---
title: "Spillway function, vertical and radial gates"
subject: "Hydro Power"
chapter: 6
key: "semester_6/hydro_power:spillway-function-vertical-radial-gate"
source: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-function-vertical-radial-gate
site: BCT Engineering
---

# Spillway function, vertical and radial gates

Hydro Power · Chapter 6: Spillways

- Asked in: 2075 Bhadra, 2+4 marks
- Video: [How Different Spillway Gates Work](https://www.youtube.com/watch?v=Ry_3icwd68o) — Practical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-function-vertical-radial-gate
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6

## Question

What is the function of a spillway? Describe with sketches the vertical and radius gate use in spillway.

## Short answer

A spillway safely passes surplus flood water to prevent dam overtopping and keep the reservoir within safe limits. Vertical lift gates are rectangular gates moving in pier slots; they need large hoisting force and disturb flow. Radial gates have a curved skin plate on arms pivoted at trunnions; water thrust passes through trunnion, requiring only a small hoisting force.

## Answer

#### Function of a spillway
A **spillway** safely passes surplus flood water from the reservoir to the downstream river. Its functions are:
- to prevent **overtopping** and failure of the dam during floods;
- to keep the reservoir below the maximum water level;
- to release water without eroding the dam toe or riverbed (with an energy dissipator).

#### Vertical lift gate
A **vertical lift gate** is a rectangular steel gate that moves vertically in grooves (slots) in the piers.

```text
     hoist / gantry crane
     ______|______
    | |    |    | |  pier
    | |=========| |  gate (raised to open)
 ~~~| |  skin   | |
    | |  plate  | |
 ___|_|_________|_|___ crest
      wheels in pier slots
```
- **Types:** plain sliding gate (slides on steel faces, high friction), **fixed-wheel gate** (wheels on axles run on rails in the piers), **Stoney gate** (free roller train between gate and pier).
- The full water thrust is carried by the wheels to the piers, so the lifting force must overcome gate weight + friction of wheels/seals; a high hoist bridge or gantry crane is needed.
- **Advantages:** simple, compact in the flow direction, can be lifted out completely for maintenance, suits stop-log/emergency use.
- **Disadvantages:** large hoisting force, tall piers and hoist structure, slots disturb flow and may cause cavitation.

#### Radial (Tainter) gate
A **radial gate** has a curved skin plate (a segment of a cylinder) carried by radial arms on trunnion pins anchored in the piers.

```text
            hoist
              |
   ~~~~~~~~~  |\  curved skin plate
   ~~~~~~~~~  | \___ arms ___ o trunnion
              | /         _/
   ___________|/_________/_____
       crest         flow -->
```
- Water pressure on the curved plate passes through the trunnion, so **no moment** about the pin; the hoist lifts only the gate weight and friction—**very small hoisting force**.
- It rotates upward to open; flow passes under the gate.
- **Advantages:** light hoist, smooth flow (no deep slots), suits large spans, easy automatic control; most used on large spillways.
- **Disadvantages:** long piers needed for trunnions, heavy concentrated trunnion loads.

## Check yourself

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

1. What is the main function of a spillway?
   A. To generate power from surplus water
   B. Safely pass surplus flood water to the downstream river and prevent overtopping of the dam
   C. Store sediment carried by the river
   D. Supply water to the penstock at constant head
   Answer: B. A spillway passes surplus flood water downstream so the reservoir stays below maximum level and the dam is not overtopped. (Key point: Function of a spillway)

2. Why does a radial (Tainter) gate need only a small hoisting force?
   A. Its skin plate is made very thin
   B. Water pressure on the curved plate passes through the trunnion pin, giving no moment about it, so the hoist lifts only the gate weight and friction
   C. It is lifted by wheels running on rails in the piers
   D. It opens by sliding on steel faces with low friction
   Answer: B. Pressure acts radially through the trunnion, so the water load causes no moment; the hoist handles only weight and friction. (Key point: Radial gate hoisting force)

3. In a fixed-wheel vertical lift gate, how is the water thrust carried and what must the hoist overcome?
   A. Thrust goes to the trunnion; hoist overcomes only gate weight
   B. Thrust goes to the crest; hoist overcomes only seal leakage
   C. Thrust is carried by the wheels to the piers; hoist must overcome gate weight plus wheel and seal friction
   D. Thrust is carried by the hoist cable; hoist overcomes only buoyancy
   Answer: C. The wheels transfer the full thrust to the piers, and lifting must overcome weight plus friction, so a large hoist is needed. (Key point: Vertical lift gate loads)

4. Which is a disadvantage of a radial gate compared with a vertical lift gate?
   A. Large hoisting force required
   B. Deep slots that disturb flow
   C. Cannot be used on large spans
   D. Long piers needed for trunnions and heavy concentrated trunnion loads
   Answer: D. Radial gates need long piers to hold the trunnions and these carry heavy concentrated loads; vertical gates have slots and need large hoisting force. (Key point: Gate comparison)

## Related in Hydro Power

- [Spillway gates and radial gate](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-gates-radial-gate) — Hydro Power, chapter 6
- [Types of spillway gates](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-gate-types) — Hydro Power, chapter 6, asked 2×

## Related in other subjects

- [Purposes of spillway; types of gates](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purposes-and-gate-types) — Hydropower Engineering, chapter 4
- [Types of spillway crest gates](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-crest-gate-types) — Hydropower Engineering, chapter 4, asked 2×
- [XOR gate using minimum NAND gates](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Axor-using-nand-only) — Digital Logic, chapter 2, asked 2×
- [X-OR gate using NAND gates only](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Axor-using-nand) — Basic Electronics Engineering, chapter 6