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What’s the science behind turning our cities into sponges?

At first glance, the numbers in this infographic might seem unrelated. What connects mental health with climate change, and what does either of them have to do with flooding and water pollution?

Quite a lot! They are all affected in one way or another by the green spaces in our towns and cities.

Especially in urban areas, our green spaces play as much of an important role in our lives as the roads, services and buildings we use everyday. Green spaces provide habitats for pollinators, places to reconnect with nature, areas to soak up rainwater, cool shady areas in summer, a store for carbon, and spaces where we can take a little time out from our busy lives.

On this page we’re going to explore three issues, and how more and better green spaces could help with all of them:

  • Climate change, and adapting to its effects
  • Traditional drainage and sewer systems – the impact of pressures on these systems
  • Healthy people and wildlife

A changing climate in the UK

In a country like the UK, rain is – for the most part – just a part of daily life! However, due to climate change, the number and impact of ‘extreme rainfall events’ is expected to increase.

There is no fixed definition of what an ‘extreme rainfall event’ is, but you can understand it as a downpour that is heavier than you would expect to happen in a number of years in your area, for example a rainfall event that would only have a one percent chance of occurring in any given year, or a 1-in-100 type event.

The link between climate change, the increase in heavy downpours of rain, and the risk of flooding is not always clearcut (have a look here if you want to understand more about why this is the case). However, we do know that heavy rainfall puts a lot of pressure on our drainage and sewer systems. This can lead to these systems, which manage our water and the rain that falls on our towns, becoming overwhelmed. As you can read below, this risks both flooding and the pollution of our rivers and streams.

Therefore, while efforts to reduce greenhouse gas emissions (known as climate change mitigation) are very important, so is adaptation.

Why is green space important?

We need to transform our cities into resilient places that can adapt to the changes that are taking place. Well managed, high quality green spaces are crucial to enable this transformation. Green spaces keep urban areas cool and allow water to soak into the ground, reducing floods and droughts. It is this impact on water management which will be investigated next…

Where does rain go and what happens to it?

Most of the rain that falls on our towns falls onto hard surfaces like roofs and roads. These surfaces cannot soak up water, they are ‘impermeable’, and so drains and pipes quickly take this water to the drainage system. Surface water flooding happens when local drainage systems are overwhelmed by the amount of water they have to cope with and you end up with huge puddles building up at the surface.

There are two types of systems to deal with water from homes and driveways.

Combined Sewer

Older buildings often have combined sewers. Dirty water from our homes, including toilets and washing machines, goes into the same pipes as the rainwater from our roofs and driveways. This combined water is taken to the treatment plant.

This means that the water from our roofs and driveways, which is fairly clean, is treated in the same way as sewage, which is a costly and energy intensive process.

There are more local negative impacts too. If too much water rushes to the sewer, the system is overwhelmed and an emergency outflow discharges into a river or stream.

This means that when there is heavy rain, not only is a lot of water rushing to the rivers, increasing the risk of flooding, but this water is also contaminated with sewage, cleaning products and anything else which goes down our toilets and sinks!

Seperate System

In newer buildings, these systems are often separated, with only rainwater flowing into streams.

This has less impact on water quality, but it can carry chemicals, like oils, metals, pesticides and fertilisers straight to the stream. Many people don’t realise that the surface water drains on their driveways and roads lead straight to the river! Try to think about what might end up going down these drains when gardening or washing your car.

This system takes water very quickly to the stream during a storm, which can make flooding more likely, especially as storms become more extreme. 

There can also be issues when sewer pipes are connected the wrong way – these are called misconnections – and pipes that should lead to the sewage treatment plants are mistakenly connected to the surface water system. This can lead to severe pollution in rivers and have detrimental effects on wildlife. If you suspect that your property might be affected by this, you should get in touch with a plumber who can check and correct this.

Why is green space important?

On a natural surface, like a meadow or woodland, rain would be slowed down by plants before soaking into the soil.

This reduces pressure on drainage systems, reducing the risk of surface water flooding.

It also means the peak flow (that is the highest amount of water flowing through a watercourse in a flood) is lower in streams fed by mainly natural land uses compared to those fed by land covered in hard surfaces. A lower peak flow means a lower risk of a river breaking its banks and causing river flooding.

Furthermore, some types of plants can to help filter and clean up water before it reaches the rivers.

Green spaces for people & wildlife

In 2013, less than a quarter of waterbodies in the UK were able to support viable ecosystems, and many of them were failing due to pollution caused by the urban environment.

And the growth of urban areas isn’t just affecting water habitats & wildlife. Over a tenth of the UK’s area is built up or ‘urban’. While some species are very well adapted to the city environment and can make use of modified spaces, fragmentation of habitats or lack of connectivity between existing spots can make it difficult for species moving around to find new sources of food or breeding areas. Loss of high quality green spaces in cities or degradation thereof are the biggest threats to biodiversity in urban areas and can impact the surrounding environment too.

Of course, it’s not just wildlife that needs to find a home in urban areas. Over 80% of the UK population live in cities, and that number is expected to increase. The need for housing is also pressing, and in Taunton alone a 40% increase in housing stock is proposed over the next few years.

Development of this kind has in the past often meant a loss of high quality urban environment: in 2001, almost 40% of urban parks were degrading in quality as a result of reduced spending – and the worst declines were reported in areas of social deprivation, indicating that those who could benefit most from imrpoved green spaces and neighbourhoods have the least access to them. In a 2016 Heritage Lottery Fund study, only a quarter of park managers reported their parks to be improving. 


Why is green space important?

Visits to urban parks are increasing, and the benefits people gain from spending time in a greener environment are significant.

In particular, each year, one in four people experience a mental health problem. One factor that can influence our mental well-being is access to and connection with nature, as research from Exeter University shows.

At the same time, wildlife can benefit from being able to move between green spaces, linking up habitats within cities and to the wider countryside.


So, to return to an infographic – this time showing the many amazing benefits of green spaces. SPONGE2020 is all about using nature to adapt to climate change, but is also about much more than this. It is about creating spaces which people will love, and which will positively impact our lives in both visible and less visible ways.

But its not just big parks and nature reserves which make a difference! Raingardens, ponds, wetlands, and other ‘Sustainable Drainage’ (SuDS) features are green spaces where rainwater can collect & slowly soak into the ground. They can be built at any scale – even a water butt will make a contribution!

By creating lots of these features, all over our towns and cities, together we can reduce the risk of flooding, clean up our local streams, make more space for wildlife, and create attractive, green places for us all to enjoy.

If you want to be part of it, have a look at how you can get involved or what you can do at home.