---
title: "Flood versus GLOF"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:flood-vs-glof"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/flood-vs-glof
site: BCT Engineering
---

# Flood versus GLOF

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2078 Poush
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/flood-vs-glof
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Differentiate between flood and GLOF.

## Short answer

A flood is the overflow of a river due to heavy rain or snowmelt, with gradual onset and moderate peak discharge. A GLOF is a sudden, catastrophic release from a glacial lake when its moraine or ice dam fails, generating extremely high discharge and debris.

## Answer

A **flood** is the overflow of a river onto normally dry land, mostly caused by prolonged or intense rainfall (monsoon) or snowmelt. A **GLOF (Glacial Lake Outburst Flood)** is a sudden release of a large volume of water from a glacial lake when its moraine or ice dam fails.

| Basis | Flood | GLOF |
|---|---|---|
| Cause | Heavy monsoon rain, cloudburst, rapid snowmelt, dam or embankment breach | Failure of moraine/ice dam of a glacial lake (ice or rock avalanche into lake, piping, earthquake, overtopping) |
| Source of water | Rainfall runoff in the catchment | Water stored in a high-altitude glacial lake |
| Onset | Gradual to rapid (hours to days) | Very sudden, often within minutes to hours |
| Peak discharge | Moderate, rises slowly | Extremely high, many times the normal flow |
| Warning time | Can be forecast from rainfall and river gauges | Very little; needs lake monitoring and early-warning systems |
| Sediment and debris | Mainly suspended sediment and some debris | Huge load of boulders, moraine debris and ice; behaves like a debris flow |
| Season | Mostly June to September | Any time, but more common in summer (July to September) |
| Affected area | Wide floodplains, mostly in lower reaches | Narrow valley floor, travels very far downstream (tens to over 100 km) |
| Nepal example | Koshi flood 2008, Melamchi flood 2021 | Dig Tsho 1985 (Bhote Koshi, Namche hydropower destroyed), Tam Pokhari 1998 |

**Control:** floods are controlled by embankments, spurs, dams and early warning. GLOF hazard is reduced by lowering lake level (siphons, tunnels, open channels), strengthening the moraine dam and installing monitoring and warning systems (e.g. Tsho Rolpa).

## Check yourself

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

1. What causes a GLOF?
   A. Prolonged monsoon rainfall over the catchment
   B. Failure of the moraine or ice dam of a glacial lake
   C. Overflow of a river onto floodplains by snowmelt
   D. Rising groundwater in the Terai
   Answer: B. A GLOF is the sudden release of water when a glacial lake's moraine or ice dam fails. (Key point: Cause of GLOF)

2. How does the onset of a GLOF compare with an ordinary flood?
   A. GLOF is slower, taking days
   B. Both rise equally slowly
   C. GLOF is very sudden, often within minutes to hours; flood is gradual to rapid over hours to days
   D. Flood is sudden, GLOF is gradual
   Answer: C. GLOFs release stored water abruptly, whereas rainfall floods build over hours to days. (Key point: Onset)

3. Why is warning time for a GLOF very short compared with a flood?
   A. Floods cannot be forecast at all
   B. GLOFs occur only at night
   C. GLOF sources are in the lowlands
   D. Floods can be forecast from rainfall and river gauges, while GLOFs need lake monitoring and early-warning systems
   Answer: D. Rain-driven floods follow gauge and rainfall data; GLOFs are sudden and need dedicated lake monitoring. (Key point: Warning)

4. Which statement about load and travel of a GLOF is correct?
   A. It carries only fine suspended sediment
   B. It carries huge boulders, moraine debris and ice like a debris flow and travels tens to over 100 km down a narrow valley
   C. It spreads over wide floodplains in lower reaches only
   D. It stops within a few kilometres
   Answer: B. GLOFs behave like debris flows and travel far along narrow valleys. (Key point: Debris and reach)

## Related in Engineering Geology II

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

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- [Flood frequency graph, 30-year flood, 1981-2000](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Areturn-period-graph-30yr-1981-2000) — Engineering Hydrology, chapter 6
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