Flash floods are no longer just a threat to India’s known flood-prone areas. A new study by researchers from IIT Gandhinagar shows that places once considered “safe” are now facing sudden, intense floods — with little warning.
This shift is being driven by a changing climate. Warmer air holds more moisture, which means when it rains, it can pour harder and faster than ever before. In areas unprepared for such events, this surge of water can tear through homes, farms, and roads within minutes, upending lives and livelihoods.
Flash floods claim more lives than riverine or coastal floods. They strike quickly, with little time to react, often leaving behind a trail of destruction. In the past two decades, India has seen devastating examples, from Mumbai in 2005 to Leh in 2010, Uttarakhand in 2013, Chennai in 2015, Kerala in 2018, and Assam as recently as 2024. The new findings show that as climate patterns shift, the boundaries of danger are shifting too , making it urgent to rethink what we mean by “safe zones.”
Flash floods are fast, furious and deadly - one of the most devastating natural hazards caused due to extreme rainfall events that occur in a very short span of time. This leads to sudden rise and recession of water levels in local areas, making them extremely unpredictable and disastrous. Flash floods cause the highest number of deaths as compared to riverine and coastal flooding.
Several river basins and urban areas such as Mumbai, 2005; Leh, 2010; Uttarakhand, 2013; Jammu and Kashmir, 2014; Chennai, 2015; Kerala, 2018; Bihar, 2019; Hyderabad, 2020; Assam, Meghalaya, Arunachal Pradesh, 2022 and Assam, 2024 have experienced flash floods in the past decade, which severely impacted infrastructure and agriculture. The low-lying river basins in India are highly susceptible to flash floods.
A combination of meteorological, geomorphological, and anthropogenic factors cause flash floods. Intense rainfall associated with tropical cyclones and the summer monsoon are the top factors causing flash floods. Saturated soil or impervious surfaces like urban landscapes with concrete and asphalt prevent water absorption, leading to sudden runoff. Soil that is already saturated from previous rainfall cannot absorb more, acting like a spillway.
In arid regions such as desert landscapes, dry soil can lead to quick surface runoff generating flash floods. Cloudbursts that involve extreme rainfall events that cross 100 mm/hour intensity, can also cause flash floods in hilly terrains. Dam breach or sudden release from dams can also result in flash floods.
Land and river basin shape can affect occurrence of flash floods. If the land is steep and hilly, water rushes down faster, making floods more intense. Water moves quickly in the presence of streams and channels making the flood peak arrive sooner. If the river basin (the area that collects rainwater) is round, all the water reaches one point quickly, like a flash flood, while if the basin is long and stretched out, water takes more time to travel, so floods may be less sudden. Heavy rain on steep land creates powerful floods fast because water does not have time to soak into the ground.
The study titled ‘Drivers of flash floods in the Indian subcontinental river basins’ by Nandana Dilip K, Urmin Vegad and Vimal Mishra published in npj Natural Hazards used hydrological and geomorphological characteristics to map the flash flood prone river sub-basins in the Indian sub-continent by looking at:
Weather patterns – like how much and how fast rain falls and temperature changes
Land shape – such as steep hills or valleys
River and drainage features – like how water moves through an area
The results challenge old assumptions about where floods are most likely. The analysis revealed:
Emerging risks in once low-risk basins
About half of the sub-basins that were earlier considered low-risk are now seeing heavier rainfall and faster river flows. In these basins, “wet hours”, which are the number of hours with rainfall, are increasing, pointing to a growing potential for flash floods. The amount of extreme rain has increased by 56 percent during the monsoon, 40 percent after the monsoon and 12.5 percent in winter. These changes are happening because the climate is warming. Warmer air holds more moisture, leading to sudden, intense rainfall.
Monsoon seasons drive most flash floods
More than 75 percent of flash floods occur between June and September (summer monsoon). The south-east coast sees flash floods later in the year, between October and December, due to the winter monsoon. Heavy rains and cyclones linked to the south-west monsoon in summer and the north-east monsoon in winter are the major drivers of flash floods.
When heavy rain meets saturated ground
Most (~ 3/4th) of the flash floods happen when there's heavy rain and the land is already soaked from earlier rainfall while some floods (~ 1/4th) happen from extreme rain alone, even if the ground was dry before. Rainfall shows an increasing trend in peninsular India, western India, and lower Himalayas along with increasing temperatures.
Rising hourly river flows signal faster flood onset
Hourly streamflow is increasing more than daily streamflow in several river areas—like parts of the Brahmaputra, Ganga, and rivers on the east and west coasts (Sabarmati, Narmada, Tapi, Krishna). This means water flow is changing more quickly hour by hour, not just day by day. Also areas prone to flash floods are seeing more rain and faster-flowing water, which makes sudden floods more likely and dangerous.
Regional drivers of flash floods differ
Flash flood hotspots are mainly centered in the Himalayas, West Coast, and Central India, with geomorphological factors (such as temperature, rainfall, humidity, wind patterns) driving flash floods in the Himalayas and hydrological factors (factors affecting movement and storage of water) in the West Coast and Central India.
Some river basins in India are extremely prone to sudden flooding, especially during intense rains or cloudbursts. Many sub basins in the Brahmaputra, Narmada, Tapi, Mahanadi, Brahmani, West Coast face frequent flash floods while those in the Godavari, Ganga, Mahi, Indus basins also vulnerable, but risk varies.
Most flood-prone sub-basins are in:
The Brahmaputra (but only 45 percent of its total sub-basins)
The Ganga (only 21percent of its total sub-basins)
Godavari (36 percent of its sub-basins)
On the West Coast, 5 out of 6 sub-basins are highly flood-prone, especially due to cloudbursts and long wet spells.
High-risk sub-basins show declining rainfall and streamflow
Many currently high-risk sub-basins show declining rainfall and streamflow, hinting at a spatial reorganisation of flood risk under a warming climate. For example, 51 percent of non-flash flood prone sub-basins are now showing increasing rainfall patterns while 66.5 percent are showing increasing streamflow hinting at potential emergence of new flash flood zones.
Changing flood hotspots across India
Even wet hours are increasing in these lower-risk sub-basins, while some established hotspots (in basins like Indus, Ganga, Brahmaputra, Mahanadi, Godavari) are experiencing a decline in wet spells, signaling a drying tendency.
The findings send a clear message: India’s flood preparedness must expand beyond known danger zones. Areas considered safe in the past can no longer be assumed to be risk-free.
For communities, this means staying aware of local weather alerts, understanding the signs of sudden flooding, and knowing where to move in an emergency, even if floods have never happened there before. For planners and policymakers, it means updating flood hazard maps, strengthening early warning systems in emerging hotspots, and designing infrastructure that can handle sudden surges in water flow.
Flash floods will always be part of India’s climate reality, but the pattern of where and how they strike is changing. By recognising that “safe zones” can become the next flashpoints and acting on that knowledge now — we can save lives, protect livelihoods, and build resilience in a warming, wetter world.