Urbanisation and Urban floods 

Context: There was unprecedented flooding in India’s tech capital Bengaluru.

• Urban Flooding is an inundation of land in a constructed setting, particularly in densely populated areas.
• Previous such flooding incidents: Kochi, Mumbai, Hyderabad, and Chennai.

Reasons

Natural:

• Higher rainfall: As per the IMD, monsoon has become frequent and unpredictable.
• Storm surges (for coastal cities):g., Cyclone Amphan in 2020 flooded the streets of Kolkata. Within eastern India, the storm killed 98 people and caused $13.8 billion (2020 USD).
• Groundwater levels: In Chennai, the replenished groundwater table across the city after rains becomes a challenge for several buildings with basements.

Anthropogenic:

• Encroachment on Drainage Channels: In Indian cities and towns, due to increased land prices and less availability of land in the city centre.
• New developments are coming up in low-lying areas, usually as encroachments over lakes, wetlands and riverbeds.
• A large number of wetlands that soaked up the rainwater has encroached. Bangalore had 262 lakes in the 1960s; now only 10 of them hold water.
• Climate Change: Exacerbated by changing climate, resulting in extreme events. The climate change has caused an increase in the frequency of short duration heavy rainfall leading to higher water run-off.
• Rapid and unplanned urbanization: In Bengaluru, stormwater drains were not directly connected to its water bodies. In some places, the runoff water was flowing into constructed deviation canals. CAG found that this increased the chances of flash floods.
• Destruction of mangroves: Mumbai lost about 40% of its mangroves between 1995 and 2005.
• Poor civic management of storm water drainage: CAG report (last year) pulled up Bengaluru municipality for this poor management.
• Lack of data: CAG also found that the Bengaluru municipality did not maintain proper records of the stormwater management funds allotted to it under the JNNURM.
• Lack of coordination: CAG report also noted the lack of coordination between the municipality and the Bangalore Development Authority on drainage-related matters.

Steps to mitigate urban flooding:

• Sponge Cities Mission: The idea of a sponge city is to make cities more permeable so as to hold and use the water which falls upon them.
• Implementation of Atal Mission for Rejuvenation and Urban Transformation (AMRUT) in a planned manner.
• Stormwater disposal system: The Brihanmumbai Storm Water Disposal System or BRIMSTOWAD, the project to overhaul Mumbai city’s old stormwater drainage system was started after the 2005 deluge.
• Water-sensitive urban design (WSUD in Australia): regards urban stormwater runoff, and waste water as a resource rather than a nuisance or liability.
• Bioswales or ‘Rain Garden’ (New York): are landscape features that collect polluted stormwater runoff, soak it into the ground, and filter out pollution.
• Rainwater harvesting:
  • People must invest heavily in rainwater harvesting.
  • Need to make sure that collection of the rainwater as much as possible, use that to recharge our aquifers and try to design our plots and apartments and gated communities as zero rainfall discharge areas.
• Design on a watershed basis:
  • Every lake is a micro or mini watershed. Authorities must understand that a watershed is a hydrological unit, understand how water flows and ensure that it is drained into channels and flows into the lake.
  • When the lake overflows, it connects to the next downstream lake, for which they will need sluice gates that they can operate.

For a city like Bengaluru, it takes a special kind of skill to mitigate floods. The city has an average elevation of 920 metres and cascades down into valleys and rivers on all sides. Unlike Chennai, Mumbai or Kolkata, this city is on a hill or a ridge line. It calls for great incompetence if the city can create floods.

Therefore, there is a need to include public open spaces within the urban fabric in the form of storm management infrastructure, which could help our cities transform into water-sensitive cities.

Source: The Hindu