Why in 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.

What’s in Today’s Article?

• Unclear Cause of Dharali Flash Flood Highlights Rising Risks
• Understanding Rainfall Patterns and Cloudburst Concerns in Uttarkashi
• Multiple Triggers Behind Flash Floods in the Himalayas
• Early Warning Limitations and Mitigation for Flash Floods

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.

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.