Klimaatslimme stad heeft meer groen

In the spotlight

Climate-smart city needs more green spaces

In the Netherlands, we are almost starting to get used to seeing cars getting stranded in flooded tunnels and the fire service pumping out cellars. Sometimes it rains so hard that the water has nowhere to go and the lowest-lying streets and neighbourhoods bear the brunt.

Of course, it's peanuts compared to the damage wreaked by a hurricane like Irma, the biggest hurricane ever to hit the Caribbean. But climate change means we can also expect more weather extremes on the European mainland: heavy showers or prolonged hot or dry periods. ‘As a result, cities will suffer more flooding and heat stress,’ says Tim van Hattum, programme manager for Climate at Wageningen University & Research.

How do we prevent flooding of streets and cellars and the attendant damage every time the heavens open? How do we ensure that in hot periods, the city is still a pleasant place to be and that people stay healthy? How do we keep the city liveable amid climate change? WUR is developing know-how around climate-smart, liveable cities.

An important part of the solution lies in making the city greener. Put bluntly: fewer bricks, more greenery. But why does green work?

Sewers

In some cities, up to 70 per cent of the surface area is built up and paved, and in some parts that figure can rise to 90 per cent. The soil basically acts as a sponge. But with all those hard surfaces, the water barely gets a chance to sink into the soil. ‘Cities are currently designed to dispose of the water’, explains Van Hattum. ‘However, the water peaks are getting bigger, and the drainage capacity is often insufficient to cope with that much water. With flooding as a result.’ Even if it is technically feasible to adapt the sewers to cope with the higher peaks, it remains questionable whether it is cost-effective.

Retention, storage, disposal

The approach to preventing flooding therefore consists of three steps: Retention, storage and disposal. More greenery in gardens, on industrial sites and in public spaces means more raindrops can infiltrate where they fall. Green roofs and wadis – lower-lying pieces of land such as a ditches or pits at the bottom of a slope to which rainwater is diverted before slowly sinking into the soil – can help. Step two, storage, is necessary if sufficient rainwater cannot be retained. Open water can be used for storage: ditches, canals and ponds.

Disposal is the final option, says Van Hattum. But instead of channelling this rainwater to the sewers, it can also be diverted to green areas around the city. One example is Het Waterrijk (Only available in Dutch) in Lingezegen park, a landscape park between Arnhem and Nijmegen. Just as a secondary channel relieves the river at high tide and provides scope for recreation and nature, redevelopment of city edges offers space for a combination of nature, recreation, water purification and water storage.

Heat

The problem with heat is that the city acts as a heat island due to all the buildings and streets and pavements. All that concrete and asphalt hold onto the heat. This might be nice in winter, but in summer it means the city barely cools down. And what's more, with every hot day the city heats up even more. Sustained hot weather in particular represents a health risk to the elderly, the chronically ill and people who are overweight. Children and others who are less able to look after themselves are also vulnerable.

Greenery has a cooling effect. Trees provide shade, while greenery results in evaporation, causing the local temperature to drop a little. It is no coincidence that people like going to the park when it is hot in summer.

More benefits

Besides fewer problems with water and heat, greenery has other benefits too. Greenery can retain particulate matter, which can improve air quality and absorb CO2, which is necessary for limiting climate change. Greenery also contributes to a positive experience and well-being on the part of residents and visitors and it invites you to exercise, play or do sports. In addition, research shows that houses overlooking greenery have a higher economic value. Finally, more green space naturally offers more opportunities for bees, birds and other urban nature.

The greening of cities improves their climate-resilience and liveability
Tim van Hattum, WUR

Small-scale solutions

In order to make the city more climate-smart, it really isn't necessary to dig it up and start again. ‘Actually, lots of improvements are possible on a small scale and every little helps. Everyone can contribute,’ states Van Hattum. ‘If you have a garden or a commercial building, replace paving with planting wherever possible, make your downspout discharge into a pit with plants that can handle a soaking, and when replacing a flat roof consider a green roof that can retain rainwater. There are also various types of water-permeable surfaces for car parks, for instance. As a neighbourhood, you might be able to persuade the municipal authorities to reconfigure a square as a neighbourhood garden or make a playground greener and more natural. That is also good for contacts between neighbours and the general atmosphere.

In new neighbourhoods and industrial sites, it makes sense to include greenery into the design from the outset. It would be a good thing if government agencies, companies and organisations were to include climate-resistance and suitability for the circular economy as standard requirements in procurement.

Every contribution counts

Naturally there are also incentive programmes and subsidies for climate-smart modifications, such as disconnecting the downspout from the sewers, says Van Hattum. ‘But the most important thing is to remember that every little contribution adds up. All the efforts put together can have a big impact.’