Context:
Urban flooding in Indian cities is increasingly influenced by a phenomenon called hydrological hysteresis, where landscapes retain a “memory” of past rainfall events, affecting how water flows, accumulates, and drains. This phenomenon has significant implications for urban flood management and planning.
Key Highlights:
Hydrological Hysteresis
- Refers to the phenomenon where a landscape’s response to rainfall depends on both current rainfall and past rainfall events.
- This creates non-linear relationships between rainfall and runoff.
Landscape ‘Memory’
- Soil, wetlands, and aquifers store water from previous rainfall events.
- When the land becomes saturated, additional rainfall leads to rapid runoff and flooding.
Urban Flooding Dynamics
- Floodwaters behave differently during:
- Rising flood stages
- Receding flood stages
- Stored water in floodplains and soils drains slowly, prolonging flooding.
Case Study – Bengaluru Flooding (October 2024)
- Kogilu and Doddabommasandra lakes overflowed during heavy rainfall.
- Saturated soils and blocked drainage systems caused persistent waterlogging.
Impact of Urbanization
- Bengaluru historically had interconnected lakes and wetlands built in the 16th century.
- These natural drainage systems were replaced by concrete stormwater channels, causing:
- Rapid filling of lakes
- Abrupt overflow
- Slower drainage.
Significance
- Highlights the importance of ecosystem-based urban flood management.
- Emphasizes basin-level planning and protection of wetlands and floodplains.
Relevant Prelims Points:
- Hydrological Hysteresis
- Occurs when runoff response to rainfall differs during rising and falling stages of storms.
- Influenced by soil saturation, groundwater levels, and landscape characteristics.
- Floodplain
- A flat area adjacent to a river that is periodically flooded.
- Plays a key role in natural flood control and groundwater recharge.
- Catchment Area
- The geographical area from which rainfall flows into a river, lake, or reservoir.
- Urban Lakes in Bengaluru
- Historically interconnected tank systems built during the Vijayanagara period.
- Designed for water storage, irrigation, and flood control.
Relevant Mains Points:
Causes of Urban Flooding in India
- Loss of wetlands and floodplains due to urban expansion.
- Encroachment of drainage channels and lakes.
- Impervious surfaces like concrete reducing infiltration.
- Climate change leading to intense rainfall events.
Importance of Hydrological Understanding in Urban Planning
- Recognizing landscape memory helps predict flood behaviour.
- Enables better drainage design and flood management systems.
Policy Implications
- Need for basin-scale urban planning instead of fragmented city-level approaches.
- Integrating natural infrastructure such as wetlands and lakes into urban design.
Way Forward
- Protect and restore urban wetlands and floodplains.
- Adopt nature-based solutions for flood mitigation.
- Improve urban stormwater drainage systems.
- Incorporate hydrological modelling in city planning.
UPSC Relevance:
- Prelims: Hydrological hysteresis, floodplain, catchment area.
- Mains: Urban flooding, sustainable urban planning, ecosystem-based disaster management.