Bangkok is sinking – fast. As urban development continues unabated, this city of more than 10 million people is getting lower by 2cm a year, according to Greenpeace estimates. Meanwhile, the surface of the Gulf of Thailand is rising by 4mm a year – above the global average.

With the Thai capital currently approximately 1.5 metres (5ft) above sea level, the spectres of the 2011 floods that inundated the city and those of 2017 that killed 1,200 people in India, Nepal and Bangladesh loom large. Recent rainwater floods, plus a United Nations preparatory meeting on climate change hosted in Bangkok, pushed concerns to the surface once more.

“When I was young I liked floods,” says the architect Kotchakorn Voraakhom, who was born in the city. “I pushed my little boat out and the road became a canal, it was such fun. But after 2011 everyone was like, ‘Oh. What used to be childhood fun has become a disaster.’ And it’s getting worse.”

In 2011, Thailand suffered its worst flooding in 50 years. Climate scientist Seri Suptharathit predicts the city will be mostly underwater by the end of the century. Photographs: Sukree Sukplang/Reuters/Paula Bronstein/Getty Images

Voraakhom’s ingenious answer was the 11-acre Centenary Park at Chulalongkorn University in the centre of the city. Hidden beneath the trees and grass lies its most interesting feature: vast underground water containers that, along with a large pond, can hold a million gallons of water.

Under normal conditions, water that is not absorbed by plants flows into these receptacles, where it is stored for watering during dry periods. When severe floods hit, the containers hold water and release it into the public sewage system after flooding has subsided.

What used to be childhood fun has become a disaster. And it’s getting worse Kotchakorn Voraakhom

Voraakhom and her architecture firm Landprocess will open a 36-acre park with similar water retention functions at Bangkok’s Thammasat University next year.

Centenary Park is also a welcome glimpse of grass in a metropolis that, according to the Economist Intelligence Unit’s Green City Index, has just 3.3 sq m (35.5 sq ft) of green space per resident. That compares with 27 sq m (290 sq ft) in London and 66 sq m (710 sq ft) in Singapore.

Dr Seri Suptharathit, director of the Centre on Climate Change and Disaster at Bangkok’s Rangsit University, says even more green has been turned grey since that survey took place in 2011. He says that in the past 20 years the amount of green space in Bangkok has dropped from 40% of total land to less than 10% – exacerbating flood risk.

Walking around Centenary Park, which opened last year on a site on the Chao Phraya delta previously occupied by university residential buildings, Voraakhom points out trees, and an education centre with a lawn roof and herb garden.

“One time last year we had six hours of heavy rainfall and all the roads around here were flooded,” says Voraakhom. “But the park still held the water.”

Chulalongkorn Centenary Park. During flooding underground containers retain water, which is released into the sewage system once flooding has subsided. Images: Landprocess

Her design ethos fits with the “monkey cheeks” water retention initiative pushed by Thailand’s King Bhumibol Adulyadej before he died in 2016. Just as monkeys stuff their cheeks with banana, saving the fruit mush for later, the monarch encouraged Thailand’s Ministry of Agriculture and Cooperatives to help prevent floods by utilising land areas that could temporarily store water.

Suptharathit believes current flood prevention in Bangkok is too reliant on hard structures such as dams and canals. Voraakhom’s park helps, and Suptharathit proposes paying more farmers to use their land for water retention during the July-October rainy season. “We don’t think about the use of nature enough,” he says. “Low-elevation rice fields, things like that. Before flowing to Bangkok much of the area [that floodwater travels through] is rural.”

For its part, the city government insists it is responding to increased concern about flooding. It recently announced 28 new flood prevention projects at a cost of 26bn baht (£610m). Flood barriers and underground tunnels are being built, and canals are being dredged and expanded.

Chulalongkorn Centenary Park. Bangkok has just 3.3 sq m of green space per resident. The figure compares with 27 sq m in London and 66 sq m in Singapore. Images: PanoramicStudio/Landprocess

Sakchai Boonma, director of the Department of City Planning for the Bangkok Metropolitan Administration, says a “green zone” east of Bangkok is already used to retain water for the city. Since 2013, he says, flood concerns have been better addressed by building regulations. “There must now be space for water to absorb into the ground [of new-builds],” he says. “Bangkok has been expanding in every direction … but city planning authorities are prioritising preventing and fixing flood situations.”

However, Suptharathit says the poor will still feel the brunt of severe flooding. They are likely to live in old buildings not well protected by new anti-flood measures, and are also at risk of being displaced by flood prevention redevelopment around canals. “The strong can survive with these big structures, but what about the normal people?” he says. “The water will come to them.”

Long-term forecasts by Suptharathit’s research team suggest Bangkok is likely to be hit by serious flooding roughly once per decade, and he predicts that by 2100 the city will be mostly underwater. Tara Buakamsri, Greenpeace’s Thailand director, says: “We just don’t know when a ‘worst-case scenario’ – surge from the sea, rainfall and flooding from the north, all arriving at the same time – will happen.”

What would an entirely flood-proof city look like? Read more

Voraakhom admits parks such as hers can only be a tiny part of the solution, but believes they help raise awareness and “show society what can be done in the next 100 years”.

She climbs onto an exercise bicycle fixed to the ground in Centenary Park. It serves the dual purpose of giving users a workout while also churning pondwater to prevent stagnation. “Is it too late?” she says as she starts pedalling. “I’ve no idea, but we have to do as much as possible.”

Follow Guardian Cities on Twitter, Facebook and Instagram to join the discussion, and explore our archive here