---
title: "Methods of flood control"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:flood-control-measures"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/flood-control-measures
site: BCT Engineering
---

# Methods of flood control

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2070 Chaitra (old course), 4 marks; 2065 Shrawan (old course), 3 marks; 2063 Asoj (old course), 4 marks; 2062 Baisakh (old course), 4 marks (asked 4 times)
- Video: [How Do Flood Control Structures Work?](https://www.youtube.com/watch?v=5mCJh5SJEis) — Practical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/flood-control-measures
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Explain the different (structural) methods of flood control measures.

## Short answer

Structural flood control methods include storage reservoirs and detention basins to store flood water, embankments (levees) to confine flow, channel improvement and cut-offs to increase capacity, diversion floodways to bypass excess water, and bank protection/spurs. Watershed management measures like afforestation also reduce runoff.

## Answer

Flood control aims to reduce flood damage by storing, diverting, confining or reducing the flood flow. The structural methods are as follows.

#### 1. Storage reservoirs
A dam stores flood water and releases it gradually, cutting the peak flow downstream. It is the most effective method but also the most costly. In Nepal, large storage projects of this kind have been proposed on the Koshi and Karnali river systems.

#### 2. Detention basins and retarding basins
Low-lying areas or small ponds hold flood water temporarily.

#### 3. Embankments (levees or dykes)
Earth banks built along the river confine the flood and prevent spilling. The flood level rises between the embankments, so they need adequate free-board and must be raised periodically.

```text
   ____ levee               levee ____
  /    \_______ river _______/    \
 /  flood plain protected          \
```

#### 4. Channel improvement
Straightening, widening, deepening, desilting or removing obstacles increases the carrying capacity and lowers flood levels.

#### 5. Cut-offs
A shortcut across a meander neck steepens the slope and lowers flood levels upstream.

#### 6. Diversion and flood-ways
Excess flood water is diverted to a side channel, lake or another basin through a spillway or gated weir.

#### 7. Bank protection and river training
Spurs, revetments and guide bunds protect the banks and hold the river in its course.

#### 8. Watershed management
Afforestation, check dams and terracing in the catchment reduce run-off and sediment.

#### 9. Flood proofing and warning
Raised platforms, flood warning systems and flood-plain zoning reduce damage to people and property (these are non-structural, but are used together with the above).

## Check yourself

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

1. How does a storage reservoir reduce floods?
   A. It removes silt from the river
   B. It stores flood water and releases it gradually, cutting the peak
   C. It speeds up the flood wave
   D. It raises downstream flood levels
   Answer: B. Storing flood water and releasing it slowly lowers the peak flow downstream; it is the most effective but costly method. (Key point: Storage reservoirs)

2. What is a drawback of embankments (levees)?
   A. They lower flood level
   B. They increase the area flooded outside
   C. They cannot confine floods
   D. Flood level rises inside them, so they need raising and free-board
   Answer: D. Confining the flood raises its level within the embankments, so free-board and periodic raising are needed. (Key point: Embankment effect)

3. What does a cut-off do to flood levels?
   A. Steepens slope and lowers flood levels upstream
   B. Has no effect
   C. Lengthens the river and raises levels
   D. Reduces slope and raises levels
   Answer: A. A shortcut across a meander neck shortens the river, steepening slope and lowering flood levels upstream. (Key point: Cut-offs)

4. What do afforestation, check dams and terracing in the catchment achieve?
   A. Increase peak run-off
   B. Cut run-off and sediment
   C. Build embankments
   D. Divert floods to another basin
   Answer: B. Catchment treatment reduces run-off and sediment reaching the river. (Key point: Watershed management)

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