Was glacial lake breach over Dharali the trigger?

Geographical & Geomorphological Context

• Location: Dharali village, Uttarakhand, near Kheer Ganga River.
• Terrain:
  • Glaciated region in upper catchment.
  • Presence of deglaciated valleys bounded by end moraines (glacially deposited sediment ridges).
  • Alluvial fan at Dharali — formed from debris deposition of past floods/landslides.
• River Characteristics: Steep gradient — increases velocity and destructive potential of floods.

Relevance : GS 1(Geography ), GS 3(Disaster Management)

Possible Causes of Disaster (Hypotheses from Experts)

• Glacial Lake Outburst Flood (GLOF) Scenario:
  • Evidence:
    • September 2022 satellite images show past lake formation signatures above end moraine.
    • Meandering stream above end moraine suggests low-gradient, ponding-prone terrain.
    • Deglaciated valley features indicate possible historical impoundment.
  • Trigger: Landslide from end moraine blocking drainage, forming lake; later breach due to glacier mass movement or instability.
  • Outcome: Sudden release of water and debris → flash flood and mudflow.
• Glacial Snout Detachment:
  • NDMA’s alternative hypothesis: Partial collapse of glacier nose carrying large sediment load downstream.
• Landslide Reactivation:
  • Landslide 2 km upstream reportedly reactivated, potentially blocking and then breaching river channel.

Contributing Factors

• Geological Vulnerability:
  • Unstable moraines and steep slopes.
  • Past flood and debris flow evidence in Kheer Ganga basin.
• Anthropogenic Stress:
  • Rapid commercialisation & tourism infrastructure on alluvial fan (unstable landform).
  • Construction of Dharasu–Gangotri highway increasing slope instability and altering drainage.
• Climate Link:
  • Possible link to glacier retreat and increased meltwater ponding due to warming.

Disaster Dynamics

• GLOF Mechanics:
  • Dam formation (by moraine/landslide) → water accumulation → dam breach.
  • Steep channel → high energy flood with sediment and debris entrainment.
• Alluvial Fan Vulnerability:
  • Channels can shift during floods, spreading destruction beyond main river course.
  • Floodwaters in steep terrain travel rapidly, giving minimal early warning.

Remote Sensing & Investigation

• Current Effort: NDMA coordinating with National Remote Sensing Centre (NRSC) for sharper pre-disaster imagery to confirm lake presence and breach dynamics.
• Indicators to be checked:
  • Pre-disaster water body size and location.
  • Landslide activity on moraines or adjacent slopes.
  • Glacier snout changes (fracture, retreat).

Broader Significance

• Early Warning Needs:
  • Regular monitoring of high-risk glacial lakes in Uttarakhand using satellites.
  • Mapping of unstable moraines and slope movement zones.
• Land-use Regulation:
  • Avoiding infrastructure and dense settlements on alluvial fans in Himalayan valleys.
• Climate Adaptation:
  • Integrating glacial hazard mapping into tourism and highway development plans.