Credit: Scientific American

In low-lying, coastal Auckland, it rains a lot even in the driest months. There are three flood risk variables there, all at once.

The New Zealand metropolis is more resistant to floods and other hydrological catastrophes than many other metropolitan areas due to a number of natural features that allow excess water to flow away rapidly.

According to experts, this “sponginess” or capacity to suck water away from the surface has to be a crucial factor in urban development. Sponge cities will be better equipped to handle increased harsh weather conditions and the resulting rise in sea levels.

What is a sponge city?

According to a study by the professional services company Arup on 10 significant cities, Auckland tops a list of “spongy” cities. Instead of merely relying on old structures like pipes and pumps, sponge cities operate in harmony with nature to swiftly absorb heavy rains.

The proportion of blue (ponds, lakes), green (grass, trees), and grey (buildings, hard surfaces) architecture in a city determines how pliable it is. The possibility for water runoff, vegetation, and soil types all play a part.

The level of the soil and the distance to the water table can have an effect. Sandy soils are often softer than heavier clay-based soils. The soil’s ability to act as a sponge is diminished if the level of groundwater is near to the surface.

Sydney, Australia, which has barely 24% of its area covered by green or blue space and several parks outside of the city, is the least pliable city that Arup examined. The majority of the city’s centre is constructed, and as a result, its surfaces are made of impervious concrete. Sydney also has a reasonably high potential for runoff due to its relatively clay-rich soil.

How can sponge cities avoid floods?

Sponge cities have the capacity to absorb more water and distribute it more gradually into rivers and other waterways.

Our weather systems are already being affected by climate change, which is resulting in greater flooding, higher rainfall, and droughts. The consequences of this catastrophic weather will increase as global warming continues.

Flooding is a factor in 44% of all weather-related disasters worldwide, according to the World Meteorological Organisation.

Arup claims that we should quantify and value green and blue infrastructure, such as trees, grass, and ponds, and that we should even plan cities with sponginess in consideration. Compared to constructed alternatives, nature-based climate change solutions are often 50% more affordable and provide 28% more value added than grey infrastructure.

Urban greenspaces are under growing strain due to housing demand, which affects how pliable cities are. However, metropolitan areas from Cardiff in Wales to Shanghai in China are designing with floodwater control in mind. Shanghai is using its rivers to assist it better and to address its issues with urban drainage, while Cardiff has implemented “rain gardens” to help stop rainfall from overflowing sewage systems.

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