Context

London is vulnerable to flooding from the North Sea (tidal flooding), the freshwater Thames and the tributaries to the Thames (fluvial flooding) and from heavy rainfall (surface water flooding). Flood risk in London is managed by a system of flood defences (walls, gates and the Thames Barrier) and drainage networks.

Probability - Currently, there is a LOW likelihood of tidal flooding, a MEDIUM probability of river flooding and HIGH probability of surface water flooding. The risk is INCREASING as sea levels rise, tidal surges increase in height and winter rainfall increases in volume and intensity. Increasing impermeability of the urban landscape (loss of front gardens, for example) and limited drainage capacity also increase the likelihood of flooding.

Consequence - The consequences of a major flood in London would be HIGH because London is heavily urbanised and 15 per cent of London lies on the floodplains of London’s rivers, an area that includes 1.25 million people, 481,180 properties, and a substantial proportion of the capital’s schools, transport network, and emergency services. More than 680,000 properties lie at risk of surface water flooding. The consequences of flooding are INCREASING as London’s population is set to grow by approximately 800,000 people over the next two decades.

Vulnerability - HIGH as there are a large number of flood-vulnerable communities and assets at risk. Warning times for fluvial and surface water flooding are short and public awareness and capacity to act are low.

Summary assessment

Despite the very low probability of a catastrophic tidal flood in London, the impact of a tidal flood would be significant. River and/or surface water flooding are much more likely and are difficult to predict and manage. The Mayor assesses that flood risk should be considered a HIGH RISK to London

Response

The Mayor believes that London should be resilient to all but the most extreme floods and should have robust emergency plans to respond to, and recover from, flooding.

To reduce and manage current and future flood risk in London, the Mayor will work with partners to:

• a) improve the understanding of flood risk in London and how climate change will alter the risks, to improve our ability to manage flood risk
• b)reduce flood risk to the most critical assets and vulnerable communities, to target the greatest effort on London’s most vulnerable assets
• c)raise public awareness of flooding and individual and community capacity to cope and recover from a flood, to improve London’s resilience to flood events.

There is a good understanding of current tidal and fluvial flood risk in London, but a poor understanding of surface water flood risk. To improve our ability to predict and manage flood risk, further work is required to understand surface water flood risk and how climate change will increase all forms of flood risk.

• Action 1 - The Mayor will work with the Environment Agency, boroughs and other partners to improve the mapping of who and what is at risk from all sources of flooding today, and to predict future flood risk for all flood sources.
• Action 2 - The Drain London Forum will develop a surface water management plan for London which identifies and prioritises areas at risk and develops more detailed plans for the priority areas.
• Action 3 - The Drain London Forum will create an online data portal to allow flood risk management partners to more effectively share information and data analysis.
• Action 4 - The Drain London Forum will create a flood incident reporting system that is adopted throughout London to improve our understanding of flood risk today.
• Action 5 - The Mayor will work with boroughs and the Local Resilience Forums to ensure that flood risk management is integrated across borough boundaries and within borough teams.

In order to prioritise flood risk management actions that target the most important and the most needy, we need to identify the most vulnerable communities and critical assets.

• Action 6 - The Mayor will work with the Environment Agency, London Resilience and the London Climate Change Partnership to identify and prioritise critical infrastructure and vulnerable communities at flood risk.
• Action 7 - To reduce the risk of local surface water flooding, the Mayor will work with Transport for London (TfL), the London boroughs and Thames Water to review their drain and gully maintenance programmes, particularly in high-risk areas.

In order to increase our capacity to cope and recover from a flood, we will seek to raise individual and community-level awareness of flooding.

• Action 8 - The Mayor will work with the Environment Agency to increase the number of Londoners signing up to the Floodline Warning Direct scheme and to raise awareness of the measures that individuals and communities can undertake to reduce the risks and manage the consequences of flooding.
• Action 9 – The Drain London Forum will identify two communities at significant flood risk and work with them to develop bespoke community flood plans to build their capacity to manage flood risk.