Utilising nature to increase the sponginess of cities for flood resilience

A potential answer lies in the ‘sponge city’ concept. This idea is not to physically use sponges but take inspiration from how they soak up water, creating cities filled with natural spaces and features capable of absorbing, storing and cleaning rainwater.

Sponge cities incorporate design elements that hold and slow rainwater between hitting the ground and re-entering water highways, thereby alleviating situations where heavy rainfall leads to overflow in those water highways, creating flooding events. This involves the implementation of green and blue infrastructure in the built environment, covering elements such as permeable pavements, green roofs and swales.

This model was first seen in China, but adoption of the sponge city model is increasing across the globe – and can be found in the UK already, with more than 100 ‘rain gardens’ having been built in Cardiff, which are designed to soak up 40,000 square metres of rainwater every year.

Increasing the sponginess of cities

With a name like ‘sponge cities’, it may be easy to assume this practice is a higher-level, more inaccessible one. However, it’s a much more common concept and very much something we’re already doing up and down the UK, albeit under different guises.

Today, whether looking at green or brownfield development, all development is regulated and without implementing measures that serve to reduce peak water run-off, planning simply would not be granted.

The ways to manage rainfall without causing flooding is two-fold – how can we return more water into the ground, and how can we slow the water returning to rivers and waterways?

Those working in development will be familiar with a variety of means such as SuDS and attenuation, but we need to find new ways of thinking too. There also needs to be focus on looking to utilise every available opportunity to retrofit suitable features within existing developed areas. For example, hammerhead turning areas in residential developments can be reconstructed in permeable surfacing to allow the surface water to soak away, without impacting on the robustness and usability of the road itself.

Elsewhere, storage and greenery can be implemented as effective tools that also serve to increase biodiversity and attractiveness simultaneously, such as the network of ‘Living Roof’ bus shelters in Leicester. These storage elements again reflect the ‘sponge’ name, with the idea being that once rain has landed on a feature, it is held and drip-fed back into the system – as opposed to traditional drainage that directs water back into the system very quickly.

The natural challenges

The biggest challenge lies – as always – in existing infrastructure and how we approach development across project teams.

While retrofitting solutions is difficult, it is not impossible. The trick lies in having a range of features and tools at your disposal to enable the selection of the most appropriate solution for any given situation, which thanks to more extreme weather is much more in flux than ever before.

The way land ownership works means multi-agency buy-in is needed. Wider teams need to engage early, fostering proper collaboration to agree and deliver the universal benefits without falling foul of squabbling over smaller elements. Crucially, this includes agreeing levels and responsibility of ongoing maintenance.

The measures most suited for this active water management are not ‘fit and forget’ elements – they need looking after. For example, rain gardens will need regular maintenance and replanting to remain as effective as possible.

An example of the importance of maintenance can be found at the redevelopment of St George Street in Leicester, which has created greener and more sustainable public realm in the heart of the city’s Cultural Quarter. The kerbs in this area incorporate small gaps at regular intervals to assist with water flow to the permeable areas behind – but these will only work if they remain unblocked by litter or leaves.

This also ties in an important education element with the public on water management features, what they do and how they are meant to work. People are quick to panic or assume something has gone wrong if they see water at ground level – particular with climate change seeing different and more extreme weather patterns. However, with greater implementation of SuDS this is more and more often by design, helping to effectively manage water systems and alleviate flooding or infrastructure damage.

By introducing more and more of these blue/green infrastructure elements in development projects – particularly storage ones – the public will become more familiar. But we have a job to do in communicating what the benefits are, along with the critical safety measures and considerations that come with introducing water features in public spaces while balancing how they help tackle the impacts of climate change.