Surface water flooding poses a risk to thousands of Central London buildings

If you thought Central London was safe from flooding, think again. Heavy rain puts critical infrastructure and thousands of buildings throughout Central London at risk from surface water flooding (also known as flash flooding), caused by excess surface water run-off. Understanding the causes of this flooding, identifying vulnerable areas, and finding ways to improve resilience are critical for protecting buildings from disruption and damage.

What is surface water flooding?

Surface water flooding occurs when rain falls so fast that the underlying ground cannot absorb it quickly enough. In urban areas, this can be exacerbated by issues such as a lower proportion of permeable ground and blockages to drainage, causing water to accumulate in areas it is not meant to. In England, 3.4 million properties are at risk of surface water flooding, more than those at risk from river and coastal floods. The duration of surface water floods varies, but flood water can remain for days. The resulting damage, however, can take much longer to recover from.

Surface water flooding is a growing risk. Due to climate change, extreme rainfall events are predicted to become increasingly frequent and severe in the UK. This change is already underway with 13% more rainfall recorded during UK summers between 2009-2018 compared to 1961-1990.

Surface water flooding is a threat to London

Surface water flooding is not caused by proximity to rivers or the coast, so inland locations, protected by traditional flood defences, can still be exposed. In London, areas protected by the Thames barrier can still flood after heavy rain.

In 2021 some parts of London received a month’s rainfall in only a few hours, leading to over 1,500 properties flooding. Portobello Road in West London, for example, recorded 7.6cm of rainfall in only 90 minutes. A more widespread rainfall event of a similar magnitude would have far more severe consequences. While the intense rainfall that causes surface water flooding is unlikely to occur across the whole of Central London at once, flooding could still be widespread, affecting multiple boroughs.

The Environment Agency provides a flood risk service which maps areas vulnerable to surface water flooding. CBRE has reviewed the mapping for a flood with a 1% chance of occurring in any year to assess the potential impact on real estate and infrastructure. Flood events of this magnitude could also occur multiple years in a row or even multiple times in the same year.

How would surface water flooding affect buildings in Central London?

While the flooding may be shallow in many locations, surface runoff will cause water to collect in the lowest areas. Properties with basements can be particularly vulnerable, with deep internal flooding possible even when flooding at street level is shallow or absent.

This has the potential to flood domestic basements across Central London, posing a risk to life as well as damage to property.

Commercial property will also be affected. Environment Agency data highlights many streets and buildings across the West End and Midtown which are exposed to surface water flooding. Office and Retail buildings in these markets may be vulnerable, particularly those with basements or shopfronts with level access from the street.

In the City, far fewer areas are highlighted as at risk from flash flooding, but there are some areas at higher risk of flooding to the west, particularly around Blackfriars and Holborn Viaduct.

Surface water flooding not only poses a danger to occupants but also any stock or equipment kept at or beneath ground floor level. Besides this immediate risk, internal flooding can cause structural damage, threatening the safety of the entire building, as well as damage to building plant and electrical systems. Flooded areas may also suffer from mould and damp long after the water has been removed, making the space unusable without further treatment.

For buildings at higher risk of surface water flooding, insurance is likely to become more expensive, or may even not be available at all. Policies for higher risk sites can have higher premiums, higher excess and a lower limit of indemnity, i.e. a lower maximum pay out.

It is important to note that an appropriately designed building can be resilient to surface water flooding. However, the potential losses for investors and occupiers in Central London could be significant. Many of those at risk may be unprepared if they assess the risk of flooding based only on data relating to river flooding.

How to improve flash flood resilience

1. Occupiers and investors must understand their exposure to surface water flooding. How frequently and severe could floods be, how this will change in the future due to climate change, and how would buildings at risk be affected?
2. Develop a flood action plan in order to maximise building user safety and minimise building damage and disruption in the event of a flood.
3. If at risk from surface water flooding, investigate how building level flood protection or adaptations could prevent flooding or limit its damage:

• Storms associated with surface water flooding can arrive with little warning. Therefore, passive flood protection measures such as non-return valves, flood-proof airbricks, doors, and windows can be useful.
• Flood resilience is important. While preventing surface water flooding may not always be possible, buildings can become more resilient with easy to clean and dry surfaces, or raising electric equipment to limit damage if flooding occurs.

Flooding’s effect on infrastructure

Whilst resilience can be incorporated at the building level, surface water flooding can also have indirect impacts that prevent a building or occupier from functioning properly. Flooding may affect transport routes, such as roads or railways, with a number of stations in London shown to be at risk. Similarly, surface water flooding may affect power infrastructure, with data showing some substations in inner London would be flooded in a 1 in 100-year event, which could result in power cuts, leading to potential operational disruption.

Prepare for a material risk that is likely to get worse

A 1% chance event may not seem like a high risk, but the consequences of such flooding could be severe, disrupting lives, businesses, and the wider economy.

As the climate warms and heavy precipitation becomes more frequent and severe in the UK, extreme flood events are likely to become more common, increasing the need to understand and prepare for the risk of surface water flooding.

Investors and occupiers can protect themselves from the worst effects of flooding by undertaking appropriate risk assessments, developing flood action plans, and focusing on building resilience.

CBRE’s Risk and Resilience team has the expertise and insight to assess the risks posed to individual buildings by all types of flooding, including surface water flooding. It provides clear, commercial, and practical advice on effectively identifying, managing, and mitigating risks, both now and in the future in the face of a changing climate.