Disasters, Pandemics & Economic Recovery
Floods and Cities
Why urban floods are becoming more damaging, how building over wetlands and drains worsens them, and what cities can do.
Floods are among the most common and costly disasters. As cities grow, urban flooding has become a major economic problem, damaging homes, businesses and transport, and disrupting daily life.
Why cities flood
- Paved surfaces: roads, roofs and car parks stop rain soaking into the ground, so water runs off quickly.
- Blocked or inadequate drains: drainage systems built for smaller cities cannot cope, and rubbish clogs them.
- Building on wetlands and floodplains: lakes, marshes and river banks that once absorbed water are filled in for housing.
- Heavier rainfall: extreme rainfall events are becoming more intense in many regions.
Indian examples
On 26 July 2005, Mumbai received over 900 millimetres of rain in a single day. Much of the city flooded, hundreds of people died, and transport and business were paralysed. Investigations pointed to encroachment on the Mithi River and loss of mangroves as factors.
In late 2015, Chennai suffered severe floods after extreme rainfall. Studies highlighted how the city’s lakes and wetlands had shrunk dramatically as land was built on, removing natural flood storage.
A wetland on a city's edge may look like empty land ripe for development. But during heavy rain it acts like a sponge, storing water and releasing it slowly. If it is filled in for housing, the water has nowhere to go, and it floods streets and homes downstream. The wetland's value in preventing flood damage never appears on any price tag until it is gone.
Solutions
Cities use a mix of approaches:
- Grey infrastructure: larger drains, pumps, embankments and flood barriers.
- Green infrastructure: protecting wetlands, restoring rivers and adding parks that absorb water.
- Flood zoning: restricting building in the highest-risk areas.
- Sponge city designs: China launched a “sponge city” programme in 2015 to help cities absorb and reuse rainwater.
- Maintenance: regularly clearing drains before rainy seasons.
Who pays
Poorer residents often live in the most flood-prone areas, such as low-lying informal settlements, because land there is cheaper. They suffer repeated losses and have the least ability to recover. Flood protection decisions therefore raise questions of fairness as well as efficiency.
Engineering helps, but cities that pave over natural drainage and wetlands can overwhelm even large drains. Protecting natural systems and controlling where people build are often cheaper and more effective than pipes and pumps alone.
- Urban floods are worsened by paved surfaces, poor drainage and building on wetlands.
- Mumbai in 2005 and Chennai in 2015 suffered severe, costly floods.
- Solutions combine drains and pumps with wetlands, flood zoning and sponge city designs.
- Poorer residents often live in the most flood-prone areas and suffer most.
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