Failure of the moraine or ice dam of a glacial lake

Prolonged monsoon rainfall over the catchment

Overflow of a river onto floodplains by snowmelt

Rising groundwater in the Terai

How does the onset of a GLOF compare with an ordinary flood?

GLOF is very sudden, often within minutes to hours; flood is gradual to rapid over hours to days

Flood is sudden, GLOF is gradual

Why is warning time for a GLOF very short compared with a flood?

Floods can be forecast from rainfall and river gauges, while GLOFs need lake monitoring and early-warning systems

Floods cannot be forecast at all

GLOF sources are in the lowlands

Which statement about load and travel of a GLOF is correct?

It carries huge boulders, moraine debris and ice like a debris flow and travels tens to over 100 km down a narrow valley

It carries only fine suspended sediment

It spreads over wide floodplains in lower reaches only

It stops within a few kilometres

2078 Poush

Flood versus 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.

Differentiate between flood and GLOF.

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).

✓ Answer checked by Claude Opus 5.5 (high effort)

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