Himalayan cascades: avalanches, GLOFs and cloudbursts

Detail

What a "cascade" is

• Cascade hazard: one hazard sets off the next, like a chain of falling dominoes. For example, a slope collapses, the falling mass hits a lake, the lake bursts, and a flood carrying debris rushes down the valley.
• In the Himalayas, three hazards often join up in this way: avalanches, GLOFs and cloudbursts. Together they cause flash floods and debris flows.
• Flash flood: a flood that rises within minutes or hours, with very little warning.
• Debris flow: a fast, thick mix of water, mud, rock and boulders that pours down a steep channel.

Feature	Avalanche	GLOF	Cloudburst
What moves	Snow (sometimes with rock and ice)	Water stored in a glacial lake	Rainwater
Main season	Winter and early spring	Summer and monsoon, when there is most meltwater	Monsoon (June-September)
Key trigger	Weak snow layers, warming, quakes	Dam breach by overfilling, a landslide, an avalanche or a quake	Monsoon air pushed up by the mountains
Indian example	Siachen, Zojila, Gurez	South Lhonak 2023; Chorabari 2013	Leh 2010; Kedarnath 2013

Avalanches

• Avalanche: a sudden fall of a large mass of snow down a steep slope.
• Triggers:
• Heavy snow falls in a short spell on top of a weak layer (a loose, poorly bonded layer of older snow). The new load is too heavy for it and the slab slides.
• Sudden warming and partial melting. Meltwater weakens the bonds between snow grains.
• Wind piles snow into deep drifts on the slopes facing away from the wind. These wind-loaded drifts are unstable.
• Earthquakes and vibration.

• Human activity: skiing, trekking, and road or tunnel construction.

• Prone areas in India:

• J&K: Gurez (Kishanganga valley, near the Line of Control).
• Ladakh: Siachen (Karakoram range) and Zojila (the pass on the Srinagar-Leh road).
• Himachal Pradesh: Lahaul-Spiti and Kinnaur.

• Uttarakhand and Sikkim.

• Warning agency: the Defence Geoinformatics Research Establishment (DGRE), part of DRDO and formerly called SASE (Snow and Avalanche Study Establishment), issues avalanche warnings.

• Who is most exposed: troops at high-altitude posts such as Siachen, and highways and passes such as Zojila.

Glacial lake outburst floods (GLOFs)

• GLOF (glacial lake outburst flood): a sudden release of water from a lake held back by a glacier or by glacier debris. Class 7 calls it a "glacial burst".
• Moraine: the heap of rock, gravel and soil that a glacier carries and dumps. A moraine dam is loose and weak.

• How a GLOF happens: 1. Lake grows. Warming melts the glacier. The meltwater collects behind a moraine dam or an ice dam, and the lake gets bigger every year. 2. Dam breaks. One of these triggers breaks the dam:

  • overfilling;
  • heavy rain or snow;
  • an avalanche or landslide into the lake, which pushes up a big wave;
  • an earthquake. 3. Flood rushes down. A huge volume of water and debris surges down the valley within minutes or hours.

• Policy response:

• NDMA Guidelines on Management of GLOFs (2020).
• A national GLOF risk-mitigation programme (2024) for high-risk lakes (verify current). Its aims are to assess lakes, put in automatic monitoring and early-warning systems, and lower lake levels.
• NRSC (National Remote Sensing Centre, ISRO) keeps an inventory of glacial lakes mapped from satellite images.
• CWC (Central Water Commission) monitors glacial lakes and water bodies.

South Lhonak GLOF 2023 (north Sikkim)

• When and where: 3-4 October 2023, at South Lhonak lake, a moraine-dammed lake in north Sikkim.
• Course of the flood: the surge went down the Teesta.
• The Teesta rises in the glaciers of north Sikkim.
• It flows south through Sikkim and West Bengal.

• It then enters Bangladesh and joins the Brahmaputra (Jamuna).

• Damage:

• It destroyed the Teesta III dam at Chungthang.

• It killed many people, including soldiers.

• Lesson: a dam built downstream of a known high-risk lake became part of the disaster.

Cloudbursts

• Cloudburst: the IMD defines it as 100 mm or more of rain in one hour over an area of about 20-30 sq km.
• Why cloudbursts strike the Himalayas:
• Moist monsoon winds hit the mountain wall.
• The air is forced up. This is called orographic lifting.
• Rising air cools, its water vapour condenses, and tall clouds build up quickly.

