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The new London Mayor must make London a ‘Sponge City’

Alice Roberts
By Alice Roberts
1st May 2024

The next four years are CRITICAL to tackling flooding and water pollution in London.

To address these problems, London must URGENTLY become a ‘SPONGE CITY’.

But how – and why? And how can Londoners with front or back gardens help?

  • FLOODING. London used to be like a sponge, soaking up rainwater. But building and paving mean rainwater now runs into the drains – and they are failing to cope. BBC News from 2021.
  • SEWAGE POLLUTION. Rainwater and sewage drains are combined in most of central/inner London. Unable to cope in heavy rainfall, they overflow into rivers, taking raw sewage with them. (Here’s a great summary from London Waterkeeper on sewage pollution.)
  • ROAD RUN-OFF POLLUTION. Rainwater from roads and parking lots picks up oil and heavy metals from vehicles, and microplastics from tyres and road markings. In much of London this enters ‘road drains’, flushing pollutants straight into rivers. (Here’s London Waterkeeper again on run-off pollution.)

A SPONGE CITY manages these problems with greenery instead of relying solely on drains. ‘Rain gardens’ and wetlands capture, retain and absorb excess storm water, and filter it. (BONUS – they also support wildlife, make our streets more beautiful and help with urban cooling). Here’s Arup on sponginess and why it matters.

Cities as diverse as Shanghai, New York, Berlin and Cardiff are becoming sponge cities. Read about Cardiff.

RAIN GARDENS are a super-effective way to create a sponge city: they stop water flowing into sewers and filter road run-off pollution. They are engineered to capture, hold on to and filter water from an area roughly ten times the size of the rain garden itself.

Roadside rain garden, designed to capture, retain and filter rain water run-off. Meristem Design
Roadside rain garden: Meristem Design

London needs a massive drive to put in beautiful new rain gardens. Thames Water say we need a staggering 357,000 to ensure sewers aren’t overwhelmed. The new London Mayor must coordinate an emergency programme of rain garden construction. This video is a great summary.

AND we need many more URBAN WETLANDS – which are of course also a massive boon for wildlife. Here’s Thames21 on what and why.

Thames 21 explain why wetlands are a key part of the solution


FRONT GARDENS also need to be reinstated, so water doesn’t run-off into the sewers, but is absorbed instead. Read more from London Waterkeeper. LONDONERS – you can help by de-paving your front garden.

Image: Ealing Front Gardens Project
Image: Ealing Front Gardens Project

AND Londoners can help by installing ‘SUDS planters’, to capture and hold rainwater from roofs, in their front and back gardens.  Installing water butts will help too. Here’s Meristem Design explaining ‘SUDS planters

SUDS planter, designed to capture rainwater from roofs: Meristem Design

The next four years are critical to making London a resilient SPONGE CITY – tackling flooding and water pollution. It’s a journey … this is just the start. Here’s a short video on how and why Berlin is also collecting rainwater for times of drought.