Context: Flash floods were recently reported in several districts of Himachal Pradesh due to intense rainfall over a short duration.
Relevance of the Topic: Prelims: Concept of Flash Floods and affected regions. Mains: Key drivers of Flash Floods and mitigation strategies.
What Are Flash Floods?
- According to the National Disaster Management Authority (NDMA): Flash floods are sudden and high-intensity floods that occur within six hours of a triggering event such as heavy rainfall, dam breach, cloudburst, or landslide. These floods are highly localised, short-lived, and destructive.
Key Drivers of Flash Floods:
- Extreme Precipitation: 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.
- Pre-wet Soil Conditions: Saturated soil cannot absorb new rainfall, leading to instant runoff.
- Geomorphological Factors: In the Himalayas, geomorphological factors such as steep slopes, and high relief contribute to flash floods.
- Flashiness of Sub-basins: River basins that react quickly to rainfall lead to sudden floods. In the West Coast and Central India, flash floods are driven by the high flashiness of sub-basins, meaning water levels peak quickly after heavy rainfall. Prevailing soil conditions significantly influence how fast water infiltrates these sub-basins.
- Multi-day Rainfall Events: The study 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.
- Climate Change: With rising global temperatures, extreme weather events such as flash floods are increasing in frequency and intensity across the world. 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.
Increasing Frequency and Regional Spread of Flash Floods:
Recent flash floods in Himachal Pradesh, Assam, Goa, and parts of Central India highlight their increasing frequency and regional spread across diverse terrains. A study by researchers based at the Indian Institute of Technology (IIT)- Gandhinagar has found:
- Notable increase in flash flood events since 1995. Flash floods are mainly centred in the Himalayas, the west coast, and Central India.
- Most of the flash floods occurred in the Brahmaputra River basin, followed by the Ganga and Krishna River basins.
- 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.
Way Forward
- Adopt Region-Specific Adaptation strategies 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.
- Identify potential flash flood hotspots and take measures like building climate-resilient infrastructure to limit the impact of the extreme weather event.
- Enhance land-use and flood planning in the wake of rising temperatures and changing rainfall patterns.