How flash floods are affecting India
Although these events took place in different parts of the country, they had one common trigger: flash floods. Every year, more than 5,000 people die, and significant damage is sustained by infrastructure, agricultural land, and the environment due to these sudden floods, which are caused by extreme rainfall. To make matters worse, with rising global temperatures, there has been an increase in the frequency of flash floods. For instance, between 2020 and 2022, the number of flash flood events increased from 132 to 184, according to the Union Ministry of Jal Shakti's written reply in the Lok Sabha in 2023.
Despite the increasing threat posed by flash floods, there has been insufficient research to identify areas that are most vulnerable to this extreme weather event. This has posed a challenge in implementing adaptation strategies such as early warning systems.
A new study by researchers based at the Indian Institute of Technology (IIT)- Gandhinagar has tried to address this gap. Published in the journal Nature Hazards on July 13, the study, 'Drivers of flash floods in the Indian sub-continental river basins', has found that flash floods are mainly centred in the Himalayas, the west coast, and Central India (see box).
It has also revealed that several regions in the country have become more vulnerable to flash floods than before, due to soaring temperatures.
What factors are driving flash floods? How is the warming climate making flash floods more frequent and intense? How can authorities prepare to limit the impact of flash floods in India? Here is a look.
According to the study, only 25% of the flash floods that occur in India are directly caused by extreme precipitation. The rest of them are a result of a combination of extreme rainfall and the condition of the soil before precipitation.
Vismal Misra, who co-authored the study with Nandana Dilip K and Urmin Vegad, told The Indian Express, 'If the ground is already wet before a heavy rainfall event, chances are the soil would be saturated or near saturated, which can lead to instant runoff. This increases the risk of flash floods.'
The study also notes that an extreme rainfall event leads to immediate flash floods (within six hours) only 23% of the time. Usually, it is the prolonged (multi-day) low-intensity and high-intensity rainfall which results in these floods.
At the most vulnerable regions, other factors also contribute to flash flood events. For instance, in the west coast and Central India, flash floods are driven by the high flashiness of sub-basins (part of large river basins). This means that their water levels quickly reach the peak after a heavy rainfall event. Prevailing soil conditions play a crucial role in how rapidly water infiltrates a sub-basin.
In the Himalayas, geomorphological factors — such as steep slopes, and high relief (when a landscape has a significant difference between a high point and a low point) — contribute to flash floods.
'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,' the study said.
With rising global temperatures, extreme weather events such as flash floods are increasing in frequency and intensity across the world. That is because for every 1 degree Celsius rise in average temperature, the atmosphere can hold about 7% more moisture, leading to more intense precipitation, which exacerbates the risk of flash floods.
In India, between 1981 and 2020, the yearly frequency of extreme precipitation events doubled during the pre-monsoon season. Extreme rainfall during the monsoon, post-monsoon, and winter seasons has increased by 56%, 40%, and 12.5% respectively, according to the study. Note that more than 75% of the total flash flood events that occurred between 1980 and 2018 took place during the monsoon season (June-September).
There has been a notable increase in flash flood events since 1995, with most of them occurring in the Brahmaputra River basin, followed by the Ganga and Krishna River basins, the study said.
Misra and his colleagues have also found that rising temperatures are leading to more wet hours in most of the sub-basins, which are not currently flash flood-prone.
'Across all the Indian river basins, 51% and 66.5% of the non-flash flood-prone sub-basins show an increase in precipitation and streamflow, respectively. On the contrary, we observed that a few flash flood-prone basins show a decline in the wet hours,' the study said.
The findings indicate that authorities need to adopt region-specific adaptation strategies, which are based on factors such as topography and soil conditions, and not just extreme rainfall events. This can help in developing better early warning systems, targeted disaster preparedness, and long-term adaptation plans, according to the researchers.
Authorities also need to identify new potential flash flood hotspots and take measures like building climate-resilient infrastructure to limit the impact of the extreme weather event.
With rising temperatures and changing rainfall patterns, there is a requirement to enhance land-use and flood planning as well.
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