oscillation) |
| Shape | Open, wide, shallow pond | Tall vertical shaft or chamber |
| Spillway | Has overflow spillway | Usually none; sized for max upsurge |
| Sediment | Acts as final settling basin with flushing | No settling function |
| Design basis | Storage volume (2–3 min of flow), submergence | Max upsurge, min downsurge, Thoma stability area |

## Check yourself

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

1. A forebay is used with ___ while a surge tank is used with ___.
   A. an open canal; a pressure tunnel or long pipeline
   B. a pressure tunnel; an open canal
   C. a weir; an intake
   D. a penstock; a tailrace
   Answer: A. Forebay ends an open canal; a surge tank is at the end of a pressure conduit. (Key point: Where each is used)

2. Which statement about water level is correct?
   A. Forebay level oscillates; surge tank level is constant
   B. Both are constant
   C. Forebay level is nearly constant; surge tank level oscillates (mass oscillation)
   D. Both oscillate equally
   Answer: C. The surge tank water level swings up and down; the forebay stays near canal level. (Key point: Water level behaviour)

3. The main purpose of a surge tank is to:
   A. Absorb water hammer so only the short penstock below sees high pressure
   B. Settle sediment
   C. Spill excess canal flow
   D. Increase gross head
   Answer: A. Its free surface reflects the wave, protecting the tunnel. (Key point: Main purpose of surge tank)

4. The surge tank design is based on:
   A. Settling velocity
   B. Trap efficiency
   C. Maximum upsurge, minimum downsurge and Thoma stability
   D. Storage of 2-3 minutes of flow
   Answer: C. Forebay is sized by storage and submergence; surge tank by surge limits and Thoma stability. (Key point: Design basis)

## Related in Hydro Power

- [Forebay function and surge tank types](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aforebay-function-surge-tank-types) — Hydro Power, chapter 5
- [Surge tank objectives and economic penstock diameter factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-objectives-economic-diameter) — Hydro Power, chapter 5
- [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×
- [Functions and layout of forebay](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aforebay-functions-layout) — Hydro Power, chapter 5, asked 2×

## Related in other subjects

- [Function of surge tank in hydro plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asurge-tank-function) — Power Plant Equipment, chapter 1
- [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×
- [Surge and water hammer: 60 m³/s, 30 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-60cumec-3-penstocks-30m-tank) — Hydropower Engineering, chapter 6, asked 2×