UPSC Issue at a Glance | Assam Floods: The anatomy of flash floods, from sudden disasters to smarter preparedness

People cross a flood-affected area on a makeshift raft after heavy rainfall in Golaghat on Wednesday. (PTI)

A scientist at the Meteorological Centre Kohima, however, told The Indian Express that the area had not experienced a cloudburst but rather heavy rainfall of 137 mm over the course of eight to nine hours on that day.  In this context, let’s know about the floods from a broader perspective. 

Question 1: What are flash floods, and how are they different from floods in general?

Excessive or continuous rainfall over a period of days, or during particular seasons can lead to stagnation of water and cause flooding. Flash floods refer to such a situation, but occurring in a much shorter span of time. For instance, the US’s meteorological agency, the National Weather Service, says flash floods are caused when rainfall creates flooding in less than 6 hours. It adds that flash floods can also be caused by factors apart from rainfall, like when water goes beyond the levels of a dam.

In India, flash floods are often associated with cloudbursts – sudden, intense rainfall in a short period of time. Himalayan states further face the challenge of overflowing glacial lakes, formed due to the melting of glaciers, and their numbers have been increasing in the last few years.

Flash flooding commonly happens more where rivers are narrow and steep, so they flow more quickly, as per the Met Office, UK’s national weather service. They can occur in urban areas located near small rivers, since hard surfaces such as roads and concrete do not allow the water to absorb into the ground.

Question 2: How common are flash floods and floods?

According to government data from a project by the Assam State Disaster Management Authority, India is the worst flood-affected country in the world after Bangladesh and accounts for one-fifth of the global death count due to floods. Flash floods have been commonly witnessed in cities like Chennai and Mumbai. Depression and cyclonic storms in the coastal areas of Orissa, West Bengal, Andhra Pradesh, and others also cause flash floods.

Further, data from the National Disaster Management Authority states that one of the reasons for flood situations occurring so frequently, is that nearly 75 per cent of the total Indian rainfall is concentrated over a short monsoon season of four months (June to September). As a result, the rivers witness a heavy discharge during these months. About 40 million hectares of land in the country are liable to floods according to the National Flood Commission, and an average of 18.6 million hectares of land are affected annually.

Flash floods may in the future, begin to take place after wildfires that have been taking place more frequently. This is because wildfires destroy forests and other vegetation, which in turn weakens the soil and makes it less permeable for water to seep through.

Andrew Hoell, a meteorologist at the US’s National Oceanic and Atmospheric Administration’s Physical Sciences Lab, told the New York Times, “If heavy rains occur on land damaged by a fire, the water does not get absorbed by the land surface as effectively as it once did”.

Question 3: What are the driving factors of flash floods?

Every year, more than 5,000 people die, and significant damage is sustained by infrastructure, agricultural land, and the environment due to the 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.

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, 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. It has also revealed that several regions in the country have become more vulnerable to flash floods than before, due to soaring temperatures.

Driving factors of flash floods

(i) 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.

(ii) 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.

(iii) 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.

(iv) 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.

(v) Climate change is an important driving factor of flash floods. 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.

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.

Question 4: What should India do to reduce the risk of future catastrophe?

An alarming increase in extreme weather phenomena — sudden cloudbursts, flash floods, and landslides — that threaten lives, livelihoods, and ecosystems presents the accelerated impacts of climate change. In this context, it becomes essential to take the right measures in the right direction to reduce the risk of future catastrophe.

As changing climate has destabilised weather patterns, in order to saving lives and reduce damage during natural disasters effective early warning systems are critical. For the same expanding and modernising weather monitoring infrastructure, especially in the Himalayas and fragile mountain regions, is essential. Establishing more Automatic Weather Stations (AWS) and utilising satellite-based observation systems can provide real-time data critical for early warnings.

Anjal Prakash explains it: “AWS and advanced meteorological models can provide real-time data that forecasts extreme weather events well in advance when fed into predictive algorithms. This will allow authorities to issue timely alerts, evacuate vulnerable populations, and mobilise relief efforts before catastrophe strikes. International examples, such as Nepal’s pioneering use of early warning systems for mountain disasters, illustrate how technology can mitigate risks when integrated into a comprehensive disaster preparedness plan.”

Integrating ecological solutions into broader infrastructure planning is vital. As Anjal Prakash explains “Instead of relying solely on grey infrastructure (such as dams and embankments), India must embrace ecological solutions — restoring wetlands, mangroves, and natural sponge zones that absorb floodwaters and reduce runoff. These nature-based solutions are cost-effective, sustainable, and adaptable to changing climate conditions.

(Infographic: AI-generated)

In the mountains, land-use planning must prioritise the preservation of forests and natural buffers that minimise landslides and stabilise slopes. Reforestation and afforestation programs should be expanded, especially in areas prone to erosion. These measures increase the resilience of local ecosystems and communities, helping them withstand climate change shocks.”

Climate change adaptation must be embedded within policy frameworks. This involves mainstreaming climate resilience into urban and rural planning, allocating dedicated funds for climate risk assessments, investing in resilient infrastructure, and empowering local communities with knowledge and resources.

Community-based disaster management should also be prioritised, particularly in vulnerable rural and hill areas. As residents of particular areas often possess valuable traditional knowledge about weather patterns and natural warning signs, which can be integrated into official early warning systems for more effective responses.

Furthermore, strengthening climate resilience through collaboration and innovation is the need of the hour. As climate change is a systemic challenge, it requires coordinated action at all local, national, and international levels; thus, investing in research, fostering innovation in climate resilience technologies, and partnering with regional neighbours can bolster adaptive capacity across the Himalayas and beyond.

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. Updated hazard mapping can help identify emerging flash flood hotspots and guide land-use planning, infrastructure development, and disaster preparedness. Investing in climate-resilient infrastructure and risk-informed development will help reduce losses as extreme weather events become more frequent.

Post Read Questions

Prelims

Which of the following statements with regard to cloudburst is/are correct? (UPSC CDS 2017)

1. It is defined as sudden localized very heavy downpour with cloud thunder and lightning.

2. It mostly occurs in the hilly areas.

3. It results into very high intensity of rainfall, i.e., 250 mm-300 mm in a couple of hours.

4. It occurs only during the daytime.

Select the correct answer using the code given below.

(a) 1, 2, and 3 only

(b) 1, 3, and 4 only

(c) 2 and 3 only

(d) 2 only

Mains

1. The interlinking of rivers can provide viable solutions to the multi-dimensional inter-related problems of droughts, floods, and interrupted navigation. Critically examine. (UPSC CSE 2020)

2. Account for the huge flooding of million cities in India including the smart ones like Hyderabad and Pune. Suggest lasting remedial measures. (UPSC CSE 2020)

(Sources: Why Assam is experiencing unprecedented flooding this year, How flash floods are affecting India)

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