Global Trends by Martin Khor

Star, Monday 29 January 2018

Create sponge cities to beat the floods

With floods now hitting Petaling Jaya as well, it is time to consider turning our towns into “Sponge Cities”, a trend ignited by China to absorb rainwater into roads, parks, rooftop gardens and other green spaces.


Last week it was the turn of Petaling Jaya, Gombak and Sungai Buloh to be the latest major urban centre in Malaysia to suffer flash floods. Scenes of cars and buildings submerged in muddy water are now occurring almost everyday. The focus should now shift from the bad situation to the solutions.

Last week I attended a briefing on the recent floods by civil society groups to Penang and Seberang Perai municipal officials and members.

Later, I came across several articles on how China is turning 30 of its flood-prone areas into “sponge cities” to prevent floods and retain rainwater.

The Chinese plan big and fast.  It launched the sponge city project only in 2015, but they target that 70% of rain will be retained in 80% of urban areas by 2020.

The “sponge” concept will spread rapidly as key to the global efforts to reduce the impact of increased rainfall and floods and climate change.

The sponge figured prominently at the briefing, chaired by Penang state exco member Chow Kon Yeow. Scientist Dr Kam Suan Pheng introduced it when explaining the floods.

She contrasted the original with the present situation when rain falls.  In the past, 50% of the rain seeps through the natural ground cover (trees, grass, etc) into the ground, there is 10% water runoff (to rivers and drains) and 40% evapotranspiration (water goes back to the atmosphere).  

The trees and green spaces act as a sponge to absorb the rainwater which infiltrates the soil, preventing the water from becoming flash floods.

Due to urbanisation, the green spaces have been paved over by cement and concrete.  Only 15% of the rain infiltrates to the soil, while the runoff expands to 55% and evapotranspiration is 30% of the total. The sponge now absorbs 15% of the rainwater compared to the previous 50%.

Dr Kam quoted the former Penang Water Authority general manager Kam U-Tee as saying that the October 2008 Penang floods were caused by conversion of the valleys into “concrete aprons that do not retain water which immediately flows into streams causing flash floods with moderate rainfall.”    

Given this analysis, a key part of tackling the floods is to reverse the loss of the sponge.  In recent decades, Malaysian has seen the conversion of a lot of farms, parks, trees and grass areas into “concrete jungles” of roads, houses, commercial buildings and car-parks.

There now has to be high sensitivity to the valuable environmental and economic roles of trees, gardens, fields and grass-lands, and parks. The aim of garden cities is not just to be pleasing to the eye but a very important part of development as well.

Now comes the role of “sponge cities.”   The world is applauding the Chinese initiative to counter its floods and increase water security by building up the natural cover or sponge in its cities.

In 2010, landslides from flooding killed 700 in three-quarters of China’s provinces and last year rains flooded South China, destroying homes and killing around 60 people.

In 2015, China launched the Sponge City initiative which now covers 30 cities, including Shanghai, Xiamen and Wuhan.  The target: by 2020, 80% of its urban areas will absorb and re-use 70% of rainwater.

The many types of projects include:

·         Constructing permeable roads that enable water to infiltrate into the ground;

·         Replacing pavements on roads and parks to make them permeable;

·         Building wetlands to absorb and store rainwater;

·         Constructing rooftop gardens (for example, 4.3 mil square feet in  Shanghai);

·         Plant trees on streets and public squares;

·         Build community gardens and parks to expand green spaces;

·         Build manmade lakes and preserve agricultural land to hold water.

“In the natural environment, most precipitation infiltrates the ground or is received by surface water, but this is disrupted when there are large-scale hard pavements,” said Wen Mei Dubbelaar, water management director at China Arcadis, in words similar to Dr Kam’s.

“Now only about 20-30% of rainwater infiltrates the ground in urban areas so it breaks the natural water circulation and causes water logging and surface water pollution,” said Wen in an interview with The Guardian.

In Shanghai’s Lingang district, the streets are built with permeable pavements, there are rain gardens filled with soil and plants, buildings feature green rooftops and water tanks, and a manmade lake controls water flows, reports The Guardian.

Prof Hui Li at Tongji University said the first thing is to preserve or restore natural waterways as that is the natural way to reduce flooding risk.  In Wuhan, the problem is that a lot of small rivers were filled in during building.  But Lingang still has agriculture land and a lake to hold more water during heavy rain.

What about the cost factor?  So far the cities have received over US$12 bil for sponge projects.

The central government funds 15-20% of costs, the rest is funded by local governments and private developers.

But compare this to the US$100 bil of direct economic losses due to floods in China in 2011-2014, plus the human lives lost.

Sponge cities are the way to go for the future.  Our own governments – federal, state and municipal – should study this option seriously, as the public braces itself for more floods ahead.