---
title: "Short note: Debris flow"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:short-note-debris-flow"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-debris-flow
site: BCT Engineering
---

# Short note: Debris flow

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2065 Shrawan (old course), 2 marks
- Video: [Debris Flow Events in Colorado](https://www.youtube.com/watch?v=EXtBCR9ySyA) — Mark Newby
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-debris-flow
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

Write a short note on debris flow.

## Short answer

A debris flow is a rapid, water-saturated mixture of soil, rock and boulders (50–80% solids) moving down a steep channel like wet concrete. It has an initiation zone, transport channel with levees, and a deposition fan. Control measures include check dams, debris basins, channel lining, and early warning systems.

## Answer

A **debris flow** is a rapid flow of a water-saturated mixture of soil, rock fragments, boulders and organic matter down a steep channel, under gravity. It contains 50-80% solids by volume and behaves like wet concrete.

- **Causes:** intense, prolonged rainfall; abundant loose debris from old landslides; steep gullies; sudden release of landslide dams or glacial lakes; earthquakes.
- **Features:** three parts: initiation zone (source), transport channel with levees, and a deposition fan at the foot. Boulders concentrate at the front.
- **Speed:** from 1 m/s up to over 10 m/s, so it is destructive.
- **Occurrence:** frequent in the Siwalik and Lesser Himalaya during the monsoon.
- **Control:** check dams, debris basins, channel lining, deflection walls, planting, early warning, and keeping settlement away from fans and gully mouths.

## Check yourself

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

1. A debris flow is:
   A. Slow creep of dry soil on gentle slope
   B. Fall of single boulders through air
   C. River flow carrying only clear water
   D. Rapid flow of a water-saturated mixture of soil, rock, boulders and organic matter down a steep channel
   Answer: D. It behaves like wet concrete with 50-80% solids by volume. (Key point: Definition)

2. The three parts of a debris flow path are:
   A. Initiation zone, transport channel with levees, and deposition fan
   B. Crown, scarp and toe
   C. Source, delta and lake
   D. Head, body and foot of a landslide
   Answer: A. Boulders concentrate at the front. (Key point: Parts)

3. Which is a control measure for debris flow?
   A. Removing check dams
   B. Check dams and debris basins
   C. Building houses on fans
   D. Cutting more slope toes
   Answer: B. Settlement should be kept away from fans and gully mouths. (Key point: Control)

4. Debris flows are frequent in:
   A. The Terai plain in winter
   B. The Tethys zone in dry season only
   C. The Siwalik and Lesser Himalaya during monsoon
   D. The ocean floor
   Answer: C. Steep gullies and loose debris plus intense rain favour them. (Key point: Occurrence)

## Related in Engineering Geology I

- [Landslide versus debris flow](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Alandslide-versus-debris-flow) — Engineering Geology I, chapter 6

## Related in other subjects

- [Landslide versus debris flow](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Alandslide-vs-debris-flow) — Engineering Geology II, chapter 5, asked 2×