Flash Flooding in London – Why it is likely to become a more frequent occurrence?
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Monday 12th July and Sunday 25th July 2021 saw many parts of London experience horrendous flash flooding. The preceding heatwaves provided the perfect conditions for intense summer storms to develop, and the resulting deluges of rainfall caused many streets to become rivers, left basements inundated, and sewers overflowing.
Although the flood water will have subsided within a matter of hours, the impacts for the unfortunate residents and businesses affected will be felt for many months to come.
Policy makers and the media often only consider the financial impacts, however there is a huge emotional impact of having your home and belongings damaged and destroyed, along with the unexpected and unwarranted stress of organising and overseeing the repairs, and in some instances needing to temporarily move elsewhere or close a business, can be a huge burden.
So how do we ensure future flooding is less of a burden? Firstly, we need to understand how and why this flooding is occurring, and why the impacts of these recent floods seem greater than previous flood events.
The recent flooding in London has been a result of two combined factors; surface water flooding and sewer flooding.
Surface water flooding occurs when run-off following heavy rainfall cannot drain away, causing water to accumulate at the surface.
Sewer flooding occurs when drainage systems become overwhelmed, causing flows to backup and surcharge at the surface through gullies, manholes, and internal private systems such as toilets, sinks, and showers. The volume of surface water run-off can cause sewers to become overwhelmed, and therefore these flood mechanisms often need to be considered together.
The risks of these types are flooding are typically far greater in urban areas for a number of reasons:
Impermeable surfaces. Urban areas are predominantly impermeable, meaning rainfall cannot drain into the ground. It runs off and collects on hard surfaces; roofs, tarmacked and concrete roads and pavements. The greater the impermeable area, the greater the volumes of surface water run-off following a storm event. Many urban areas have grown significantly over the past century, resulting in increased impermeable surfacing.
The problem is not just at the surface. In London particularly, huge basement developments have removed large volumes of soil which would previously allowed some run-off to percolate into the ground, but can no longer do so.
At the same time areas of existing urban green spaces have also declined, only adding to the problem.
The nature of the sewer systems can also present a problem. Modern developments must separate surface water sewers from foul water sewer systems, and demonstrate that the systems have sufficient capacity to manage flows both now and in the future. However, the historic Victorian systems in many areas of central London particularly are combined systems, meaning they drain both surface water run-off and foul water together. These sewer systems were not originally designed to cope with the volumes of run-off now being demanded of them, as a result of both increases in rainfall and an increase in run-off from the greater impermeable areas.
Poor maintenance. The extent and number of drains and gullies within urban areas means that clearance and maintenance may not occur as regularly as required. Blocked and silted up drains prevent water entering sewers and draining away, exacerbating flooding at the surface.
All of these factors are amplified by climate change. It is now widely acknowledged that the increased frequency and severity of intense rainfall events experienced across the world is almost certainly a result of changes to the climate.
On 12th July, parts of London experienced 48mm of rainfall within 24 hours, greater than the region’s average monthly rainfall for July. 48mm may not sound like much, but over an area of 1km2 this equates to 48,000m3 of water. And these rainfall totals were dwarfed by those recorded elsewhere across the globe in July 2021:
In western Germany some gauges recorded 182mm of rainfall across a three-day period, the equivalent of two months of rainfall. Dozens of people have lost their lives, with many more still unaccounted for. The resulting flooding has been described by some scientists as 1 in 10,000 year event. (https://www.theguardian.com/environment/2021/jul/16/climate-scientists-shocked-by-scale-of-floods-in-germany).
In Henan province in China, over 600mm fell in some areas within a single day, and 200mm within a single hour. Streets and subways were flooded and the death toll now stands at more than 30. (https://www.theguardian.com/world/2021/jul/20/heavy-flooding-hits-central-china-affecting-tens-of-millions) .
As the impacts of climate change accelerate over the coming decades the flooding problem in urban areas is only going to worsen unless the changes required to better manage rainfall and surface water run-off are implemented. Increasing sewer capacity is extremely expensive and disruptive, and may not have the desired effect if upgrades are also not provided holistically to gutters, downpipes and gullies, as well as regular, proactive and ongoing maintenance to ensure the systems remain clear. Increasing capacity may also only move the problem further downstream.
There is therefore an increasing desire and acceptance to utilise Sustainable Drainage Systems to slow the flow and manage rainfall more naturally. Residents and businesses must also play their part by managing and maintaining their own existing drainage systems. By slowing the flow of water into the sewer network, the sewers are less likely to be overwhelmed. The key change will be using and implementing green infrastructure to work with nature, rather than against it. This could have huge benefits for cities, not just in reducing flood risk, but also wider environmental benefits.
Flooding of the nature experienced in July 2021 is likely to become a more regular occurrence in urban areas worldwide. Flash flooding can occur anywhere with little warning, and therefore the time to act is now to ensure we’re prepared for the next event.
67% of the UK Population do not know their flood risk. It is vital everyone checks their flood risk (this can be done on the Environment Agency’s free website). Once the flood risk is known, property flood resilience measures can be implemented to mitigate the flood risk at a property level. Non-return valves (which allow water to flow out and not back) should be utilised, certainly in basement flats.
Article by Flood Specialist Simon Crowther BEng (Hons) MCIWEM C.WEM from Flood Protection Solutions Ltd. For further assistance with property flood resilience, Flood Protection Solutions Ltd are able to assist from concept through to completion. www.Floodprotectionsolutions.co.uk
