How punching holes in pavements could prevent flash flooding

Not just one, but two days in July 2021 saw over 2,000 homes and properties in London flooded with stormwater and sewage after biblical rainstorms. Newham Hospital and Whipps Cross Hospital were forced to evacuate wards, while Chalk Farm and Pudding Mill Lane were among the 30 stations deluged with floodwater.

As the climate warms, rainstorms with effects as extreme as this will only become more likely. Upgrading our Victorian-era sewers is one solution, although it’s a long-term and costly one. In the meantime, engineers are exploring other ways to drain waterlogged streets – such as punctuating pavements with holes. 

“At the moment, our cities are covered in impermeable surfaces,” says Dr Alalea Kia, an Advanced Research Fellow at Imperial College London in civil engineering and materials. This means rather than seeping into the ground, rainwater either pools on roads and pavements or flows directly into drains and overwhelms the often ageing sewers. The more intense the storm, the more likely that the sewers become backed up, causing flooding. “At the time [cities] were being planned, this effect of the climate emergency was not taken into account.”

When she started her PhD in 2014, Kia knew she wanted to do something with her research to help deal with the increasing flooding around the world. At the same time, she was fascinated by the material properties of concrete and cement that make them so ubiquitous. Concrete is the second most used material in the world after water, contained in most structures somewhere because of its strength, durability and ability to be cast into almost any shape. She had come across permeable concretes, designed to allow water to drain through, but noticed they weren’t widely used. So, she decided to develop her own.

Twisty flow

The main problem with permeable concretes is that they are easily clogged, explains Kia. Composition is key to this. Standard concrete is made by binding together small stones and sand with cement. In permeable concretes, omitting the sand creates small voids. It’s a little like filling a jar full of small pebbles: you would see gaps of varying sizes between the stones. If you were to add sand into the mix, it would fill much of the available space, making it harder for water to travel through. 

In practice, when rain falls on permeable concrete, rainwater seeps down through the gaps in the material, flowing in a twisting, convoluted path. Relatively quickly, in as little as a few months, sediment washed from the surface can block these tiny voids. And once blocked, there’s no unblocking them: pressure washing, vacuuming and sweeping can only clean the top couple of centimetres, says Kia. That’s not the only problem with these types of permeable concretes. Once inside, water can’t easily escape, so if it gets cold enough to freeze, it can expand and crack the concrete.

Kia’s idea, called Kiacrete, was to cast regular concrete around a grid of vertical recycled plastic tubes to create drainage holes. More recently, Kia and her research group have developed a version without the plastic tubes. These six millimetre-wide holes channel water away from the surface, allowing it to be quickly captured and then diverted to the sewer system or even reused. Importantly, any clogs are easily cleaned by a pressure washer, says Kia, noting that enough rain will also do the trick.

The exact way that rainwater is then reused depends on the soil underneath. If it is permeable, collected rain will flow into the groundwater (which is helpful for drought-prone regions). If the soil is already saturated, or impermeable – which might be the case if it’s heavily compacted, or rich in clay – the water will sit in a gravelly underlayer until the flooding has subsided. On new sites, a storage tank can be installed underneath, so that the rainwater can be funnelled elsewhere to flush toilets or irrigate gardens. 

The design benefits other types of extreme weather event, too. Thanks to the improved airflow through the channels, the concrete is less likely to crack in a heatwave.

You might think that filling a block of concrete with holes would make it weaker. And technically, you’d be right. However, Kia and her team have carefully tested different diameters and densities of holes to achieve a puddle-free pavement surface with high strength. This means it’s strong enough to turn it into footpaths, cycleways, roads and even the paving for airport taxiways. Any type of concrete can be used to form Kiacrete, with recent trials using low carbon and zero cement mixes, as well as pigmented concretes.

Piece by piece

Thankfully, digging up and replacing all of London’s roads and pavements – and the associated carbon footprint – is not on the cards. The idea is a more targeted approach, replacing sections such as the surface flooding hotspots that are typically lower in the landscape of a road or pedestrian footpath. In a recent trial at Imperial’s White City campus, Kia explains that a 10-square-metre section of Kiacrete created drainage for the whole front entrance of the site.

However, since roads require maintenance and resurfacing every seven years, there will be plenty of opportunities to install Kiacrete. In the long term, Kia says we should see a change in the pattern of where water flows, and fewer accidents relating to heavy rainfall and flooding, such as when cars hydroplane. Eventually, the plan is to take it to beyond the UK to other countries that regularly experience flooding. 

So, watch this space. Aside from the successful White City campus trial, the team is working with engineering consultancies, contractors and local authorities in the UK and internationally who are interested in testing Kiacrete. (But it’s a little too early to name them, says Kia. “Obviously it’s not concrete yet.”)