Cloudbursts in Uttarkashi trigger flash floods; Whats behind these extreme weather events and how are they spelling havoc?
Recent flash floods in Uttarakhand highlight a growing crisis in India. A new IIT Gandhinagar study reveals that while extreme rainfall contributes, soil conditions and terrain significantly influence flash flood susceptibility. The study identifies hotspots, primarily in the Himalayas and along the west coast, with climate change exacerbating the risk through increased precipitation intensity and frequency. Understanding these factors is crucial for mitigation and preparedness.
On Tuesday, at least four people were killed and several others went missing after a cloudburst in Uttarkashi district of Uttarakhand sparked menacing flash floods in Dharali village.
Two separate cloudbursts in Dharali and Sukhi Top triggered widespread chaos across the region, with Dharali in Uttarkashi bearing the brunt. The cloudbursts have left a trail of destruction in their wake.
On July 29, a flash flood claimed the lives of three people and buried over 20 vehicles while submerging a number of houses in Himachal's Mandi. The flash flood was triggered by a cloudburst. In July, more than 100 people died in the hilly state. In July last year, 373 people were killed when landslides following incessant rains and flash flood hit Wayanad in Kerala. In June that year, five soldiers died in Ladakh after a flash flood in the Shyok River washed away a tank. Dozens of people lost their lives in a Glacial Lake Outburst Flood (GLOF) in Sikkim.
Flash floods are becoming a common phenomenon in India. And they are taking a huge toll. It is estimated that every year in India, over 5,000 people die due to flash floods after extreme rainfall events. Flash floods cause huge damage to infrastructure, farms, and the environment.
These weather events have seen a surge in recent times due to rising global temperatures. Union Ministry of Jal Shakti told the Lok Sabha in 2023 that between 2020 and 2022, flash flood incidents grew from 132 to 184.
Very little research and study has gone into understanding and assessing the damage caused flash floods and taking measures to contain them. However, a new study by researchers based at the Indian Institute of Technology (IIT)- Gandhinagar has made an attempt to delve into the issue. The study was published in the journal Nature Hazards on July 13. It's titled: ‘Drivers of flash floods in the Indian sub-continental river basins’.
What did the study find
The study reported that most flash floods are centred in the Himalayas, the west coast, and Central India. It has also pointed out that spike in temperatures has caused several regions in the country to become more vulnerable to flash floods than in the past.
The analysis revolved around sub-basins (parts of large river basins) to identify flash flood hotspots. It was found that sub-basins in the Brahmaputra, Narmada, Tapti, Mahanadi, Brahmani, and west coast river basins have become extremely prone to flash floods.
Some areas of the Godavari, Ganga, Mahi and Indus river basins are "highly susceptible" to flash flooding. Since 1995, these extreme weather events are increasing, and most of them have occurred in the Brahmaputra river basin, followed by Ganga and Krishna basins.
What's driving them
Only 25% of flash floods in India are directly caused by extreme precipitation, the study said. The remaining cases are caused due to a combination of extreme rain and the soil’s condition before the precipitation.
The study also underlined that extreme rainfall leads to flash floods within six hours in only 23% of cases. Most flash floods are usually caused by prolonged rainfall — either low or high intensity — over a number of days.
It is the high flashiness of sub-basins (part of large river basins) that cause flash floods in the west coast and Central India. Here, the water levels quickly reach a peak after a heavy rainfall event. The prevailing condition of the soil plays a key role in how quickly water can soak into a sub-basin. In the Himalayas, steep slopes go a long way in contributing to flash floods.
The study said: "Flash flood susceptibility varies widely within the major river basins. For instance, the sub-basins located in the Himalayan regions and the southern parts of the Ganga River basin are highly prone to flash floods, whereas the sub-basins in the central regions of the Ganga River basin exhibit low flash flood susceptibility. This variation in flash flood susceptibility underscores the influence of terrain and climatic factors on flood risk within different parts of the same river basin.”
How climate change is playing a part
As global temperatures see a surge, extreme weather events like flash floods are becoming more frequent and intense. The reason behind this is that with every 1°C increase in average temperature, the atmosphere can hold about 7% more moisture, leading to heavier rainfall and a greater risk of flash floods.
Between 1981 and 2020, the annual frequency of extreme precipitation events doubled during the pre-monsoon season in India. Extreme rainfall during the monsoon, post-monsoon, and winter seasons has increased by 56%, 40%, and 12.5% respectively, the study said. It is to be noted that over 75% of the total flash flood events that took place between 1980 and 2018 occurred during the monsoon season (June-September). Rising temperatures are causing more wet hours in many sub-basins that were not prone to flash floods earlier.