How can green infrastructure reduce surface water flood risks?




There is no question that the traditional urban drainage systems present in the majority of the UK’s towns and cities are simply not up to coping with surface water flood risks.

It’s no news that the progression of climate change will bring about more frequent instances of pluvial flooding, which will continue to overwhelm our old-fashioned Victorian pipe systems and grey infrastructure.

Last month, Steve Wilson, managing director of Welsh Water’s wastewater services, revealed that reducing surface water flooding was the company’s biggest challenge and that the installation of more green infrastructure was the only way of combating the problem.

Yet, despite the immediacy of surface water flood risks, the urgency to implement more sustainable drainage systems and other forms of green infrastructure is not being seen at an administrative level.

For thousands of years, traditional drainage systems have proved successful, but as urbanised, impermeable landscapes grow alongside more intense climate events, the need for natural flood management solutions is greater than ever.


What is the problem with traditional urban drainage systems?

Traditional surface water drainage systems date back to the Babylonian and Mesopotamian eras in Iraq, between 4,500 and 6,000 years ago.

Sustainable drainage systems (SuDS) are not a recent solution to urban runoff either.

According to Colin A. Booth and Susanne M. Charlesworth in their text, Sustainable Surface Water Management, examples of SuDS such as “water harvesting, storage and conveyance were all well-known and efficiently carried out by ancient cultures as long ago as the Early Bronze Age in Crete.”

We still use an advanced form of these early methods several millennia later.

But in that time, civilisation has changed, and with it, the rapid development of urban areas where paving and impermeable surfaces are frequent.

“The high urban density within cities and resultant soil sealing have led to a reduction in the potential of water infiltration in the ground, which increases water run-off and flood risk”, Horst Korn and Nadja Kabisch state in their book, Nature-Based Solutions to Climate Change Adaptation in Urban Areas.

Combine intense urbanisation with out-of-date drainage infrastructure and climate change, and you have a recipe for contaminated surface water flooding.

London is a perfect example of this issue; 1,000 buildings were partially engulfed last year when heavy flash floods overwhelmed the city’s drainage systems.

A recent assessment by City Hall has indicated that flooding will affect one-quarter of the capital’s rail stations, one in five schools, half the city hospitals and many thousands of homes and businesses.

As a result, London Mayor Sadiq Khan is writing to 45,000 Londoners living in basement properties to raise awareness of flooding and help them prepare for future events.

The inability of current urban drainage systems to deal with excessive amounts of water cannot be overstated; without green infrastructure, the damage caused by flooding will only increase.

This includes:

  • Polluted flooding of buildings and transport routes
  • A build-up of sewage water in drainage systems, toilets and showers leads to flooded basements and streets
  • The contamination of nearby water courses by water run-off
  • The pollution of natural spaces in the vicinity of surface water flooding

With 1.77 million hectares of urban area in Great Britain and counting, a shift towards natural drainage solutions is paramount.


What is green infrastructure, and why is it important?

Green infrastructure refers to a network of environmental spaces, features and watercourses that can be present in both rural and urban areas and can be used to store and control the flow of excess water during flood events.

Natural flood management systems come under the umbrella of green infrastructure and denote a set of techniques which imitate or restore the natural functions of rivers, floodplains, and the wider catchment.

Other examples of green infrastructure include vegetation corridors, parks and gardens, coastal habitats, green roofs and sustainable urban drainage systems like linear wetlands, permeable paving and retention ponds.

In the UK, the development of green infrastructure is progressing somewhat, evident in Essex County Council’s pledge to double the amount of natural flood management solutions by 2040.

However, these decisions need to be implemented nationwide and not just by a couple of local councils to make a difference.

With few downsides to green infrastructure, there’s no reason for our slow approach to creating more natural drainage solutions, especially considering the benefits they provide to health and biodiversity.

Reflecting this sentiment, an independent briefing carried out last month by the Climate Change Committee (CCC) revealed that installing natural flood management solutions by CCC experts on the land of 14 farmers improved wildlife habitat, reduced downstream flood risks and saved on hard-engineering costs.

“Natural flood management is only part of the solution, as retrofitting traditional drainage solutions with Sustainable Drainage Scheme (SuDS) features and investment into traditional harder localised flood defence schemes are also important to ensuring drainage systems are more resilient to flooding”, said Mike Piotrowski, Principal Hydrologist at GeoSmart.


How does green infrastructure reduce surface water flooding?

“You may not be interested in surface water flooding, but it is interested in you,” Chief Executive of the Environment Agency, Sir James Bevan stated in his 2018 CIWEM conference speech.

In other words, there’s no escaping it.

But through green infrastructure, there is a way to reduce the harm caused by surface water flooding.

Allowing water to soak into the ground, infiltration-based sustainable drainage systems control excess water more efficiently than gullies and conventional underground pipe systems.

Porous substances such as sand and gravel form the best SuDS by enabling water to seep through air pockets and spaces.

Other forms of green infrastructure such as green roofs and walls also help to decrease stormwater runoff by absorbing and storing rainwater and are beneficial in their provision of a small natural environment.

Making up 40-50 per cent of impermeable surfaces in urban areas, roofs provide a huge opportunity to manage water more effectively if they are adapted to absorb instead of repelling rainfall.

Water pollution is another huge problem caused by urbanisation and something which can be remedied by the green infrastructure, which behaves as a buffer by lessening the runoff of contaminated stormwater flowing into rivers, natural environments and sewers.

For more information on sustainable drainage systems and how GeoSmart’s SuDSmart drainage reports can help ensure your development or property is prepared to deal with surface water flooding, please visit our SuDSmart page.