‘Memory of Rain’ Flooding Indian Cities

Context

The flooding of cities in India is not only caused by the absence of drainage, but also by hydrological hysteresis, which is the recall of the past precipitation by the landscape about the storage and discharge of water.

Key Highlights

• Waterlogging in Indian cities such as Bengaluru often occurs even when it has stopped pouring.
• This cannot be attributed to drainage failure as such; hydrology introduces the concept of hydrological hysteresis that defines the slowness of water response in landscapes.

What is the Hydrological Hysteresis?

• Hydrological hysteresis can be defined as the sensitivity of a landscape to rainfall, which is conditional upon future and past rainfall.
• Water is stored in:
  • Soil
  • Wetlands
  • Aquifers
  • Floodplains
• Due to this storage, runoff and flooding are determined by the level of saturation that has occurred before and not only by rainfall.
• How It Works
  • A dry landscape takes in water like a sponge.
  • When the landscape becomes saturated, it will not be able to take in more water and this causes:
    • Increased surface runoff
    • Faster flooding
  • The light rain also has the ability to cause floods when the soil is already full.

Indian Context: Flooding in Urban areas

• The urbanisation in India has been rapid and has led to:
  • Less natural drainage systems.
  • Intruded floodplains and wetlands.
  • More impermeable surfaces such as concrete.
• This causes a loss of the natural capacity of cities to accumulate and release water slowly.

Case Study: Bengaluru Floods (2024)

• Some of the lakes, which overflowed due to persistent rainfall, included Kogilu and Doddabommasandra in areas such as Yelahanka.
• Key observations:
  • Lakes used to absorb runoffs.
  • Water flooded the roads and lower places when capacity was reached.
  • Flooding did not end even by the fact that rainfall decreased.
• This is a hysterical demonstration of:
  • Flooding does not solely rely on the present precipitation but on the previous buildup.

Role of Natural Systems

• Cities such as Bengaluru had historically interlinked lake systems that had been built during the Kempegowda rule.
• These systems:
  • Stored excess rainwater
  • Allowed gradual drainage
• Development has also broken these connections thereby exacerbating the effects of floods in urban areas.

Wet Droughts

• Soils and floodplains are inundated.
• The groundwater levels increase and drainage is retarded.
• Obstructed or flooded drains cut down the outflow.
• Water when it is kept in urban areas takes time to recede.

India Policy implications

• Flood management needs to shift to basin-level planning instead of drainage engineering.
• Key measures include:
  • Wetlands and floodplains protection.
  • Reinstating the urban lakes networks.
  • Restricting the building in flood-prone regions.
• The use of hydrological information in urban design.

Conclusion

• The concept of hydrological hysteresis describes why the floods in urban areas in India are late, abrupt, and prolonged.
• The acknowledgment of the memory of water is indispensable to the sustainable urban planning area, particularly under the circumstances of climate change and increasing levels of extreme rainfall occurrences.

Source: The Hindu

Mains PYQ

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

• Flooding is caused by the previous rain.
• Urbanisation interferes with the natural water storage systems.
• Hysteresis is in action in Bengaluru floods.
• The focus of long-term planning should be on the natural infrastructure in addition to engineered solutions.

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