Flash Floods Rising Across India: New Study Maps High-Risk Zones

In July 2025, flash floods and landslides killed 112 people in Himachal Pradesh. The rain came fast and heavy. Roads collapsed, water and power supply systems broke down, and damage crossed ₹883 crore. Mandi, Kangra, and Kullu were among the worst-hit districts.

Many of these deaths were caused by flash floods. These are different from regular floods. Normal floods rise slowly, over days, often from steady rain or melting snow. Flash floods are sudden. They come within minutes or hours, usually after intense rainfall. There is little or no time to react. This makes flash floods far more dangerous.

Globally, flash floods kill over 5,000 people every year. India is especially at risk. Over 40 million hectares of its land are flood-prone. Major flash floods in Mumbai (2005), Leh (2010), and Kerala (2018) are still fresh in public memory. But now, they are becoming common, damaging homes, roads, and crops, and costing human lives. 

Flash floods are often concentrated in hilly areas and are becoming common in other regions too. Now, a new study shows where flash floods are likely to happen next. The study was led by Professor Vimal Mishra, Civil Engineering, Indian Institute of Technology (IIT) Gandhinagar, and two PhD research scholars, Nandana Dilip K. and Urmin Vegad, also from IIT Gandhinagar. It was published in the journal npj (Nature Partner Journal)Natural Hazards. The team mapped flash flood risks across India by looking at how rainwater behaves in different areas.

Researchers wanted to figure out what makes some places more likely to have flash floods. So, the study focused on two main things: how fast water flows in a basin after rainfall and the shape and slope of the land. If rivers rise quickly after rain, the basin is considered flashy and at risk. This fast response to rainfall is called "flashiness." The second factor is “geomorphology,” the natural features of the land, like hills, valleys, soil type, and how rivers are shaped.

The researchers used satellite data, past rainfall records, and river maps. They studied 395 river sub-basins across India using a computer model that simulates water flow. This helped them see which areas are more likely to flood quickly after rain.

What Increases the Risk?

They found that flash flood risks vary depending on the region. In the Himalayas, steep slopes and rough terrain make flash floods more likely. Here, the shape of the land is the main cause. In contrast, places like the West Coast and Central India are more affected by fast drainage systems. Rainwater gathers and flows quickly through connected streams and rivers, raising flood risks even in flatter areas.

One key insight from the study was that large basins are not always more dangerous than small ones. Some small basins were safer because of slower water movement. Some larger ones had a higher risk due to sharp slopes or fast runoff. So, size alone doesn’t predict risk. The flow pattern and land shape matter more.

The study also looked at the connection between rain and soil. In about 75% of flash floods, the cause was not just heavy rain but also already wet soil. When the ground is soaked from earlier rainfall, even moderate rain can cause flooding. This means that not all flash floods are caused by one big storm. Many are the result of several days of steady rainfall.

In places like the Western Ghats or northeast India, the weather is humid and it rains often. Because of this, the soil stays wet most of the time. When more rain falls, the ground can’t absorb much of it, because it’s already soaked. So the water flows over the surface. This can quickly lead to flash floods.

Now take a place like Rajasthan. It’s dry, and the soil there is hard and packed. When it finally rains, the water doesn’t soak in easily. Instead, it runs off fast, like water on concrete. That sudden flow of water can also cause flash floods. So whether the soil is too wet or too dry, both can lead to floods, just in different ways.

The team also found signs of change. Places that were not prone to flash floods earlier are now facing a higher risk. Climate change, rising temperatures, and more short, intense rainfalls are part of the reason. The lower Himalayas, parts of western India, and some areas in peninsular India are now showing signs of increased flood danger.

Some specific regions are at very high risk. In the Brahmaputra basin, 45% of sub-basins were marked as highly flood-prone. On the West Coast, five out of six sub-basins ranked “very high” in flood risk. Even the Ganga and Godavari basins, known more for slow river floods, are now facing short, sudden flash floods.

Many towns and cities are expanding into floodplains, riverbeds, and low-lying zones. More concrete means less water is absorbed into the ground, and more flows quickly into drains and rivers. They also called for better planning. Buildings, roads, and public infrastructure must take flood risks into account. Local governments must update flood maps and prepare emergency plans. Finally, they stressed the need for public awareness. People who have never faced flash floods may not know what to expect. That makes the impact worse when floods happen. Awareness campaigns, drills, and early alerts can save lives.

The researchers said that early warning systems must improve. Right now, most systems only track rain. That’s not enough. We also need to monitor soil moisture, the shape of the land, and how quickly rivers respond to rain. Warnings must be local and fast.

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