ROADS that trap water like a baby’s nappy could help keep the sweltering
streets of Tokyo cool, say the city’s engineers.
In the countryside, the air is cooled by water evaporating from leaves and
soil, and the vegetation also provides shade, keeping down temperatures. But
like many cities, built-up Tokyo doesn’t have much vegetation and as a result
suffers from the “urban heat island” effect.
In July, the city can be a few degrees hotter than the surrounding
countryside—often exceeding 30 °C. The materials used to build roofs,
roads and pavements compound the effect by absorbing more of the Sun’s rays,
causing both surface and air temperatures to rise.
Advertisement
So the Tokyo Metropolitan Government (TMG) is looking into ways to make the
city’s roads and paving produce less heat. Its engineers are developing road
surfaces that stop rainwater from seeping quickly away or gushing down drains.
Instead the road will soak up some of the rainwater and use it to dissipate
excess heat.
The TMG engineers are undertaking trials of two types of wet road surface.
The first is a semipermeable structure that slows down water loss. A highly
adhesive substance is used to bind the asphalt aggregate together, leaving
water-holding crevices within the asphalt. By containing the water in this way,
instead of steering it to the drains, it remains available for evaporation.
The second type of road surface contains water-trapping polymers similar to
those used in disposable nappies or diapers, says Yuko Nishida of TMG’s
environmental department. The road is cooled as water held in the top five
centimetres of the surface evaporates.
The heat island effect doesn’t just make cities uncomfortable to live in, it
can also trigger storms and floods. A rapid increase in daytime temperature
causes clouds to form, leading to heavy evening rain. So Yoshiro Tanaka of the
Japan Meteorological Agency says the wet road surface trials will also indicate
whether lowered temperatures can help prevent flash floods. “We have come to
experience severe flooding in metropolitan areas in Japan partly because of the
heat island effect,” Tanaka says.
Nishida says this is the first time any major city has considered using
water-permeable materials to cool down its roads and pavements.