Hard surfaces, like pavements and roofs, prevent rainwater from soaking into the soil. The results can range from inconvenient puddles to severe surface water flooding.

Sustainable drainage in London

What's the problem?

Even light rainfall can cause flooding and water quality problems across the whole city. London has two main types of sewer system, each with different problems. If rainfall overwhelms our sewer system, it can have serious impacts:

  • surface water flooding of buildings and transport routes
  • toilets and showers backing-up with sewage and flooding basements
  • pollution of rivers and canals with contaminated surface water from roads
  • release of raw sewage into the Thames

On this page, you can:

  • use the Environment Agency's website to check if you're at risk of surface water flooding
  • find out how small changes to your home or business can help
  • learn about sustainable drainage and find out where it's been installed in your area 
  • submit your own sustainable drainage project to our interactive map
  • download the London Sustainable Drainage Action Plan
  • watch a short video summarising the problem with surface water flooding and how it can be reduced


Inner London. Many parts of inner London have sewer systems that carry rainwater as well as sewage. These Victorian 'combined sewer systems' were designed for a city of four milion people. But with sewage from almost nine million people, the network is now at 80 per cent or more of capacity, even in dry weather.

Outer London. Even in parts of outer London where there are separate pipes for carrying rainwater, there are significant capacity and water quality problems.

To see which type of sewer system is in your area, view the London Sewer System Map.

The main problem is too much rainwater reaching the sewer system. That's why the Thames Tideway Tunnel is being built. It will help reduce the amount of untreated sewage that is released into the Thames. But there are three main reasons why it won't solve the flooding and water quality problems completely.


London's growing population

London's population is set to reach around 11 million people by 2050. This will have two main impacts:

  1. More people means that there will be even more sewage in the sewer systems.
  2. Building new homes, schools and transport routes means that more rainwater could reach the drainage systems. This is because hard surfaces (like roofs and roads) reduce the amount of rainwater that can be absorbed into the soil or by plants.

Climate change

Climate projections predict that London will experience warmer and wetter winters, as well as more frequent and intense rainfall events. This will increase the risk of the drainage system being overwhelmed. 

Paving of gardens

Almost a quarter of London is garden. But this is disappearing under paving slabs, decking and concrete to provide car parking or to reduce maintenance. We're losing gardens at the rate of an area two and a half times the size of Hyde Park every year. Paving slabs, decking and concrete reduce the amount of rainwater that can reach the soil, meaning that more of it will reach the drainage systems.

What is sustainable drainage and how can it help?

Sustainable drainage measures (often known as SuDS) try to mimic natural drainage as much as possible. They can perform several different functions:

  • reduce the amount of rainwater that will reach the drainage system
  • slow water down to give the drainage system time to empty
  • re-using rainwater, for example, for irrigation or toilet flushing
  • filter rainwater of pollution
  • a combination of these functions

There is a wide range of different measures that can be used in different circumstances. Some are hard-engineered (‘grey’ SuDS) solutions. Others include plants (‘green’ SuDS).

Examples of different types of sustainable drainage measures
'Grey' SuDS 'Green' SuDS
Attenuation tank Pond / basin
Permeable paving Rain garden
Rainwater harvesting Swale / channel / rill
Soakaway Green roof
Geocellular storage Wetlands

Both types of SuDS measures will be needed in London. But the biggest advantage of green over grey SuDS measures is that they can also provide lots of other benefits, as shown in the diagram below.

Benefits of 'Green' SuDS

How can you get involved?

There are some simple ways to do your bit for London.

  1. You can take up some of the concrete, decking or paving in your garden and replace it with gravel or plants. This will allow rainwater to soak into the soil, instead of overwhelming our drainage systems. The Royal Horticultural Society website has lots of advice on how a front garden can be used for car parking and also be good for the environment.
  2. You can install a water butt to take rainwater from your roof to help water your garden.
  3. Or if you know of a SuDS project that's been installed in your area, you can add it to our interactive map using the Submit Feedback button.


If you have large numbers of SuDS projects you'd like to let us know about, or if you'd like to find out more about the map, please get in touch with George Warren ([email protected]).

What are we doing to encourage sustainable drainage?


The London Plan has a sustainable drainage policy that new developments must follow. But each year, this will only affect 0.5 per cent of London's area. That's why we've produced the London Sustainable Drainage Action Plan.

London Sustainable Drainage Action Plan

To make a real difference, we need to add sustainable drainage measures to existing buildings and transport routes (SuDS retrofit). The aim of the London Sustainable Drainage Action Plan is for all future building, maintenance, repair and improvement works to think about including sustainable drainage measures as part of normal working practice. 

Over time, many of these small-scale actions can bring about a big difference in surface water flood risk and water quality. To achieve this, the Action Plan provides actions tailored to each of the main land-use sectors in London, such as education, retail, housing and transport.

Partnership - Drain London

We can't do everything by ourselves and that's where our partners come in. The Drain London partnership aims to:

  • improve our knowledge of the surface water drainage system
  • identify areas most at risk of flooding
  • find ways to reduce flood risk

It was set up in 2010 with funding from the Department for Environment, Food & Rural Affairs. The work programme is managed by the Drain London Board, which is chaired by City Hall and includes representatives from:

  • Thames Water
  • Environment Agency
  • London Councils

Drain London has supported some important and innovative work on surface water flood risk and sustainable drainage. This is split into two main categories: projects and data.

Sustainable Drainage Projects

The Drain London Partnership has helped fund several sustainable drainage projects. these include five SuDS retrofit projects across London. These good practice projects have shown how SuDS retrofit can be cost-effective, can improve the way an area looks, and be great for wildlife.

We hope that they'll inspire Boroughs, private organisations, landowners and residents to think differently about their land.


An important part of dealing with flood risk and water quality is knowing where the risks are highest, which SuDS measures are likely to be most cost-effective, and where SuDS project have already been installed. The Drain London Partnership has helped fund several data projects to find out just that. 

  1. City Hall is collecting information on different SuDS schemes that have been installed across London. You can use our interactive map to find out where SuDS have been installed in your area, and even submit your own projects.
  2. Drain London has funded SuDS Opportunity Modelling to identify where different measures can be installed, and which are most cost-effective in different places. We'll provide more information once this work is finished. 
  3. Drain London also funded flood risk modelling to help the Boroughs better understand risks in their area. This has informed the Boroughs' Surface Water Management Plans, which help manage and reduce those risks.

Ever wondered where the rain goes?

This short Susdrain video explains why surface water is a problem in cities, and what can be done about it.

SuDS not Floods!

“SuDS not Floods” by Year 3 students at Harris Junior Academy Carshalton and South East Rivers Trust as part of the SuDS in Sutton Schools project

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