UK Floods – London Launches Sustainable Drainage Action Plan

The London Mayor, Boris Johnson, in partnership with Thames Water, the Environment Agency and London Councils have published the first London Sustainable Drainage Action Plan (LSDAP) for consultation.

LSDAP is a long-term plan intended to facilitate a step-change in how rainwater in London is managed. It proposes that by 2040, London will manage its rainwater sustainably to reduce flood risk and improve water security, maximising the benefits for people, the environment and the economy.

Population Growth, Changing Land Use, Changing Climate

The plan says that, whilst London is currently well protected against flooding from the tidal Thames, the city has a relatively lower standard of protection against surface water flooding.

London is outgrowing its drains and sewers. The combined sewer system originally built over 150 years ago was designed for a smaller, more permeable city. The growth of London’s population, along with changing land use and changing climate means that London could be facing an increasing risk of surface water flooding.

Surface water flooding happens when rainwater can’t soak into the ground and overwhelms the drainage system.

London, like many cities, has a high proportion of impermeable surfaces which prevent water from soaking into the ground. LSDAP says that:

if London continues to seal off its permeable surfaces, effectively ‘waterproofing’ the city, even relatively light rainfall may overcome the drains and sewers.

Flooding Crisis Points

Thames Water has modelled the impact of London’s projected population growth and climate change on its drains and sewers to understand their ability to cope with these future challenges.

The modelling shows that for a relatively common rainfall event in 2050 (one that would be expected on average once every other year), some areas of London would not have sufficient drainage or sewerage capacity to manage the expected flows, leading to an increasing risk of surface water and sewer flooding.

london floods
Areas highlighted in red on the map below are where the projected flows in the system exceed its capacity and some flooding should be expected. Image credit: Thames Water / Mayor of London

Traditional Drainage v Sustainable Drainage

This situation has left London with a stark choice: to build further enhanced storm drains and sewers across large parts of London; or steadily increase London’s resilience by adopting more sustainable drainage (SuDS) techniques.

However, the options to significantly increase the capacity of London’s drainage system using traditional underground piped networks are becoming increasingly complex and prohibitively expensive.

According to LSDAP, the choice, therefore is an obvious one. London must develop a new approach in dealing with its water and embrace sustainable drainage techniques such as the increased use of permeable hard surfaces for paving, the wider adoption of greenroofs and the storage of surface water run-off in ponds, tanks, rain gardens and swales.

The draft of the plan for public consultation can be found here.

Other SuDS Cities

Developing a Sustainable Drainage Action Plan means that London will follow other cities around the world that have developed long-term plans to increase the use of sustainable drainage techniques.

Portland, Oregon in the USA, has a sustainable drainage plan dating back to the early 1990s that has been shown to be a great success. Philadelphia and New York City are also developing sustainable drainage plans. In Europe, Copenhagen in Denmark has developed the “Cloudburst Plan” to manage severe thunderstorm events. In Australia, Melbourne has also undertaken a wide range of sustainable drainage projects.