• All the rain falls on a small area in a short time.

• Result: flash floods, landslides and debris flows in narrow valleys.

• Examples:
• Leh, Ladakh (2010): a cloudburst in a normally cold-desert area set off debris flows through the town.
• Himachal Pradesh (2023 and 2025) (verify current).

Case study 1: Kedarnath disaster 2013 (Uttarakhand)

• When: 16-17 June 2013, early in the monsoon.
• Cause chain:
• The monsoon arrived early.
• A western disturbance joined it. A western disturbance is a low-pressure system that comes from the Mediterranean region and travels east.
• Together they produced extreme rain.

• Chorabari lake, above Kedarnath at the snout of the Chorabari glacier, filled up and breached.

• Course of the flood:

• Floods and debris rushed down the Mandakini.

• The Mandakini rises near Kedarnath and joins the Alaknanda at Rudraprayag.

• Toll: about 6,000 dead, many of them pilgrims of the Char Dham yatra.

Case study 2: Chamoli flood 2021 (Uttarakhand)

• When: 7 February 2021. It came in winter, so it was not a monsoon event.
• Cause chain:
• A rock-and-ice avalanche broke away from Ronti peak.
• About 27 million m³ of rock and glacier ice collapsed from the steep north face of Ronti Peak [2].
• The falling mass quickly turned into a very large, fast debris flow [2].
• The flow carried boulders more than 20 m across [2].

• It scraped the valley walls up to 220 m above the valley floor [2].

• Course of the flood: Ronti Gad → Rishiganga → Dhauliganga valleys [2]. The Dhauliganga then joins the Alaknanda.

• Damage:
• It wrecked the Rishiganga and Tapovan-Vishnugad hydropower projects.
• About 200 people were killed or went missing [2].

Common thread

• Warming: lakes grow, permafrost and glacier ice weaken, and rain becomes more extreme. (The climate-change note covers warming as a driver.)
• Fragile slopes: the Himalayas are young, steep, earthquake-prone mountains with loose debris.
• Exposure: hydropower dams, roads and large crowds of pilgrims sit inside the hazard zone. This links to the Char Dham road debate.
• Result: one trigger high up can kill people tens of kilometres downstream.

Prelims Hooks

• IMD cloudburst threshold: 100 mm or more of rain in 1 hour over about 20-30 sq km.
• DGRE (formerly SASE, under DRDO) issues avalanche warnings. It is not IMD.
• Chorabari lake → Mandakini → Kedarnath disaster (June 2013). Trap: it was not the Bhagirathi.
• Ronti peak → Ronti Gad → Rishiganga → Dhauliganga → Chamoli flood (7 February 2021). It was a rock-and-ice avalanche, not a lake outburst.
• South Lhonak lake (north Sikkim) → Teesta → Teesta III dam at Chungthang destroyed (October 2023).
• Avalanche-prone places: Siachen and Zojila are in Ladakh, Gurez is in J&K, and Lahaul-Spiti and Kinnaur are in Himachal Pradesh.
• "Glacial burst" (Class 7) means the same thing as a GLOF.
• NDMA GLOF Guidelines came out in 2020.
• Leh 2010 was a cloudburst in a cold desert.

Mains Points

• Cascading risk (GS-III): in Kedarnath 2013, Chamoli 2021 and South Lhonak 2023, a trigger high in the mountains turned into a debris flood far downstream. Planning has to cover the whole valley, not just the single hazard site.
• Development in hazard zones: hydropower (Tapovan-Vishnugad, Teesta III), Char Dham roads and pilgrim crowds raise exposure. Possible responses:
• carry out cumulative impact assessment for each river basin;
• keep dams away from the paths of high-risk lakes;

• cap pilgrim numbers.

• Early warning gap:

• What is in place: NRSC's glacial-lake inventory, CWC monitoring, the NDMA 2020 guidelines and a national programme for high-risk lakes.

• What is still needed: automatic sensors, lake lowering, and sirens and evacuation drills in villages. This follows the Sendai Framework's push for "early warning for all".

• Climate link (GS-I/III): warming is enlarging glacial lakes and making rain more extreme. Cloudbursts and GLOFs are therefore becoming more frequent. This calls for climate-resilient infrastructure and cooperation with Nepal, China and Bhutan on shared river basins.