Rain, Landslides, and Flash Floods in the Himalayas: Causes, Risks, and Mitigation

Himalayan Flash Floods Latest News

• A flash flood in Dharali, Uttarkashi on August 5 killed at least four, left many missing, and caused widespread destruction. 
• Hundreds were displaced, and similar extreme weather events in Himachal Pradesh last month killed dozens and affected thousands.

Unclear Cause of Dharali Flash Flood Highlights Rising Risks

• Flash floods, often triggered by extreme rainfall causing landslides or mudslides, sweep debris into rivers, destroying everything in their path. 
• The exact cause of the August 5 Dharali flood remains uncertain, though a glacial lake breach is a possibility yet to be confirmed. 
• Despite normal monsoon levels in the area, the incident underscores growing disaster risks in ecologically fragile regions and the increasing difficulty of establishing effective early warning and mitigation systems.

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Understanding Rainfall Patterns and Cloudburst Concerns in Uttarkashi

• Despite heavy and continuous rainfall across Uttarakhand this month, Uttarkashi district experienced relatively low precipitation before the recent flash flood. 
• On August 3 and 4, when Haridwar and Udham Singh Nagar saw very heavy rains, Uttarkashi received below-normal rainfall. 
• According to IMD data, on August 5 — the flood day — Uttarkashi recorded 32 mm of rain, double the daily normal but far from an extreme rainfall event. 
• Cloudbursts, defined by IMD as at least 100 mm of rain in about an hour over a 10×10 km area, are highly localised and often go unrecorded if measuring instruments are absent in the affected zone. 
• Such intense, short-duration rain can trigger landslides and flash floods, as seen in Himachal Pradesh recently. 
• However, in Uttarkashi’s case, there is no evidence of a cloudburst or cloudburst-like event, even in remote areas.

Multiple Triggers Behind Flash Floods in the Himalayas

• Flash floods in the Himalayas are rarely caused by a single factor and do not always result directly from heavy rainfall or cloudbursts. 
• Such events typically require a combination of conditions, such as 
  • intense rain leading to landslides 
  • or mudslides that channel debris into rivers, steep slopes that facilitate rapid water flow, 
  • or sudden glacial breaks releasing large volumes of water, as seen in Chamoli in 2021. 
• In fragile terrains with eroding slopes and saturated soil, even low-intensity rainfall can trigger landslides or floods. 
• The Himalayas, being young, geologically weak, and earthquake-prone, are inherently more vulnerable compared to older ranges like the Aravalis. 
• Added to this natural fragility, large-scale construction and heavy vehicular traffic further heighten the risk during extreme rainfall events.

Early Warning Limitations and Mitigation for Flash Floods

• The Dharali incident highlights the challenge of predicting flash floods, as even moderate rainfall can trigger them. 
• While heavy rain forecasts and improving landslide predictions exist, determining whether such events will cause flash floods remains highly uncertain. 
• Mitigation measures can help reduce risks, including: 
  • restricting construction near riverbanks, 
  • relocating vulnerable settlements away from rivers, and 

• managing large boulders and construction debris to prevent them from being swept into waterways during high-flow events.

Source: IE