What Caused the Uttarkashi Flash Floods? Experts Offer Multiple Theories
The recent flash floods in Uttarkashi’s Dharali village, located in the Indian state of Uttarakhand, have triggered deep concern and speculation about what caused the disaster. While the exact cause is still under investigation, several experts and glaciologists have come forward with different theories to explain the massive mudslide and flooding that severely impacted the region.
Dharali, the impacted area, is situated on an alluvial fan, a fan-shaped landform formed over time by the deposition of sediment from past floods. This sediment-rich feature was specifically formed as a result of floods at the meeting point of the Bhagirathi River and the Kheer Ganga stream. Despite being only 200 meters from the river, the village is vulnerable because of the increased commercialization and development on this alluvial fan in recent years.
Understanding the Terrain: Alluvial Fan and Human Activity
Experts believe that the alteration of the natural geography may have been a major factor in this catastrophe. New establishments have been built directly on the alluvial fan as a result of infrastructure development, such as the Dharasu-Gangotri highway, and tourism-related construction over the years. Despite its seemingly stable appearance, the fan is actually a deposit of loose sediment, which makes it vulnerable to erosion and sliding, particularly during severe weather conditions.
The village’s position, although seemingly safe due to its distance from the main river channel, became risky due to unchecked development, increasing the impact when water and debris rushed through the area.
Reactivated Landslides: A Possible Trigger?
A potential link to a previous landslide that happened two kilometers uphill from Dharali was noted by a glaciologist researching the area. He believes that recent rainfall, seismic activity, or glacial melting may have reactivated this landslide. A sudden surge of water, rocks, and mud could have resulted from such reactivation, and it would have crashed into the village.
This theory finds support in earlier studies, including a paper by Bhambri R et al, titled “High Mountain Hazards in Uttarakhand”, which identified a range of natural disasters — including glacier lake outburst floods, rainfall-induced landslides, and flash floods — that frequently affect downstream areas.
The Bhagirathi Basin, where numerous landslides have been reported, is specifically mentioned in the paper. Researchers saw 1,434 new landslides and 533 reactivated landslides following the June 2013 extreme rainfall event, especially in the areas close to Bhatwari, Pilang, Dharali, and the Tehri dam. These results imply that reactivation of past landslide locations is a persistent risk in the area, especially during severe weather conditions.
Glacial Activity: Melting and Hidden Dangers
Another angle being examined is glacial activity further upslope. Using satellite imagery and terrain analysis, the glaciologist noted that around 7 kilometers above Dharali, there are two large glaciers with deep crevasses. These glaciers are avalanche-fed, meaning they constantly receive new snow and ice through falling snow and small avalanches.
Just below these glaciers lie seven small glacial lakes. With rising temperatures, these glaciers may be melting faster, and if the lakes breach or overflow, they can release huge volumes of water, carrying with them moraines, sediments, and debris.
“From the videos circulating online, we can tell that the debris content was very high,” said the glaciologist. “Such flows can be extremely destructive because they not only flood but also erode everything in their path.”
The presence of such high debris content in the floodwaters suggests that glacial meltwater mixed with sediment could be a contributing factor, making this a potential glacier lake outburst flood (GLOF) scenario.
The Role of Rainfall: Orographic Barriers and Microclimates
Interestingly, data from the India Meteorological Department (IMD) showed that rainfall in the area was moderate. However, scientists believe that localized weather phenomena, particularly those influenced by orographic barriers, may have led to unusual and intense precipitation in higher elevations.
An orographic barrier is a geographic feature — like a mountain — that blocks moisture-laden air, causing it to rise, cool, and condense, often resulting in heavy rainfall on the windward side and reduced rainfall on the leeward side.
In this case, a peak about 5,700 meters above sea level may have trapped a moisture-heavy cloud, preventing it from moving forward. This resulted in a sudden, localized downpour in the upper regions like Kheer Ganga, Harsil, and Sukhi Top, rather than widespread rainfall in lower areas.
Because of higher temperatures, the precipitation fell as rain rather than snow, accelerating the runoff and triggering floods in multiple tributaries of the Bhagirathi River.
“This particular rainfall pattern rules out the glacier breach theory for some locations,” the scientist said. “If it was just a glacier breach, only Kheer Ganga would have been affected. But the fact that Harsil and Sukhi Top also flooded points to a common atmospheric cause — likely orographic precipitation and condensation.”
Multiple Catchments, One Catastrophe
The glaciologist explained that the affected areas belong to different catchment zones, meaning they are not directly connected by the same water flow. However, the simultaneous flooding of these three areas — Kheer Ganga, Harsil, and Sukhi Top — suggests a widespread meteorological trigger, most likely linked to orographic rainfall and rapid runoff.
These tributaries all eventually join the Bhagirathi River downstream, but their independent flooding due to localized rain further supports the theory that natural barriers altered rainfall patterns, leading to intense rainfall in the high-altitude zones and relatively dry conditions in lower regions.
A Complex Puzzle with No Easy Answers
Honoring Nature’s Warning: Seeking Science and Solutions
The Uttarkashi flash floods stand as yet another tragic reminder of the fragility of mountain ecosystems. While no single cause can be definitively pinpointed yet, a combination of human interference, geological vulnerability, glacial changes, and climate-influenced rainfall patterns seem to be at play.
From the construction on alluvial fans to the reawakening of ancient landslides, and from the melting glaciers above to orographic effects creating isolated heavy rain, the disaster was likely the result of multiple forces acting simultaneously.
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