Flash Flooding in London Explained: Causes, Impacts and Solutions

The latest NaFRA2 update shows that nearly a third of a million properties in London (319,800) are at high risk from surface water flooding. This total represents almost 30% of all such at-risk properties in the country and is more than eleven times the number of London properties that are at high risk from river and sea flooding.

This high potential for surface water buildup creates a severe danger of flash flooding, a threat which is particularly acute for the city’s most vulnerable properties.

More than 50,000 basement properties in London are at an increased risk of flash floods due to climate change, according to an analysis by City Hall. The Mayor of London has issued a warning, and guidance is being sent to households in basement properties on how to prepare for and deal with a flash flood.

The analysis also found that flood risk could affect nearly half of London’s hospitals, a quarter of its railway stations, and one in five schools.

This follows previous instances of severe flooding, such as in July 2021, when heavy rainfall caused thousands of properties to be flooded with stormwater and sewage.

Causes of Flash Flooding in London

Flash flooding in London is becoming a growing concern, and it is largely driven by how the city has developed over time. Due to the increase of hardstanding and impervious surfaces throughout the city, rainwater which would previously have infiltrated into the subsurface remains above ground as runoff, overwhelming drains and accumulates in low-lying areas. Instead of soaking into the ground, it runs quickly across these surfaces, overwhelming drains and pooling in low-lying areas.

An additional consideration is London’s ageing Victorian drainage system. These combined sewer systems were designed in the 19th century for a much smaller population and very different rainfall patterns. These assets convey both sewage (foul flows) and rainwater, so when a heavy downpour hits, they can overflow. This often leads to untreated wastewater being released into rivers or even backing up into streets.

Climate change adds another consideration. London is already experiencing more intense and unpredictable rainfall events. Short, sharp storms are now more common, and when they hit a city built largely on impermeable ground with stressed drainage, flash flooding is almost inevitable.

The Severe Storms of 2021

The storms of July 25th, 2021, and August 2022 brought exceptionally intense rainfall to the London Borough of Redbridge. During the July 2021 storm, the sheer volume was staggering, with local gauges recording 54.1mm of rain in just two hours and fifteen minutes. This incredible downpour led official Section 19 reports to classify the event as exceeding a 1-in-100 year storm.

This exposed a fundamental vulnerability: London’s Victorian-era sewer network was never engineered for such events. However, the sheer volume of water meant that even modern drainage infrastructure would have been overwhelmed, making widespread surface water flooding much more likely.

This highlights a key reality of flood management. While we cannot always prevent flooding from such rare and extreme events, implementing solutions like retrofitted SuDS is crucial. These schemes may not stop another 1-in-100 year event, but they are vital for protecting homes and businesses from the impact of more common, lower-intensity storms.

Impacts of Flash Flooding in London

Combined Sewage Overflows (CSO)

London’s vast network of combined sewers, built in the Victorian era, channels both wastewater and surface water runoff through the same pipes. During heavy rainfall, the system can become overloaded, leading to overflow discharges of untreated sewage into the Thames and other water bodies. These overflows are safety valves to prevent sewage from backing into homes, but they pose serious pollution and public health risks — with the Environment Agency reporting in July 2025 that serious water pollution incidents are up 60% in England over the past year.

How Can We Reduce The Risk Of Flash Flooding In London?

Critical Drainage Areas

To combat the risk of surface water flooding, authorities have identified over 160 Critical Drainage Areas (CDAs) across the capital. These areas are hotspots for surface water flooding — areas where impervious surfaces and overloaded sewers are most prominent and are most likely to turn a summer storm into a flood event.

Because of this heightened risk, any new development proposed within a CDA must demonstrate that its project won’t make the situation worse, often through mandatory Flood Risk Assessments and the integration of Sustainable Drainage Systems (SuDS).

The Thames Tideway Tunnel

The Thames Tideway Tunnel, or super sewer, was completed in 2025 and is a 25 km tunnel running from west to east London, largely beneath the River Thames. It is designed to intercept and divert wastewater and rainwater from 34 of London’s worst Combined Sewer Overflow (CSO) sites, reducing CSOs in a typical year by 95%. Instead of discharging untreated sewage into the Thames or other watercourses during heavy rainfall, the tunnel intercepts, stores, and conveys the flows to Beckton Sewage Treatment Works for proper treatment.

Sustainable Drainage Systems

Sustainable Drainage Systems, more commonly known as SuDS, are designed to manage the flood and pollution risk from urban runoff and to contribute wherever possible to environmental enhancement and place-making. SuDS are designed to manage storm water locally (as close to its source as possible) to mimic natural drainage and encourage infiltration where possible and then attenuate and passively treat it.

Examples of SuDS:

• Green Roofs: Plants on roofs absorb water.
• Permeable Paving: Used for driveways and car parks, allowing water to soak through into the ground.
• Rain Gardens & Swales: Specially designed gardens or grassy channels that collect runoff and allow it to infiltrate slowly.
• De-paving: Removing unnecessary concrete or asphalt to create green space.
• Parks and Wetlands: Designing public spaces to temporarily hold floodwater during a storm.

London Flood Risk Policies & SuDS

Policy 5.13 of The London Plan (formerly 5.12 under the 2016 Plan) requires all new developments to incorporate sustainable urban drainage systems (SuDS) unless it’s proven impractical. The goal is to mimic greenfield runoff rates and manage surface water at the source.

The SuDS hierarchy according to The London Plan is:

1. Store rainwater for later use
2. Use infiltration techniques, such as porous surfaces in non-clay areas
3. Attenuate rainwater in ponds or open water features for gradual release
4. Attenuate rainwater by storing in tanks or sealed water features for gradual release
5. Discharge rainwater direct to a watercourse
6. Discharge rainwater to a surface water sewer/drain
7. Discharge rainwater to the combined sewer

London Sustainable Drainage Action Plan

Published in 2016, the London Sustainable Drainage Action Plan (LSDAP) outlined 40 actionable steps over 5 years and aimed to set the direction for the next 20 years. The LSDAP marked a shift from reliance on conventional piped drainage to implementing SuDS retrofit and integration, alongside promoting rainwater reuse and ecological functionality.

“The main focus of the action plan is on the retrofitting of sustainable drainage to existing buildings, land and infrastructure. It is recognised that funding pressures mean there will not be funds specifically for a large-scale drainage improvement programme. Instead, the key is to identify when and where other planned maintenance, repair or improvement works are scheduled and then to identify opportunities to retrofit sustainable drainage as part of those works.”

The London Surface Water Strategy

The London Surface Water Strategy is published by the Flood Ready London partnership, made up of the Environment Agency, London Councils, London Fire Brigade, Mayor of London, Thames Water and Transport for London. The vision of the strategy is “increased resilience to surface water flooding through collective action that benefits people, places and the environment.”

Underpinning the strategy are six guiding principles:

1. Prioritise the most vulnerable
2. Prioritise nature-based solutions
3. Develop evidence-based actions informed by hydrology
4. Work in more effective partnerships
5. Enable change through strong leadership
6. Manage surface water flood risk at the right scales

London is divided into 10 Surface Water Catchment Partnerships (SWCPs), dividing London into hydrological catchments, defined by water flow rather than political boundaries. Two pilot catchments (Central London and Lee Valley) are already underway, preparing tailored Surface Water Catchment Action Plans (SWCAPs). £1.5 million has been committed for Sustainable Drainage Systems (SuDS), with £500,000 each announced from the Mayor of London, Transport for London and Thames Water.